JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. C2, PAGES , FEBRUARY 15, 1987

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1 -- JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. C2, PAGES , FEBRUARY 15, 1987 Large-Scale Structure f the Spring Transitin in the Castal Ocean ff Western Nrth America P. TED STRUB, J. S. ALLEN, A. HUYER, AND R. L. SMITH Cllege f Oceangraphy, Oregn State University, Crvallis Past measurements ff the cast f central Oregn and Washingtn have shwn that the rapid change frm nrthward mnthly mean winter winds t suthward summer winds frces a "spring transitin" f the castal cean: sea levels and temperatures drp, and mean surface currents shift frm nrthward t suthward. Current and water temperature data frm 35øN t 48øN frm 1981 and 1982, and sea level and wind stress data frm and , shw the transitin t have a large alngshre scale, typically 5 t 2 km; the large-scale wind stress appears t be th frcing mechanism at latitudes nrth f apprximately 37øN. Suth f 37øN, sea level usually falls mre gradually befre the nrthern transitin event. Bth wind and sea level events generally prgress frm suth t nrth ver a 3- t 1-day perid, but this is nt always true. Several aspects f the spring transitin reflect castal trapped wave dynamics. Previus studies at 45øN fund persistent vertical shear f the suthward summer current, assciated with a crss-shelf density gradient. During 1982 the shear and the density frnt develp ver the shelf break immediately after the transitin at 43øN and t the suth, but they are much less persistent than at 48øN. The strnger winds between 38øN and 42øN and the narrwer shelf result in an ffshre displacement f the density frnt and vertical shear past the shelf break, leaving the water ver the shelf less stratified and mre subject t bartrpic reversals f the current than that farther nrth, where the frnt stays clser t the cast. INTRODUCTION AND BACKGROUND Off the west cast f Nrth America the transitin frm winter t summer wind regimes ccurs as the Aleutian lwpressure system weakens and mves nrthwestward and the Nrth Pacific high-pressure system strengthens and mves t the nrth, resulting in suthward winds alng the west cast f Nrth America [Huyer, 1983]. Off central Oregn the respnse f the castal cean t this large-scale change in winds is quite rapid, as is dcumented in detail by Huyer et al, [1979]. At 45øN in 1973 and 1975 the winter regime in the castal cean cnsisted f high sea levels, mnthly mean nrthward flw ver the shelf, little mnthly mean vertical shear in the alngshre current, crrespndingly small crssshelf density gradients, and large barclinic fluctuatins in alngshre velcity with perids less than a mnth; the summer regime cnsisted f lw sea levels and suthward mnthly mean surface flw ver a nrthward (r weaker suthward) undercurrent, resulting in substantial mnthly mean vertical shear, with a strng mean crss-shelf density gradient. The fluctuatins with perids less than a mnth in summer were mstly bartrpic and weaker than in the winter [Kundu et al., 1975; Huyer et al., 1978]. The transitin between winter and summer regimes ccurred within apprximately 1 week; the sea level and currents changed mst rapidly fllwed by the establishment f the barclinic shear and crss-shelf density gradient ver a 5- t 1-day perid. Huyer et al. [1979] cnclude that the castal cean transitin is cntrlled by the lcal suthward winds, which cause suthward alngshre surface flw and ffshre surface Ekman transprt. This draws denser water nt the slping shelf and establishes the crss-shelf density gradient, which is assciated with the vertical shear f the alngshre current thrugh the thermal wind relatin. At 45øN in 1972, 1973, and 1975 the Cpyright 1987 by the American Gephysical Unin. Paper number 6C /87/6C-52 $5. summer regime f lw sea level and suthward midshelf surface currents (dwn t 4 m) was mre persistent than the suthward winds; i.e., sea levels remained lw, and currents usually remained suthward even during nrthward wind events frm late March t late July rkundu et al., 1975; Huyer et al., 1979]. Recently, data cllected frm 35øN t 48øN have been used t examine the nrth-suth differences in the seasnal cycles 125øW BARKLEY " (S ' SOUND V COOS BAY CRESCENT CITY EUREKA CCY CODE RY SAN FRANCISCO : RY ½SFO - ---WE-2' -I I 5 ø 5øN - 45 ø PURISIMA POINT i i % i k,/ X / Fi$. 1. Lc tins f curr nt m t r mrings (pen quar s) and s a 1 1 statins (slid ckcl s). S a 1 1 statins af (nrth t suth) N ah Bay (NBA; 48.4 N), Suth B ach (SBC; 44.6øN), Charlestn (CHR; 43.3øN), Crescent City (CCY; 41.8øN), Pint R y $ (PRY; 38. N), San Francisc (SFO; 37.8 N), Mnt r (MRY; 36.6øN), Prt San Luis (PSL; 3S.2 N), Ls An l s (LOS; 33.? N), and San Di $ (SDO; 32.8 N). Current m t r mrin lcatins ar Barkl y Sund (s named since it as th mst ffshre mrin n a lin ff Barkl y Sund, thugh it is sm dist nc frm Barkl y sund), British Clumbia, Canada (48.YN), Cs Bay, Or n (43.1øN), Crescent City, Califrnia (41.9 N), ur ka, Califrnia (4.9øN), Castal Ocean Dynamics Exp fim nt (COD ) (38.6 N), S n Francisc, C lifmi (37.4 N), d Pufisim Ptre, C lifmi (M.IøN). 4 ø 1527

2 I DAILY ALONGSHORE WIND STRESS I JUN DEC JUN D C JUN DEC JUN DEC JUN DEC IIIIIIIIIllllllllllllllll IIIIIIIIIIIIIIIIIIIIIIIII1,11111 IIII 'IT""-r ", '"',.,l; l,..,.,,...,. L N ' i...,.,i,. I,l..,..,.L..,.., -.I.,,. "ql/q "..."r' r" I "' ' "."'..t..-'"''"'"1- F-' 'r ',l,-!-pr r-- r,.," Fr','-' -. --,..'.r r r,,-[-ii.,,-.t',',... -"/ I. t.ll,,,... 4,,, f -.,l '. ø" I,,L,.[,., I..,,,.,k., i,,1 ' -4 4 s6'øø,,.,,.,l,,,,.i,,. 1,, 1, &l,,, i -4 F I '""'""' 'r ' "* '"'... "'r" 'r -4. N MM JUN DEC MM JUN SEP DEC MM JUN SEP DEC MM JUN SEP DEC MM JUN DEC 6 t! 4O 2, 48.4ON '6øN øN øN -'., N.,.,.,.,,,,,,....,.,.. tk, --' ß ß" ',.,,,.. V I'-w' -2 lr' '"'- 'r',w. '1 '-'" ",,,'r' 1' -2 2 ß,, '11' W' *'! " -- r, -,, MAR JUN DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN DEC DETRENDED DAILY SEA LEVEL Fig. 2. (tp) Alngshre wind stress, derived frm 6-hurly pressure fields, and (bttm) sea level, after the remval f the mean values and lng-term trends determined frm 9 t 34 years f mnthly data fr (left) and (right)

3 . STRUB ET AL.' LARGE-SCALE SPRING TRANSITION 1529 DAILY ALONGSHORE WIND STRESS ,.3 JUN SE:P DEC JUN SE:P DEC JUN SEP DEC JUN SEP DEC II II II I II II III II II I II I I I II II II I I I I I I II I I II I II I I 4 E --4 e 4 ' O I I I I I I I I I I I I I I I I I I I I I I I I II I I I I I I I I I I II I I I I I I I I I MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC MAR JUN SEP DEC "N '6øN -2 E øN 2-2 2: 36-6øN,.A,,.,,, k -2 2 C N C C N C DETRENDED DAILY SEA LEVEL Fig. 2. (cntinued)

4 153 STRUB ET AL.: LARGE-SCALE SPRING TRANSITION f castal wind stress, sea level, currents, and temperatures (Strub et al. [-this issue], hereinafter referred t as SAHSB). In the present paper the same data are used t lk at the largescale structure f the transitin event and the first few mnths f the summer regime. Fr the purpses f this paper the spring transitin is defined as a large-scale change frm the winter t the summer regimes, as is indicated by the rapid lwering f sea level, the switch t suthward surface currents with a mean vertical shear that is mre persistent than the several-day fluctuatins in alngshre wind stress and currents, and the upwelling f cld water that results in a persistent crss-shelf temperature difference. Since sea level and wind stress data are available fr 9 years and current and temperature data are availabe fr nly 2, castal sea level is used as the primary indicatr f the transitin. The date f the transitin is taken t be the time that sea level drps rapidly, ver a several-day perid, and stays lw at 41.8øN, a lcatin where the event is seen clearly in the sea level n mst years; fr the 9 years examined, the dates f the transitin in sea level at 41.8øN ranged frm March 22 t April 18. Questins abut the large-scale nature f the spring transitin include the fllwing: 1. What is the alngshre scale f the event as is seen in sea level, current, and water temperature bservatins, and hw is that scale related t the scale f the wind frcing? 2. Is the persistence in castal respnse greater than in the wind frcing, as was seen at 45øN previusly? 3. Is the sequence f events that define the transitin similar everywhere alng the cast, i.e., the tempral relatin between the nset r strengthening f suthward wind stress, the drp in sea level, the upwelling f clder water and develpment f the crss-shelf temperature difference, and the develpment f the suthward current and barclinic shear? 4. Is the castal cean's preference fr pleward prpagatin imprtant r necessary in driving the event, r can the event prgress equatrward as easily as pleward? sured winds, such as pr expsure and data gaps, and allws cmparisn f the 1981 and 1982 events t events in ther years when measured winds are nt available. The sea levels cme frm tide gages at sites with lnger histrical recrds that were used t detrend the data. Six such sites are available fr , and 1 are available fr (Figure 1). The atmspheric pressure has been added t the sea level height measurements t frm a subsurface pressure; the result is referred t simply as sea level. Current and temperature data are available frm mrings at midshelf (near the 9-m isbath) and/r the shelf break (13-m t 155-m isbath) between 35øN and 43øN (Figure 1), with nminal sensr depths f 35 m, 7 m, and 11 m. AT 48øN the data are frm 5 m, 1 m, and 15 m n a shelf break mring ver the 21-m isbath that is much farther frm shre than the ther mr- ings (Figure 1 f SAHSB). Currents and wind stresses have been prjected nt their principal axes, and the majr axis cmpnent is referred t as the "alngshre" cmpnent. Fr the 1981 and 1982 events, "limited fine mesh" (LFM) wind fields, cmputed by the Natinal Weather Service frm surface pressure fields with grid spacing f 19.5 km, were btained frm Dynalysis f Princetn [Blumberg et al., 1984]. The mring data frm 1981 sampled the transitin event (judged t be March frm the sea level data) at three lcatins (48øN, 42øN, and 35øN). Data frm 39øN began in early April, data frm 43øN and 37øN began in late April, and the 42øN mring failed in mid-april. Thus we have three widely spaced mrings at the time f the event and five mrings during mst f the summer. In 1982 we have data frm mrings at six lcatins (48øN, 43øN, 41øN, 39øN, 37øN, and 35øN) at the time f the event (April 15-2) and during the summer, thugh perids f missing data ccur. At 35øN, 39øN, and 43øN, data frm bth midshelf and shelf break mrings are available in 1982; at the ther lcatins, data are available frm nly a midshelf mring (shelf break at 48øN). DATA SETS RESULTS The data used t examine the large-scale alngshre structure f the spring transitin cnsist f lw-passed (c <.5 cpd) wind stress, sea level, current, and temperature recrds. The lcatins f the tide gages and mrings are seen in Figure 1, and all the data are mre cmpletely described by SAHSB, Winant and Bratkvich [1983], Brwn et al. [1983], Halliwell and Allen [1983], Denb et al. [1984], Winant et al. [1985], Irish [1985], Halliwell and Allen [1985], and Thmsn et al. [1985]. Nine years ( and ) f wind stress and sea level data were available fr analysis and are used in this study. The wind stresses were calculated frm wind velcities derived frm 6-hurly surface pressures n a 3 ø grid [Bakun, 1973], using bulk frmula [Large and Pnd, 1981] at 1 castal lcatins between 33øN and 48øN. The 18-km castal separatin f the lcatins is much less than the decrrelatin length scales fr these winds fund by Halliwell and Allen [this issue], wh als reprt high levels f statistical agreement between these pressure-derived winds and measured winds frm castal lcatins. Cmparisns f the pressure-derived and measured winds during the spring transitins indicate that the pressure-derived winds accurately reprduce the large-scale features f the wind frcing during these events. Use f these winds avids prblems encuntered with mea- Figure 2 shws the wind stress and sea level data at six lcatins fr the 9 years f data at lcatins between 34øN and 48øN. The mean values and lng-term trends determined frm 9-34 years f mnthly data have been remved frm the sea level shwn here and elsewhere. The spring transitin can be seen as the large-scale drp in sea level which generally ccurs in March r April. At 38øN and t the nrth, variability f the sea level with perids less than a mnth is reduced after the transitin; i.e., the lw sea levels are mre persistent in summer than the high sea levels are in winter. This can be seen in the mnthly standard deviatins f the 6-hurly sea level measurements presented in Figure 16 f SAHSB. The length f the lw sea level summer regime is greater in the nrth than in the suth, and the transitin is mst abrupt in the middle latitudes, 38ø-45øN, justifying the use f the time f the sea level transitin at 41.8øN t define the date f the verall transitin. The transitin can als be seen in the switch frm nrthward t suthward wind stress; nrth f 38øN, the variability f the wind stress with perids less than a mnth is greater in the winter than in the summer, as it is in sea level (Figure 16 f SAHSB). The shrt-term variability is relatively greater fr wind stress than fr sea level, making it difficult t chse the exact date f the transitin n the basis f the wind alne. The summer suthward wind regime lasts lnger in the

5 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION CALCULATED "r (dyne cm -2) FEB MAR APR MAY JUN , I 2 3, ' " II t 45.øN 41.8øN 4.2ON 56.øN 54.6øN CALCULATED "r (dyne cm -2) 1 2 MAR 25 3 APR 5 I I I I I I I I I I I I I I I -1 I I I I I I I I I-I I I i 48.2øN 45.øN 45.4øN 41.8øN 4.2øN 58.7øN 56.øN 54.6øN ill iiii, iiii, iiiil!l,llililll i i 48.4øN i 43.3øN 1981 SEA LEVEL 33.øN 44.6øN 41.8øN,11 i øN 35.2øN I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i i i i i i i i i i i i i i i! i i i! I I! i! i i i i , 5 MAR 1981 SEA LEVEL Fig. 3. Cnturs f (tp) alngshre calculated wind stress and (bttm) adjusted sea level. Statin lcatins are indicated by the latitudes n the right axes. (left) February-June 1981; cntur intervals are 1 dyn cm -2 and 1 cm (with an additinal dashed 5-cm cntur included); areas f psitive values are indicated by hatching. (right) One mnth centered arund the transitin: cntur interval are 1 dyn cm -2 and 5 cm; areas f psitive values are indicated by hatching. The nminal date determined fr the transitin (March 26) is indicated by the arrw at the tp f each panel. APR 33.øN 48.4øN 44.6øN 45.3øN 41.8øN 38.øN 36.6øN 55.2øN 55.7øN 32.8øN suth than in the nrth, ppsite t the pattern seen in the sea level. The magnitude f the wind reversal is greatest in the middle latitudes, 39ø-42øN, similar t the sea level. Interannual differences can be seen in the abruptness f the transitin, in nrth-suth timing and the alngshre scale f the transitin, and in the length f the summer regime. At 45øN the lw sea level values f the summer regimes are mst persistent in years 1973 and 1975, the years f the lng histrical current meter recrds used by Huyer et al. [1979] in their discussin f the transitin. The current and temperature data used in this study cme frm 1981 and During 1981, summer sea levels appear similar t the persistently lw 1973 and 1975 recrds, while the 1982 summer data appear less persistently lw. The high winter sea levels assciated with the and E1 Nifis can be seen in the data, especially at 38øN and t the suth. The transitin and summer regimes fllwing the tw E1 Nifis are quite different, hwever. The 1973 transitin was large-scale, and the lw sea level persists thrugh the summer, whereas the 1983 transitin was weak, and the lw sea levels were shrt lived, thugh winds remained primarily suthward. The reasn fr the difference in behavirs is nt knwn. Alngshre Structure 1981 AND 1982 SPRING TRANSITIONS Figure 3 shws cnturs f sea level and alngshre wind stress, ver the dmain frm 33øN t 48øN, fr the February- June perid f 1981 and an expanded view f the perid frm March 1 t April 9. On this year the date f the transitin at 41.8øN was March 26. Figure 3 shws this as the 16th day f a 3-day perid, a pltting prcedure fllwed fr the ther 8 years in Figures 5 and 16a-16g. The alngshre distance in these figures is measured alng a smthed castline [Halliwell and Allen,!983, 1985] and thus is lnger than indicated by the latitudes alne. In Figure 3 the fluctuating winds f February and March include a perid f suthward wind stress f 1-2 dyn cm -2 fr several days arund February 2 (35ø-41øN) and a mre extensive ne arund March 5 (34 ø- 43øN). These wind events temprarily lwer sea levels t 5 and 1 cm belw the mean, respectively. Between March 1 and 25 the winds nrth f 35øN cntinue t alternate between nrthward and suthward, with extremes f + 2 dyn cm-2 (Figure 3)' sea level fluctuates in respnse t the wind, with values f 1 cm arund the mean. Suth f 35øN, winds remain weakly suthward (except fr a brief reversal n March 14), and sea level slwly lwers t 1 cm belw the mean. The final transitin at 41.8øN cmes during a wind event n March that lwers sea levels ver a large (apprximately 15 km) dmain frm 33øN t 44øN, with a slwer and weaker respnse in the far nrth. The winds fields frm the time arund the transitin are presented in Figure 4, which shws LFM wind fields [Blurnberg et al., 1984] fr the regin between 3ø-42øN and 117 ø- 129øW fr March These indicate a diverging wind field, characteristic f the high pressure in the suth and the lw

6 1532 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION 42 WIND STRESS /25/81 1 DynelC' I. I. I, I,, WIND STRESS 83127/81 1 Dyn /C ' Fig. 4. Limited fine mesh (LFM) wind stress fields fr March 24-27, 1981, frm 3øN t 42øN, frm Blumberg et al. [1984]. pressure in the nrth. Daily surface weather charts shw a lw-pressure system passing thrugh the nrthern part f the dmain n March 24-25, fllwed by a nrthward spread f the high ver the entire castal regin frm Canada t Baja Califrnia, while the lw-pressure system stays ver the western United States fr several days. This high remains ffshre mst f the time fr the next several mnths, thugh lwpressure systems d pass thrugh the nrthern end f the dmain. The result is the summer regime f alternating winds in the nrth and suthward winds with fluctuating magnitude in the middle and suthern dmain, as is seen in Figure 3. The LFM wind fields during the earlier suthward wind event n February 15-2 initially resembled thse f Figure 3 but were weaker and ended n February 22 when the highpressure system mved nshre t a lcatin ver Nevada and Utah, a cmmn psitin fr high pressure in winter. The LFM wind fields during early March were different, with cyclnic curvature and winds that were suthward in the nrth and mstly nshre in the suth between 3ø-35øN. These suthward winds were caused by the western edge f a lwpressure system which remained ver the western United States fr several days, then mved sutheastward. In the suth, increasing high pressure ver the cean and lw pressure ver land kept winds there weakly suthward frm early March n, while strm systems cntinued t pass thrugh the middle and nrth f the dmain. The transitin event invlves the last lw t cause nrthward winds in the middle f the castal dmain, fllwed by the rapid spread f the highpressure system while the lw remains ver a large regin f the western cntinent, creating a several-day perid f strngly suthward winds ver the large-scale castal dmain. After the transitin in late March, maximum suthward

7 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION CALCULATED T (dyne cm -2) t FEB 2 t MAR 2 3 I MAY 2 3 t JUN 2 t 3 t 48.2øN CALCULATED T (dyne cm -2) APR 5!! *N 8OO 6OO 'N ON ON ' 4 4,2ON øN -2 56'øN -4 54'6øN øN -8 loo III I I I I! IIv!! *N 43.4"N 41.8øN 4.2øN 38.7øN 33.øN 34.6øN 48.4øN 8OO øN ON øN SEA LEVEL i i i i i i i i i I i i i i! i 5! 2 2, APR 1982 SEA LEVEL - 38.ON ON ON 33.7øN øN Fig. 5. Cnturs f alngshre calculated wind stress and sea level, as is in Figure 3, except that the right-hand panel cvers the perid April 3 t May 2, 1982, centered n the April 18 transitin. stresses f 2 dyn cm -2 usually ccur suth f 41øN, while maximum depressins f sea levels ccur farther nrth (Figure 3). At 45øN the wind alternates between suthward and nrthward, while the sea level stays lw, as is reprted fr spring and summer 1973 by Huyer et al. [1979]. Halliwell and Allen [1984] and Allen and Denb [1984] have shwn bth bservatinally and theretically that sea levels and currents at a lcatin ver the cntinental shelf respnd t winds that are 1 t 4 km suth f that lcatin. Thus the persistence f the lw sea levels (and suthward currents) seen at 45øN is cnsistent with the persistently suthward winds int.egrated ver sme distance suth f 45øN. Figure 5 shws cnturs f alngshre wind stress and sea level fr The transitin date fr this year is April 18. Again, there were suthward wind events during the February-April perid befre the transitin, specifically arund February 2 and March 1-25 (Figure 5). Nrth f 35øN, these were fllwed by nrthward events that were strnger than thse in Sea levels suth f 35øN drpped slwly during the light, mstly suthward winds f March, as they did in A lnger perid elapsed in 1982 between the lwering f sea levels suth f 35øN in March and the nrthern wind event that accmpanied the seasnal strengthening f the high-pressure system in mid-april. By the time this ccurred, the shrt, lw sea level perid was nearly ver in the suth, and the drp t -1 cm was nly seen nrth f 39øN (Figure 5). The LFM wind fields during the 1982 transitin perid were very similar t Figure 4, except that the mvement f the area f suthward winds t the nrth was nly half as fast. Surface pressure maps shw a lw-pressure system passing slwly thrugh the regin between April 1 and 15, fllwed by an enlargement f the high-pressure regin alng the cast, as ccurred in The picture afterward is mre cnfusing than that in 1981: the high mved nshre, causing the light nrthward winds arund April 2, but reestablished itself ffshre after a cuple f days, where it stayed farther nrth than it did in Figure 5 shws that the strngest (2 dyn cm-2) May and June suthward wind stresses were nrth f 38øN in 1982, as were the regins f lwest sea levels. Suth f the regin f maximum wind stress, the sea level at 38øN rse abve its mean value several times, and the lw sea level summer regime suth f 38øN was less persistent than in In the nrth (45ø-48øN), hwever, there were fewer perids f wind reversal, and the summer regime was strnger and mre persistent that that in 1981, at least thrugh June. Figure 6 (tp left panel) shws cnturs f the alngshre current fr February-June 1982 frm the upper current meter (35-4 m deep) ver midshelf (data frm 48øN cme frm 5 m deep ver the shelf break). Figure 6 (tp f right pane!) shws an expanded view cvering April 3 t May 2. These figures illustrate that the summer regime cnsists f persistently suthward currents nly at the 5-m shelf break lcatin at 48øN. Freeland et al. [1984] shw that nearby midshelf mnthly mean currents frm 48øN are weakly nrthward in summer, indicating that the data shwn here frm 48øN are characteristic nly f the shelf break. At 39øN, current reversals resume shrtly after the April 15-2 transitin event, as is dcumented in detail by Lentz [this issue]. Suth f

8 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION 35 rn ALONGSHORE V (crn/s) 1 FFB 2 1 MAR MAY JUN øN 1 35 rn ALONGSHORE V (crn/s) APR I I I I I I../I I I I I I I I ; I I I I I I I I I I I 48.5øN 8 8O 6 4OO øn 6 4OO 4.9øN < øN 45.1øN 4.9øN 58.6øN ON øN 45.1 øn 41.9øN 4.9øN 58.6øN 57.4øN 1 6O I I I I I I I I I I I I I I I I III II 54.7øN 48.5øN -45.1ON -41.9ON -4.9ON øN ON I 2.3 I øN -4 i i i i i i i i i I i i i i i i i i i i i 5 I APR -54.7ON m TEMPERATURE (øc) m TEMPERATURE Fig. 6. Cnturs f (tp) alngshre current frm the upper (35-4 m) current meter at each midshelf mring (except at 48øN, where the meter is 5 m deep ver the shelf break) and (bttm) temperature frm the lwer (65-7 m) midshelf temperature sensr (except at 48øN, where the meter is 15 m deep in 21 m f water). Mring lcatins are indicated by the latitudes n the right. Areas f psitive values are indicated by hatching. (left) Values fr February-June, 1982; (right) values fr April 3 t May 2. 43øN, fluctuating midshelf currents are typical f the 1982 summer regime, rather than atypical. Data frm after the 1981 transitin [SAHSB, Figure 1] als shw fluctuating currents at 43øN and t the suth in that year. Thus current reversals are seen frm 35øN t 43øN whether sea levels remain lw (1981) r nt (1982). Huyer et al. [1979] suggest that the spring transitin event ccurs at a given lcatin when the lcal winds cause enugh ffshre Ekman transprt t draw dense water ver the shelf frm deeper and farther ffshre and set up the crss-shelf density gradient. Thugh we d nt have salinity measurements, the temper.ature recrds serve as an indicatr f density variatins. Figure 6 (bttm) presents cnturs f the 1982 temperature data frm the lwer (65-7 m) instrument ver midshelf (15 m ver the shelf break at 48øN). The annual means frm the harmnic fits fund in SAHSB have been remved frm the recrds befre cnturing. Perids f suthward winds (February 5 and 2 and March 1-15) befre the transitin event result in water temperatures that becme slightly cler (.5øC) than the annual mean. The transitin event is characterized by a dramatic, large-scale 1ø-2øC drp in temperature. The mst persistent characteristic f the summer regime seems t be the presence f cld water ver the shelf. Figures 6 and 5 shw that alngshre velcity and sea level respnd rapidly t the wind stress in like manners. After the transitin, perids f nrthward currents crrespnd t rises in sea level, while temperatures remain lw. This is generally true f the entire April-June perid in 1982 (nrthward currents in Figure 6 crrespnd t perids f higher sea level in Figure 5). Figures 5 and 6 shw that the temperature drp at 7 m lags the wind, alngshre current, and sea level by 3-5 days, with the largest lag bserved in the nrth. This temper- ature drp marks the nshre mvement f clder bttm water. Repeated cnductivity, temperature, and depth (CTD) sectins alng a line extending ffshre acrss the shelf and upper slpe at 39øN shw this seasnal change in the temperature and density distributin ver the shelf [Huyer, 1984]. In 1982 this sectin was ccupied repeatedly befre and after the spring transitin [Fleishbein et al., 1983a, b, c]. Selected sectins (Figure 7) shw that the midshelf temperature at 7 m fell by 1. ø t 1.5øC between March 1 and April 24, while the density anmaly a, increased frm 26. t 26.5 mg cm-3. the 7-m density difference acrss the shelf was apprximately.9 mg cm- 3 n April 24, in cmparisn t apprximately.2 mg cm-3 n March 1. A mnth later, n May 26 and 27, bttm water ver the shelf still had abut the same density (fit 26.5) while shallwer waters ver the midshelf had increased in density (Figure 7), presumably because cntinued upwelling had advected the lighter waters ffshre well beynd the upper slpe. The regin f greatest hrizntal density gradient had als mved farther ffshre, leaving a crss-shelf density difference f nly.2-.4 mg cm-3. the midshelf regin had lst mst f its hrizntal and vertical density gradients. Figures 6 and 7 cnfirm that the transitin event draws water that is 1ø-2øC clder up nt the shelf all alng the cast t a much greater extent than any f the previus suthward wind events and that this cler water remains ver the shelf, at least thrugh June. The crss-shelf density difference at 39øN (Figure 7) immediately after the transitin is the same rder f magnitude (1. mg cm-3) as that reprted by Huyer et al. [1979] at 45øN in In 1981 after the transitin, bttm temperatures als stayed 1ø-2øC lwer than the annual mean until late summer, while alngshre currents alternated between nrthward and suthward [SAHSB,

9 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION Feb - I Mc r April Mc]y \ \ }1/ /i / 1 - JlOO ZGG /, KM FROM SHORE 2 4 / / O' Fig. 7. Crss-shelf vertical sectins f (tp) temperature and (bttm) a t at 39øN fr (left) February 28 t March 1, (middle) April 23 and 24, and (right) May 26 and 27, Cntur intervals are in degrees Celsius and milligrams per cubic centimeter [frm Fleischbein et al., 1983a, b, c]. Statin lcatins are indicated by the tick marks at the tp f each figure and the perpendicular signs ver the shelfi Figure 12]. Thus the current and temperature data frm the 2 years present a cnsistent picture f bttm temperatures that stay lw while alngshre velcities fluctuate. Time Histries f the 1982 Event at Individual Lcatins Figure 8 presents time histries f the wind stress, sea level, alngshre currents, and temperatures frm the shelf break and midshelf at 39øN fr February thrugh June The mring and instrument depths presented here are similar t thse at ther latitudes shwn belw and allw a cmparisn f the nrth-suth structure f the 1982 transitin. Lentz [this issue] gives a mre detailed accunt f the transitin at the 39øN lcatin. At 39øN the wind event at the time f the transitin (April 15-2) is quite similar t the earlier event arund February 2-25, as are the currents generated ver the midshelfi The drp in sea level and water temperatures are slightly greater fllwing the April transitin than after the earlier February event and may reflect the larger alngshre scale f the suthward winds during the transitin. In Figure 5 it can be seen that the -1 and -2 dyn cm-2 cnturs extend ver apprximately 14 and 5 km during the transitin, in cmparisn t 8 and 2 km, respectively, during the February 2 event. Strng nrthward winds after the February event reverse the upwelling prcess and return cnditins t the winter regime. At bth midshelf and shelf break mrings the lwest instrument is apprximately 2 m abve the bttm, and bth shw the rapid drp t belw 8øC during the transitin (April 15-2) in Figure 8, als seen in the April 24 sectin in Figure 7. At a given depth the decrease in temperature is greater at midshelf than ver the shelf break, and crss-shelf temperature differences f the rder f IøC develp at 35 m and 7 m. Vertical shear in the alngshre current ver the shelf break lasts at least until May 25, but there is nly a brief perid f vertical shear ver the midshelf befre the currents reverse immediately after the maximum suthward wind stress n April 18. Figure 7 shws the regin f maximum crss-shelf density difference t have mved ffshre past the midshelf by April 24, althugh it is still strng enugh at the shelf break t maintain the vertical shear seen there in Figure 8. The 35-m and 7-m currents at the shelf break d nt reverse and becme nrthward during the wind relaxatin f April 2-3, as they d ver midshelf where the vertical shear is absent. During much f the April 2 t May 2 perid the crssshelf temperature difference is greater at 25 m than at 7 m, and the vertical shear at the shelf break appears mre cnstant between the 35-m and 7-m currents than between 7 and 11 m. This is reflected in the April 24 density sectin (Figure 7), which shws the main crss-shelf density difference t be cncentrated abve 7 m. The vertical shear and the crss-shelf temperature difference drp t zer during a perid f lw wind stress and nrthward bartrpic currents in early May, then return and last until the end f May. Subsequently, the hrizntal temperature difference and the vertical shear becme intermittent, and the entire shelf experiences surface current reversals [Beardsley and Alessi, 1985]. Figure 9 shws data frm the midshelf mring at 41øN,

10 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION H! FEB MAR APR MAY dun ,3 1 2.,3 1 2.,3 1 2.,3 q E u 2 ß... A 2 SHELF BREAK V,,, ' :"' I ' / 1 SHELF BREAK T 8 MIDSHELF T [ hi. /'".., M I.D, E, b,: V//'g' I -'"-/'"'-' ' x.. v.-.. v MIDSHELF T c,ss-s,l, OT q., F -2 -! 2! 2 3! 2 3! 2 3! EUREKA 4.9øN - lo PT REYE S CODE 58.6øN Fig. 8. Data frm the mrings at 38.6øN (CODE) and nearby alngshre wind stress and sea level fr February-June Frm tp t bttm: alngshre calculated wind stress; alngshre currents frm the shelf break mring; alngshre currents frm the midshelf mring; temperatures frm the shelf break mring; temperatures frm the midshelf mring; crss-shelf temperature differences (midshelf-shelf break); and adjusted sea level frm Pint Reyes (38:'N). Mring data cme frm 35 m (slid line), 7 m (lng-dashed line), and 11 m (shrt-dashed line). apprximately 22 km nrth f the 39øN mring. The 41øN mring, althugh at the same isbath as the midshelf mring at 39 N, is farther frm shre (14 km cmpared t 8 km) and clser t the lcal shelf break than the lcal midshelf. The wind frcing here is very similar t that at 39øN. Lw sea levels persist fr the first 2 mnths after the transitin n April The difference between the strng sea level respnse t this transitin event cmpared t the weak respnse t the February 2-25 event is mre prnunced at 41øN than at 39øN. Vertical shear in the alngshre current persists fr at least a mnth after the transitin, and the currents resemble thse frm the shelf break at 39øN. The vertical shear dimin- ishes gradually and becmes negligible by late May, when currents are nearly bartrpic with nrthward fluctuatins. The perid f vertical shear frm April 18 t May 28 is characterized by initial rapid cling at 7 m (as is seen at 39øN) and mre gradual cling at 35 m. Data frm 43øN (26 km t the nrth f the 41øN mring) are shwn in Figure 1; the midshelf and shelf break mrings there are 11 km and 17 km frm shre, abut 3 km farther ffshre than thse at 39øN. Lcal wind frcing f the transitin event appears t be weaker than farther suth, but the sea level respnse is greater. Lcal suthward winds during the February 2-25 wind event are very weak, but there is a drp in the temperature at the deepest instrument ver the shelf Fig. 9. Data frm the midshelf mring at 4.9øN in Ntatin is the same as is in Figure 8. break and suthward currents are bserved ver bth the midshelf and shelf break; these may be respnses t the strnger frcing farther suth, indicated in Figure 5. The data frm the shelf break are cmplete until May 19 and shw that the vertical shear after the April 18 transitin appears first in the lwer water clumn (between 7 m and 11 m) and later... I,, , 2., ,... 2,,......,...,......,%,,,.',%,,lllll, 6-2,, 2-4,, 2 -I,/ 2, I J t v ' </ ' / ,... -? t ' v,- MIDSHELF V 7 g'--'-- SHELF BREAK 9 MIDSHELF T 7 N, -.4-, 7 m DT NA COOS BAY 43.3øN Fig. 1. Data frm the midshelf and shelf break mrings at 43.3øN in Ntatin is the same as is in Figure 8.

11 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION E 4 -r r -4 '-" , ' v vx / \// ""'""¾.: lo BARKLEY SOUND 48.5øN Fig. 11. Data frm the shelf break mring at 48.3øN in Meter depths fr this mring are 5 m (slid),! m (lng-dashed line). and 15 m (shrt-dashed line) Jn 21 m f water. Other ntatin is the same as is in Figure 8. at middepth (between 35 m and 7 m). N decrease f vertical shear r f crss-shelf temperature difference at 7 m ccurs by the end f the shelf break recrd n May 19. The vertical shear at 43øN develped mre slwly than at 39øN, and nce established, the vertical shear and crss-shelf temperature difference were mre persistent at 43øN than at 39øN. The current reversal at 35 m and 7 m seen at 39øN in early May is nt seen at 43øN. After May 19 the midshelf recrds shw a gradual decrease in the vertical shear in late May and early June as the 35-m temperature drps t within.5øc f the 7-m temperature. Unlike the midshelf currents farther suth, this weak vertical shear persists until the end f June. A CTD sectin ff 43øN n May 2 [Fleischbein et al., 1983c] shws a regin f strng crss-shelf density gradient ver the shelf break, unlike the diffuse gradients fund farther ffshre n May 26 and 27 at 39øN (Figure 7). The ffshre water at 43øN is much fresher and lighter than at 39øN, suggesting the imprtance f the Clumbia River plume in maintaining the crss-shelf density gradient at 43øN. At the 48øN mring (58 km nrth f 43øN), data frm the shelf break shw that the summer regime in currents and temperatures takes lnger t becme established, but cnditins then remain much mre cnstant (Figure 11). The shelf near this mring is much wider than at any f the ther mring lcatins, and the mring is 75 km frm shre [SAHSB, Figure 1]. As is nted abve, measurements frm midshelf at this latitude during previus summers shwed nrthward mnthly mean midshelf currents [Freeland et al., 1984], similar t the mnthly mean currents in summer at 43øN and 39øN seen in Midshelf currents at 48øN were nt availble during 1982 fr cmparisn t the ther currents presented here. Wind stresses at 48øN are similar t thse at 43øN, and the sea level respnse between April 15 and 2 is rapid, leading t persistently lw levels. The equatrward velcities at the time f the transitin are nt as strng as farther suth, and an immediate signal is nt seen in even the lwest temperatures (15 m in water 21 m deep), pssibly because f the time required t draw water up ver the large shelf and be mixed upward and utward t the 15-m shelf break lcatin. Temperatures drp very gradually, beginning first at 15 m in late April. Persistent vertical shear develps first between the lwer (15 m and 1 m) alngshre currents in early May and later between the upper (1 m and 5 m) currents in the secnd half f May' this vertical shear is very steady. Current and temperature measurements frm the mring at 37øN, which are nt shwn, cme nly frm the 7-m depth n the midshelf mring and are very similar t the 7-m midshelf data at 39øN. The picture in the suth at 35øN in 1982 (Figure 12) is different frm that between 37øN and 48øN. Winds in February thrugh mid-april are alternating but suthward in the mean, becming mre strngly suthward at the time f the nrthern April 15-2 transitin. During mst f April there is persistent suthward flw ver bth the midshelf and shelf break, persistent shear at the shelf break, and a persistent crss-shelf temperature gradient. Hwever, this perid f sheared suthward flw begins 1-15 days earlier than in the nrth, its nset des nt cincide with a drp in sea level (which is aleady belw the annual mean by April), and the crss-shelf temperature difference is caused by a rise in the upper shelf break temperatures, rather than a drp in the lwer midshelf temperatures. Warm water appears t mve nshre with a suthward velcity cmpnent befre the strnger suthward winds f the mid-april spring transitin and disappears afterward, leaving alternating nrthward and suthward currents during the strnger suthward winds f May and June. At midshelf there is a gradual cling by a.,1.,lo ' E ' ' -1-2 Fig. 12.,,,,,,,,,,,,,,,,,,I,,,,,,,,,,,,,,,,,, '...,I,,,,,,,;,,...,,,,,,,,,I,,,,,,,,,;, ' ,,... %,,,,,,,,,...,,,,,,;,... MIDSHELF V I-..,",,...,-. -'.,.,, " --.., , iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i PURISIIdA PT. 34.7øN Data frm the midshelf and shelf break mrings at 34.7øN in Ntatin is the same as is in Figure 8.

12 1538 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION ; Denb el al., 1984] indicate that the crss-shelf ature difference and vertical shear at the shelf break lasted? 2-1,,.,,,, * b w --2 v I 2 W' '-" V.!,3, -2 - SHELF BREAK 12 - SHELF BREAK - 4,. ---, i. r\/ - ß......,J , 1-,"... '""",,,.,,~ ,. -.,'...,..., 12 - MIDSHELF T -.., lo -.,..-, v " ¾" v TM ''' -. -,. d PORT SAN LUIS ASL ' PURISIMA PT. 54.7øN Fig. 13. Data frm the midshelf and shelf break mrings at 34.7øN in 1981, similar t Figure 12. ] -].5øC ver the 5-mnth perid, interrupted by the warming assciated with the suthward currents in April. Data frm the same lcatin in 1951 (Figure 13) shw mre nrmal castal upwelling picture, at least fr a brief perid arund the late March spring transitin. When winds becme strngly suthward abut March 25, currents at the shelf break becme suthward with a vertical shear between 7 m and 11 m, and sea levels and temperatures drp, with mre rapid cling at midshelf and a slight crss-shelf temperature gradient. 'This lasts nly until April 1, when currents resume alternating abut a nrthward mean in ppsitin t strnger suthward winds, while temperatures and sea level risc but d nt return t their prtransitin values. 'Thus in! 981 the behavir at 35 N culd bc said t be a briefer versin f that seen at 39øN in 1982: rapid nset and then decay œ a crss-shelf temperature gradient and vertical shear f the suthward alngshre currents alter the transitin, returning t alternating currents alter the shell' break temperatures have cled t apprximately the same values as thse at midshelf. Data frm mrings at 42øN and 48øN are als available at the time œ the 198] transitin and are cmpared t the data in Figure 14. The suthward currents increase and sea ]eve] and bttm temperatures drp at 35øN in apparent respnsc t suthward wind stress between March 21 and 24, while nrthward winds at 42øN and 48øN (Figure 3) cntinue t frce nrthward currents and high sea levels and temperaturcs there. Large-scale suthward winds n March (Figure 3) frce strng suthward currents and lwer sea levels at bth 35øN and 42øN (separated by 83 kin) and sn after at 48øN (anther 74 km t the nrth). Psttransitin current and temperature data frm these mrings and thers at 39øN and 43øN EWin m nd Br k ch, 1983; Brwn ½ l., temper- lnger than in 1982 (until the end f June r lnger) and were again strnger and mre lasting at 43øN than at 39øN. It appears that the 1981 spring transitin was f a larger scale (15 km, extending t 35øN), as is nted abve frm the sea level and wind data in Figure 3, and that the early summer regime was mre persistent in suthward currents and in vertical shear ver the shelf break than in In bth years the respnse was slwer and mre persistent in the nrth. DISCUSSION T extend the discussin beynd the 1981 and 1982 perids, alngshre wind stress and sea level data fr 9 years ( and ) have been used t frm a "cmpsite" mnth f data, shwing the average sequence f events frm 15 days befre t 15 days after the spring transitin. The date f each year's transitin was based n sea level data frm 41.8øN; that date was designated day 16 f a 3-day perid, and the 3-day recrds l'r all 9 years at each lcatin were ensemble-averaged t frm cmpsite time series, centered n the transitin. Cnturs f these wind stress and sea level time series arc presented in Figure 15. Cnturs f the 7 individual years nt previusly shwn arc presented in Figure 16. Alngshre Scale the Spring Transitin On the basis f the drp frm the - t the -5-cm cntur l' the cmpsite sea level data in Figure 15 as an indicatin f MAR t ' I ' I I I I I I I I I I I I I i ' 2-..,. - '/' -- '. / "-." -'" /'", /,' / '-., V APR...,,,,.,, ß,'. )',,, ,. '--- ' / NN ' '. T 1 'x /- -..,-- ''---.-./- ' -.N i i I i i i i i i i i i i i i i i i i i i i i i i i i i i / MAR Fig. 14. Cmparisn f alngshre wind stress (tp), alngshre velcity at 35-5 m depth (secnd frm tp), water temperature frm the bttm instrument (third frm tp), and sea level (bttm) fr March 11 t April 9, The mrings cme frm the shelf break at 48øN (shrt-dashed line), midshelf at 42øN (slid line), and midshelf at 35øN (lng- and shrt-dashed line) APR

13 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION E 4 2OO z IIIIIIIIII CALCULATED 'r (dyne cm -2) IIIl-IIII--IIIII--IIIIJ-- / 48.2øN 45.øN 45.4øN 41.8øN - 4.2ON 58.7øN smthed pressure fields n a 3 ø grid, they cannt represent small alngshre scales, and it is natural t ask if measured winds frm castal lcatins shw the same alngshre structure. Examinatin f measured winds frm [e.g., SAHSB, Figure 4b; Halliwell and Allen, 1985] shw that the wind events assciated with the spring transitin in thse years had alngshre scales f the rder f 1 km in 1981 and 1982 and 13 km in 1983 and were similar in structure t the crrespnding events in the pressure-derived winds OO IIIIIIIIIIIIIIIIIIIIIii IIIIIIIIIiiiiiiiiiiiiii!1iiii MAR g YEAR COIdPOSITE -- SEA LEVEL - 36.ON 34.6øN 33.øN 48.4øN 44.6øN 41.8øN 37.8øN 36.6øN 33.7øN Fig. 15. Cnturs f the cmpsite average time series f alngshre calculated wind stress and adjusted sea level, frmed frm 9 years f data ( and ), centered n a date chsen fr each years's transitin event at 41.8øN, which appears here as March 16 (nt the real date). Areas f psitive values are indicated by hatching. Cntur intervals are.5 dyn cm -2 and 5 cm. Statin lcatins are indicated n the right by their latitudes. the spring transitin, the alngshre scale f the respnse is f the rder f 12 km r mre. The strngest respnse, indicated by the -- 1-cm cnturs, has a smaller scale f rughly 8 km. The alngshre scale f the wind stress assciated with the transitin is larger than the sea level respnse defined abve' 18 km fr the --1 dyn cm -2 cntur and 15 km t 13 km fr the -1.5 and -2 dyn cm-2 cnturs. Using the 9 individual years seen in Figures 3, 5, and 16, the scales f the -1 dyn cm-2 and -2 dyn cm-2 cnturs have ranges f 15-2 km and 5-2 km, respectively' the scales f the --1-cm and -15-cm sea level cnturs have ranges f 2-2 km and -15 km, respectively. The 1981 and 1982 data (Figures 3 and 5) indicate that the alngshre scale f the strng suthward winds (-2 dyn cm -2) was 13 km in 1981 and 5 km in 1982, while the scale f the rapid drp in sea level t 15 cm belw the annual mean was 11 km in 1981 and 4 km in Thus differences between 1981 (strnger, larger-scale frcing and respnse) and 1982 (weak, smallerscale frcing and respnse) represent the nrmal variability seen in the 9 years f data, thugh nt the extremes f variability seen by cmparing the weak frcing and respnse in 1974 t the extreme frcing and respnse f The current and temperature data fr these 2 years als have alngshre scales in the 1-15 km range, indicated by the respnse frm 35øN t 48øN (ver 15 km)in 1981 (Figure 14)and 39øN t 48øN (abut 1 km) in 1982 (Figure 6). Since the winds used in this analysis were calculated frm Sequence f Events and Persistence f the Features The cmpsite picture f wind stress and sea level in Figure 15 shws the typical sequence f events seen in the individual years. Mnthly mean wind stress is climatlgically suthward all year suth f 35ø-37øN [SAHSB] and becmes mre strngly suthward sme time befre the main transitin, while wind stress in the nrth alternates between brief perids f suthward wind stress (usually less than 2 dyn cm-2) and lnger perids f strnger nrthward wind stress. Sea levels drp slwly in the suth and are frced dwn briefly in the nrth by the suthward events, nly t be frced up again by the nrthward events. The large-scale transitin nrth f 37øN ccurs as the regin f suthward wind stress mves prgressively nrth ver a 3- t 1-day perid, usually including a final nrthward event in the nrth. Fllwing the nrthward mvement f the suthward winds, a several-day perid f large-scale suthward wind stress f 2 dyn cm -2 r mre prduces the large-scale respnse in sea level. Wind fields fr the 1981 (Figure 4) and 1982 events have the same character, shwing nrthward (cyclnic) winds in the nrth and suthward (anticyclnic) winds in the suth and the nrthward mvement f the suthward winds ver the large-scale castal dmain. The cmpsite f 9 years f alngshre wind stresse shwn in Figure 15 indicates that a majr change ccurs in the castal alngshre winds and that this is fllwed rapidly by a large-scale drp f castal sea levels. This pattern f winds is cnsistent with the strengthening and nrthward spread f the Nrth Pacific high, fllwing passage f a late winter strm thrugh the nrthern part f the dmain. A detailed descriptin f the cause f this large-scale atmspheric transitin is nt readily available in the literature. Lahey et al. [1958] present 5-day mean sea level pressure maps frm thrughut the year, frmed by cmpsiting 2 years f histrical data accrding t calendar dates. These shw the weakening, splitting, and nrthward displacment f the Aleutian lw-pressure center (frm apprximately 5øN t 55øN) and the nrthward mvement f the Nrth Pacific high-pressure system in early April, in agreement with the range f dates fr the spring transitin determined here frm 9 years f sea level data at 41.8øN (March 22 t April 18). Lentz [this issue] als describes the mvement f these pressure systems (his Figure 21), n the basis f Brysn and Lahey's [1958] analysis f the 5-day mean sea level pressure maps shwn by Lahey et al. [1958]. The 5-day mean climatlgical pressure maps als shw the grwth f a lw-pressure regin ver Mexic in February that mves int the suthwest United States during March, causing the increase in suthward winds ver the cast ff Baja Califrnia that may be linked t the gradual drp in sea level ff suthern Califrnia. A clearer descriptin f the atmspheric transitin might be frmed by cmpsiting

14

15 STRUB ET AL.' LARGE-SCALE SPRING TRANSITION 1541 CALCULATED "r (dyne cm -2) MAR APR 'N CALCULATED -r (dyne cm -2) 15 MAR APr ' ' I I I I I I I I I I I I I I I I I I I I I I I! I I ', I I I I I I I i i i i i i i i i I I I I I I I I i I i i I MAR APR 1975 SEA LEVEL e 45.'N 45.4øN ON 4 v 4.2ON m ON 'N ON 'N 48.4øN 44.6'N 41.8øN 37.8øN 36.6øN 33.7øN v E 4 2 z in -2-4O -6-8,,,,,,,,, 'i' ;'i i i 1 i i I i1 i i i i.1 i i i i.1 i i i i i i MAR APR 198 SEA LEVEL / 45.øN 45.4øN 41.8øN 4.2øN 58.7øN 56.øN 54.6øN 33.øN 48,4øN 44.6øN 45.5øN øN - 58.ON ON ON ON øN CALCULATED -r (dyne cm -2) MAR APR I I I I I i i i i i i i i i i i-i i,,u i i i i i I i i 48.2øN,, 45.øN 45'4øN 4.2øN % - 38,7N / /- 36,øN 34.6ON I I I I I I I I I I I I I I I I I I I I I I I I l I i 48.4øN !. ; -36,6N ON ON i i i i i i I i i i i i i i i i i i i i i1 i i i i ON lg85 SEA LEVEL Fig. 16. (cntinued)

16 1542 STRUB ET AL..' LARGE-SCALE SPRING TRANSITION the large-scale fields f histrical atmspheric variables arund the time f the ceanic transitin (using sea level t indicate the timing f the ceanic transitin) as is dne in frming Figure 15. This wuld avid the smearing f features related t the event caused by averages based n calendar dates, since the date f the event varies frm year t year. The current and water temperature data indicate that suthward currents, vertical shear f thse currents ver the shelf break, upwelling f cld water, and a crss-shelf temperature gradient persist fr at least a few weeks fllwing the transitin in sea level. This is qualitatively similar t the sequence f events seen at 45øN in 1973 and 1975 I-Huyer et al., 1979]. The amunt f time it takes t set up the barclinic summer regime ver the shelf break and the length f time it persists there seem t increase prgressively frm 39øN (r frm 35øN in 1981) t 48øN. These time differences may be due t differences in the shelf width and/r the strength f the lcal wind. At 39øN the crss-shelf temperature gradient and vertical shear ver the shelf break nly persist fr the first mnth after the transitin r less and are variable even during this perid. CTD data frm 39øN in 1982 (Figure 7) shw that a crss-shelf density difference f the rder f 1. mg cm-3 seen immediately after the transitin in late April is reduced t values f the rder f.2 mg cm- 3 by the end f May, similar t values bserved befre the transitin. The May CTD sectin shws the density frnt t have mved ffshre f the shelf break and weakened. In 1981 at 39øN, crss-shelf density differences f.6 mg cm-3 in mid-april (after the late March transitin) are reduced t apprximately.2 mg cm-3 r less by late April, when the density frnt is again fund farther ffshre I-Huyer, 1984]. With the lss f the mean hrizntal density gradient and vertical shear, the bartrpic fluctuatins in the alngshre current result in surface current reversals ver the shelf. At lcatins farther nrth the crss-shelf den- sity gradient and mean vertical shear take prgressively lnger t develp at the shelf break, as is indicated by the current and temperature data frm 39øN and 43øN presented in Figures 8 and 1; nce established, the density frnt appears t be maintained lnger ver the shelf break. At the suthern end f the dmain (35øN), the current and temperature behavirs in 1982 are quite different frm thse in the dmain nrth f 3T N. During the larger-scale transitin in 1981 this regin des appear t be included in the nrthern regime fr a very brief perid lasting less than a mnth. The sea level data shwn in Figures 3, 5, 15, and 16 indicate that nly in a few years des the sea level suth f 37øN shw a sharp drp at the same time as the nrthern sites, thugh there are ften strng suthward winds in the suth. The lack f crrespndence between lcal wind stress frcing and sea level and current respnse arund 35øN has been nted by ther authrs [Brink et al., 1984; Brink and Muench, 1986; Denb and Allen, this issue], but the dynamics respnsible fr this behavir have nt been determined. Using simple mdels f sustained castal upwelling in the presence f wind-generated mixing and surface heating, de Szeke and Richman [1981, 1984] have shwn that the upwelling frnt can be advected several Rssby radii ffshre, leaving a regin f lw-density gradients between the cast and the frnt. This is the simplest explanatin fr the bservatins ff nrthern Califrnia, where strng suthward winds are ften sustained fr mre than a week and the shelf is usually narrwer than 25 km. The presence f the upwelling frnt ffshre f the shelf break is suppted by data frm an upper slpe mring ver the 5-m isbath at 39øN in 1982 [Winant et al., 1985]. At that lcatin, vertical shear between the 7-m and 15-m alngshre currents develped at the time f the transitin and lasted until at least the end f June, mre than a mnth lnger than the shear at the shelf break. In the extreme nrth (48øN) the shelf is 75 km wide, and the weaker alternating Ekman transprt makes it easier t maintain the density gradient and vertical shear ver the shelf thrugh the summer. An additinal factr ff central Oregn is the presence f lighter Clumbia River water ffshre in the spring and summer; its influence is strngest in the regin between 42øN and 46øN [Huyer, 1983]. This lighter water, alng with the alternating winds fund in the nrth, may help keep the density frnt clser t the cast ff Oregn than farther suth. Characteristics f the Lar te-scale Wind Frcin t and Respnse The cmpsite picture f wind stress and sea level in Figure 15 indicates that the spring transitin wind event is generally f large alngshre scale. The histrical data shw, hwever, that subregins f the dmain between 33øN and 48øN (apprximately 2 km)can be frced separately. In 1971 (Figure 2), winds became suthward and sea levels lwered suth f 38øN in February (the earliest date bserved), while they cntinued t alternate nrth f 38øN until April, dividing the cast int tw large-scale regins f 5 t 1 km. In 198 (Figure 16f) the sea level transitin ccurred at prgressively nrthward lcatins as a respnse t a sequence f three r fur 1- t 2-day events with alngshre scales f 8 km and larger, ver a 12-day perid. The transitin in sea level thus appears t be able t respnd t persistent wind frcing in regins f 5 km and larger. Temperature data frm 1982 (Figure 6) prvide evidence that large-scale wind frcing is mre effective in drawing up the denser water and setting up the spring-summer regime than smaller-scale events f similar magnitude. Suthward wind events f 1 dyn cm-2 result in temperature drps, f.5 ø t 1.øC arund February 5 and The alngshre scales f these wind events were f the rder f 4 km and 8 km, respectively. These temperatures increased again when the suthward winds relaxed but befre strng nrthward winds appeared, shwing their transient nature. This can als be seen in Figure 8, where the transient February 2-25 and final April 15-2 events can be cmpared at 39øN. The final transitin invlves much larger scale frcing and results in a mre rapid and permanent respnse in water temperature. Several features f the large-scale spring transitin suggest that it shuld be cnsidered a nnlcal event, with characteristics cnsistent with castal trapped wave thery. In Figure 15 the maximum respnse in sea level ccurs several hundred kilmeters nrth f the maximum in the suthward wind stress. This is similar t the characteristics f sea level re- spnse t smaller-scale wind stress events and is cnsistent with results frm castal trapped wave thery [Halliwell and Allen, 1981]. These small-scale suthward wind stress events cause upwelling and raise the lcal ispycnals ver the shelf, but the small-scale density anmaly created by the lcal upwelling can (accrding t shelf wave thery) prpagate away pleward and alngshre when the wind relaxes. Such an event is suggested in Figure 1 by the drp in bttm temperature (and increase in suthward velcities) ver the shelf

17 STRUB ET AL.: LARGE-SCALE SPRING TRANSITION 1543 break at 43øN in late February, apparently in respnse t the small-scale suthward wind event centered farther suth arund 39øN (Figures 5 and 8). In cntrast, a lage-scale (15-2 km) suthward wind event causes lcal upwelling and ispycnal displacements ver a large dmain. Nrthward prpagatin then maintains the upwelling regime in the nrthern regin f the wind frcing. If the wind als prpagates t the nrth, the respnse t the lcal wind may cmbine with the nrthward prpagating respnse t the winds farther suth t drive an even mre efficient upwelling prcess. With the relaxatin f the winds the water is mre unifrmly cld and dense ver a large regin alng the cast, and the predminantly alngshre currents and wave prpagatin will nt as easily alter the characteristics f the shelf water. Thus large-scale suthward wind events prpagating nrthward theretically prvide the mst efficient mechanism fr drawing denser water ver the shelf and shuld result in a mre persistent lcal upwelling regime than wuld winds f similar magnitude with smaller alngshre scales. K. H. Brink (persnal cmmunicatin, 1986) has pinted ut that ther characteristics f the spring transitin seen in this data are als cnsistent with cntinental shelf wave dynamics. If the spring transitin is thught f as a respnse invlving a cmbinatin f the first several castal trapped wave mdes, it can be shwn that the signal in pressure (sea level) will be dminated by the lwest mde. The higher mdes will cntribute smewhat mre t the alngshre velcity signal and prprtinately even mre t the density (temperature) signal. The higher mdes travel mre slwly and tend t smear the signal ver a greater distance and, cnsequently, ver a lnger perid f time at a given lcatin alng the cast. This is cnsistent with the lnger time required t see the spring and summer regimes in the temperature (Figure 6) nrth f the regin f strngest wind frcing, in cmparisn t the rapid respnse in sea level. Thugh large-scale nrthward prpagating wind systems are theretically mre efficient in frcing strng upwelling, the bservatins shw that they are nt necessary fr the creatin f a strng transitin. Examples f the transitin beginning in the nrth and extending t the suth can be seen in the histrical data. In 1973 (Figure 16c), strng suthward winds begin n March 25 in the nrth and extend t the suth ver the next 2 days, and the drp f sea level t 1 and 15 cm belw the mean als prgresses t the suth. In 1975 (Figure 16e) the transitin n March invlves the strngest wind frcing seen in the spring transitin events, with suthward wind stresses f 4-6 dyn cm-2 beginning slightly earlier in the nrth. Sea levels als drp slightly earlier in the nrth in this year, as is pinted ut by Huyer et al. [1979]. Examples can als be fund, hwever, f wind events prgressing frm nrth t suth and sea level respnding frm suth t nrth, as is seen April 15-2, 1972, and April 28-3, 1972 (Figure 16d). During many f the events in varius years the drp t the lwest sea levels appears t prgress frm Suth t nrth, even if the initial drp t 5-1 cm belw the mean prgresses frm nrth t suth (fr example, n March 26, 1975, and March 14, 1973). Thus the castal cean's preference fr pleward prpagatin can ften be seen in the sea level data, even when absent in the wind stress. CONCLUSIONS 1. The spring transitin in castal sea level, currents, and temperatures is a large-scale phenmenn, driven by the large- scale wind system. The alngshre scale f the suthward wind frcing f magnitude 2 dyn cm- 2 is typically f the rder f 15 km, as is seen in the 9-year cmpsite (Figure 15), with strngest winds between 38øN and 42øN; the scale f the frcing ranges frm 5 t 2 km in the individual years (Figures 3, 5, and 16). The alngshre scale f the rapid sea level drp t 1 cm belw the annual mean is typically f the rder f 8 km, as is seen in the cmpsite, centered arund 42øN; the scale ranges frm 2 t 2 km in individual years, with the scale f the sea level respnse varying in the same sense as the scale f the wind stress frcing. Suth f 37øN, the respnse t the wind event is weak, if present at all. 2. The initial sequence f events is similar at the lcatins sampled nrth f 37øN, fllwing the pattern seen at 45øN in 1973 and 1975 [Huyer et al., 1979]. Suthward winds ver a several-day perid frce lw sea levels and upwelling f denser water alng the shelf, which results in a hrizntal density gradient acrss the shelf and a mean vertical shear in the alngshre (suthward) current. The difference between lcatins is that this regime is less persistent ver the shelf and shelf break in the regin frm 38øN t 42øN, where the wind stress is greatest and the shelf is narrw. At these latitudes the upwelling frnt is mved ffshre past the shelf break in the first 1 t 2 mnths, leaving a mre hmgeneus water mass ver the shelf, with little mean vertical shear. Bartrpic fluctuatins then allw nrthward surface currents ver the entire shelf area. Nrth f 42øN, the weaker wind stress alternates between suthward and nrthward, the hrizntal density gradient and vertical shear persist ver the wider shelf and shelf break fr lnger perids (3-6 mnths), and surface currents remain predminantly suthward. Fresh water frm the Clumbia River plume may help t maintain the upwelling frnt clser t the cast nrth f 42øN. Suth f 37øN, the suthward wind stress becmes strnger, and sea levels begin lwering during the mnth preceding the strnger transitin wind event. A rapid lwering f sea level there ccurs at the same time as the nrthern transitin event nly in a few years. The current and temperature data frm 35øN shw the transitin event in 1981 but nt in 1982; the perid f suthward flw and lw sea levels lasts fr less than a mnth in 1981, thugh winds remain suthward. 3. The behavir f the surface winds during the transitin is related t changes in the large-scale atmspheric sea level pressure systems. The appearance f a lw-pressure system ver Mexic and its mvement int the suthwest in February and March, while high pressure builds ver the cean at the same latitude t the west, may result in the suthward winds and lwer sea levels suth f 37øN befre the nrthern transi- tin. The weakening and nrthward displacement f the Aleutian lw frm apprximately 5øN t 55øN in late March r early April accmpanies a strengthening and nrthward mvement f the Nrth Pacific high. The transitin event ften ccurs when a late winter strm passes thrugh the middle f the 33øN t 48øN dmain and creates an episde f strng nrthward winds there, fllwed by a spreading f the high-pressure regin and suthward winds alng the cast as the strm mves eastward. The strngest transitins may ccur when this strm remains ver the western cntinental United States fr several days, subjecting the castal regin t strng pressure gradients and suthward winds, as ccurred in The ability fr disturbances t prpagate pleward as castal trapped waves appears t give a nnlcal nature t the

18 1544 STRUB ET AL.: LARGE-SCALE SPRING TRANSITION large-scale spring transitin. Evidence fr this is prvided by the general ccurrence f the maximum respnse f sea level at an alngshre lcatin several hundred kilmeters nrth f the maximum in wind stress frcing and by the general nrthward prgressin f the sea level fluctuatins invlved in the adjustment ver several days fllwing the initial respnse. In 1982 the alngshre velcity and the temperature signal als prgress nrthward, and temperature prgresses mre slwly than velcity. The slwer respnse f temperature is als cnsistent with results frm mdels f castal trapped waves (K. H. Brink, persnal cmmunicatin, 1986). 5. It is nt necessary fr the wind event t prgress nrthward, and examples f suthward prgressin f frcing and respnse can be fund in the histrical wind and sea level data. The castal cean's preference fr pleward prpagatin, hwever, can be seen in the sea level data, in examples f events which prgress suthward in wind stress and nrthward in sea level respnse. This again demnstrates the nnlcal nature f the transitin. Acknwledgments. This wrk was supprted by NSF under grants OCE and OCE Cnversatins with S. Lentz and K. Brink were especially helpful during the writing f the paper. H. Freeland kindly supplied the Canadian data, and R. C. Beardsley supplied data frm 39øN. D. Denb, B. Halliwell, H. Pittck, P. Newberger, and C. Allessi helped prcess the current meter, wind, and sea level data. K. Daniels helped with the graphics. REFERENCES Allen, J. S., and D. W. Denb, Statistical characteristics f the largescale respnse f the castal sea level t atmspheric frcing, J. Phys. Oceangr., 14, , Bakun, A., Castal upwelling indices, west cast f Nrth America, , Tech. Rep. NNFS SSRF-671, 13 pp., Natl. Oceangr. and Atms. Admin., Seattle, Wash., Beardsley, R. C., and C. A. Alessi, CODE-2: An array descriptin f the surface wind and near-surface currents, in CODE-2: Mred Array and Large-Scale Data Reprt, Tech. Rep , edited by R. Limeburner, pp , Wds Hle Oceangr. Inst., Wds Hle, Mass., Blumberg, A. F., L. H. Kantha, J. H. Herring, and G. L. Mellr, Califrnia shelf physical ceangraphy circulatin mdel, appendix C.1, Atmspheric frcing, Rep. 88 C.1, 171 pp., Dynalysis f Princetn, Princetn, N.J., Brink, K. H., and R. D. Muench, Circulatin in the Pint Cnceptin-Santa Barbara channel regin, J. Gephys. Res., 91, , Brink, K. H., D. W. Stewart, and J. C. Van Leer, Observatins f the castal upwelling regin near 34ø3'N ff Califrnia: Spring 1981, J. Phys. Oceangr., 14, , Brwn, W. S., J. D. Irish, and A. W. Bratkvich, CODE-l: Mred temperature and cnductivity bservatins, in CODE-l: Mred Array and Large-Scale Data Reprt, Tech. Rep , edited by L. K. Rsenreid, pp , Wds Hle Oceangr. Inst., Wds Hle, Mass., Brysn, R. A., and J. F. Lahey, The march f seasns, Rep. 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Ocean Sci. 4, 12 pp., Inst. f Ocean Sci., Sidney, British Clumbia, Canada, Winant, C. D., and A. W. Bratkvich, CODE-l: Mred current bservatins, in CODE-l: Mred Array and Large-Scale Data Reprt, Tech. Rep , edited by L. K. Rsenreid, pp. 55-8, Wds Hle Oceangr. Inst., Wds Hle, Mass., Winant, C. D., U. Send, and S. J. Kentz, CODE-2: Mred current bservatins, in CODE-2: Mred Array and Large-Scale Data Reprt, Tech. Rep , edited by R. Limeburner, pp , Wds Hle Oceangr. Inst., Wds Hle, Mass., J. S. Allen, A. Huyer, R. L. Smith, and P. T. Strub, Cllege f Oceangraphy, Oregn State University, Crvallis, OR (Received April 28, 1986; accepted July 1, 1986.)