Analysis of the seasonal and interannual variability of the sea

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. C10, PAGES 22,937-22,946, OCTOBER 15, 1997 Analysis of the seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea from AVHRR data ( ) M. Ga id, S. Marullo, 2 R. Santoleri, 3 and A. Bergamasco 4 Abstract. Seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea is analyzed from the low-resolution advanced very high resolution radiometer data. The spatial resolution of 18 km allowed analysis of only basin and subbasin scale features. Average monthly and seasonal sea surface temperature fields for the entire studied period ( ) are discussed. The analysi shows the absence of any permanent sea surface thermal features in the Adriatic Sea. The south Adriatic sea surface temperature minimum presumably associated to the cyclonic gyre, previously considered as one of the permanent features, appears to be recurrent, being prominent only in late autumn and early winter, i.e., in the preconditioning and a deepwater formation phases. The major Ionian water inflow is documented in autumn while the thermal signature of the western surface outflow of Adriatic water appears most prominent in winter. The variability of the basin-wide thermal pattern in the Adriatic reveals four distinct seasons, which is different from both the eastern and western Mediterranean, where only two major patterns are recognized. A prominent interannual signal occurs in a northward extension of the warm water plume along the eastern coast, which in some years reaches the northernmost corner of the Adriatic, while in other situations it remains trapped in the south Adriatic cyclonic gyre. The surface thermal signature of the south Adriatic gyre also varies on an interannual timescale, and it was weak or completely absent during the period while it was rather prominent in the period A constantrend of sea surface temperature decrease in the center of the south Adriatic gyre and in the northernmost corner of the Adriatic was evidenced over the studied period. 1. Introduction The Adriatic Sea forms the extreme northernmost area of the Mediterranean Sea (Figure 1). The major part, especially the northern and central portions, is a shelf area with a depth inferior to 100 m. The southern part of the basin is characterized by a circular pit with a maximum depth of 1200 m. The Adriatic Sea communicates with the adjacent Ionian Sea via the 75-kin-wide and about 350-m-deep Otranto Strait. Climatic conditions in the area are characterized by relatively severe winters with violent outbreaks of cold and dry continental air associated with the Bora events. These events contribute to the net annualatent heat loss of about 20 W/m 2 [Artegiani et al., 1997]. These Bora outbreaks also stir up the entire water column over the most of the northern Adriatic area having a depth inferior to 50 m. Thus in winter the water column becomes vertically homogeneous because of direct wind mixing and a vertical convection associated with the surface buoyancy loss. Severe climatic winter conditions, subsequent surface buoyancy losses, and the presence of the relatively saline Levantine Intermediate Water (LIW) in the subsurface layer lead to bottom water formation processes. The water formed in the south Adriatic Pit outflows in the bottom layer of the Otranto Strait. The outflow is probably strongly linked to the LIW and Ionian Surface Water (ISW) inflow. Surface water, coming from the Ionian Sea, has a distinct thermal signature with respect to the Adriatic water, and thus its spreading can be easily traced from the satellite IR imagery. During the winter, ISW is warmer than Adriatic surface water while during the summer these differences are less remarkable, but ISW is still recognizable, being colder than the Adriatic water. The surface circulation pattern consists of a basin-wide cyclonic meander with several smaller subbasin scale cells embedded in it. Thus Ionian water inflow and spreading take place along the eastern coast, while Adriatic water outflow is pressed against the western coast and continental slope (see Orlid et al. [1992] for a detailed review). The Adriatic Sea is a dilution basin, with a total freshwater Osservatorio Geofisico Sperimentale, Trieste, Italy. gain being recently estimated at about 1.2 m/yr [Artegiani et al., 2Ente per le Nuove Tecnologie l'energia e l'ambiente, Centro Ricerche, Casaccia, Via Anguillarese, Rome, Italy. 1997]. The freshwater buoyancy input is associated with several 3Istituto di Fisica dell'atmosfera, Consiglio Nazionale delle major rivers. The strongest input comes from the Po river, Ricerche, Rome, Italy. which brings in average 1500 m3/s and represents about 30% of 4Istituto per lo Studio della Dinamica delle Grandi Masse, Consiglio the total riverine input. Another important freshwater source Nazionale delle Ricerche, Venice, Italy. is the Albanian rivers, which altogether have a discharge rate Copyright 1997 by the American Geophysical Union. of about 1000 m3/s. Riverine inflow influences to a large extent the surface thermal structure of the coastal area and some- Paper number 97JC /97/97J C times even of the basin interior of the northern Adriatic [Zore- 22,937

2 ß 22,938 GA(2I( ET AL.: SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA o N ANCONA /2' JABUKA'; P T MT CARCANe ' ø%% SOUTH ADRIATIC PIT tion of the large-scale climatic conditions [Zore-Armanda, 1974]. An important driving force of the exchange between the Ionian and the Adriatic is very likely the freshwater buoyancy input [Ga i et al., 1996], which generates horizontal density gradients and intensifies the Adriatic cyclonic circulation. The present study is part of a series of climatological studies on the Mediterranean Sea [Santoled et al., 1994; Marullo et al., 1997] and represents the first systematic analysis of long-term and seasonal variations of the basin-wide surface thermal structure of the Adriatic Sea using the AVHRR data. The NASA Ocean Data System multichannel sea surface temperature (NODS/MCSST) data used are coarser in scale (18 km, weekly averaged image) in comparison with the raw AVHRR, and thus only features of subbasin spatial scale can be resolved. The primary objectives of this paper are (1) to identify major subbasin scale thermal features and to analyze their timedependent nature; and (2) to discuss the basin-wide and subbasin scale upper thermocline circulation patterns and their seasonal and year-to-year variability in terms of the associated sea surface thermal signature. The paper is organized as follows. In section 2 the data set and processing are described. Section 3 describes in detail the seasonal variability of the Adriatic sea surface thermal structure and discusses the results of the empirical orthogonal function (EOF) analysis. Finally, section 4 contains the conclusions. IONIAN SEA I øE Figure 1. Adriatic Sea bathymetric map and its geographic position. Depths are given in meters. 2. Data Processing The NODS/MCSST used in the present work are 18 x 18 km weekly averaged sea surface temperature (SST) maps for the period January 1984 to December The data were obtained from the Distributed Active Archive Center (DAAC) of the NASA National Oceanographic Data Center (NODC) at the Jet Propulsion Laboratory. These weekly maps were produced by the Rosenstiel School of Marine and Atmospheric Sciences (RSMAS) processing the global area coverage (GAC) AVHRR data, which have been available since the beginning of the National Oceanic and Atmospheric Administration (NOAA) mission (September 1981). The GAC data, with a nominal resolution of 4 km, were processed with a multichannel algorithm to obtain an SST estimation for each location [McClain et al., 1985]. Cloud detection and tests to reduce the presence of anomalously warm pixels were applied. The weekly data produced by RSMAS were obtained by averaging all the available cloud-free daytime MCSST values. Because of the effect of cloudiness or other environmental Armanda and Ga6i, 1987; Bergamasco et al., 1996]. The impact of the riverine inflow in the surface thermal structure is prominent during the winter months (January, February, and March), when water of coastal origin has appreciably lower temperatures than the ambient seawater; the Po water can be colder by as much as several degrees than the ambient seawater [Sturm et al., 1992]. Less saline surface waters of continental origin, which have at the same time distinct thermal characteristics, flow southward in a narrow coastal layer whose width is of the order of 10 km along the Italian coastline. This flow is driven by the horizontal pressure gradient associated with the riverine freshwater discharge [Malanotte-Rizzoli and Bergamasco, 1983]. Mainly as a consequence of the temperature differences between the riverine and seawater, during the winter the coastal boundary layer has lower temperatures while during summer the coastal boundary layer has temperatures higher than the open sea surface waters. In addition, in summer the coastal boundary layer is thinner than in winter. However, even during the winter, when the freshwater is colder than the ambient sea water, the pressure gradient in the coastal boundary layer is dictated by the salinity distribution since the temperature gradient cannot compensate for the salinity contribution. Oceanographic conditions in the Adriatic Sea are subject to strong seasonal and interannual variations as a consequence of the meteorological forcing variability. During the summer season, water exchange with the Ionian gives rise to a net advective heat loss from the Adriatic. However, the winter period is characterized by an advective heat gain since the Mediterrafactors the resulting field was not completely filled. A Laplaclan relaxation technique to interpolate values into the data voids was used. For each weekly SST map an equivalent file containing information on the number of data utilized to estimate the averaged SST in each single pixel is given with the data. This enabled us to make all subsequent calculations taking into account the relative validity of data values. Santoled et al. [1994] found that a large portion of the Mediterranean Sea weekly maps (especially for the first 2 years) were interpolated and thus exhibited unrealistic bands of uniform SST values. They indicate that this banding was due to the application of the relaxation technique over much greater areas than the typical Mediterranean circulation features. Therefore, following Santoled et al. [1994], we decided to use only noninterpolated data in the following analysis. Discriminean waters are warmer than the Adriatic ones. Interannual nation between valid and interpolated pixel values was done on variations of the Adriatic oceanographi conditions are a func- the basis of the flag information given with the data.

3 GA I ET AL.: SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA 22,939 WINTER SPRING 84-92,(- _...ml"t&. """l ,.,..,.-'"'....,_ C13N'I'a _ %.,..,... ß -..,_,: / --.'-',... -.,.,.,,...,.,,.'-:...,'-,.,,,. / SUMMER oC ';., %;,..,.y.._... e C// hv½..., AUTUMN " 7'], ½J; ( --. ', <,.. }?...,, -... I -..,, ['e,,_ rift ,...,., ½ Figure 2. Average seasonal sea surface temperature (SST) maps for the period Temperature is given in degrees Celsius. All the valid data were grouped by months, and then monthly objective maps were produced. The monthly interval was a result of a compromise between a quite short sampling time and enough data to obtain an SST field completely filled with a reasonably low interpolation error. The objective analysis technique was used because it interpolates the field by taking into account either the SST values in neighboring pixels (as does the relaxation technique used by RSMAS to interpolate these data) or the past and future values (as does the compositing technique often used to eliminate cloud from satellite images). Moreover, this method also gave us the information on the interpolation error with which the field is known. Santoleft et al. [1991] and Leonardi et al. [1994] showed that this method, when applied to the AVHRR images of the Mediterranean Sea, reconstructs the field under clouds better than other declouding techniques. The objective analysi scheme relies upon knowledge of the structural characteristics of the field described by the correlation function. The monthly objective maps were produced using an exponential correlation function with an e-folding distance of 154 km. This function was estimated directly from the data set [Leonardi et al., 1994]. The error map was used to eliminate monthly maps with a too high interpolation error. As a result, the data from 1982 and 1983 were discarded. From January 1984 we obtained a low but rather uniform interpolation error field because of a strong increase in the number of data points available for each individual month. It was found that about 75% of the total number of points have an interpolation error smaller than 10% or more than 90% of points have an interpolation error smaller than 20%. 3. Climatological Characteristics of the Surface Thermal Structure 3.1. Seasonal Characteristics Four seasons are defined as in previous studies of the Adriatic, i.e., January, February, and March for winter; April, May, and June for spring; July, August, and September for summer; and October, November, and December for autumn. The monthly maps are averaged by season to construct seasonal maps and then averaged by years to compute the mean seasonal maps over the entire studied period (Figure 2). Winter shows large longitudinal sea surface temperature gradients with the extreme northernmost part of the basin being colder by 5øC than the Otranto Strait area. These dif- ferences are partly due to the riverine freshwater input bringing water colder than the basin interior and partly due to surface cooling. The cold water spreads along the Italian shore in a well-defined coastal plume, which is easily resolved even with a present spatial resolution of 18 km. The cold coastal

4 22,940 GA(2I( ET AL.' SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA CONTOIJR 21.4 TO 24.2 BY Figure 3. Average winter and summer temperature maps at 10-m depth from the in situ historical data set (reconstructed from Brasseur et al. [1996]). Temperature is given in degrees Celsius. boundary layer is evident as far as immediately to the south of Mount Gargano. Maximum horizontal temperature gradients are along the Italian shore and at the southern limit of the northern Adriatic, separating the cold coastal boundary layer together with the northernmost portion from the rest of the sea. The entire southern portion of the basin is much warmer because of the inflow of the ISW of higher temperatures than the Adriatic surface waters [Poulain et al., 1996]. In winter the surface thermal pattern appears smoother than in the other seasons without prominent subbasin scale features, probably because horizontal baroclinic scales (internal Rossby radius of deformation) are at their yearly minimum in that season (of the order of a kilometer according to Bergamasco et al. [1996]), and the present spatial resolution is appreciably larger than the mesoscale features. Spring is characterized by inversion of the horizontal temperature gradient and the occurrence of the warm water core in the area adjacent to the Po river mouth. Another warm pool occurs in the Jabuka Pit area, which is probably associated with an anticyclonic gyre of the upper thermocline circulation. Most of the previous studies (mainly from in situ temperaturesalinity measurements) gave evidence of a cyclonic gyre in that area [Zore, 1956; Mosetti and Lavenia, 1969; Paschini et al., 1993]. Only Limid and Orlid [1986] documented the presence of an anticyclone over the western Jabuka Pit. The south Adriatic Pit area shows smooth longitudinal isotherms, with the eastern shelf being warmer than the western coastal waters by IøC. The summer thermal pattern in the northern Adriatic remains very similar to the spring one, with a warm water pool situated in front of the Po river estuary. The warm gyre in the Jabuka Pit area is displaced westward with respect to the spring situation and becomes a meander. Colder water pools occur along the eastern shore in the central Adriatic and along the Albanian coast. In autumn, as a consequence of the net heat loss, the northernmost corner of the basin again becomes colder than the rest of the basin. Two cold pools, which are probably associated with the closed cyclonic circulation, are situated in the central and southern Adriatic. As opposed to the winter situation, the area of the maximum transversal thermal gradient is to the east of the two cyclonic gyres and separates the warm water plume along the eastern shore from colder open sea waters. In the northwestern portion of the basin, there are signs of the formation of the cold coastal plume, which, subsequently, in winter becomes the most prominent. Thus, from the above discussion of seasonal SST maps it follows that the south Adriatic gyre is not a permanent feature in the upper thermocline circulation but a recurrent one occurring only in autumn, probably as a preconditioning phase of deepwater formation. Signs of the cyclonic isotherm curvature and probably of the cyclonic circulation pattern occur also in winter, but no closed structure is evidenced, at least from the mean seasonal SST maps. This, however, does not preclude its presence in winter but only suggests that it cannot be evidenced from the average seasonal SST field and with the present spatial resolution. The SST pattern in the Jabuka Pit undergoes the strongest seasonal variation, changing from a closed local minimum in autumn to a local maximum in spring both being situated exactly above the pit. The remotely sensed infrared data provide estimates of the ocean temperature in the surface skin layer. To what extent these data are representative of a thicker surface layer and to what extent they give evidence of subsurface features can only be determined by comparing the satellite surface temperature field with in situ temperature data. For this comparison we use seasonal SST gridded data from a historical in situ data set [Brasseur et al., 1996], showing the winter and summer in situ temperature fields (Figure 3). One can see that the winter satellite-derived and in situ fields are indeed very similar and that the range of temperatures in both cases is almost identical. Certainly, this is related to the fact that during the winter in the Adriatic the water column is vertically homogeneous. In the summer one sees that both in situ and satellite data show the same subbasin features mentioned earlier, i.e., cold water patches along the east coast, a warm pool in front of the Po river mouth, etc., but the range of the in situ temperature is larger than that of the satellite data. This is due to the fact that the water column in summer is highly stratified. In that context one should bear in mind that the in situ SST maps are really obtained from the upper 10-m layer. This is the reason for the larger temperature range in the historical SST field. The comparison of satellite and in situ SST data for the other two seasons (not shown here) also gives satisfactory agreement with the main features noticed in the satellite-derived fields also being evident in the in situ fields. This is especially true of

5 GA I ET AL.' SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA 22,941 JANUARY FEBRUARY %'1 44.g 42.g 4g.g 12.o / l R,. - -,.. I.,,,,, '".,,. :.4' 7.4 I.2 14.e e 2.,_... CI 11'O. 'ro 14.4 ll¾ i l'.' l'" NARCH ß """l APRIL.. "--'T...,...,, -"-,--... "",-- _ ('i:':' : : ;"'"' Figure 4. Average monthly SST maps for the period Temperature is given in degrees Celsius. the autumn central and south Adriatic SST lows as well as of the warm water plume spreading northward along the east coast Monthly Variability From the average seasonal thermal patterns it was noted that the basin-wide longitudinal isotherm orientation occurs in winter and autumn, winter showing the presence of the western cold water plume and autumn showing the much wider area of the warm water plume along the eastern coast originating from the south Adriatic and Ionian Seas. Spring and summer, however, do not show any basin-wide correlated thermal pattern. In these two seasons the northernmost part of the Adriatic is characterized by the presence of the warm water pool caused by the net heat gain. Analysi of the monthly mean SST maps will give a possibility to analyze in more detail the yearly temperature variability. Indeed, monthly mean SST fields (Figure 4) suggest that the western coastal cold water plume is most prominent in January and February. The plume weakens in March, but it is still evident. The local SST minimum and a cyclonic curvature in isotherms in the south Adriatic is prominent January and to a lesser extent in February. For the entire period from May until August the presence of the warm water pool is evident in front of the Po river mouth. During this period the structure of the SST field appears rather fragmented. In June the eastern coast is warmer than the western one as is the case for the major part of the year. However, in July, August, and September, large patches of relatively cold water occur along the eastern shore in the central Adriatic area and at the Albanian shelf, which reverse the transversal temperature gradient, making the eastern shelf waters colder than the western ones. These patches are very likely associated to windinduced coastal upwelling [Bergamasco and Ga id, 1996] which, although being of transient nature, leaves a signal in the quasi-steady field as well. October represents an initial autumn situation since there are already signs of the formation of the warm water plume along he eastern coast and of the two temperature minimums in the Jabuka Pit and the south Adriatic Pit. In October the Jabuka Pit SST minimum is more intense than the south Ad- riatic one. Also, north of the Jabuka Pit minimum, there is an anticyclone (local SST maximum), which, subsequently, i n November and December, transforms into an anticyclonic meander. In December the south Adriatic local SST minimum strengthens and becomes more prominent than the Jabuka Pit gyre. Northward extension of the warm water plume reaches maximum in November, and the plume covers the entire length of the Adriatic Sea. Also in November, a colder water patch begiris to form in the extreme northwestern corner of the Adriatic while the northern part of the Italian shelf shows the first evidence of formation of the cold longshore plume. The occurrence of the most prominent warm water plume along the

6 22,942 GA I( ET AL.: SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA MAY JUNE i i i 46'ø1 i i i _.,'..-:":. -;"' l 'q. ih.. '"'"l i F _ g 16.g 18.g 2g.g 12.g 14.g 16.g 18.g 2g.g JULY AUGUST...- "gl 'I"N"" -i,. ' ' f" 2 ½,1 ß l.--" '"'"""1 :.-. - ; -.,. 0 " '"""-'"... '"......, i, ///,... 0 I 1171}1, ',2 I:I]N'I'I]I 24,2 1 :.8, B. 2. Figure 4. (continued) eastern coast in autumn is in a good agreement with findings a spatial pattern very similar to the winter temperature distrifrom direct current measurements in the Otranto Strait that bution, with the northern Adriatic and the western coastal suggest that the Ionian waters inflow reaches its maximum in boundary layer being out of phase with respecto the rest of autumn [Gafi et al., 1996; Poulain et al., 1996]. the basin (Figure 5a). The amplitude of the first EOF mode Summarizing, the 9-year sea surface thermal climatology shows that it mainly represents the seasonal signal with a maxclearly indicates the absence of any permanent SST structure imum in winter (Figure 5b). Values of the first mode spatial throughout the year in the Adriatic Sea. Even for the south structure in the northern Adriatic and coastal boundary layer Adriatic gyre, which presumably coincides with the found local are negative, which then results in a maximum negative tem- SST minimum, previously considered as permanent, our analperature departure from the spatial mean for that season. ysis clearly suggests its recurrent nature since it is prominent However, in the area of the south Adriatic, values of the first only in late autumn (December) in the preconditioning phase mode are positive, showing that in winter the south Adriatic is and in January in the deepwater formation phase. This, howwarmer than the basin-wide average. The temporal amplitude ever, does not preclude completely the permanent nature of of the first mode becomes negative in summer but with smaller the gyre since it can be controlled by the salinity distribution as well. absolute values than the positive ones. Thus, in summer the northernmost corner of the Adriatic Sea and nearby western 3.3. Empirical Orthogonal Function Analysis: Gradient coastal area are slightly warmer than the rest of the basin. Modes However, the central and Otranto Strait areas are colder, being EOF analysis was applied to the data set, with the spatial under the influence of the ISW. The temporal amplitudes of average from each individual image subtracted, and as a result, the first mode display rather strong interannual variations. An the so-called gradient modes [Paden et al., 1991] were ob- absolute maximum in the winter amplitudes occurs in 1987, tained. This method enables us to study seasonal and interan- with a subsequent trend of diminishing values of the peaks. nual variability and respective spatial patterns. The first four This suggests that in winter 1987 the northwest-southeast gramodes were selected for physical interpretation since it was dient of the SST was the strongest and that the value of this shown that they contain the signal which is greater than the gradient diminished subsequently. The largest value of the SST level of noise. This selection was carried out following the temperature gradient of opposite sign occurs in summer 1985 technique presented by Overland and Preisendorfer [1982]. and summer In other years, northwest-southeast summer The first EOF explains 72% of the total variance. It displays SST differences associated with the first mode are negligible.

7 GA I( ET AL.: SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA 22,943 SEPTEMBER OCTOBER _,,.,. '".., '.a '-,......>..., _ " '%.,..,._ _,,' ,, NOVEMBER 46.o DECEMBER '"'"'-'--...., _ Figure 4. (continued) TO , , The second gradient EOF mode explains about 7% of the total variance. Its spatial pattern contains the south Adriatic cyclonic gyre and a water pool in front of the Po river mouth (Figure 5a). Its amplitude as a function of time shows prominent minima in late autumn (December) or early winter (January) (Figure 5b), which is exactly the period when the south Adriatic gyre is present, as documented from mean monthly values. Second EOF values in the area of the gyre are positive, suggesting that the gyre is, as expected, colder than the rest of the sea, except for the water pool in front of the Po river mouth. One can also see that at the beginning of the studied period, from 1984 to 1986, the winter minimum is almost absent, which suggests that the gyre is very weak while in subsequent years it is well pronounced. Whether this means occur, one in May or June and the other in November. In both cases, eastern coastal waters, together with a water pool in the northernmost corner of the basin, are warmer than the rest of the basin. Thus the two maximums are associated to the early summer formation of the warm water pool in front of the Po estuary and to the autumnal intensification of the ISW inflow, respectively. The fourth mode explains only 3% of the total variance, and its amplitude shows a clearly increasing trend with rather small high-frequency variability (Figure 5b). Considering the spatial pattern of that mode (Figure 5a), the Otranto Strait area and the western part of the central Adriatic (off Ancona) are characterized by a long-term temperature increase. However, the central portion of the south Adriatic Pit shows a long-term that after 1986 the preconditiong phase and subsequent deepwater formation in the south Adriatic underwent important temperature decrease. Certainly, these long-term changes repchanges or not cannot be said. resent a very small portion of the total sea surface temperature The third mode explains about 6% of the total variance, and signal. These results can be related to a recent study of longits spatial pattern is very similar to the autumn situation when term temperature changes in deeper layers in the Adriatic Sea ISW inflow prevails (Figure 5a). The eastern shelf area is out from in situ data [Lascaratos, 1995]. A net increase in the of phase with respect to the western portion of the basin. The bottom water temperature in the south Adriatic Pit over the maximum values of the EOF spatial amplitude occur along the last three decades was documented. This may be associated Albanian coast. Another local maximum, although much with the increase of the SST along the eastern shore, as eviweaker, is evident near the Po river mouth. Amplitudes of this denced by the fourth EOF mode. More specifically, the exact mode as a function of time show half-yearly oscillations which location of the Adriatic Bottom Water (ABW) formation is not are strongly modulated (Figure 5b); they are prominent and known. However, if the bottom water is also formed by vertical are very regular in the period Two maximums convection along the eastern shelf, as can be hypothesized

8 22,944 GA I ET AL.' SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA FIRST EOF (72.glZ) 4g g i -.- ½'. -,'_ ;.,.- f..- ",_.,. \XN\.\ '"-'?;.:.'-.,, CGNTOI, -2.1 TO 1.1 BY SECOND EOF ( 6.9X). :.3 ' " -, _2' ';:,' ",,.'.., ?... _ -"; :.,...,. -- "'..,_., ß -,.,.,..x: '. -' ' ' ß,,2q,, "-Cg--'5-" : ' '-,.;;'- :' j..., %. ß - : c... ),,.,, ;,., ""'..'R, i '--..,... ' i "'"',,... FI I I -,4 TO.& BY THIRD EOF ( 5. I I FOURTH EOF ( 3.2X1 ½,,...:,._..,., ),...I,. " '..-'/' c_",.. 't";/' e 2"' _.,",,'/ ",.'" I [[!l(((q.. ¾.',-',.,:'.,.,, :.' ,, -..-.?.... _ 40. I -- ' K'.'-,,"-,L'---., "' ' '"--- :F... -.) 1 %.. r.'-,:z i.'_-.':... 5; ' -..._ / J ', --..,..,...,, Figure 5a. Empirical orthogonal functions of the first four modes, percentage of explained variance is given above each figure. from the results of the paper byartegiani et al. [1989], then the two phenomena can be associated with each other. Also, the net cooling trend of the bottom water in the Jabuka Pit, documented again by Lascaratos [1995], can be explained in terms of the SST decrease observed in the formation area of that water, i.e., in the northernmost corner of the Adriatic Sea. This feature is again evident from the spatial structure of the fourth EOF mode calculated in this paper. 4. Conclusions In the present paper the climatological surface thermal conditions of the Adriatic Sea are studied by analyzing AVHRR imagery for the period The spatial resolution of the SST data is 18 km, allowing studies of basin-wide and subbasin thermal patterns only. The seasonal temperature variations and some characteristics of the interannual signal are discussed. It is shown that the surface thermal pattern is highly variable on a seasonal timescale and that there are no permanent subbasin scale features persisting throughout the year. SST variations within a year clearly show the existence of four distinct seasons, which is not the case for either the eastern or western Mediterranean, where only two main patterns are evidenced (see Marullo et al. [1994] for the case of the western Mediter- ranean). The south Adriatic SST minimum, which presumably coin- cides with the cyclonic gyre, is only evident in late autumn (December) and early winter (January), i.e., in both preconditioning and deepwater formation phase. Also in autumn, another local temperature minimum of smaller dimensions is evident in the Jabuka Pit area (middle Adriatic). The surface thermal pattern over the Jabuka Pit undergoes especially large seasonal variations, changing from a local SST maximum in spring to a minimum in autumn. The warm water plume presumably of the Ionian Surface Water meanders to the east of the south Adriatic and Jabuka Pit local SST minimums and is most prominent in autumn, more precisely in November. However, the cold coastal plume along the western shore is most pronounced in the surface thermal structure during winter, especially in January. This feature was explained in terms of the Po river water spreading along the western coast. In the winter season, as shown earlier, the Po water is colder for several degrees than the ambient sea water and thus can be easily traced from the satellite IR data. The summer surface thermal structure (clearly evident in all three mean monthly SST patterns) is characterized by relatively cold and distinct water patches along the eastern shore in the middle and south Adriatic, which can be associated with the wind-induced coastal upwelling [Bergamasco and Ga i, 1996]. Results of the EOF analysi suggesthat the intensity of the western coastal cold water plume and the northern Adriatic thermal front vary from year to year. Other subbasin features

9 GAt I(2 ET AL.: SEA SURFACE TEMPERATURE VARIABILITY IN THE ADRIATIC SEA 22,945 FIRST EOF MOOE SECOND EOF MODE THIRD EOF MODE FOURTH EOF MODE -2 TIME IN MONTHS Figure 5b. The time-dependent amplitudes of the empirical orthogonal functions of the first four modes. also display interannual variability. The south Adriatic gyre was very weak or completely absent in the first 3 years of the studied period while it was very prominent in the period Whether this indicates changes in the regime of the deepwater formation processes cannot be said on the basis of the present data set but should be a subject of a separate study using in situ data. EOF analysis also suggests a long-term trend of temperature decrease over the 9-year study period, in the center of the south Adriatic gyre and in the northernmost shallow corner of the Adriatic Sea. Acknowledgments. This study was partially supported by the Italian national project PRISMA2. We also acknowledge the support from the European Commission's Marine Science and Technology Programme (MAST III) under contract MAS3-CT The data used in this work were obtained from the NASA Ocean Data System at the Jet Propulsion Laborato, California. We extend our appreciation to reviewers for very valuable comments. We also thank Corrado Fragiacomo for help in producing the figures. References Artegiani, A., R. Azzolini, and E. Salusti, On the dense water in the Adriatic Sea, Oceanol. Acta, 12(2), , Artegiani, A., D. Bregant, E. Paschini, N. Pinardi, F. Raicich, and A. Russo, The Adriatic Sea general circulation, I, Air-sea interactions and water mass structure, J. Phys. Oceanogr., 27(8), , Bergamasco, A., and M. Gaai6, Baroclinic response of the Adriatic Sea to an episode of bora-wind, J. Phys. Oceanogr., 26(7), , Bergamasco, A., M. Gaai6, R. Boscolo, and G. Umgiesser, Winter oceanographic conditions and water mass balance in the northern Adriatic (February 1993), J. Mar. Syst., 7, 67-94, Brasseur, P., J. M. Beckers, J. M. Brankart, and P. Schoenauen, Seasonal temperature and salinity fields in the Mediterranean Sea: Climatological analyses of a historical data set, Deep Sea Res., Part I, 43(2), , 1996.

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