Physical Processes of Shelf-Open Ocean Exchange and their Influence on Upwelling Ecosystems

Transcription

1 Physical Processes of Shelf-Open Ocean Exchange and their Influence on Upwelling Ecosystems Jack Barth College of Oceanic & Atmospheric Sciences Oregon State University Marine Ecosystems and Climate: Modeling and Analysis of Observed Variability NCAR, Boulder, CO August 5, 2009

2 Outline coastal ocean fronts physical processes of shelf-deep ocean exchange wind-driven Ekman transport offshore, onshore intrinsic hydrodynamic instability eddies, jets and filaments flow-topography interaction wind stress curl high-frequency motions (internal tide) ecosystem impacts shelf hypoxia carbon export larval connectivity

3 Coastal Upwelling Ecosystems California Current Canary Current Humboldt Current Benguela Current 1% of surface area, but > 20% of wild caught seafood

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5 Two types of fronts isopycnal Retrograde e.g., buoyancy driven, shelf-break front, downwelling, water-mass boundary Prograde e.g., wind-driven, upwelling, western boundary current (e.g., Gulf Stream)

6 Courtesy of Jane Huyer (OSU)

7 Coastal Upwelling Front and Jet Barth et al. (JGR, 2005)

8 Coastal downwelling front and jet σ 23 Jan Jan 2003 E-W vel well-mixed S contours of north(red)-south(blue) current speed in cm/s T downwelling front downwelling jet N-S vel Courtesy of J. Barth (OSU) Distance (km)

9 Tidal mixing fronts in the Irish Sea Satellite SST on 12 July 1999 Sharples and Simpson (2001)

10 Tidal Mixing Fronts Deep water, tidal ellipses small shallow water, tidal ellipses large H/u 3 large H/u 3 small Tidal fronts occur when surface heating (or other source of stratification) is overcome by mixing these boundaries between cold and warm water can be seen from space

11 SST fronts h/u 3 = constant Simpson and James (1986)

12 satellite SST and chlorophyll on 12 July 1999 Sharples and Simpson (2001)

13 Probability density of SST fronts. (Ullman and Cornillon, 1999) Tidal Mixing Front on Georges Bank Shelfbreak front all along Middle Atlantic Bight

14 Shelfbreak Front Barth et al. (1998)

15 Wind Forcing and Large-Scale Circulation North Pacific High & Continental Low Huyer (1983)

16 upwelling Seasonal cycle of winds spring transition Sea-surface temperature

17 Seasonal cycle of winds Sea-surface temperature wind variability

18 wind-driven upwelling drives ocean productivity chlorophyll temperature Plankton bloom near Port Orford, Oregon Courtesy B. Menge of Ted (OSU) Strub (OSU)

19 2006 Massive phytoplankton blooms Courtesy of B. Menge (OSU)

20 The continental margin km ecosystem impacts on mid- to innershelf

21 What happens on the inner shelf? Adapted from Hart and Currie (1960)

22 Cross-shelf shelf circulation - Relationship between cross-shelf shelf flow and wind stress Given steady, linear flow with no pressure gradient, the along-shelf momentum balance can be approximated as (m 2 s -1 ) (Ekman, 1905) In shallow waters, where Ekman transport is not fully developed, this becomes: (m 2 s -1 ) (Lentz, 2001) Where: And: U = measured transport (m 2 s - 1 ) o ) = stress (Nm - 2 ) f = Coriolis (1.03x10-4 s - 1 at 45 o N) a = fraction of full Ekman transport present b = time-series mean transport (m 2 s -1 )

23 Lentz (2001); Kirincich et al. (2005)

24 but inner shelf can be efficiently flushed by time-dependent wind forcing u t fv A V 2 u 2 z v t fu A V 2 v 2 z Kirincich and Barth (JPO, 2009)

25 Average summer SST and SSH Strub and James (2000)

26 Submesoscale frontal instabilities and eddies 46 (relative vorticity)/f from numerical circulation model X. Capet and co-workers

27 Horizontal scales of variability Coastal capes and bays km Baroclinic instability traditional ~ mesoscale km 2 L L R R Rossby radius frontal ~ submesoscale km

28 Probability of detecting a SST front (4-yr average) from GOES imagery Castelao et al. (2006)

29 Probability of detecting a SST front (4-yr average) from GOES imagery Castelao et al. (2006)

30 Cross-shelf transport (mass, heat, salt, nutrients, carbon, larvae, eggs, ) ~20% of shelf production 5 events like this per year = benthic carbon mineralization rate Barth et al. (2002)

31 Flow-topography interaction off Oregon Heceta Bank

32 Flow-topography interaction off Oregon Barth et al. (2005)

33 The CoOP-sponsored Coastal Ocean Advances in Shelf Transport (COAST) project ( ) HF radars Moorings (ADP, T, S) graphic design: A. Kurapov lat, N Data Assimilation: Model + Data = Optimized Solution (3D+Time)

34 T ( C) ADCP velocity chl (mg/m³) 29 May 1 June 2001 plankton incubator contributes to hypoxia! Barth et al. (2005)

35 Flow-topography Interaction Astoria Hypoxia Zone Newport Coastal upwelling jet Heceta Bank Florence Coos Bay

36 Significant fish and Dungeness crab die-offs in 2002 Normal Inner-Shelf Rockfish Community B 44.7 A NH-Line Newport B Stonewall Bank C Latitude ( N) SH-Line Crab Mortality Classes 0% 1-25% 44.1 Heceta Bank HH-Line 26-50% 51-75% % Florence July Longitude ( W) Percentage of Pots Grantham et al. (2004)

37 Mesoscale activity and larval connectivity Barnacles: genetic similarity drifters released here drifters released here Sotka et al. (2004)

38 Numerical Model Princeton Ocean Model (POM) Domain: Extension: 250 x 400 km Resolution: 1.5 km Vertical resolution: 31 σ-layers N-S: periodic; W: open wind D Castelao and Barth (2006)

39 Several topographies used, differing in the value of D 2 2 N H Bu ~ 2 2 f D Rossby radius bank radius 2 2 If Bu small (Rossby radius > bank radius), flow follows topography If Bu large (Rossby radius < bank radius), flow can t follow topography Model forced by constant winds Ro v x u y f ~ nonlinear terms Coriolis acceleration If Ro small (weak flow), flow follows topography If Ro large, flow inertially overshoots topographic bends Castelao and Barth (2006)

40 Flow response to steady upwelling favorable winds Surface velocities and temperature Bu = 1.10 Bu = 0.38 o C = 0.12Pa larger bank D Sx Castelao and Barth (2006)

41 Sx Internal Rossby radius Sx Stronger upwelling winds Rossby number Oregon values D ~ 15 km Ro ~ Bu ~ Burger number Castelao and Barth (2006) Increase bank radius

42 Goal: Investigate the importance of curl-driven upwelling for jet separation 4-year average Chelton et al. (2004) (Aug 99 Jul 03)

43 Satellite wind curl w ^ k.( f ) Chelton et al. (2004) Perlin et al. (2004)

44 Nearshore wind curl influences coastal jet separation and fronts in the California Current System modeled winds 450 modeled circulation Day: 70 Day: C y km 250 Sx [ Note: angle of wind maximum to coast does not matter] x km x km Castelao and Barth (2007)

45 Need the cape to get jet separation; wind maximum with curl alone won t do it 80 basic case no cape Amount of separation (Sx) (km) S x cape + wind curl No cape Time (days) Time (days) Castelao and Barth (2007)

46 Wind maximum with no curl or uniform wind = delayed separation Amount of separation (Sx) (km) S x y km y km Basic case x km no curl x km uniform x km x 10 6 N m Pa cape + wind curl basic case no curl uniform Time (days) Time (days) Castelao and Barth (2007)

47 How might winds change? Seasonal cycle of winds spring transition Sea-surface temperature strength* direction seasonality* persistence *examples in next slides

48 Cumulative Wind Stress as measure of amount of upwelling ac-cumulate wind Spring Transition Fall Transition Huyer and Smith (1978)

49 Interannual variability in wind stress Spring transition 2005 Equatorward, Upwelling favorable Cumulative wind stress since Spring Transition 2000 Fall transition Barth/Pierce (OSU)

50 Interannual variability in wind stress Spring transition Equatorward, Upwelling favorable Cumulative wind stress since Spring Transition Fall transition Barth et al. (2007)

51 late, weak upwelling in 2005 led to warm ocean temperatures PISCO Barth et al. (2007)

52 late, weak upwelling in 2005 led to low nutrients and chlorophyll long-term average PISCO Barth et al. (2007)

53 and unprecedented low recruitment! mussels (Mytilus spp.) Cape Meares N Boiler Bay N Mytilus spp Fogarty Creek N Seal Rock N 2005 Long-term recruits/day Yachats Beach N Strawberry Hill N Jun-Aug: -83% Tokatee Klootchman N Cape Arago N Cape Blanco N Rocky Point N PISCO Barth et al. (2007) Month Long-term Month

54 and unprecedented low recruitment! barnacles (Balanus glandula) PISCO 2005 Long-term May-Jul: -66% Barth et al. (2007) recruits/day Cape Meares year x season p = Boiler Bay NS Yachats Beach year x season p< Tokatee Klootchman year x season p=0.045 Cape Blanco year x season p= Month Balanus glandula Fogarty Creek year p = season p< Seal Rock year x season p< Strawberry Hill year x season p=0.027 Cape Arago year x season p< Rocky Point NS Long-term Month

55 Lack of upwelling early in 2005 Upwelling favorable 2005 Led to poor plankton growth and marine dieoffs and poor salmon returns in 2008 March 14, 2008 Barth et al. (2007)

56 The culprit? Strong intraseasonal wind oscillations and an anomalously southern Jet Stream location Wind Stress (N m -2 ) day oscillations 44.6N = Oregon Barth et al. (2007)

57 The culprit? Strong intraseasonal wind oscillations and an anomalously southern Jet Stream location Jet Stream Position May 2005 July 2005 Barth et al. (2007)

58 Supercharged upwelling of two extremes in the last several years! Upwelling favorable 2006 Led to massive plankton blooms and hypoxia

59 Summary - Shelf-Deep Ocean Exchange wind forcing Ekman transport offshore, onshore curl-driven changing w/climate? intrinsic hydrodynamic instability eddies, jets and filaments flow-topography interaction ecosystem impacts shelf hypoxia carbon export larval connectivity