Indian Ocean Seasonal Cycle Jérôme Vialard (IRD) LOCEAN Paris France From Schott & McCreary (Prog. Oc.

Transcription

1 Indian Ocean Seasonal Cycle Jérôme Vialard (IRD) LOCEAN Paris France From Schott & McCreary (Prog. Oc. 2001)

2 Outline The monsoon cycle The basin-scale dynamical response Thermocline dome & South Equatorial Current Overview of IO currents (focus on Wyrtki jet, EICC) Upwellings Indian Ocean thermodynamics The Bay of Bengal: a dilution basin Seasonal cycle of SST in the northern Indian Ocean Consequences for biogeochemistry

3 The monsoon cycle Lands warms/cools much faster than the ocean => Asian landmass is much warmer than Indian ocean during NH summer & much cooler during NH winter This sets up an inter-hemispheric pressure gradient and planetary scale circulation The latent heat release by rainfall further reinforces the circulation

4 Maximum rainfall tend to occur over the sea Much more rainfall over Bay of Bengal than other Arabian Sea The monsoon cycle

5 The monsoon cycle During both inter-monsoons, maximum solar heating crosses the equator and leads to increased atmospheric convection in the eastern Indian Ocean As a result, the Walker circulation sets up again and gives rise to westerlies in along the equator

6 Outline The monsoon cycle The basin-scale dynamical response Thermocline dome Overview of IO currents (focus on Wyrtki jet, EICC) Upwellings Indian Ocean thermodynamics The Bay of Bengal: a dilution basin Seasonal cycle of SST in the northern Indian Ocean Consequences for biogeochemistry

7 Overview of IO circulation Winds completely reverse north of 10 S, due to monsoon. More steady easterlies south of 10 S. Ø While current patterns are relatively steady south of 10 S, there s a complete reversal of the circulation north of 10 S

8 The thermocline dome Winds completely reverse north of 10 S, due to monsoon. More steady easterlies south of 10 S. Ekman pumping results in 5-10 S thermocline dome. Relatively steady eastward current (SEC) to the south. Currents change direction north of 10 S (SECC during NH winter)

9 Wyrtki jets Well-defined westerly winds along the equator during both intermonsoon seasons Wyrtki jets are two well-defined eastward equatorially-trapped jets (with observed velocities > 1 m/s) that develop in October- November and May-June Wyrtki jet are associated with a K wave ( 3N3S) Currents at 0 N, 80.5 E (from Reppin et al. 1999)

10 The equator: Wyrtki jets The onset of the westerlies => K downwelling wave & eastward current

11 EICC (East India Coastal Current) Main current in the Bay of Bengal (principal pathway for exchanges between Bay of Bengal and equatorial region and Arabian Sea) A good prototype of a remotely forced current (Eigenheer & Quadfasel, 2000)

12 EICC (East India Coastal Current) Evidence for remote forcing of EICC (and WICC) from sea level and winds

13 Upwellings Large scale winds drive upwelling in the eastern Arabian Sea: Somalia & Oman upwellings

14 Upwellings Detailed structure and associated currents From (Schott & McCreary, 2001)

15 Outline The monsoon cycle The basin-scale dynamical response Thermocline dome Overview of IO currents (focus on Wyrtki jet, EICC) Upwellings Indian Ocean thermodynamics The Bay of Bengal: a dilution basin Seasonal cycle of SST in the northern Indian Ocean Consequences for biogeochemistry

16 The Bay of Bengal: a dilution basin Bay of Bengal: relatively small, semi-enclosed basin that receives a very important quantity of freshwater between June and October From (Chaitanya et al. 2014)

17 The Bay of Bengal: a dilution basin Large freshening of the northern BoB in the postmonsoon as a result Large gradient between surface very fresh water and saltier water below, due to the stabilizing effect of rainfall From (Akhil et al. 2014)

18 The Bay of Bengal: a dilution basin Barrier layer and temperature inversion: very weak cooling by vertical processes (and often even warming) Very shallow mixed layer: penetrative shortwave flux needs to be taken into acount t T ( ( )) Q 1 f h ρ 0 C P h ( ) T ( h) T + H w h + w e h ( ) +... From (Thadathil et al. 2007)

19 SST seasonal cycle in northern IO Similar surface heat fluxes into Arabian Sea and BoB Semi-annual cycle of net heat flux due to: Primarily, latent heat flux (strong evaporation during both the NW and SW monsoons) Also solar heat flux (seasonal cycle + cloud effect during SW monsoon) Notice the large part of sw flux lost below the mixed layer From (de Boyer Montégut et al. 2007)

20 Winter convection in Northern Arabian Sea Dry, cold air associated with NE monsoon wind induces strong evaporative cooling + wind stirring and Ekman pumping Ø Deep mixed layer in the Northern Arabain Sea during the NE monsoon From (de Boyer Montégut et al. 2007)

21 SST seasonal cycle in northern IO More pronounced semi-annual cycle in the Arabian Sea due to large cooling by vertical processes during the monsoon Very weak cooling by vertical processes (and even warming) in the Bay of Bengal From (de Boyer Montégut et al. 2007)

22 Outline The monsoon cycle The basin-scale dynamical response Thermocline dome Overview of IO currents (focus on Wyrtki jet, EICC) Upwellings Indian Ocean thermodynamics The Bay of Bengal: a dilution basin Seasonal cycle of SST in the northern Indian Ocean Consequences for biogeochemistry

23 Summary (1) Thermocline dome: open ocean upwelling (not coastal, not equatorial) 5-10 S in Western IO Steady circulation south of 10 S, reversal of the circulation northward Importance of remote forcing (equatorially forced waves that propagate into the coastal waveguide) for main currents in northern IO (EICC, WICC)

24 Summary (2) Bay of Bengal: the strong haline stratification tends to inhibit vertical mixing Arabian Sea: Strong upwellings in the Western Arabian Sea during the southwest monsoon Convective overturning in the Northern Arabian Sea during the NE monsoon

25 Consequences for biogeochemistry Weak exchanges between the mixed layer and nutrient-rich interior ocean lead to weak primary production in BoB Upwelling in Summer and convective overturning in winter lead to a much more productive Arabian Sea Chl From (McCreary et al. AGU monograph 2009)