Indian Ocean and Monsoon

Transcription

1 Indo-French Workshop on Atmospheric Sciences 3-5 October 2013, New Delhi (Organised by MoES and CEFIPRA) Indian Ocean and Monsoon Satheesh C. Shenoi Indian National Center for Ocean Information Services Earth System Science Organisation, Ministry of Earth Sciences, Govt. of India

2 For this talk on Indian Ocean and Monsoon, I would like to search for the cases in which the oceanic process influences the monsoonal process. Specific cases to show how the ocean influences the Monsoon. If the ocean influences the Monsoon, what is the prospect of using the ocean s inertia to predict behaviour of the Monsoon at least a few days in advance?

3 Driving mechanisms of monsoon May to October Hot Continent Giant Low Pressure Colder Ocean Sub-tropical High The fundamental driving mechanisms of the monsoon cycle are the cross-equatorial pressure gradients resulting from differential heating of land and ocean, modified by the rotation of the earth and the exchange of moisture between the ocean, atmosphere, and land (e.g., Webster 1987).

4 Driving mechanisms of monsoon Inter Tropical Convergence Zone (Jul-Aug, Jan-Feb)

5 Evolution of surface temperature in the ocean

6 SST Indian rainfall relationships Several earlier studies correlated SST in some or other part of Indian Ocean with All India Rainfall (for example, Shukla, 1975; Joseph and Pillai, 1984; Rao and Goswami, 1988; Sadhuram, 1997; Clark et al. 2000; etc.) Till recently, we believed that the role of ocean on monsoon like large scale atmospheric circulation is limited to some correlation between the SST and that the ocean plays a minor/passive role in determining the variability of SST. Clark et al (2000)

7 ITCZ hangs over the warmest areas of the tropical oceans. Distribution of sea surface temperature (SST) is determined by heat flux across the ocean surface and the processes within the oceanic surface mixed layer. Solar radiation is the most important contributor to the heat flux in the tropics.

8 Warm Bay of Bengal and cold Arabian Sea Why? SST < 28.0 C Short wave flux in August SST > 28.0 C Bay AS Mooley and Shukla 1989)

9 Shortwave Net air-sea Sensible Longwave Latent Meridional (blue) Coastal (green) Diffusion (red) Sum (black) LHS (blue) RHS (red) Shenoi et al (2002)

10 Cooler (deeper) northward moving waters upwell, mix with surface waters, turn eastward, then southward. The net result: cooling of the upper layer. The process much more active in the Arabian Sea than in the Bay of Bengal during the ISM. Shenoi et al (2002)

11 While the main difference between the budgets arises from difference in wind forcing, hence the currents in the ocean, stratification too is expected to play a role Precipitation rate during July (mm/day) Fresh Water discharge into the Bay

12 Energy required to mix the upper 50 m water column ERM in the bay, especially in the north, is much greater than in the AS 10 3 J m -2 During June-December ERM in the northern bay is ~ J m -2 ERM in the Arabian Sea is < ~ J m -2 Shenoi et al (2002)

13 Available turbulent kinetic energy ATKE for mixing in the Aarabian Sea is an order of magnitude greater than that in the bay W m -2 During summer monsoon ATKE over the Arabian Sea is > W m -2 ATKE over the bay is ~ W m -2 Shenoi et al (2002)

14 Feed back cycle that lead to warmer Bay of Bengal Arabian Sea Bay of Bengal strong winds (Findlater Jet) weak winds strong vertical transport weak vertical transport weak near-surface stratification cool SST strong near-surface stratification warm SST P-E < 0 weak convective activity P-E > 0 Strong convective activity Shenoi et al (2002)

15 This wonderful coupling between Indian Ocean and monsoon is seen in several instances. Vecchi and Harrison (2002), using new daily fields of SST and precipitation over the oceans showed that a break in the monsoon follows cooling of Bay of Bengal. Usually, the cooling in the northern Bay (1-2 C) precedes monsoon breaks by about 1 week. This raises the possibility that air sea interaction may be a significant factor in monsoon variability; the SST variability is coherent with monsoon variability with a phase relationship consistent with a coupled oscillation.

16 Meridional SST gradient over the Bay of Bengal and growth and decay of convection over the Bay Southwest Monsoon Current SST over S varies slowly and decreases almost monotonically. Over NB there are distinct oscillations. These oscillations determine the SST difference between the two boxes (and the meridional SST gradient over the bay). Shankar et al., 2007

17 Meridional SST gradient over the Bay of Bengal and growth and decay of convection over the Bay Shankar et al., 2007

18 Seesaw of winds over the Bay of Bengal Joseph and Sivakumar (2004) used other data to point out that, on average, wind speed over S increases when it decrease over NB.

19 SST in the south is controlled by the SMC, usually stays constant or keeps dropping. SST in the north oscillates: when monsoon is active it drops; when in break the SST rises. However, it is all the time above the threshold to support convection. During breaks wind in the north decreases, solar radiation increases, SST gradient over the Bay increases. Eastward geostrophic flow increases, convection increases. Monsoon revives.

20 During the years this picture seems to hold in about 80% cases: i.e. if delta-sst between NB and S exceeds 1-degree-C, there is 80% chance of monsoon revival over the Bay with precipitation exceeding 20 mm/day over NB during few days. But there are 20% cases when the picture does not hold. The time it takes for a revival to occur after buildup of SST-gradient is variable (days to weeks).

21 Francis and Gadgil (2009) 2009 Monsoon and SST gradient

22 Formation of warm pool in the Southeastern Arabian Sea Downwelling Rossby waves radiated by the coastal Kelvin waves deepens the mixed layer Arrival of low-saline water Stable stratified layer Heating due to atmospheric fluxes Shenoi et al., 19994

23 What controls the SST?. inversions x no inversions Westward propagating temperature inversions Shankar et al.,2004

24 Tendency ( C/month) SST ( C) Barrier layer thickness (m) What controls the SST? Does the ocean dynamics/thermodynamics play an active role in the heat budget of upper ocean, hence the SST? SST BLT C vs +1.1 C! J A S O N D J F M A M J Total SST tendency Forcing Horizontal advection Vertical processes The inversions in the barrier layer heat the surface layer, increasing SST Durand et al., 2004

25 Depth (m) Depth (m) Evolution of Barrier Layer in the southeastern Arabian Sea Inflow of highsalinity waters Upwelling BLT MLD ILD Temperature (ºC) Salinity (PSU) Shenoi et al., 2004

26 MoES-NERC collaboration - Impact of ocean-atmosphere processes in the Bay of Bengal on the South Asian monsoon The prime aim of this project is to understand the coupled ocean-atmosphere processes over the Bay of Bengal during different phases of the summer monsoon. This will be achieved by analysing critical new measurements, evaluation of long-term estimates of air-sea exchange and ocean storage, and investigation of processes and data assimilation in models.

27 Monsoon mission - Coupled physical processes in the Bay of Bengal and monsoon air-sea interaction The main objective is to understand the fine-scale physical processes in the ocean and atmosphere boundary layers, and their role in basin-scale air-sea interaction How does surface freshwater maintain its identity across seasons and mix with deeper layers in spite of a stable near-surface layer? What are the key mixing processes (tides, surface forcing, frontal dynamics, submesoscale processes)? The role of freshwater in basin-scale evolution of sea surface temperature (SST). Does the lateral salinity gradients force the near-surface currents forced? Why are air-sea gradients in the Bay so different from other warm oceans? Does the diurnal cycle rectify to longer scales?

28 Conclusions There are several evidences to suggest that the monsoonal phenomena precedes the variability of ocean Evidences to link monsoon variability with nonatmospheric processes Evidences to show that the ocean plays an active role in the monsoonal process Better understanding and predictive capability of oceanic variability will certainly improve our ability in predicting monsoon at various time scales

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