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 [email protected]

Indian Ocean and Monsoon

Indo-French Workshop on Atmospheric Sciences 3-5 October 2013, New Delhi

(Organised by MoES and CEFIPRA)

 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?

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).

Inter Tropical Convergence Zone (Jul-Aug, Jan-Feb)

Driving mechanisms of monsoon

Evolution of surface temperature in the ocean

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)

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.

Warm Bay of Bengal and cold Arabian Sea – Why?

SST < 28.0 °C SST > 28.0 °C

Short wave flux in August

Mooley and Shukla 1989)

AS Bay

Shortwave Net air-sea Sensible Longwave Latent

Meridional (blue)

Coastal (green)

Diffusion (red)

Sum (black)

LHS (blue)

RHS (red)

Shenoi et al (2002)

 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)

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

Energy required to mix the upper 50 m water column

10³ J m^-2

ERM in the bay, especially in the north, is much greater than in the AS

During June-December

ERM in the northern bay is ~ 12  10³ J m^-2 ERM in the Arabian Sea is < ~ 3  10³ J m^-2

Shenoi et al (2002)

Available turbulent kinetic energy

10^-5 W m^-2

ATKE for

mixing in the Aarabian Sea is an order of magnitude greater than that in the bay.

During summer monsoon

ATKE over the Arabian Sea is > 5  10^-3 W m^-2 ATKE over the bay is ~ 4  10^-4 W m^-2

Shenoi et al (2002)

Arabian Sea Bay of Bengal

strong winds (Findlater Jet)

weak winds

weak near-surface stratification

strong vertical transport

cool SST

weak convective activity

P-E < 0

strong near-surface stratification

weak vertical transport

warm SST

Strong convective activity

P-E > 0

Feed back cycle that lead to warmer Bay of Bengal

Shenoi et al (2002)

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.”

Large-amplitude basin-scale sub-seasonal sea surface

temperature (SST) variability in the Bay of Bengal (BoB), in which northern BoB cooling precedes monsoon breaks by about 1 week.

The basin wide averaged amplitude of SST changes is 1–2 C and local changes can exceed 3 C over 2 weeks. 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.

Southwest Monsoon Current

Shankar et al., 2007

Meridional SST gradient over the Bay of Bengal and growth and decay of convection over the Bay

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

Meridional SST gradient over the Bay of Bengal and

growth and decay of convection over the Bay

Joseph and Sivakumar (2004) used other data to point out that, on average, wind speed over “S” increases when it decrease over “NB”.

Seesaw of winds over the Bay of Bengal

• 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.

• During the years 1998-2005 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 build-

up of SST-gradient is variable (days to weeks).

2009 Monsoon and SST gradient

Francis and Gadgil (2009)

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

What controls the SST?

.

Shankar et al.,2004

Westward propagating temperature inversions

What controls the SST?

Does the ocean dynamics/thermodynamics play an active role in the heat budget of upper ocean, hence the SST?

Tendency (C/month)

J A S O N D J F M A M J

-1.5 -1.0 -0.5 0 0.5 1.5

Forcing

Total SST tendency

Vertical processes 1.0

Horizontal advection

SST (C)

27 28 29 30

31

SST

BLT

Barrier layer thickness (m)

0 10 20 30 40

- 0.3C vs +1.1C !

The inversions in the barrier layer heat the surface layer,

increasing SST

Durand et al., 2004

Evolution of Barrier Layer in the southeastern Arabian Sea

Temperature (ºC) Salinity (PSU)

Depth (m)Depth (m)

Upwelling

Inflow of high- salinity waters

BLT MLD

ILD

Shenoi et al., 2004

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.

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 ?

Conclusions

 There are several evidences to suggest that the

monsoonal phenomena precedes the variability of ocean

 Evidences to link monsoon variability with non- atmospheric 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