576 research outputs found
LES_data_With_and_Without_Evaporation
No full text<b>External Organisations</b><br/>Indian Institute of Science<b>Associated Persons</b><br/>Devang Falor (Creator); Bishakhdatta Gayen (Creator); Debasis Sengupta (Creator)LES simulation dataset for the cases: Oscillating wind stress with heat flux and evaporation included. Oscillating wind stress with heat flux without evaporation
Biennial Oscillation Of Indian Summer Monsoon And Global Surface Climate In The Present Decade
No full textThe ENSO-monsoon system is known to have a biennial component. Here we show using high resolution satellite data, mainly daily rainfall and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM), and daily scatterometer surface winds from QuickSCAT, that there is a clear biennial oscillation (TBO) in summer monsoon rainfall over Central India – Bay of Bengal (Cl-BoB) and the far west Pacific in the period 1999-2005. Summer (JJAS) mean rainfall oscillates between high and low values in alternate years; the rainfall is high in the odd years 1999, 2001, 2003, and 2005, and low in even years 2000, 2002 and 2004. The amplitude of the oscillation is significant, as measured against the long term standard deviation of seasonal rain based on 1979-2005 Global Precipitation Climatology Project (GPCP) data. We find that the TBO in rainfall is associated with TBO of SST over the tropical Indian, west Pacific and Atlantic Oceans in different seasons. There is no TBO in east Pacific SST, and no strong El Nino in this period. The TBO of SST is related to change in evaporation due to TBO of surface wind speed.
A TBO of the surface branch of the Walker circulation in the eastern Indian and western Pacific basins is clearest in the autumn season during 1999-2005. There is a clear relation between a large-amplitude TBO of winter surface air temperature over north Asia associated with TBO of the Arctic oscillation (AO), and the TBO of summer monsoon rainfall. High rainfall over CI-BoB lin summer is followed by a relatively high value of the AO Index, and warm air termperature over north Asia in the succeeding winter. The Inter Tropical Convergence Zone(ITCZ) over the central Pacific and Atlantic Oceans shift north by about two degrees when the northern hemisphere is warm, reminiscent of the behaviour of the climate system of ENSO, decadal and palaeoclimate time scales. In this thesis we document the biennial oscillation of monsoon rain and its spatial structure in the recent period, and its relation with biennial oscillation of surface climate over the global tropics and extratropical regions. The existence of TBO in the tropical Atlantic, and its relation with the monsoon, is a new finding. We demonstrate that the interannual variability of the summer monsoon during 1999-2005, including the drought of 2002, is part of a pervasive TBO of global surface climate
Observed Subseasonal Variability Of Temperarture And Salinity In The Tropical Indian Ocean
No full textSubseasonal variability of tropical Indian Ocean sea surface temperature is thought to influence the active-break cycle of the Asian monsoon. There are several open questions related to the role of surface fluxes, large-scale ocean circulation and subsurface ocean processes in the subseasonal variability of upper ocean temperature. We present a unified study of the subseasonal (2-90 day) variability of surface heat flux and upper ocean temperature and salinity throughout the tropical Indian Ocean in all seasons. We focus on the relation between surface fluxes and ocean response using a new satellitebased
daily heat flux. The role of ocean processes (advection, entrainment and mixing)
in determining SST variability is diagnosed from the daily satellite SST.
Before the onset of the summer monsoon, sea surface temperature (SST) of the north Indian Ocean warms to 30-32oC. Climatological mean mixed layer depth in spring (March-May) is 10-20 m, and net surface heat flux (Qnet) is 80-100 Wm2 into the ocean. It has been suggested that observed spring SST warming is small mainly due to (a) penetrative flux of solar radiation through the base of the mixed layer (Qpen), (b) advective cooling by upper ocean currents and (c) entrainment of sub-mixed layer cool water. We estimate the role of the first two processes in SST evolution from a two-week ARMEX experiment in April-May 2005 in the the southeastern Arabian Sea. The upper ocean is stratified by salinity and temperature, and mixed layer depth is shallow (6 to 12 m). Current speed at 2 m depth is high even under light winds. Currents within the mixed layer are quite distinct from those at 25 m. On subseasonal scales, SST warming is followed by rapid cooling. The cooling occurs although the ocean gains heat at the surface - Qnet is about 105 Wm2 in the warming phase, and 25 Wm2 in the cooling phase; penetrative loss Qpen, is 80 Wm2 and 70 Wm2. In the warming phase, SST rises mainly due to heat absorbed within the mixed layer, i.e. Qnet minus Qpen; Qpen, reduces the rate of SST warming by a factor of three. In the
second phase, SST cools rapidly because (a) Qpen, is larger than Qnet, and (b) advective cooling is _85 Wm2. A calculation using time-averaged heat fluxes and mixed layer depth suggests that diurnal variability of fluxes and upper ocean stratification tends to warm SST on subseasonal time scale. Buoy and satellite data suggest that a typical premonsoon intraseasonal SST cooling event occurs under clear skies and weak winds, when the ocean is gaining heat. In this respect, premonsoon SST cooling in the north Indian ocean is different from that due to MJO or monsoon ISO.
As a follow-up to ARMEX, we use a short dataset from a field campaign in the
premonsoon north Bay of Bengal to study diurnal variability of SST. In addition to the standard meteorological and hydrographic parameters measured from shipborne instruments and buoy sensors, we obtained a two-hourly record of subsurface sunlight profiles. Heat fluxes are seen to drive the SST warming during the day while both advection and entrainment/mixing are important during the night. The simple heat balance based on heat flux shows that it drives the diurnal cycle of SST, though ocean processes contribute towards night time cooling; this has been confirmed using the Price-Weller-Pinkel mixing model forced by heat flux and wind stress. A similar analysis for mixed layer salinity revealed that the salt balance in the region is dominated by advection rather than freshwater flux or entrainment/mixing.
Buoy and satellite data show pronounced subseasonal oscillations of sea surface
temperature (SST) in the summertime north Indian Ocean. The SST oscillations are forced mainly by surface heat flux associated with the active-break cycle of the south Asian summer monsoon. The input of freshwater (FW) from summer rain and rivers to the Bay is large, but not much is known about subseasonal salinity variability. We use 2002-2007 observations from Argo floats with 5-day repeat cycle to study the subseasonal response of temperature and salinity to surface heat and freshwater flux in the central Bay of Bengal and central Arabian Sea. Estimates of surface heat and freshwater flux are based on daily satellite data sampled along the float trajectory. We find that intraseasonal variability (ISV) of mixed layer temperature is mainly a response to net surface heat flux minus penetrative radiation during the summer monsoon season. In winter and spring, however, temperature variability appears to be mainly due to
ocean processes rather than local heat flux. Variability of mixed layer freshwater content is generally independent of local surface flux (precipitation minus evaporation) in all seasons. There are occasions when intense monsoon rainfall leads to local freshening, but these are rare. The large subseasonal fluctuations observed in FW appear to be due to advection, suggesting that freshwater from rivers and rain moves in eddies or filaments.
We have developed a new daily satellite-based heat flux dataset for the tropical Indian Ocean (30oE 120oE; 30oS 30oN); satellite data include surface air temperature and relative humidity from the Atmospheric Infrared Sounder (AIRS). On the seasonal scale (> 90 days) the flux compares reasonably well with climatologies and other daily data. On the subseasonal scale, our flux product has realistic behaviour relative to buoy data at validation sites. An important result is that ocean processes (advection, entrainment/detrainment, mixing at the base of the mixed layer) cool the tropical Indian Ocean SST by 8oC over the year. The largest contribution of ocean processes (_20oC SST cooling over the year) is in the western equatorial Indian Ocean. Ocean processes generally cool the upper ocean in all seasons and all regions, except in boreal winter, when they warm the north Indian Ocean. This is likely due to entrainment of
warm sub-mixed layer water in regions of inversions.
On subseasonal (2-90 days) scales, the contribution of air temperature and humidity to latent heat flux is roughly equal to the contribution from wind speed variability: Another interesting finding is that the contribution of air temperature and humidity increases away from the equator. One of the most important contributions of this thesis is the demonstration that tropical Indian Ocean SST has a coherent response to intraseasonal changes in heat flux associated with organised convection in the summer hemisphere. SST responds to flux in (i) the northeast Indian Ocean during May-October and (ii) the 15oS-5oN region during November-April. In the winter hemisphere and in regions with no organised convection, it is ocean processes and not fluxes which drive the subseasonal changes in SST. This result suggests that SST ISV feeds back to organise and sustain organised convection in the tropical atmosphere
Bay of Bengal Freshwater in the tropical Indian Ocean
No full textThe annual total continental runoff into the Bay of Bengal (BoB) is more than half the runoff into the entire tropical Indian Ocean. The net freshwater (FW) content in the Bay of Bengal mixed layer increases from a minimum of 6200 km3 in May to a maximum of 8700 km3 in November. For steady state freshwater balance, there has to be a net transport of around 0.11 Sv (1 Sv = 106 m3s−1) out of the Bay. This large transport of freshwater has a significant influence on regional hydrological balance. In this thesis, we investigate the seasonal pathways of BoB freshwater based on climatological observations. In order to trace the movement of BoB freshwater in the tropical Indian Ocean, we remove the influence of local precipitation minus evaporation by subtracting seasonal P-E from FW at each point. Although this recipe does not remove advected rainwater for simplicity we call the difference “runoff water” (RW), as the major source of this water is continental runoff as well as freshwater from the Indonesian Throughflow (ITF). The datasets used in this work are (1) World Ocean Atlas 2001 Salinity and Temperature (2) Satellite-gauge merged precipitation from GPCP and CMAP (3) SOC and COADS evaporation (4) Surface currents from WOCE drifters (5) Dai and Trenberth River Runoff Data (6)SK197 Cruise data from north Bay in October 2003 (7) NIOT Buoy observations, including DS1 thermistor chain data and (8) Sea Surface Temperature from TRMM Microwave Imager (TMI). Estimates suggest that the net annual input of freshwater into the Bay (from runoff plus rain minus evaporation) is more than 4000 km3. The upper ocean freshwater content is highest in the north Bay in the post monsoon season. We also study the effect of the upper ocean freshwater pool
on ocean cooling due to cyclones in the north Bay.
We find two principal pathways for the export of freshwater out of the northern Bay of Bengal. These pathways had been identified in previous model studies. However, most models underestimate the true reach of Bay of Bengal freshwater because model mixing is unrealistically large. The two pathways are as follows:
(1) The western pathway, during November-May. Observations, and a few model studies using passive tracers and drifters, suggest that runoff water from the north Bay flows down the east coast of India in the East India Coastal Current (EICC) and into the eastern Arabian Sea around Sri Lanka during November-December. Later in winter, water from south Bay flows past Sri Lanka in the Northeast Monsoon Current (NMC) (January-February). We see BoB freshwater in the Arabian Sea up to 15 0N along the west coast of India in February, with RW decreasing gradually to the north. Bay runoff spreads in the southern Arabian Sea up to the coast of Africa by May. Upper ocean currents around the Lakshadweep high and smaller vortices (January-April) might then carry the BoB water west.
(2) The eastern pathway, during the second half of the year, carries BoB freshwater south. The surface water flows along the Indonesian coast, joins the Indonesian Throughflow and flows west in the surface south equatorial current (SEC), in agreement with some model results.
High space and time resolution sea surface temperature (SST) from satellite shows that premonsoon cyclones cool SST in the Arabian Sea(AS) and the southern Bay of Bengal by up to 50C, but post monsoon cyclones do not cool the north Bay by more than 10C. In situ data is used to examine the possible reasons for the small SST cooling in the north Bay, even under strong post-monsoon cyclones.
The cyclone of June 1998 in the eastern AS passed within 200 km of the NIOT mooring DS1. The thermistor chain on DS1 showed strong thermal stratification in the upper ocean before the storm developed. The cyclone deepened the mixed layer from about 10 m or less to about 70 m. The potential energy input to the upper ocean is about 11,000 Jm−2. We do not have similar subsurface temperature profiles, recording the influence of a cyclone in the north Bay. We use CTD data from Sagar Kanya cruise SK197 in October 2003 and ask the question: What would happen to north Bay SST if 11,000 Jm−2 of potential energy were supplied by a cyclone to mix the upper ocean? We find that the mixed layer would deepen from about 10 m to 40 m, but this would not lead to significant SST cooling because the isothermal layer is around 40 m deep. This suggests that vertical mixing due to post monsoon cyclones does not lead to SST cooling of the north Bay because (a) salinity stratification resists deep vertical mixing, and (b) the sub mixed layer water is warm. Therefore, the observed cooling of under 10C must be mainly due to evaporation
Stirring and mixing driven by mesoscale eddies in the stratified Bay of Bengal
No full textThe stirring of passive tracers driven by altimetry-derived daily surface geostrophic currents is studied on subseasonal timescales in the Bay of Bengal. Advection of latitudinal and longitudinal bands highlights the chaotic nature of stirring in the Bay via repeated straining and filamentation of the tracer field. An immediate finding is that stirring is local, i.e., of the scale of the eddies, and does not span the entire basin. Further, stirring rates are enhanced along the coast of the Bay and are relatively higher in the pre-and post-monsoonal seasons. The spatially non-uniform stirring at the surface of the Bay is reflected in long-tailed probability density functions of Finite-Time Lyapunov Exponents (FTLEs), which become more stretched for longer time intervals. Quantitatively, advection for a week shows that mean FTLEs lie between 0.130.07 day, while extremes reach almost 0.6 day. Averaged over the Bay, relative dispersion initially grows exponentially, followed by a power-law at scales between approximately 100 and 250 km, which finally transitions to an eddy-diffusive regime. Quantitatively, below 250 km, a scale-dependent diffusion coefficient is extracted that behaves as a power-law with cluster size, while above 250 km, eddy-diffusivities range from ms in different regions of the Bay. These estimates provide a useful guide for resolution-dependent diffusivities in numerical models that hope to properly represent surface stirring in the Bay.\\
A particularly important tracer field in the Bay is the sea surface salinity; indeed, freshwater from rivers influences Indian summer monsoon rainfall and tropical cyclones by stratifying the upper layer and warming the subsurface ocean in the Bay of Bengal. We use {\it in situ} and satellite data with reanalysis to showcase how river water experiences a significant increase in salinity on sub-seasonal timescales. This involves the trapping and homogenization of freshwater by a cyclonic eddy in the Bay. Using a specific example from 2015, river water is shown to enter an eddy along its attracting manifolds within a period of two weeks. This leads to the formation of a highly stratified subsurface layer within the eddy. When freshest, the eddy has the largest sea-level anomaly, spins fastest, and supports strong lateral gradients in salinity. Subsequently, observations reveal a progressive increase in salinity inside the eddy within a month. In particular, salty water spirals in, and freshwater is pulled out across the eddy boundary. Lagrangian experiments elucidate this process, whereby horizontal chaotic mixing provides a mechanism for the rapid increase in surface salinity.\\
The eddy-freshwater interaction, or adjustment, is then studied using a high-resolution Regional Ocean Modeling System. Apart from lateral advection, a mixed layer salinity budget shows the importance of ageostrophic vertical advection during the evolution of salinity within the eddy. An analysis of the depth-integrated eddy kinetic energy indicates the development of both barotropic and baroclinic instabilities. The vertical profile associated with these conversion terms reveals that the surface freshwater was likely involved in developing baroclinic terms in the mixed layer. In addition, an eddy available potential energy (EPE) budget suggests that the entrainment of the river water raises the EPE, which is reflected in the development of gradients in salinity within the eddy. The EPE is lowered with homogenization, signifying irreversible mixing. Further, EPE rates are modulated by the correlation of buoyancy fluxes with density anomalies, which involves lateral advection of freshwater associated with surface cooling and local, regional rainfall. Finally, the adjustment of this freshwater eddy triggers submesoscale dynamics that appear to be an integral part of salinity homogenization. The observation and reanalysis data also showcase the presence of these events across different years, thus bringing out the broader impact of mixing freshwater into high salinity ambient water by eddies in the Bay. This pathway is distinct from vertical diffusive mixing and is likely to be important for the evolution of salinity in the Bay of Bengal.Ministry of Education (MoE), Indian Institute of Science, Bengaluru; University Grants Commission; National Monsoon Mission, Indian Institute of Tropical Meteorology, Pune; Divecha Centre for Climate Change, Indian Institute of Science, Bengalur
Intraseasonal Variability in Aquaplanet Configuration of Community Atmosphere Model
No full textThe Intraseasonal Oscillation (ISO) plays an important role to modulate deep convective activity
in the tropical region. In this thesis, I aim to understand the role of land and warm oceans
in ISO, using a general circulation model. For this, I conduct a series of experiments in the
Community Atmosphere Model (CAM) with various idealized and realistic surface boundary
conditions to study tropical ISO. To investigate the influence of tropical sea surface temperature
(SST) on ISO and convectively coupled equatorial waves in the global atmosphere, I conduct
experiments with idealized, zonally symmetric SST profiles having different widths of warm
ocean centered at the equator. I use the model in its basic “Aquaplanet” configuration, with
the sun at the equator, i.e. perpetual spring equinox forcing; with idealized zonally symmetric
SST, the aquaplanet model produces a double Intertropical Convergence Zone (ITCZ) on
either side of the equator, and an eastward propagating Madden Julian oscillation (MJO)like
mode with variance at intraseasonal (30 to 96 day) periods and zonal wavenumber one. In the
experiment with the narrowest meridional width of warm SST, the variance of moist convective
activity lies predominantly in equatorially trapped Kelvin wave band. As the width of the
warm equatorial SST is increased, the eastward propagating speed of the MJO-like signal decreases;
for the broadest SST profile (warm SST covering 20 degrees of latitude), the speed of
the model MJO is about 5.5 m s−1, close to the observed speed. This is because the latitudinal
extent of warm SST is comparable to the equatorial Rossby radius, and the model produces off equatorial
Rossby waves of sufficient strength to interact with the Kelvin wave and slow down
the MJO-like mode. The model also generates westward propagating waves with intraseasonal periods and zonal wavenumber 1–3; the structure of these signals, which extend well into the
mid-latitudes, projects onto equatorially trapped Rossby waves with meridional mode numbers
1, 3 and 5, associated with convection that is symmetric about the equator. In addition, the
model generates 30–80 day westward moving signals with zonal wavenumber 4–7, particularly
in the experiment with a narrow region of warm SST. Although these waves are seen in
the wavenumber-frequency spectra in the equatorial region, they have the largest amplitude in
the middle and high latitudes. Thus, our study shows that wider, meridionally symmetric SST
profiles support a strong MJO-like eastward propagation, and even in an aquaplanet setting,
westward propagating Rossby waves comprise a large portion of tropical intraseasonal variability.
In the observations (ERA-Interim daily reanalysis), the MJO signal lies in the range of
zonal wavenumbers 1 to 5. The variance of MJO at higher wavenumbers (2–5) is absent in
the aquaplanet model. For this, I design model experiments in order to study how model MJO
responds to the introduction of continents in the presence of zonally symmetric SST, and a realistic
SST distribution with the Indo-Pacific warm pool and cool SST in the eastern Pacific.
As before, the model is in the aquaplanet-like configuration, to eliminate the effects of seasonality.
Model results are compared with 21 years (1995–2015) ERA-Interim reanalysis data
and analyzed in terms of the moist static energy (MSE) budget to study the growth and propagation
of MJO. When I introduce continents with realistic orography and interactive surface
temperature, soil moisture, and albedo, the variance of model MJO is reduced due to weaker
boundary layer moisture convergence. However,MJO variance extends to higher wavenumbers.
With prescribed climatological January SST boundary condition in the presence of continents,
the variance of model MJO is enhanced by a factor of 2–3, and it is distributed across zonal
wavenumbers 1 to 5, in closer agreement with observations. Thus, I find that the presence of
land by itself is not enough to produce realistic MJO in CAM, but realistic SST distribution is
also necessary to simulate MJO with improved spacetime characteristics. Both in simulations
and ERA-Interim data, column-integrated longwave radiation plays a key role in the growth of
MSE anomaly associated with MJO; in general, meridional and vertical advection of MSE both
acts to promote eastward movement of MJO. In the model experiments, meridional advection of low-level MSE anomaly is most significant in the vicinity of the ITCZ. This indicates that
the physical processes which determine the location of (single or double) ITCZ are linked to
MJO dynamics.
The westward propagating “quasi-biweekly” oscillation (QBWO) with 10–25 day period is
an important intraseasonal mode of the Asian summer monsoon, yet very few model studies focus
on this mode. I study QBWO in the northern and southern tropics in the model and compare
it with ERA-Interim reanalysis data. The pure aquaplanet model produces a double Intertropical
Convergence Zone (ITCZ), winds that are predominantly zonal, and weak quasi-biweekly variance.
When continents are introduced in the model with zonally symmetric SST, the northern
ITCZ, as well as quasi-biweekly variance between 10◦N to 24◦N are strengthened in the Pacific
Ocean, bringing model results closer to observations. In the model with continents, the QBWO
signal dwells inside the mean envelope of high atmospheric moisture, or total precipitable water
(TPW), in agreement with observations. However, in the presence of zonally symmetric SST,
the model fails to simulate sufficiently high precipitable water in the region extending from the
north Indian Ocean to East Asia, resulting in very weak QBWO variance. When the model includes
continents and realistic (January) SST boundary conditions, the spatial structure of both
TPW and QBWO variance becomes more realistic. I study the mechanisms of propagation and
maintenance of the quasi-biweekly mode using vorticity budget and moist static energy (MSE)
budget analysis. Advection due to the effect is responsible for the northwestward propagation
of QBWO vorticity, while the propagation of column MSE anomaly is mainly due to horizontal
advection. Surface turbulent heat fluxes and vertical MSE advection are the dominant contributors
to the growth and maintenance of column MSE anomaly in observations and model
respectively. Surface heat flux makes a significant contribution to the growth of quasi-biweekly
MSE anomaly in the presence of land, in association with the enhanced meridional wind, and
vortical structures that resemble moist Rossby waves with a wavelength of about 4000 kilometers
Space-time variability of near-surface salinity in the Bay of Bengal
No full textFreshwater from monsoon rain and rivers leads to a 5-10 m deep low-salinity layer in the north Bay of Bengal from August to February. The thin fresh layer, with strong stratification at its base, is highly responsive to air-sea momentum and heat flux. Moored observations at 18N, about 500 km away from major river mouths, show a 3-8 psu drop in surface salinity within a week as water from the Ganga-Brahmaputra-Meghna (GBM) river arrives at the mooring in late August-early September each year, and from the Irrawady river in November-December. In conjunction with satellite sea surface salinity (SSS) and surface currents, the moored observations indicate that dispersal of river water in the open ocean is mainly driven by the flow in mesoscale (order 100 km) eddies during calm phases of the summer monsoon, and by a swift, shallow wind-driven Ekman flow as monsoon winds strengthen.
Six years of moored observations at 18N 89.5E show that surface salinity has a distinct quasi-biweekly (10-25 day) variability, which is not due to changes in freshwater input. Rather, changes in salinity are related to variations in surface winds associated with the quasi-biweekly mode of the Asian summer monsoon. During the active phase of the monsoon, a shallow wind-driven Ekman flow disperses river water to the north and east, leading to increased salinity at the moorings, and a rise of coastal sea level by 0.3-0.6 m within days along the eastern boundary. In situ and satellite observations show that the response of sea surface temperature (SST) to quasi-biweekly variations of surface heat flux is enhanced by a factor of two because the mixed layer is very shallow within the pool of river water, thus revealing a direct link between SST and surface salinity.
During research cruises of ORV Sagar Nidhi in August-September 2014 and 2015, upper ocean temperature (T), salinity (S) and ocean currents (V) in the Bay of Bengal were measured with 0.5-1.5 km horizontal resolution and 1-2 m vertical resolution in order to study sub-mesoscale (1-10 km) variability. Underway CTD data show numerous sub-mesoscale salinity-dominated surface density fronts. The spatial scale of 30 major fronts lies in the range 3-25 km, and net density change across the fronts exceeds 0.3 kg/m3. An east-west asymmetry in isopycnal slope is due to Ekman flow, which drives relatively saltier, denser water over lighter water on the western side. Ship-borne ADCP measurements show that flow at sub-mesoscale fronts has Rossby number of order one. Of the 30 fronts, two are associated with swift 5-10 km wide jets in the upper 20 m. Mixed layer depth is shallower at the fronts than on either side, and is less than 10 m if lateral density gradient exceeds 0.1-0.2 kg/m3 per km. The observations indicate that slumping of sub-mesoscale salinity-dominated fronts is an important mechanism sustaining near-surface stratification in the north Bay of Bengal.
Finally, basin-scale diapycnal diffusivity is estimated from freshwater balance within a control volume bounded by the 1018 kg/m3 isopycnal - T, S and V are from an eddy-permitting daily ocean analysis, and rainfall, evaporation and runoff from a continental runoff dataset and satellite observations. The amount of pure freshwater in the control volume increases from June to November each year due to net input from rain and runoff, and decreases from December to May. Water lighter than 1018 kg/m3 is not transported across the southern boundary of the Bay of Bengal, implying that the freshwater lost from the control volume is mixed to deeper layers within the basin. The freshwater balance indicates that average diapycnal diffusivity across the 1018 kg/m3 isopycnal surface in winter is nearly 5x10-5 m2/s, 3-5 times higher than in spring or summer. Winter mixing in the upper ocean is highest during episodes of cool, dry surface air, leading to enhanced evaporation and surface buoyancy loss
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