1. Formation and maintenance of the southern Bay of Bengal cold pool (2011-2015)

The seasonal cycle of sea surface temperature (SST) in the Indian Ocean is marked by a warm pool (SST> 28C) that envelopes the equatorial region, the eastern Arabian Sea and the Bay of Bengal during summer monsoon. Around Sri Lanka and to the south of India SST is cooler compared to the sorounding region. This region where the SST is relatively cooler is known as the cold pool. The cloud formation is inhibited during monsoon and rainfall shows a minimum above the cold pool. The cold pool region also presents energetic oceanographic conditions. The summer monsoon current (SMC) is strong around Sri Lanka and is associated with eddies and meanders. There exists a salt pump to the east of Sri Lanka, which brings saltier Arabian Sea water to the surface during several intraseasonal spells. Finally, this region also shows phytoplankton blooms during summer monsoon, caused by coastal upwelling along the southern coast of Sri Lanka and by Ekman pumping within Sri Lanka Dome (SLD). Owing to its possible impact on monsoon, ocean-dynamics and ocean-biogeochemistry, uderstanding the processes leading to the formation as well as maintanance of the cold pool are considered to be necessary.

M.S.(Engg.) Thesis (Download)

Publications:

a. Das et al.,2015

b. Vinayachandran et al. 2019

Collaborators:

• Prof P.N. Vinayachandran (CAOS, IISc)
• Ambika Behera (CAOS, IISc)

a. Formation of the Southern Bay of Bengal Cold pool

A pool of relatively cooler water, called here as the southern Bay of Bengal cold pool, exists around Sri Lanka and southern tip of India during the summer monsoon. This cold pool is enveloped by the larger Indian Ocean warm pool and is believed to affect the intraseasonal variations of summer monsoon rainfall. In this study, we have investigated the mechanisms responsible for the formation of the cold pool using a combination of both satellite data sets and a general circulation model of the Indian Ocean. Sea surface temperature (SST) within the cold pool, after the steady increase during the February–April period, decreases first during a pre-monsoon spell in April and then with the monsoon onset during May. The onset cooling is stronger (~1.8 ∘C) than the pre-monsoon cooling (~0.8 ∘C) and culminates in the formation of the cold pool. Analysis of the model temperature equation shows that SST decrease during both events is primarily due to a decrease in incoming solar radiation and an increase in latent heat loss. These changes in the net heat flux are brought about by the arrival of cloud bands above the cold pool during both periods. During the pre-monsoon period, a cloud band originates in the western equatorial Indian Ocean and subsequently arrives above the cold pool. Similarly, during the monsoon onset, a band of clouds originating in the eastern equatorial Indian Ocean comes over the cold pool region. A lead-lag correlation calculation between daily SST and rainfall anomalies suggest that cooling in SST occurs in response to rainfall events with a lag of 5 days. These sequence of events occur every year with certain amount of interannual variability.

b. Maintenance of the Southern Bay of Bengal cold pool

The region of relatively cooler water, sitting in the shadow zone of monsoon rainfall, around southern tip of India and around Sri Lanka, is known as the cold pool. A ship-board observations program to the east of Sri Lanka during the summer monsoon of 2009 captured two cooling events within the core of the Summer Monsoon Current (SMC). Time-series of temperature profiles within the cold pool showed a cooling of the sea surface temperature (SST) by about 0.3°C during 22–25 July 2009 and about 0.4°C during 29 July to 2 August 2009. During the cooling periods, both wind speed and mixed layer depth (MLD) increased, but the air-sea heat gain by the ocean remained positive. Efforts were made to estimate the temperature advection term by means of an Operation Advection strategy using CTD data at four locations about 6.5 km away from the time-series location and ship-board acoustic Doppler current profile measurements. Estimates of horizontal temperature advection suggest that these cooling events are induced by the advection of cooler water by the SMC. Simulation using an Indian Ocean model captures such cooling events and the model temperature equation shows that cooling events through the season are driven either by advection or by entrainment. Thus observations and model simulation concurrently demonstrate that cold pool is maintained through the summer monsoon by the SMC in spite of the ocean gaining heat from the atmosphere.

2. Influence of convective and stratiform rainfall on the seasonal monsoon rainfall bias as simulated by a high resolution regional model (2011)

Indian summer monsoon is an important component of global climate system and has vigorous intraseasonal oscillations (ISO) that manifest in the sub-seasonal active and break spells of monsoon rainfall. The Indian monsoon is characterized by convectively coupled monsoon ISO’s that manifests in the form of active and breaks phases [Goswami, 2005] and the overall mean monsoon precipitation distribution significantly depends on the manifestation of ISO’s in a season [Waliser et al., 2003]. Monsoon synoptic systems namely lows and depressions account for most of monsoon rain during the June–September monsoon season [Mooley and Shukla, 1989]. It is well established that during active and break phases the large scale organizations shows contrasting behavior in terms of formation of the weather systems, large scale instability and most importantly the rainfall [Yasunary, 1981; Murakami et al., 1984, 1986; Goswami et al., 2003, Krishnamurty and Ajaymohan, 2010]. It is well known that numerical models show significant bias in simulating monsoon seasonal cycle as well as its intra-seasonal variability. One of the reasons behind the rainfall bias could be the improper contribution of stratiform and convective rainfall component over land and ocean and their percentage contribution to the total rainfall. Although many researchers [Houze et al. 2009, Chattopadhyay et al. 2009] have showed the importance of stratiform and convective rainfall on Indian Summer Monsoon (ISM) rainfall variability, hardly any study is seen in attempting objectively quantifying the contribution of convective and stratiform rainfall in the model bias of seasonal total rainfall. Therefore in this study it is intended to use a state of the art mesoscale model e. g. Weather Research Forecast (WRF) model to run for 2000 to 2009 using NCEP/NCAR reanalyses data for May to October of each year. Further the seasonal rainfall will be analysed with an aim to bring out the percentage of convective and stratiform proportion in the seasonal total rainfall.

Summer Research fellowship Program through

Indian Academy of Sciences (Download)

Collaborators:

• Dr. Parthasarathy Mukhopadhyay (IITM, Pune)
• Prof B.N. Goswami (IITM, Pune)

Northward propagation of model simulated intraseasonal oscillation based on total rainfall

3.Features of Monsoon Depression as Simulated by a Mesoscale Model (2010)

The nonhydrostatic mesoscale model WRF is used to simulate the mesoscale features of two monsoon depressions. Even though the model is initialized with a coarser analyses e. g. NCEP/NCAR, still the model is able to reproduce many interesting features associated with the depressions. Firstly the depressions are found to reach significant intensification starting from a low pressure area. The northwestward movement is captured reasonably well. The precipitation field and associated circulation are consistent with the classical theories of monsoon. The middle tropospheric and upper tropospheric structures are also simulated realistically. The cold core structure and the southward tilt with height is reasonably brought out by the model. However there are some limitations of the model such as it has simulated a longer track of the Aug 2006 depression as compared to observation. Further research needs to be carried out to improve such model biases.

M.Sc. Tech Geophysics, Dissertation Report (Download)

Collaborators:

• Dr. Parthasarathi Mukhopadhyay (IITM, Pune)
• Prof. G.P. Singh (BHU, Varanasi)

Features of Monsoon Depression