P. Sreenivas

Estimation of Improvement in Indian Summer Monsoon Circulation by Assimilation of Satellite Retrieved Temperature Profiles in WRF Model

This study estimates the improvement in the simulation of Indian summer monsoon (ISM) circulation in the weather research and forecasting (WRF) model by assimilating temperature profiles from atmospheric infrared sounder. Two experiments are carried out from 1st May to 1st October during 2003-2011. In the first experiment control (CTRL), National Centers for Environmental Prediction final analysis forcing is used; whereas the second one (WRFAIRS) is same as CTRL but temperature profiles are assimilated. The improvements in the simulation are quantified using different statistical scores. Overall, the assimilation has improved the spatial and temporal distribution of various fields associated with ISM. Some of the major improvements are 1) elimination of asymmetric (north-south) SLP bias; 2) larger error reduction in winds; 3) reduction in the temperature biases at boundary layer and midtroposphere; 4) improvement in the vertical wind shear; 5) reduction in the water vapor mixing ratio errors by 0.3-0.6 g · Kg-1; and 6) improved simulation of monsoon circulation indices. Further improvements are noticed in dynamic and thermodynamic fields over different convective regions. This study advocates that accurate representation of the thermal structure in WRF is crucial for the simulation of realistic monsoon circulation. It may further pave way for developing/improving convective parameterization schemes for the model.

Monthly Variability of Mixed Layer over Arabian Sea Using ARGO Data

The mixed layer depths over the Arabian Sea were computed for the three successive years 2004–2006 using ARGO floats data. The large availability of ARGO floats for the above period resulted in better estimation of mixed layer depth (MLD) over the Arabian Sea. The results were compared with World Ocean Atlas 1994 MLD Climatology. Marked variability in MLD on a monthly time scale is observed and it was in accordance with the wind stress and/or net heat gain variability, which are the principal factors influencing mixed layer over Arabian Sea. With the availability of large number of ARGO profile data, an attempt is made to study the monthly variability of Mixed Layer.

Impact of tropical cyclones on the intensity and phase propagation of fall Wyrtki jets

Observations and model simulations are used to study the impact of tropical cyclones (TC) on the fall Wyrtki jets (WJ). These strong narrow equatorial currents peak during November and play a vital role in the energy and mass transport in the tropical Indian Ocean (TIO). Maximum number of TCs is observed over TIO during November with longer than normal life span (8-15 days). These TCs enhance equatorial westerly winds (surface) and amplify monthly mean WJs (both at surface and subsurface) by 0.4 ms-1 (anomalies exceed 0.7 ms-1 during TC), which is about half of the climatological amplitude. Intensified WJs increase the heat content of eastern TIO and modulate air-sea interaction. It is also shown that movement of TCs is mainly responsible for the westward phase propagation of WJs, a previously unexplored mechanism. These features are evident in ECCO2 simulations as well.

Impact of Oceanic Processes on the Life Cycle of Severe Cyclonic Storm “Jal”

The cyclonic system “Jal” initiated as a depression in the South China Sea on 31st of October, 2010, and propagated westward into the Bay of Bengal (BoB). In line with the forecast, it developed into a “severe cyclonic storm” by 5th November. It was predicted to intensify further to a “very severe cyclonic storm” and hit the east coast of India; however, it dramatically diminished to a “cyclonic storm” prior to the landfall. The best possible physical parameterizations in a numerical atmospheric model fail to simulate the intensity of Jal sytem. The analysis of satellite derived ocean surface properties revealed that the propagating Jal system encountered distinct oceanic environments in the eastern and western BoB and these have great impact on the intensity changes undergone by the system. The intense precipitation (from July to October, 2010) and the convergence associated with a downwelling coastal Kelvin wave preconditioned the eastern BoB with thick barrier layer and high cyclone heat potential (CHP) that enabled the Jal system to gradually reach a stage of severe-cyclonic-storm. However, the system encountered a region of shallow thermocline with low CHP associated with an upwelling eddy in the western BoB, which influenced the movement of the system and alleviated its intensity to cyclonic-storm. In the western BoB, even though the precipitation freshened the surface layer, the divergence associated with upwelling eddy weakened the barrier layer formation and stratification.

Role of Meso-Scale Eddies on Circulation in the South Eastern Arabian Sea in 2009

Spatial current data were collected from South eastern Arabian Sea during July 2009 (summer monsoon) and December 2009 (winter monsoon) using the vessel mounted Acoustic Doppler Current Profiler (ADCP). During July 2009, observed ADCP currents were flowing southward along 75°E longitude with a complex pattern near the coast. During winter (December 2009), an anti-clockwise rotation of currents was observed in the study region, with its eastern arm along the southwest coast of India. The circulation pattern observed during the two seasons from ADCP measured data does not seem to follow any pattern described in climatology. To investigate this aspect, the geostrophic currents were computed utilizing the multi-mission altimeter sea surface height anomaly data for the corresponding months. The analysis of geostrophic currents indicates the presence of multiple eddies embedded in the Laccadive Low (LL) /Laccadive High (LH). Our present study shows that the meso-scale eddies embedded in the LL and LH play a significant role in deviating the observed circulation from climatology in this region. In addition, generating mechanisms of these eddies are addressed to substantiate our findings. The formation of these eddies is attributed to the local wind force during July 2009 and the enhanced baroclinic instability during December 2009.

Intra-Annual Studies of Mixed Layer Depth in the Arabian Sea Using A 3 Layer Indian Ocean Model

Mixed layer is the upper layer of the ocean, where significant physical, chemical and biological activities take place. Knowledge of mixed layer depth variability is important in the studies of air-sea interaction, acoustic propagation, heat transport and fisheries. The Arabian Sea experiences extremes in atmospheric forcing that lead to intra-annual and in- ter-annual variability. Since the climatic conditions over the Arabian Sea are highly variable, the mixed layer depth also changes seasonally. Hence the intra-annual variability of mixed layer depth in the Arabian Sea has been examined using 3 � layer Indian Ocean model. The model is integrated for 8 years (1993 - 2000) with annually varying monthly averaged winds derived from ERS-2 scatterometer. In addition to this data, inter annually varying monthly averaged satellite esti- mates of precipitation from Global Precipitation Climatology were also incorporated in the model. Model results show a steady deepening of mixed layer depth in the central Arabian Sea during southwest monsoon period. The model shows its capability to predict the shallow mixed layer depths caused by coastal upwelling off Somalia during southwest monsoon period and is able to capture the Lakshadweep high/low as it predicted successfully the deeper/shallow mixed layer ob- served during these periods.

Impact of Aquarius Sea-Surface Salinity Assimilation in Improving the Ocean Analysis Over Indian Ocean

ABSTRACT The distribution of ocean salinity controls the density field and thereby plays a major role in influencing the ocean dynamics. It has been a challenging task to understand the variability of salinity structure in the regions of large fresh water discharge and high precipitation such as Bay of Bengal (BoB). Recent advancement in satellite technology has made possible the measurement of sea surface salinity (SSS). Aquarius is the satellite which measured the global SSS for the period 2011 to 2015. In the present study, we assimilated Aquarius SSS in the Global Ocean Data Assimilation System based on 3DVAR technique. The assimilation of Aquarius SSS resulted in reduced biases in salinity not only at the surface, but also in the vertical distribution of salinity and better captured the temporal variations of salinity structure in sensitive regions, such as the Bay of Bengal. In addition, the assimilation of SSS showed marginal improvement in ocean thermal structure over data sparse regions of Indian Ocean. It is also shown that the assimilation of Aquarius SSS has improved the stratification in the upper Ocean which is the key factor in the observed improvement in ocean analysis.

Contribution of Tides to Sea Level Variations Along Visakhapatnam, India

The sea level variations along Visakhapatnam coast are governed by astronomical tides and nontidal oscillations including atmospheric pressure, winds, coastal currents, Ekman Pumping, and river influx. Tidal and nontidal sea level oscillations are usually studied separately because of the vastly different ways in which they are forced. In this study the tidal oscillations along Visakhapatnam are analyzed using GOTIC2 tidal model. The correlation between monthly mean sea level and monthly mean tides is 47% (r = 0.68) and increases to 54% (r = 0.74) when applied for inverse-barometric effect. The major six partial tides are computed and presented. The tidal variations from Neap tide to Spring tide are studied.