Thermohaline structure and circulation in the upper layers of the southern Bay of Bengal during BOBMEX-Pilot (October–November 1998)

Hydrographic data collected on board ORV Sagar Kanya in the southern Bay of Bengal during the BOBMEX-Pilot programme (October–November 1998) have been used to describe the thermohaline structure and circulation in the upper 200 m water column of the study region. The presence of seasonal Inter-Tropical Convergence Zone (ITCZ) over the study area, typically characterized with enhanced cloudiness and flanked by the respective east/northeast winds on its northern part and west/southwest winds on its southern part, has led to net surface heat loss of about 55 W/m². The sea surface dynamic topography relative to 500 db shows that the upper layer circulation is characterised by a cyclonic gyre encompassing the study area. The eastward flowing Indian Monsoon Current (IMC) between 5‡N and 7‡N in the south and its northward branching along 87‡E up to 13‡N appear to feed the cyclonic gyre. The Vessel-Mounted Acoustic Doppler Current Profiler (VM-ADCP) measured currents confirm the presence of the cyclonic gyre in the southern Bay of Bengal during the withdrawing phase of the southwest monsoon from the northern/central parts of the Bay of Bengal.     

Diurnal variability of upper ocean temperature and heat budget in the southern Bay of Bengal during October–November, 1998 (BOBMEX-Pilot)

Time-series data on upper-ocean temperature, Vessel-Mounted Acoustic Doppler Current Profiler (VM-ADCP) measured currents and surface meteorological parameters have been obtained for the first time in the southern Bay of Bengal at 7‡N, 10‡N, and 13‡N locations along 87‡E during October–November, 1998 under BOBMEX-Pilot programme. These data have been analysed to examine the diurnal variability of upper oceanic heat budget and to estimate the eddy diffusivity coefficient of heat in the upper layer. Diurnal variation of near-surface temperature is typical at northern location (13‡N) with a range of 0.5‡C while the diurnal range of temperature is enhanced to 0.8‡C at the central location (10‡N) due to intense solar radiation (1050 W/m²), clear skies and low wind speeds. At the southern location (7‡N), the diurnal variation of temperature is atypical with the minimum temperature occurring at 2000 hrs instead of at early morning hours. In general, the diurnal curve of temperature penetrated up to 15 to 20 m with decreasing diurnal range with depth. The VM-ADCP measured horizontal currents in the upper ocean were predominantly easterly/northeasterly at southern location, north/northerly at central location and northwesterly at northern location, thus describing a large-scale cyclonic gyre with the northward meridional flow along 87‡E. The magnitudes of heat loss at the surface due to air-sea heat exchanges and in the upper 50 m layer due to vertical diffusion of heat are highest at the southern location where intense convective activity followed by overcast skies and synoptic disturbance prevailed in the lower atmosphere. This and the estimated higher value (0.0235 m²/s) of eddy diffusivity coefficient of heat in the upper ocean (0–50 m depth) suggest that 1-D processes controlled the upper layer heat budget at the southern location. On the other hand, during the fair weather conditions, at the central and northern locations, the upper layer gained heat energy, while the sea surface lost (gained) heat energy at northern (central) location. This and lower values of eddy diffusivity coefficient of heat (0.0045 and 0.0150 m²/s) and the northward intensification of horizontal currents at these locations suggest the greater role of horizontal heat advection over the 1-D processes in the upper ocean heat budget at these two locations.     

Synoptic weather conditions during the pilot study of Bay of Bengal and Monsoon Experiment (BOBMEX)

BOBMEX-Pilot was organised from 23rd October–11th November, 1998 when the seasonal trough had already shifted to south Bay of Bengal. The activity during this period was marked by the development of a monsoon depression from 26th–29th October that weakened over the sea; onset of northeast monsoon along the east coast of India on 29th October; a low pressure area that formed on 2nd November over southwest Bay off Sri Lanka — southTamilnadu coast; and another cyclonic circulation that formed towards the end of the BOBMEX-Pilot period. This paper describes the development of these synoptic systems through synoptic charts and satellite data.     

Marine boundary layer characteristics during a cyclonic storm over the Bay of Bengal

During the period 12–16 June 1996 a tropical cyclonic storm formed over the southwest Bay of Bengal and moved in a north-northeasterly direction. The thermodynamic characteristics of this system are investigated by utilizing the surface and upper air observations collected onboardORV Sagar Kanya over the Bay of Bengal region. The response of the cyclonic storm is clearly evident from the ship observations when the ship was within the distance of 600–800 km from the cyclonic storm. This study explores why (i) the whole atmosphere from surface to 500 hPa had become warm and moist during the cyclonic storm period as compared to before and after the formation of this system and (ii) the lower layer of the atmosphere had become stable during the formative stage of the cyclonic storm.     

Proper depiction of monsoon depression through IRS-P4 MSMR

In this paper, daily variations of satellite-derived geophysical parameters such as integrated water vapour (IWV), cloud liquid water content (CLW), sea surface temperature (SST) and sea surface wind speed (SSW) have been studied for a case of monsoon depression that formed over the Bay of Bengal during 19th-24th August 2000. For this purpose, IRS P4 MSMR satellite data have been utilized over the domain equator — 25‡N and 40‡-100‡E. An integrated approach of satellite data obtained from IRS-P4, METEOSAT-5 and INSAT was made for getting a signal for the development of monsoon depression over the Indian region. Variations in deep convective activity obtained through visible, infrared and OLR data at 06 UTC was thoroughly analyzed for the complete life cycle of monsoon depression. Geophysical parameters obtained through IRS-P4 satellite data were compared with vorticity, convergence and divergence at 850 and 200 hPa levels generated through cloud motion vectors (CMVs) and water vapour wind vectors (WVWVs) obtained from METEOSAT-5 satellite. This comparison was made for finding proper consistency of geophysical parameters with dynamical aspects of major convective activity of the depression. From the results of this study it is revealed that there was strengthening of sea surface winds to the south of low-pressure area prior to the formation of depression. This indicated the possibility of increase in cyclonic vorticity in the lower troposphere. Hence, wind field at 850 hPa with satellite input of CMVs in objective analysis of wind field using optimum interpolation (OI) scheme was computed. Maximum cyclonic vorticity field at 850 hPa was obtained in the region of depression just one day before its formation. Similarly, with the same procedure maximum anticyclonic vorticity was observed at 200 hPa with WVWVs input. Consistent convergence and divergence at 850 and 200 hPa was noticed with respect to these vorticities. In association with these developments, we could get lowest values of OLR (120 W/m² ) associated with major convective activity that was consistent with the maximum values of integrated water vapour (6-8gm/cm²) and cloud liquid water content (50-60 mg/cm² ) persisting particularly in the southwest sector of the monsoon depression.     

Some salient features of the atmosphere observed over the north Bay of Bengal during BOBMEX

This paper describes the near surface characteristics and vertical variations based on the observations made at 17.5‡N and 89‡E from ORV Sagar Kanya in the north Bay of Bengal during the Bay of Bengal Monsoon Experiment (BOBMEX) carried out in July–August 1999. BOBMEX captured both the active and weak phases of convection. SST remained above the convection threshold throughout the BOBMEX. While the response of the SST to atmospheric forcing was clearly observed, the response of the atmosphere to SST changes was not clear. SST decreased during periods of large scale precipitation, and increased during a weak phase of convection. It is shown that the latent heat flux at comparable wind speeds was about 25–50% lower over the Bay during BOBMEX compared to that over the Indian Ocean during other seasons and tropical west Pacific. On the other hand, the largest variations in the surface daily net heat flux are observed over the Bay during BOBMEX. SST predicted using observed surface fluxes showed that 1-D heat balance model works sometime but not always, and horizontal advection is important. The high resolution Vaisala radiosondes launched during BOBMEX could clearly bring out the changes in the vertical structure of the atmosphere between active and weak phases of convection. Convective Available Potential Energy of the surface air decreased by 2–3 kJ kg^-1 following convection, and recovered in a time period of one or two days. The mid tropospheric relative humidity and water vapor content, and wind direction show the major changes between the active and weak phases of convection.     

An experimental study of radiative fluxes in the south Bay of Bengal during BOBMEX 1999

Time series measurements of radiative fluxes were made onboard INS Sagardhwani (SD) in the south Bay of Bengal near DS3 (13‡N and 87‡E) during the BOBMEX field experiment. An inter-comparison experiment conducted at DS3 showed that the radiative fluxes measured by Kipp and Zonen, Albedo meter and net Pyrgeometer onboard SD and by Eppley radiometers onboard ORV Sagar Kanya (SK) are well matched. It may be mentioned that the measurements showed consistency and good agreement between SD and SK ships, even though no Gimbal mounting was used for radiation instruments onboard SD. The main aim of the experiment was collection of high quality radiation data during the monsoon period, which can give an insight into the nature of the ocean-atmosphere coupling. The data on the four radiative fluxes collected on SD are averaged at 5 minute intervals and then hourly and daily averages have been computed. The hourly shortwave albedo and the atmospheric transmission factor are also computed and the variation of albedo in relation to the solar altitude and the transmissivity factor (TF) are studied. The mean albedo over the south Bay of Bengal under clear, partly cloudy and overcast skies are found to be 0.05, 0.07 and 0.2 respectively.     

Meteorological fields variability over the Indian seas in pre and summer monsoon months during extreme monsoon seasons

In this study, the possible linkage between summer monsoon rainfall over India and surface meteorological fields (basic fields and heat budget components) over monsoon region (30‡E-120‡E, 30‡S30‡N) during the pre-monsoon month of May and summer monsoon season (June to September) are examined. For this purpose, monthly surface meteorological fields anomaly are analyzed for 42 years (1958-1999) using reanalysis data of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The statistical significance of the anomaly (difference) between the surplus and deficient monsoon years in the surface meteorological fields are also examined by Student’s t-test at 95% confidence level. Significant negative anomalies of mean sea level pressure are observed over India, Arabian Sea and Arabian Peninsular in the pre-monsoon month of May and monsoon season. Significant positive anomalies in the zonal and meridional wind (at 2 m) in the month of May are observed in the west Arabian Sea off Somali coast and for monsoon season it is in the central Arabian Sea that extends up to Somalia. Significant positive anomalies of the surface temperature and air temperature (at 2 m) in the month of May are observed over north India and adjoining Pakistan and Afghanistan region. During monsoon season this region is replaced by significant negative anomalies. In the month of May, significant positive anomalies of cloud amount are observed over Somali coast, north Bay of Bengal and adjoining West Bengal and Bangladesh. During monsoon season, cloud amount shows positive anomalies over NW India and north Arabian Sea. There is overall reduction in the incoming shortwave radiation flux during surplus monsoon years. A higher magnitude of latent heat flux is also found in surplus monsoon years for the month of May as well as the monsoon season. The significant positive anomaly of latent heat flux in May, observed over southwest Arabian Sea, may be considered as an advance indicator of the possible behavior of the subsequent monsoon season. The distribution of net heat flux is predominantly negative over eastern Arabian Sea, Bay of Bengal and Indian Ocean. Anomaly between the two extreme monsoon years in post 1980 (i.e., 1988 and 1987) shows that shortwave flux, latent heat flux and net heat flux indicate reversal in sign, particularly in south Indian Ocean. Variations of the heat budget components over four smaller sectors of Indian seas, namely Arabian Sea, Bay of Bengal and west Indian Ocean and east Indian Ocean show that a small sector of Arabian Sea is most dominant during May and other sectors showing reversal in sign of latent heat flux during monsoon season.     

Seasonal Variability of Cyclone Heat Potential in the Bay of Bengal

Monthly maps of cyclone heat potential (CHP) in the Bay of Bengalhave been prepared by using Levitus climatological data set. Seasonal variability ofCHP in the Bay of Bengal has been studied using the CTD data sets collected duringfive cruises during the period, 1993–1996. High value (>30 kcal/cm²) of CHP coincided with anticyclonic gyre (ACG) and the low value of CHP (16 kcal/cm²) coincided with thecyclonic gyre (CG). This emphasizes the importance of gyres in the distribution ofCHP, which play an important role in the intensification of cyclones/depressions.CHP is >14 kcal/cm² over Andaman Sea, southern and Central Bay of Bengal where the generation and movement of cyclones take place during post south west monsoon season (October–November). A depression formed on 07.11.95 at 11°N; 91°E and intensified into a cyclonic storm by 8th November evening and crossed Orissa Coast on 9th November 1995. A few days before its formation, the value of CHP at the origin of thiscyclone was about 20 kcal/cm². To understand the exact role of CHP in theformation and intensification of cyclones/depressions over Bay of Bengal, more intense and systematic data sets are essential.     

Thermodynamic structure of the marine atmosphere over the region 80–87°E along 13°N during August (phase II) BOBMEX-99

Thermodynamic structure of the marine atmosphere in the region between 80 and 87‡E along 13‡N over the Bay of Bengal was studied using 13 high resolution radiosonde profiles from surface-400 hPa collected onboard ORV Sagar Kanya during the period 27th–30th August, during BOBMEX-99. Saturation point concept, mixing line analysis and conserved variable diagrams have been used to identify boundary layer characteristics such as air mass movement and stability of the atmosphere. The results showed relatively dry air near the ocean surface between 1000 and 950 hPa. This feature is confirmed by the conserved teta_v structure in this layer. Further, teta_v seldom showed any inversions in this region. The teta_e and teta_es profiles showed persistent low cloud layers between 900 and 700 hPa. The conserved variable diagrams (teta_e-q) showed the existence of double mixing line structures approximately at 950 and 700 hPa levels.     

Spatial and Temporal Variation of a Cyclonic Eddy Detected Downstream of the Tsushima Islands in November 2007

Horizontal, vertical and temporal distribution of a cyclonic (counterclockwise) eddy, where biological productivity is high, downstream of the Tsushima Islands in the eastern channel of the Tsushima Straits in November 2007 was revealed using conductivity–temperature–depth and acoustic Doppler current profiler data. The eddy had a horizontal scale of approximately 40–60 km, and the accompanying baroclinic current was more than 15 cm s⁻¹ at the edge of the eddy. The island-induced cyclonic eddy moved east-northeastward at about 10 km day⁻¹ (∼10 cm s⁻¹) along the Tsushima Warm Current and was intensified by the barotropic instability in the current shear. The cyclonic eddy with high surface chlorophyll a concentrations intensified in the vicinity of the Tsushima Islands and was advected by the Tsushima Warm Current towards the southwestern Japan Sea.     

Salient features of Andhra Pradesh cyclonic storm in the Bay of Bengal during September 1997

During 23–30 September 1997, a rare cyclonic storm has developed close to the Andhra coast, and it has later travelled parallel to coastline northward and finally crossed the land at Chittagong (22°N, 91°E) on 27 September. While translating along the east coast of India, it has produced heavy to very heavy rainfall on the coastal stations causing devastating floods. In this study, we made an attempt to understand the salient causes of this unique cyclone movement. We have analyzed daily fields of wind and relative humidity for 850, 700, 500 hPa and mean daily OLR data to understand the plausible reasons for its movement. The buoy data deployed by National Institute of Ocean Technology, Chennai, Viz. DS5 (15°N, 81°E), DS4 (19°N, 88°E) and SW7 (20°N, 86°E) were analyzed to understand the ocean–atmosphere interaction processes in the west Bay of Bengal during formation of the system. Analysis of OLR over the cyclonic storm region has revealed that the heavy rainfall areas coincide with low OLR (120–180 W m^?2). The persistent southward movement of 500 hPa ridge on the eastern wedge of the system along with the steering current at 200 hPa has helped in maintaining the movement of the system parallel to the east coast of India during its life cycle.     

Impact of global warming on cyclonic disturbances over south Asian region

A state-of-the-art regional climate modelling system, known as PRECIS (Providing REgional Climates for Impacts Studies) developed by the Hadley Centre for Climate Prediction and Research, UK is applied over the Indian domain to investigate the impact of global warming on the cyclonic disturbances such as depressions and storms. The PRECIS simulations at 50 × 50 km horizontal resolution are made for two time slices, present (1961–1990) and the future (2071–2100), for two socioeconomic scenarios A2 and B2. The model simulations under the scenarios of increasing greenhouse gas concentrations and sulphate aerosols are analysed to study the likely changes in the frequency, intensity and the tracks of cyclonic disturbances forming over north Indian Ocean (Bay of Bengal and Arabian Sea) and the Indian landmass during monsoon season. The model overestimates the frequency of cyclonic disturbances over the Indian subcontinent in baseline simulations (1961–1990). The change is evaluated towards the end of present century (2071–2100) with respect to the baseline climate. The present study indicates that the storm tracks simulated by the model are southwards as compared to the observed tracks during the monsoon season, especially for the two main monsoon months, viz., July and August. The analysis suggests that the frequency of cyclonic disturbances forming over north Indian Ocean is likely to reduce by 9% towards the end of the present century in response to the global warming. However, the intensity of cyclonic disturbances is likely to increase by about 11% compared to the present.     

Intensification of Aila (May 2009) due to a warm core eddy in the north Bay of Bengal

A very severe cyclonic storm ??Aila?? hit West Bengal on 26 May 2009. The storm intensified when it encountered with a warm core (SST?=?31°C) anti-cyclonic eddy (ACE4) in the north Bay of Bengal. The storm intensity increased by 43% due to this eddy, which is comparable with that (34%) obtained from a best fit line (derived from several numerical experiments over north-west Pacific Ocean). The shallow mixed layer of the large-scale ocean and deep mixed layer inside the eddy appear to be crucial parameters besides translation speed of the storm (Uh), ambient relative humidity and thermal stratification below mixed layer, in the storm intensification. From the eddy size and Uh, the eddy feedback factor is found to be about 0.4 (i.e. 40%), which is close to the above. Since there exists an inverse relationship between Uh and UOHC (upper ocean heat content), slow (fast) moving storms require high (low) UOHC. The warm ACE4 with a high UOHC of 149?kj/cm² (300% higher than the climatological value) and deep warm layer (D26?=?126?m) opposes the cooling induced by the storm and helps for the intensification of the storm through the supply of large enthalpy (latent?+?sensible) flux.     

Inter-annual variability of summer monsoon rainfall over Orissa (India) in relation to cyclonic disturbances

The summer monsoon rainfall over Orissa, a state on the eastern coast of India, is more significantly related than Indian summer monsoon rainfall (ISMR) to the cyclonic disturbances developing over the Bay of Bengal. Orissa experiences floods and droughts very often due to variation in the characteristics of these disturbances. Hence, an attempt was made to find out the inter-annual variability in the rainfall over Orissa and the frequencies of different categories of cyclonic disturbances affecting Orissa during monsoon season (June–September). For this purpose, different statistical characteristics, such as mean, coefficient of variation, trends and periodicities in the rainfall and the frequencies of different categories of cyclonic disturbances affecting Orissa, were analysed from 100 years (1901–2000) of data. The basic objective of the study was to find out the contribution of inter-annual variability in the frequency of cyclonic disturbances to the inter-annual variability of monsoon rainfall over Orissa. The relationship between summer monsoon rainfall over Orissa and the frequency of cyclonic disturbances affecting Orissa shows temporal variation. The correlation between them has significantly decreased since the 1950s. The variation in their relationship is mainly due to the variation in the frequency of cyclonic disturbances affecting Orissa. The variability of both rainfall and total cyclonic disturbances has been above normal since the 1960s, leading to more floods and droughts over Orissa during recent years. The inter-annual variability of seasonal rainfall over Orissa and the frequency of cyclonic disturbances affecting Orissa during monsoon season show a quasi-biennial oscillation period of 2–2.8 years. There is least impact of El Nino southern oscillation (ENSO) on inter-annual variability of both the seasonal rainfall over Orissa and the frequencies of monsoon depressions/total cyclonic disturbances affecting Orissa.     

Impact of various synthetic vortices on cyclone track prediction

Sensitivity experiments are conducted for three cases of cyclones for investigating the impact of different vortex initialization schemes on the structure and track prediction of the cyclone using India Meteorological Department’s Limited Area Model. The surface wind and pressure profiles generated using Holland and Rankine initialization schemes differ from each other. These different generated profiles are compared with the actual data and the root mean square error (RMSE) was calculated between them. In case of the Holland vortex, ‘b’ is found to be equal to 1.5 and 2.0 respectively for two cases of very severe cyclonic storms in the Arabian Sea, namely 6–10 June 1998 and 16–20 May 1999 and 2.25 for the severe cyclonic storm in the Bay of Bengal. The ‘α’ parameter in Rankine’s scheme was found to be 0.5 for two cases and 0.4 for the third system. This shows that cyclones differ even if they attain the same intensity. The values of these parameters i.e. ‘b’ and ‘α’ are used for generating the synthetic wind data for individual cyclones and the same is used in the data assimilation system. The analysis and forecast generated for the above cases using the Holland scheme show that the simulated structure has characteristics closer to the actual storm; however, the Rankine scheme shows a weaker circulation. The mean track error for three cases in the Holland scheme is 93, 149, 257 and 307 km in 12-, 24-, 36- and 48-h forecast. The mean track errors for the Rankine scheme are 152, 274, 345 and 327 km, respectively, for the same period.     

Impact of sea surface temperature in modulating movement and intensity of tropical cyclones

It is well recognized that sea surface temperature (SST) plays a dominant role in the formation and intensification of tropical cyclones. A number of observational/empirical studies were conducted at different basins to investigate the influence of SST on the intensification of tropical cyclones and in turn, modification in SST by the cyclone itself. Although a few modeling studies confirmed the sensitivity of model simulation/forecast to SST, it is not well quantified, particularly for Bay of Bengal cyclones. The present study is designed to quantify the sensitivity of SST on mesoscale simulation of an explosively deepening storm over the Bay of Bengal, i.e., Orissa super cyclone (1999). Three numerical experiments are conducted with climatological SST, NCEP (National Center for Environmental Prediction) skin temperature as SST, and observed SST (satellite derived) toward 5-day simulation of the storm using mesoscale model MM5. At model initial state, NCEP skin temperature and observed SST over the Bay of Bengal are 1–2°C warmer than climatological SST, but cooler by nearly 1°C along the coastline. Observed SST shows a number of warm patches in the Bay of Bengal compared with NCEP skin temperature. The simulation results indicate that the sea surface temperature has a significant impact on model-simulated track and intensity of the cyclonic storm. The track and intensity of the storm is better simulated with the use of satellite-observed SST.     

Inertial-Dissipation flux measurements over south Bay of Bengal during BOBMEX-Pilot experiment

This paper describes measurement of air-sea parameters and estimation of sensible and latent heat fluxes by the “Inertial-Dissipation” technique over south Bay of Bengal. The data were collected on ORV Sagar Kanya during BOBMEX-Pilot cruise during the period 23rd October 1998 to 12th November 1998 over south Bay of Bengal. The fluxes are estimated using the data collected through fast response sensors namely Gill anemometer, Sonic anemometer and IR Hygrometer. In this paper the analyses carried out for two days, one relatively cloud free day on November 3rd and the other cloudy with rain on November 1st, are presented. Sea surface and air temperatures are higher on November 3rd than on November 1st. Sensible heat flux for both the days does not show any significant variation over the period of estimation, whereas latent heat flux is more for November 3rd than November 1st. An attempt is made to explain the variation of latent heat flux with a parameter called thermal stability on the vapor transfer from the water surface, which depends on wind speed and air to sea surface temperature difference.     

Movement of seasonal eddies and its relation with cyclonic heat potential and cyclogenesis points in the Bay of Bengal

Movement of seasonal eddies in the Bay of Bengal (BOB) and its relation with cyclonic heat potential (CHP) and cyclogenesis points have been investigated in this study using 6 years (2002–2007) of global ocean monthly analysis datasets based on the Simple Ocean Data Assimilation (SODA) package (SODA v2.0.4) of Carton et al. (2005) and Indian Meteorological Department cyclogenesis points. The region dominated by anticyclonic eddies with CHP greater than 70 × 10⁷ J/m² as well as good correlations (>0.9) with sea surface height (SSH) and 26°C isothermal depth (D ₂₆) can be a potential region of cyclogenesis. The region dominated by cyclonic eddies with CHP greater than 50 × 10⁷ J/m² and good correlation (>0.9) with both SSH and D ₂₆ can serve as a potential region of high-level depression. Potential cyclogenesis regions are the southern BOB (5°N–12°N) for the post-monsoon season and the head of BOB (north of 15°N) during southwest monsoon. Seven potential regions are identified for the eddy formation for different seasons, which are consistent with the cyclogenesis points. The CHP distributions alone are able to explain the cyclone tracks for the pre-monsoon and post-monsoon seasons but not for the monsoon season.     

Weakening of lower tropospheric temperature gradient between Indian landmass and neighbouring oceans and its impact on Indian monsoon

The study shows that in the scenario of global warming temperature gradient (TG) between Indian landmass and Arabian Sea/Bay of Bengal is significantly decreasing in the lower troposphere with maxima around 850 hPa. TG during pre-monsoon (March to May) is reducing at a significant rate of 0.036°/year (Arabian Sea) and 0.030°/year (Bay of Bengal). The above alarming results are based on sixty years (1948–2007) of daily temperature and wind data extracted from CDAS-NCEP/NCAR reanalysis datasets. TG based on ERA-40 data also indicates a decreasing trend of 0.0229°/year and 0.0397°/year for Arabian Sea and Bay of Bengal respectively. As TG is not governed by any type of significant oscillation, there is a possibility of TG tending to zero. It is further observed that the rate of warming over the oceans is more than that over the land which has resulted into the weakening of TG. Pre-monsoon TG has significant correlations with