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 onboard ORV 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

Upper ocean variability in the Bay of Bengal during the tropical cyclones Nargis and Laila

Upper ocean variability at different stages in the evolution of the tropical cyclones Nargis and Laila is evaluated over the Bay of Bengal (BoB) during May 2008 and May 2010 respectively. Nargis initially developed on 24 April 2008; intensified twice on 27–28 April and 1 May, and eventually made landfall at Myanmar on 2 May 2008. Laila developed over the western BoB in May 2010 and moved westward towards the east coast of India. Data from the Argo Profiling floats, the Research Moored Array for African–Asian–Australian Monsoon Analysis and prediction (RAMA), and various satellite products are analyzed to evaluate upper ocean variability due to Nargis and Laila. The analysis reveals pre-conditioning of the central BoB prior to Nargis with warm (>30 °C) Sea Surface Temperature (SST), low (<33 psu) Sea Surface Salinity (SSS) and shallow (<30 m) mixed layer depths during March–April 2008. Enhanced ocean response to the right of the storm track due to Nargis includes a large SST drop by ∼1.76 °C, SSS increase up to 0.74 psu, mixed layer deepening of 32 m, shoaling of the 26 °C isotherm by 36 m and high net heat loss at the sea surface. During Nargis, strong inertial currents (up to 0.9 ms−1) were generated to the right of storm track as measured at a RAMA buoy located at 15 °N, 90 °E, producing strong turbulent mixing that lead to the deepening of mixed layer. This mixing facilitated entrainment of cold waters from as deep as 75 m and, together with net heat loss at sea surface and cyclone-induced subsurface upwelling, contributed to the observed SST cooling in the wake of the storm. A similar upper ocean response occurs during Laila, though it was a significantly weaker storm than Nargi

Ocean- Atmosphere Interactions During Cyclone Nargis

Cyclone Nargis (Figure 1a) made landfall in Myanmar (formerly Burma) on 2 May 2008 with sustained winds of approximately 210 kilometers per hour, equivalent to a category 3– 4 hurricane. In addition, Nargis brought approximately 600 millimeters of rain and a storm surge of 3– 4 meters to the low- lying and densely populated Irrawaddy River delta. In its wake, the storm left an estimated 130,000 dead or missing and more than $10 billion in economic losses. It was the worst natural disaster to strike the Indian Ocean region since the 26 December 2004 tsunami and the worst recorded natural disaster ever to affect Myanmar

On the warm pool dynamics in the southeastern Arabian Sea during April - May 2005 based on the satellite remote sensing and ARGO float data

Observational data from the Arabian Sea Monsoon Experiment (ARMEX-Phase IIA) in the southeastern Arabian Sea (SEAS) showed intense warming with the SST up to 31.5°C during April-May 2005. Analysis of 5-day repeat cycles of temperature and salinity profiles from an ARGO float (ID No. 2900345) in a 3°x1° box closer to ARMEX-II buoy (8.3°N, 72.68°E) in the SEAS during January-September 2005 revealed evolution of warm pool (SST>28°C) in spring 2005. The Argo data derived D20 (depth of 20°C isotherm) showed the influence of remote forcing during January-May, and local wind forcing during southwest monsoon. Low salinity waters (<34.0) occupied the top 30 m during January-February followed by temperature inversions (up to 0.5°C) in the 30-60 m depth range. From the peak spring warming, the SST dropped gradually by 3.5°C by end-July with the advent of southwest monsoon followed by a decrease in net heat gain upto 100 W/m^2. The merged weekly products of sea surface height anomalies and the NLOM simulated surface currents showed complex surface circulation consisting of seasonal Lakshadweep High/Low in winter/summer. The examined oceanic and atmospheric variables showed an intraseasonal variability with 41 to 63 day period, coinciding with the Madden-Julian Oscillatio

RAMA : the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 90 (2009):459-480, doi:10.1175/2008BAMS2608.1.The Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25°N by the Asian landmass. Seasonal heating and cooling of the land sets the stage for dramatic monsoon wind reversals, strong ocean–atmosphere interactions, and intense seasonal rains over the Indian subcontinent, Southeast Asia, East Africa, and Australia. Recurrence of these monsoon rains is critical to agricultural production that supports a third of the world's population. The Indian Ocean also remotely influences the evolution of El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), North American weather, and hurricane activity. Despite its importance in the regional and global climate system though, the Indian Ocean is the most poorly observed and least well understood of the three tropical oceans. This article describes the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA), a new observational network designed to address outstanding scientific questions related to Indian Ocean variability and the monsoons. RAMA is a multinationally supported element of the Indian Ocean Observing System (IndOOS), a combination of complementary satellite and in situ measurement platforms for climate research and forecasting. The article discusses the scientific rationale, design criteria, and implementation of the array. Initial RAMA data are presented to illustrate how they contribute to improved documentation and understanding of phenomena in the region. Applications of the data for societal benefit are also described

Supplement to RAMA : the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 90 (2009): ES5-ES8, doi:10.1175/2008BAMS2608.2

Influence of the monsoon trough on air-sea interaction in the head of the bay of bengal during the southwest monsoon of 1990 (monsoon trough boundary layer experiment-90)

The analysis of 3-hourly time-series data on surface meteorological parameters collected at 20° N, 89° E in the head of the Bay of Bengal during the southwest monsoon period (18 August–19 September) of 1990 under the MONTBLEX-90 programme reveals considerable temporal variability in sea-level pressure, sea-surface temperature (SST) and the fluxes of heat and momentum at the air-sea interface. This variability is related closely to the north-south movement of the monsoon trough and the formation and development of synoptic weather systems during this period. A rapid increase in wind speed, cloudiness, instability, momentum flux, sensible heat flux and moisture flux (by 80 Wm-2), and a decrease of SST (by 0.3 °C) and net surface heat flux by 80 Wm-2, was associated with the development of a depression when the monsoon trough moved southwards. At the peak of the depression, values of the latent heat flux and evaporation reached up to 270 Wm-2 and 1.0 cm day-1 respectively. During the depression period the heat loss across the air-sea interface matched well with the heat loss in the upper (≈100 m) ocean. With the northward movement of the monsoon trough, the momentum and surface heat fluxes decreased rapidly while the sea surface gained heat energy at rates up to 195 Wm-

Observations of barrier layer formation in the Bay of Bengal during summer monsoon

Time series of temperature and salinity in the upper ocean,measured at 17degrees30'N, 89degreesE in the northern Bay of Bengal,from 27 July to 6 August 1999 captured an event of upper layer freshening. Initially, the upper layer that is homogeneous in both temperature and salinity was about 30 m deep. Subsequently, the arrival of a freshwater plume caused the depth of the mixed layer to decrease to about 10 m and the salinity in the surface layer by about 4 psu. The plume led to the formation of a new halocline and hence a barrier layer within the upper 30 m of the water column. The ensuing ocean-atmosphere interaction was restricted to the new thinner mixed layer. The cooling that was restricted to the mixed layer led to an inversion in temperature amounting to 0.5degreesC just below the mixed layer. The source of the plume is traced to freshwater from river discharge and rainfall that was advected by Ekman flow as a 15 m thick layer. This study suggests that wind-driven circulation is crucial in determining the path of freshwater in the Bay of Bengal. The fresh water affects the sea surface temperature and ocean-atmosphere coupling through the dependence of the depth of the mixed layer on salinity

A biweekly mode in the equatorial Indian Ocean

The National Institute of Oceanography, Goa deployed moorings with several subsurface current meters at $0^{\mathrm{0}}, 93^{\mathrm{0}}E$ (in February 2000) and $0^{\mathrm{0}}, 83^{\mathrm{0}}$E (in December 2000) in the eastern Indian Ocean. Observed meridional current at all depths has a 10-20 day (or biweekly) variability that is distinct from longer period (20-60 day) subseasonal variability. Lags between dierent instruments suggest the presence of groups of westward and vertically propagating biweekly waves with zonal wavelength in the range 2100 to 6100 km. We use an ocean model forced by high resolution scatterometer wind stress to show that the observed biweekly variability is due to equatorially trapped mixed Rossby-gravity waves generated by subseasonal variability of winds. We demonstrate that quasi-biweekly fluctuations of surface meridional wind stress resonantly excite ocean waves with westward and upward phase propagation, with typical period of 14 days and zonal wavelength of 3000-4500 km. The biweekly wave is associated with fluctuating upwelling/downwelling in the equatorial Indian Ocean, with amplitude of 2-3 meters per day located $2^{\mathrm{0}}-3^{\mathrm{0}}$ away from the equator. Possible reasons for eastward intensification of biweekly energy is discussed

Chemical characteristics of Central Indian Basin waters during the southern summer

Chemical properties of the water column were examined at the Indian Deep-sea Environment Experiment (INDEX) site in the Central Indian Basin (CIB), as a part of baseline studies prior to the benthic disturbance experiment for the environmental impact assessment of mining of polymetallic nodules. The study shows three equatorward moving water masses. (a) The Subsurface Salinity Maximum in the depth range 125–200 m, characterized by high salinity (34.74–34.77 psu) and oxygen minimum associated with weak maxima in nutrients. (b) The Deep Oxygen Maximum (234–245 mM) in the depth range 250–750 m, associated with minima in nutrients and relatively high pH. (c) The Salinity Minimum Water (34.714–34.718 psu) corresponding to the Antarctic Intermediate Water (AAIW) at depths 800–1200m in the density (sy) range 27.2–27.5. Progressive changes in these characteristics are attributed to mixing with waters above and below, and to oxidation of organic detritus en route. Among the three water masses, the oxygen maximum water shows the lowest changes in its properties, which may suggest that this water mass is moving the fastest