BOBMEX: The Bay of Bengal Monsoon Experiment

The first observational experiment under the Indian Climate Research Programme, called the Bay of Bengal Monsoon Experiment (BOBMEX), was carried out during July-August 1999. BOBMEX was aimed at measurements of important variables of the atmosphere, ocean, and their interface to gain deeper insight into some of the processes that govern the variability of organized convection over the bay. Simultaneous time series observations were carried out in the northern and southern Bay of Bengal from ships and moored buoys. About 80 scientists from 15 different institutions in India collaborated during BOBMEX to make observations in most-hostile conditions of the raging monsoon. In this paper, the objectives and the design of BOBMEX are described and some initial results presented. During the BOBMEX field phase there were several active spells of convection over the bay, separated by weak spells. Observation with high-resolution radiosondes, launched for the first time over the northern bay, showed that the magnitudes of the convective available potential energy (CA-PE) and the convective inhibition energy were comparable to those for the atmosphere over the west Pacific warm pool. CAPE decreased by 2-3 kJ kg(-1) following convection, and recovered in a time period of 1-2 days. The surface wind speed was generally higher than 8 m. s(-1). The thermohaline structure as well as its time evolution during the BOBMEX field phase were found to be different in the northern bay than in the southern bay. Over both the regions, the SST decreased during rain events and increased in cloud-free conditions. Over the season as a whole, the upper-layer salinity decreased for the north bay and increased for the south bay. The variation in SST during 1999 was found to be of smaller amplitude than in 1998. Further analysis of the surface fluxes and currents is expected to give insight into the nature of coupling.

[1]  D. Shankar,et al.  Hydrography and circulation in the western Bay of Bengal during , 1996 .

[2]  S. Gadgil.  Monsoon-ocean coupling , 2000 .

[3]  M. Goel,et al.  Monsoon Trough Boundary Layer Experiment (MONTBLEX) , 1990 .

[4]  T. N. Krishnamurti,et al.  Summer Monsoon Experiment—A Review , 1985 .

[5]  Debasis Sengupta,et al.  First results from a new observational system over the Indian seas , 2000 .

[6]  V. Murty,et al.  Thermohaline structure and circulation in the upper layers of the southern Bay of Bengal during BOBMEX-Pilot (October–November 1998) , 2000 .

[7]  T. Yasunari.  Structure of an Indian Summer Monsoon System with around 40-Day Period , 1981 .

[8]  Roger Lukas,et al.  The mixed layer of the western equatorial Pacific Ocean , 1991 .

[9]  T. N. Krishnamurti,et al.  Oscillations of a Monsoon System. Part I. Observational Aspects , 1976 .

[10]  J. S. Godfrey,et al.  Coupled Ocean-Atmosphere Response Experiment (COARE): An interim report , 1998 .

[11]  Richard E. Payne,et al.  The IMET (Improved Meteorology) Ship and Buoy Systems , 1995 .

[12]  M. Moncrieff,et al.  The dynamics and simulation of tropical cumulonimbus and squall lines , 1976 .

[13]  A. Suryanarayana,et al.  Physical oceanography of the Bay of Bengal and Andaman Sea , 1996 .

[14]  N. Mohankumar,et al.  Genesis of meteorological disturbances and thermohaline variability of the upper layers in the head of the Bay of Bengal during MONsoon Trough Boundary Layer EXperiment (MONTBLEX-90) , 1994 .

[15]  P. Webster,et al.  TOGA COARE: The Coupled Ocean-Atmosphere Response Experiment. , 1992 .

[16]  T. Barnett,et al.  Sea Surface Temperature, Surface Wind Divergence, and Convection over Tropical Oceans , 1987, Science.

[17]  J. F. Festa,et al.  Evolution of the climatological near‐surface thermal structure of the tropical Indian Ocean: 1. Description of mean monthly mixed layer depth, and sea surface temperature, surface current, and surface meteorological fields , 1989 .

[18]  W. Liu,et al.  Bulk Parameterization of Air-Sea Exchanges of Heat and Water Vapor Including the Molecular Constraints at the Interface , 1979 .

[19]  Sulochana Gadgil,et al.  On the Maximum Cloud Zone and the ITCZ over Indian, Longitudes during the Southwest Monsoon , 1980 .

[20]  P. Vinayachandran,et al.  The warm pool in the Indian Ocean , 1991, Journal of Earth System Science.

[21]  M. Ravichandran,et al.  Oscillations of Bay of Bengal sea surface temperature during the 1998 Summer Monsoon , 2001 .

[22]  S. P. Anderson,et al.  Surface meteorology and air-sea fluxes in the western equatorial Pacific warm pool during the TOGA c , 1996 .

[23]  S. Gadgil,et al.  Ocean–atmosphere coupling over monsoon regions , 1984, Nature.

[24]  Janet Sprintall,et al.  Evidence of the barrier layer in the surface layer of the tropics , 1992 .

[25]  A. Gouveia,et al.  Coastal circulation in the North Indian Ocean: Coastal segment (14,S-W) , 1998 .

[26]  V. Murty,et al.  Thermal response of upper layers of Bay of Bengal to forcing of a severe cyclonic storm: a case study , 1993 .

[27]  E. Williams,et al.  An Analysis of the Conditional Instability of the Tropical Atmosphere , 1993 .

[28]  Harry H. Hendon,et al.  Intraseasonal Variability of Surface Fluxes and Sea Surface Temperature in the Tropical Western Pacific and Indian Oceans , 1998 .

[29]  V. Murty,et al.  Variability of the oceanic boundary layer characteristics in the northern Bay of Bengal during MONTBLEX-90 , 1996 .

[30]  J. Kuettner,et al.  Report on the summer MONEX field phase , 1980 .

[31]  R. Lukas,et al.  Surface Buoyancy Forcing and the Mixed Layer of the Western Pacific Warm Pool: Observations and 1D Model Results , 1996 .

[32]  V. Murty,et al.  Water characteristics, mixing and circulation in the Bay of Bengal during southwest monsoon , 1992 .

[33]  Thermodynamic characteristics of air flowing into and out of precipitating convection over the west Pacific warm pool , 1999 .

[34]  J. S. Godfrey,et al.  The heat budget of the equatorial western Pacific surface mixed layer , 1989 .

[35]  P. Kumar,et al.  A summary of results on thermohaline variability in the upper layers of the east central Arabian Sea and Bay of Bengal during summer monsoon experiments , 1993 .