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 (CAPE) 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

Response of the upper layers of the northern and central Bay of Bengal to the atmospheric events during summer monsoon of 1999

715-723Bay of Bengal Monsoon Experiment (BOBMEX-99) provided valuable information on the influence of atmospheric systems on the upper ocean thermohaline structure at 13oN, 87oE (TS1) and 17.5oN, 89oE (TS2). Two stations, separated by 545 km, were selected based on the fact that the occurrence of organized convection was high at both locations. From oceanographic point of view, TS1 and TS2 were located within anti-cyclonic and cyclonic gyres respectively, that shifted westward at a rate of 6.3 cm/sec. During the BOBMEX-99 observational period, three systems were formed over the northern BoB. Because of the proximity of TS2 to various systems, its influence was more reflected on the atmospheric and oceanic parameters at this location than at TS1. During the initial stages, atmospheric pressure at TS2 dropped by 3-6 mb and the atmosphere and the ocean surface cooled by 1.5 to 3oC and 0.35 to 0.85oC respectively. Associated with the systems, thermal inversions of 0.1oC to 0.15oC were observed at the base of isothermal layer. Surface salinity dropped by 0.8 psu and 1.2 psu with the rainfall during 31 July-1 August (70 mm) and 15-6 August (>90 mm) respectively. The freshwater influx resulted in the formation of a barrier layer at TS2, which showed a positive relation thermal inversions. During BOBMEX-99 period, the systems were formed whenever there was large positive salinity gradient; sea surface temperature exceeded 28.5oC and heat content with respect to 28.3oC exceeded ~3107 Jm-2. </span

Long period waves in the coastal regions of north Indian Ocean

150-154The sea surface height (SSH) anomaly derived from TOPEX/POSEIDON altimeter was used to study the characteristics of long period waves in the coastal belt of north Indian Ocean. Wavelet analysis at typical locations revealed that the most dominant signals were in the bands of intra-seasonal oscillation (ISO), i.e. 30-60 days, 3-4 months, semi-annual and annual. Among them, the annual signal contained maximum energy at all the locations. But, inside the Andaman Sea, and off the Gulf of Aden, the ISO component is of nearly equal magnitude as that of annual. In the regions of large SSH variability, low frequency components (more than 3 months) dominated, whereas they contained less energy in the region of minimum SSH variability. Hovmullor diagrams revealed large inter- and intra-annual variability in the SSH fields. In general, along the eastern boundary of the ocean, high sea level was observed during April-June and October-December in association with equatorial Kelvin waves. Another notable observation was the formation of high SSH in the western Indian Ocean during February-May and August-October. Among them, the former one propagated northward while latter one was found stationary. The Markan coast was found as the region of meeting place of two waves, one from the western Indian Ocean and the other from the eastern Bay. The results can be utilized to enhance the understanding of various coastal processes controlled by long period waves, viz. upwelling, sinking, circulation and meso-scale features

Mixing in the Arabian Sea off Calicut during February 1985

194-197Short time variability in the temperature field at two stations off Calicut was analysed. Lower diurnal range of temperature was noticed in the surface layers at the shallow station (0.4-degrees-C) than at the deep station (0.8-degrees-C). This feature was explained based on surface heat exchanges, eddy diffusivity and Monin-Obukhov length. Monin-Obukhov length revealed the dominance of buoyant mixing over mechanical mixing at both the stations. Greater turbulence within the mixed layer at the shallow station (93 cm2. sec-1) than at deep station (78 cm2. sec-1) as revealed by the coefficient of eddy diffusivity reduced the amplitude of surface temperature at the shallow station. Two layers of thermal inversions were observed at the deep station while at the shallow station these two layers merged to form a single inversion layer. It is concluded that these thermal inversions are caused by the intrusion of the Arabian Sea High Salinity Watermass

Diurnal scale variability in vertical thermal structure of coastal waters off southwest coast of India during May 1985

71-76Diurnal scale variations observed in the vertical thermal structure at 3 stations of the continental shelf (off Cape Comorin, Vizhinjam and Cochin) during May 1985 are analysed with short time series data sets collected onboard R V Gaveshani (cruise 154). Heating and cooling cycles observed in the surface layer are qualitatively discussed in terms of surface heat exchange processes. Large amplitude variations noticed in the thermocline suggest the prominence of internal waves. Vertical eddy diffusivity coefficient evaluated at one of the stations showed a value of 72 cm2 sec-1

A case study on the short-term variability in the observed temperature and currents in the upper layers of the northeastern Arabian Sea during the pre-onset phase of ISMEX-73

189-194The observed short-term variability in the temperature records of the top 200 m water column and in the current meter records at 50 and 75 m depths was investigated with the aid of short time series measurements made at a single station (18ᵒN, 67ᵒE) in the NE Arabian Sea (29 May to 5 June 1973). The sea surface temperature increased by about 0.5 ᵒ C and the temperature at 75 and 100 m depths did not show any perceptible changes. However, a remarkable feature was a temperature drop of 2ᵒC at 50 m depth during this period. Both vertical and lateral advection appeared to have contributed towards cooling of 1.2ᵒC and 0.8ᵒC respectively in association with the readjustment conditions produced in the upper layers by a westward moving meteorological disturbance during the preceding week

Short term responses in ocean thermal characteristics during winter to atmospheric forcing and advection off Bombay, west coast of India

60-65The observed variability of meteorological forcing parameters and ocean thermal characteristics was examined in relation to MLD(mixed layer depth) and heat content based on time series data collected off Bombay during winter at 2 sites(deep and shallow) and vertical temperature sections. Both surface meteorological forcing and thermal variability were predominantly of semidiurnal and intra-semidiurnal periods. Delay for MLD shoaling and deepening in response to hearing and wind stirring was more at deep site. Increased thermal variability and inversions between 80 and 150m characterised the deep site on the slope while well mixed warmer waters were present at shallow site on the shelf Differential heat advection of cold and warm waters above and below 80m at deep site and progressive mild cooling of shelf waters at shallow site were supported by probable warm and cold core eddy interaction and tidal flows

Multiple subsurface maxima in vertical salinity structure: A case study

77-81Temperature and salinity data collected in the coastal waters off Cochin during the pre-onset phase of summer monsoon 1992 revealed the occurrence of some fine scale structures. Vertical salinity structure is characterised by subsurface salinity maxima with salinities 35.55-35.75 x 10(-3). Transient multiple high salinity maxima separated by low saline waters are observed in the halocline. Associated with these fine scale features, either step like structures or inversions in the temperature field and large fluctuations in sound speed gradients are noticed. Low values of Richardson number (<0.25) between 50 and 70 m suggest the dominance of shear mixing over buoyant mixing. Like any other coastal upwelling system a prominent northward flowing undercurrent is noticed below 50 m

An empirical model for estimating hourly solar radiation over the Indian seas during summer monsoon season

123-131 Performance of the widely used Octa model [Quart. J. R. Met. Soc., 114 (1988) 165-182] for the estimation of hourly solar radiation over the Indian seas during summer monsoon season is evaluated. This model was found to overestimate radiation for all sky conditions, in particular for 7 and 8 octa cloud cover by 300 W.m-2. For overcast sky conditions, observed radiation was found to be independent of the altitude of the sun and dependent only on cloud thickness. The dominant cloud types observed during this period are thick cumulus and cumulonimbus with an average base height of 300-1000 m. With a view to improve hourly estimation of radiation for all cloud categories, a new set of regression equations was developed based on transmission of radiation in the atmosphere and the altitude of the sun. With the new model, estimation has considerably improved for all cloud amounts as indicated by the reduction in the standard error estimates, in particular for 7 and 8 octa cloud cover. Application of these two models to three independent data sets also suggests the suitability of the new model for the hourly estimation of radiation for monsoon cloud conditions over the Indian Seas. </smarttagtype

Thermohaline and current structure off Cochin during December 1986

244-248Time series measurements of surface meteorological elements, vertical temperature and salinity profiles and subsurface currents at selected depths, made at a station off Cochin (depth 65 m), were utilised to describe short-term variability of these parameters during 4-8 December 1986. The vertical temperature profiles were nearly isothermal but showed diurnal perturbations of about 0.5-degrees-C in the upper layers. The salinity structure showed influx of Bay of Bengal/Equatorial Indian Ocean waters carried by the prevailing northerly current. The currents at all depths were northerly against the local wind direction, with speeds over 40 cm.sec-1 at 10 and 20 m depths during the first 2 d. The currents exhibited semidiurnal periodicity indicating predominance of semi-diurnal tide. The vertical current shear was maximum between 10 and 20 m depths which also decreased with the weakening of the flow from 6 to 8 December