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Ocean Mixing and Monsoon(OMM)

Science Quest Of The Week

Latent heat losses from the ocean are primarily determined by the wind and atmospheric humidity in the near surface layer. Although, the wind speed is relatively higher (6 to 12 ms-1) during the summer monsoon compared to spring, latent heat losses from the ocean are evenly comparable with the magnitude of latent heat loss during spring where the lowest wind speeds are observed (Figure i). During summer, the relative humidity field (80 to 90%) is high and uniform throughout the BoB. Hence, during the summer monsoon, specific humidity difference between the sea surface and the air (qs-qa) is very small due to the presence of high humid boundary layer air that leads to a small variability in the latent heat flux (-40 to -120 Wm-2) with a large range of wind speed (3 to 12 ms-1; Figure i). This result is consistent with the earlier study by Bhat and Narasimha [2007]. They suggested that during the summer monsoon, latent heat flux dependency to the wind is small in the BoB due to the high humidity in the BoB. During winter, a clear north-south difference in the humidity fields are observed in the BoB and it is due to the relatively drier boundary layer air (relative humidity 50 to 60%) in the northern BoB compared to the southern BoB (70 to 80%). Hence, during winter, particularly in the northern BoB, the specific humidity difference between the sea surface and the air are higher due to the presence of dry boundary layer air that leads to a large variability of latent heat flux (-40 to -280 Wm-2), even with a small range of wind speeds (4 to 8 ms-1; Figure i). During spring and winter (fall and summer), latent heat flux and wind speed shows similar relationship with steep (gentle) slope. The seasonal variability of the humidity field in the BoB is primarily associated with seasonal variability of the near-surface wind field in the Indian Ocean region. During summer, the large fetch of south-west monsoon winds over the Indian Ocean carries high humid air into the BoB and during winter, the northeast wind carries dry continental air into the BoB, leading to a reduction of humidity content.