Abstract
Sea surface temperature (SST) reaches its annual maximum just before the summer monsoon onset and collapses soon after in the central areas of the Bay of Bengal (BoB). Here, the impact of the peak in the pre-monsoon SST on triggering the earliest monsoon onset in the BoB is investigated, with a focus on the role they play in driving the first-branch northward-propagating intra-seasonal oscillations (FNISOs) over the equatorial Eastern Indian Ocean (EIO). During the calm pre-monsoon period, sea surface warming in the BoB could increase the surface equivalent potential temperature (θe) in several ways. Firstly, warming of the sea surface heats the surface air through sensible heating, which forces the air temperature to follow the SST. The elevated air surface temperature accounts for 30 % of the surface θe growth. Furthermore, the elevated air temperature raises the water vapor capacity of the surface air to accommodate more water vapor. Constrained by the observation that the surface relative humidity is maintained nearly constant during the monsoon transition period, the surface specific humidity exhibits a significant increase, according to the Clausius–Clapeyron relationship. Budget analysis indicates that the additional moisture is primarily obtained from sea surface evaporation, which also exhibits a weak increasing trend due to the sea surface warming. In this way, it contributes about 70 % to the surface θe growth. The rapid SST increase during the pre-monsoon period preconditions the summer monsoon onset over the BoB through its contributions to significantly increase the surface θe, which eventually establishes the meridional asymmetry of the atmospheric convective instability in the EIO. The pre-established greater convective instability leads to the FNISO convections, and the summer monsoon is triggered in the BoB region.
Highlights
The Asian monsoon is one of the most significant climate systems worldwide
We focus on the premonsoon period (PMP), defined as a calm period between day −40 and day −10 (Fig. 1)
During the monsoon transitional period, complex ocean– atmosphere–land interactions around South Asia lead to a striking sea surface warming in the central areas of the Bay of Bengal (BoB)
Summary
The Asian monsoon is one of the most significant climate systems worldwide. A distinct seasonal contrast occurs, resulting from complex ocean–atmosphere–land interactions that respond to the seasonal variations of solar radiation (Webster 2006). Before the summer monsoon onset, strong sea surface heating makes the central region of the BoB the warmest in the tropical Indian Ocean. This high sea surface temperature (SST) is considered to be responsible for the development of convective systems, which trigger the summer monsoon. Wu et al (2012) attributed the critical convective systems to strong onset vortices that could be identified in about half of the years between 1979 and 2009 They argued that the surface sensible heating induced by the high SST over the warm southeastern BoB is of vital importance for the genesis of the vortices. Possible role of pre-monsoon sea surface warming in driving the summer monsoon onset over
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