Articles written in Journal of Earth System Science

    • Relationship between summer monsoon rainfall and cyclogenesis over Bay of Bengal during post-monsoon (October–December) season

      Y Sadhuram K Maneesha

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      In this study, an attempt has been made to examine the relationship between summer monsoon rainfall (June–September) and the total number of depressions, cyclones and severe cyclones (TNDC) over Bay of Bengal during the post-monsoon (October–December) season. The seasonal rainfall of the subdivisions (located in south India) (referred as rainfall index – RI), is positively and significantly correlated(r = 0.59; significant at >99% level) with the TNDC during the period, 1984–2013. By using the first differences (current season minus previous season), the correlations are enhanced and a remarkably high correlation of 0.87 is observed between TNDC and RI for the recent period, 1993–2013. The average seasonalgenesis potential parameter (GPP) showed a very high correlation of 0.84 with the TNDC. A very high correlation of 0.83 is observed between GPP and RI for the period, 1993–2013. The relative vorticity and mid-tropospheric relative humidity are found to be the dominant terms in GPP. The GPP was 3.5 times higher in above (below) normal RI in which TNDC was 4 (2). It is inferred that RI is playing a keyrole in TNDC by modulating the environmental conditions (low level vorticity and relative humidity) over Bay of Bengal during post-monsoon season which could be seen from the very high correlation of 0.87 (which explains 76% variability in TNDC). For the first time, we show that RI is a precursor for the TNDC over Bay of Bengal during post-monsoon season. Strong westerlies after the SW monsoon seasontransport moisture over the subdivisions towards Bay of Bengal due to cyclonic circulation. This circulation favours upward motion and hence transport moisture vertically to mid-troposphere which causes convective instability and this in turn favour more number of TNDC, under above-normal RI year.

    • Ocean impact on the intensification of cyclone Titli


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      Tropical cyclones are the most devastating weather events which still needs to understand much due to the uniqueness in their paths and intensification locations. One such recent very severe cyclonic storm, Titli, is perfectly fit for the individual case study. The cyclone Titli originated from a low-pressure area formed over the southeast Bay of Bengal (BoB) and adjoining north of the Andaman Sea on 7th October 2018 and intensified into a very severe cyclonic storm on 10th over a region of low saline water pool in the western Bay of Bengal. The addition of low salinity water in this region is from the lower Mahanadi basin and surroundings that received excess rainfall in September 2018. The low saline waters developed high stratification and suppressed the upwelling at the cold-core eddy center and restricted to 50 m. Persistent high heat content and high internal energy are the primary sources of intensification of cyclone Titli. Generally, intense cyclones enrich biomass's enhancement in the ocean's surface layer after their passage. But in the case of Titli, biomass enrichment did not happen due to persistent high stratification, which is about 3–4. Even the 80 knots (150 km/h) winds cannot break up the upper layer stratification, suppressing the cyclone-induced upwelling. Observations show only a 0.2–0.4 mg/m$^{3}$ rise in chlorophyll on the surface and it may be due to the land drove nutrients and lightning. Data analyzed from the high-resolution global NEMO model also shows the intrusion of low saline waters off India's east coast. Though the model is slightly overestimating the upwelling in the cold core eddy's surface layers, it represents the barrier caused by stratified waters. Both the observations and model datasets are well correlating, daily analyzed model data is well capturing the seasonal stratification caused by the low saline waters in the northern Bay of Bengal. These high-resolution global ocean model datasets are useful and essential for the accurate forecast of cyclones over the Bay of Bengal.

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