Back to Back Cyclogenesis over Arabian Sea in 2015: Dynamics and Biogeochemistry
Abstract
In the year 2015 two extremely severe cyclonic storms formed back to back in the post monsoon (October-November) season in the Arabian Sea. Usually, after the passage of cyclone as the upper ocean cools down, the chances of occurrence of another cyclone is rare. Hence, to understand the mechanism of such back to back occurrence of cyclones we analyzed the trend and studied the inter-annual variability of oceanic and atmospheric parameters that are crucial for the cyclogenesis such as sea surface temperature (SST), low level relative vorticity, vertical wind shear and mid tropospheric relative humidity from 1979 to 2017 in the region of genesis of cyclones Chapala (28 October- 4 November) and Megh (5 November - 10 November). Our results show linear increasing trends in SST and relative humidity during the study period. In particular, the year 2015 had the highest SST, second highest relative humidity, and second lowest vertical wind shear, all of which were pointing towards the prevalence of conditions congenial for the formation of both cyclones. From the wavelet analysis of outgoing longwave radiation (OLR), three prominent and significant modes of variability were inferred in the semi-annual, inter-annual and intra-seasonal bands. Further, based on the analysis of indices associated with El Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Madden Julian Oscillation (MJO) we suggest that the in-phase occurrence of strong El Nino, and positive IOD in 2015, combined with the occurrence of MJO during October-November led to the creation of atmospheric conditions congenial for the genesis of back to back cyclones. To understand the upper ocean physical and biogeochemical response we have analyzed the along track variation of SST, chlorophyll a, net primary production and CO2 flux for the two cyclones. SST cooling of 1.5 OC and 1.0 OC were observed due to the passage of cyclones Chapala and Megh respectively. Satellite-derived chlorophyll a showed 9-fold and 2-fold increases associated with the two cyclones. This was consistent with the surface chlorophyll a obtained from in situ Bio-Argo which showed a 10-fold increase. We further document the ocean dynamics and its strong coupling to the ocean biogeochemistry including CO2 out-gassing associated with both cyclones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMOS23F1682R
- Keywords:
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- 3364 Synoptic-scale meteorology;
- ATMOSPHERIC PROCESSESDE: 4215 Climate and interannual variability;
- OCEANOGRAPHY: GENERALDE: 4263 Ocean predictability and prediction;
- OCEANOGRAPHY: GENERALDE: 4504 Air/sea interactions;
- OCEANOGRAPHY: PHYSICAL