Variability of Ocean Features and their Impact on Cyclogenesis over Arabian Sea During Post Monsoon Season

Arabian Sea (AS), the western sector of North Indian Ocean (NIO) produce smaller number of 9 tropical cyclones as compared to Bay of Bengal. Though limited in numbers, the cyclones over 10 Arabian sea are catastrophic by character. This make west coast of Indian subcontinent 11 vulnerable to these hazards. The post-monsoon cyclogenesis over this region is known to be 12 modulated by both monsoon rainfall and the El-Niño accompanied with positive Indian Ocean 13 Dipole events. No single phenomena, however, can fully explain the variability observed in 14 AS region. 15 In this study, it is observed that apart from several known atmospheric forcings, inter-annual 16 variability of ocean heat content (OHC) influence the post-monsoon AS cyclogenesis. The 17 OHC of this region is partially modulated by the changes in salinity. Heat exchanges between 18 the South West Indian Ocean (SWIO) and AS also modulates the OHC over AS. This remote 19 influence is facilitated largely by the variability in the equatorial currents. Further it is seen that 20 the recent trend of increased OHC post-2011 matches with the enhanced sea surface carbon 21 over AS. 22

October-December. During both these phases the cyclones causes the storm surges 52 accompanied with large amplitude wind waves and tides. On an average 1-2 cyclones form 53 over AS, most are intense enough to cause an impactful landfall. These makes AS cyclones 54 hazardous for the west coast of India. (Murthy and Sabh 1984). In an interesting study Evan 55 and Camargo (2011) showed that cyclones of May and June over AS are associated with early 56 and late monsoon onset, respectively. The cyclones in November are associated with high sea 57 level pressure over BoB. Thus, an active cyclonic season in AS implies non-occurrence in BoB.

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Off late in the post monsoon months, AS has shown a significant rise in the cyclonic activities.

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The persistence of AS cyclones from 2011 onwards makes the perception about its innerness 60 quite precarious. Increasing cyclonic activity over AS was also linked to global warming by cyclogenesis, yet studies discussing the inter-annual variability of physical and chemical 64 properties of underlying oceans that may be important for changing behaviour of the AS is rare.

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In this study, we observe the inter-annual changes of the Ocean Heat Content (OHC) are

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We explore best track data from 1979-2019 available from U.S. Navy's Joint Typhoon 77 Warning Centre (JTWC) to look into long term record of cyclones over AS. The mean 78 precipitation over AS and adjoining SWIO was analysed to observe its dependence on ENSO 79 events. This is done using Climate Prediction Centre (CPC) Merged Analysis of Precipitation 80 (CMAP) data. Ocean salinity and temperature fields are taken from Ocean Re-Analysis 81 (ORAS4) data provided by European Centre for Medium Range Weather Forecast (ECMWF).

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Using ORAS4, ocean heat content and its freshening is studied. Prior to its use, ORAS4 data 83 is validated using temperature/salinity profiles from in-situ ARGOs. Observation based gridded 84 monthly SPCO2 is used to analyse ocean carbon content and its correlation with SST. SST data used 85 here is collected from ECMWF Reanalysis (ERA) interim datasets. A brief description of the data 86 and methodology is provided below.  surface salinity. Therefore, this data has been validated using the Argo profiles prior to its use.

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ORAS4 temperature profiles has been used to compute the ocean heat content over the SWIO 129 and AS which is mathematically given as: Here ρ is the density. Usually the density is salinity dependent, however for simplicity in 132 calculation this is taken constant over AS. Cp is the specific heat at a constant pressure and T 133 is the temperature of dz is infinitesimal depth of water. The calculation is limited between the 134 surface to a depth D. One may consider D=2000m or even more. However, Häkkinen et al. between AS and SWIO, the meridional transport has been calculated using the ORAS4 data. April, May and August; however, ORA is able to capture the largescale variability of the 174 salinity over time. Therefore, while analysing the inter-annual variability of the salinity data 175 from ORAs4, in this study we restrict ourselves to a depth between surface to 100m.  2011. The figure 6b shows the correlation coefficient between the OHC and ACE over AS.

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The correlation here is generated using a moving window of five consecutive years or a pentad.  1982, 1994, 1997 and 2015. 218 In all these years salinity was less implying fresh upper layer of the ocean that can trap heat.

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This freshening could be a probable impact of good monsoon rainfall.

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To a contrary, few noteworthy years of exceptions are also there. In these years freshening   heat transport and is facilitated by weakening of the zonal equatorial currents. is local and the other is a remote ocean process.

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A reduced salinity due to good monsoon rainfall traps heat in upper parts of the ocean due to 295 stratification, increasing the OHC. This is the first local process. Further, after 2011 the 296 monotonic enhancement of partial pressure of carbon is found to be highly correlated to the 297 SST of AS and therefore certainly modulates the local OHC.

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When we discuss the remote impact, heat exchanges between AS and South Western Indian