Response of Upper Ocean during passage of MALA cyclone utilizing ARGO data

Abstract

In the present study an attempt has been made to study the response of the upper ocean atmospheric interactions during the passage of a very severe cyclonic storm (VSCS) ‘MALA’ formed over the Bay of Bengal (BoB) on 24 April 2006. Deepening of mixed layer depth (MLD), weakening of barrier layer thickness (BLT) associated with a deeper 26 °C isotherm level (D26) is observed after the MALA passage. Tropical cyclone heat potential (TCHP) and depth averaged temperature ($T_{\overline{100}}$) exhibit a good degree of correlation for higher values. The passage of MALA cyclone also resulted in cooling the sea surface temperature (SST) by 4–5 °C. The findings suggest that turbulent and diapycnal mixing are responsible for cooler SSTs. Turbulent air–sea fluxes are analyzed using Objectively Analyzed air–sea Fluxes (OAFlux) daily products. During the mature stage of MALA higher latent heat flux (LHF), sensible heat flux (SHF), and enthalpy (LHF + SHF) are observed in the right side of this extreme event.

Introduction

Tropical cyclones are considered as one of the major natural hazards which inflict severe threat to human life and property having implications on socio-economic aspects in the affected regions. It is considered as the most intense case in air–sea interaction studies where energy from the warm ocean waters is supplied through surface heat flux (Emanuel, 1986). In a previous study (Emanuel, 1999) used a simple numerical model to demonstrate the evolution of hurricane intensity. The results advocate that in most cases the intensity depends on three factors viz.; initial intensity of cyclone, the thermodynamic state of atmosphere through which the cyclone propagate, and finally the heat exchange with the upper layer of the ocean underlying the core of the cyclone. The size of a tropical cyclone can vary from about 200 to 500 km during its entire life cycle (Liu and Chan, 1999). The primary mechanism that lowers the SST beneath a moving cyclone is accountable due to entrainment by turbulent mixing of irreversible heat flux from the ocean mixed layer, whereas air–sea heat exchange plays a minor role. In case of a slow moving cyclone, upwelling can significantly affect the SST, which is however reported negligible for fast moving cyclones (Price, 1981). Vertical mixing at the base of the mixed layer (ML) is another contribution for the upper ocean cooling (Black, 1983). In this context, Mahapatra et al. (2007) found the shift of the maximum sea surface cooling on the left side of the track prior to landfall. From observational support (Bender et al., 1993) it can be seen that magnitude of sea surface cooling (high, medium, and slow) depends upon the speed the cyclone (slow, medium, and fast). The response of ocean to cyclone utilizing numerical model for the Orissa super cyclone event was investigated by Rao et al. (2007). They found an inverse relationship between the sea surface cooling and translation speed.

In recent years, satellite observations have been widely used in understanding the upper ocean response to tropical cyclones (e.g. Lin et al., 2003, Goni and Trinanes, 2003). Subrahmanyam et al. (2005) utilized satellite and model simulated data to examine the thermal, salinity and circulation responses of the upper ocean during severe cyclone passage over the Bay of Bengal (BoB) and Arabian Sea (AS). It was reported that SST cooling is more in the AS compared to BoB due to contrasting hydrographic features viz. salinity stratification and associated mixing processes. High resolution SST data from TRMM satellite revealed that for pre-monsoon tropical cyclone tracks, the SST cools by 3 °C in the north Indian Ocean. Interestingly the strongest post-monsoon cyclones do not cool the north BoB, attributable due to shallow layer of freshwater layer from river runoff and torrential rainfall (Sengupta et al., 2008). In another study, Lin et al. (2009) used in situ ocean temperature measurements and satellite altimetry products to address the critical issue of rapid intensification of NARGIS cyclone (defined as ≥30 kts intensification in 24 h) just prior to the land fall. The implementation of international ARGO project is very useful to understand the variability and distribution of important air sea exchange parameters, viz. sea surface heat flux, freshwater storage, and transport during the passage of the tropical cyclone. Three dimensional temperature and salinity fields from ARGO floats are utilized to understand the upper ocean response to tropical cyclones in the northwestern Pacific by Liu et al. (2007) who examined dependence of the deepening in MLD, cooling of mixed layer temperature (MLT), and freshening of mixed layer salinity (MLS). In a recent study, Prasad et al. (2009) analyzed the water properties and geostrophic currents in Fiji waters during tropical cyclone GENE passage utilizing ARGO and satellite data. After the vaning away of GENE, thermocline depth was found to increase by 20 m, temperature drop by 3 °C and salinity profiles of 0.42 psu to a depth of 35 m were reported. Ramesh Kumar and Byju (2010) conducted a multi-sensor study to understand the formation of a cyclone over North Indian Ocean.

Park et al. (2005) studied the upper ocean response during typhoon passage and found high correlation between MLT and SST in the northern Pacific. In the present study an attempt has been made to understand the upper ocean atmospheric interactions and associated air–sea fluxes during the passage of MALA cyclone over the Bay of Bengal.