Diagnosis of tropical cyclone activity through gravity wave energy density in the South West Indian Ocean

Tropical cyclones (TC) are the most severe meteorological phenomena in the tropical South-West Indian Oceann (SWIO) between 30-90°E and 0-40°S. Each year, an average of 10.5 convective tropical disturbances including 4.5 cyclones is observed in this basin. A good knowledge of the multiscale wave processes involved in the dynamics of TCs, is important to improve tropical cyclone numerical modeling and forecasting as well as to understand their impact as convective sources of gravity waves (GW) on the atmosphere (Chane Ming et al., 2002). The present study analyzed convective GW energy density in the Upper Troposphere/Lower Stratosphere (UT/LS) in relation with TC activity using daily radiosonde data at Tromelin (15.53 S, 54.31 E), Outgoing Longwave Radiation (OLR) and best track dataset for the SWIO from 1997 to 2008. TC hours derived best tracks in the vicinity of Tromelin is used as an index to characterize the activity of TCs such duration and intensity (10-min sustained surface winds > 64 kt). Chane Ming et al. (2009) have first investigated GW energy activity related to cases of intenses TCs. They suggested a relationship between the maximum surface wind speeds and the daily total energy densities. First, we examine the cyclone season 2001-2002 which was reported as the second most active season over the past 30 years with 4 TCs including 2 intense TCs and 1 very intense TC in SWIO. Evolution of GW energy density is analyzed with regards to TC induced convection in the UT/LS. Two types of GW activity can be distinguished in the LS: those produced by TCs and local deep convection.Very low OLR values (< 240 W m^-2) and large Et values (> 12 J kg^-1) are observed during intense and very intense TCs (Guillaume, Dina and Hary). Secondly, we present a climatogical diagnosis of TC activity through GW energy density from 1997 to 2008. Monthly and weekly OLR values and GW total energy densities are compared to TC hour index. Results show a linear relationship between monthly total, kinetic and potential GW energy density and TC hours index above a threshold of 150 hours. It is also confirms using weekly GW total density energy in the LS and the activity of intense TCs above a threshold of 45 hours. In conclusion, we suggest a new index based on GW energy density to diagnosis of TC activity.