Diagnosis of tropical cyclone activity in the UT/LS 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 ocean (SWIO, 30-90°E ; 0-40°S). Each year, an average of 9 convective tropical disturbances including 5 tropical cyclones is observed in this basin. A good knowledge of the multi-scale 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. The present study analyzes 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 from best tracks in the vicinity of Tromelin are used as an index to characterize the TCs activity such as duration and intensity. Only systems with 10-min sustained surface winds higher than 64 kt are considered. First, we examine the cyclone season 2001-2002 which was reported as the second most active season over the past 30 years in the SWIO, with 4 TCs including 2 intense TCs and 1 very intense TC. Evolution of GW energy density in the UT/LS is analyzed with regards to deep convection. Two types of GW activity can be distinguished: those produced by TCs and those generated by local deep convection. Very low OLR values (< 240 W m-2) and large GW energy density values (> 12 J kg-1) are observed during intense and very intense TCs. 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 hours index. Results show a linear relationship between monthly total, kinetic and potential GW energy density and TC hours above a threshold of 150 hours per month. This is confirmed at a smaller time scale: a linear relationship is found between the weekly GW total density energy in the LS and the activity of intense TCs above a threshold of 45 hours per week. In conclusion, we suggest a new index based on GW energy density as a diagnosis of TC activity.