Abstract
Abstract. Tropical cyclogenesis climatology over the South Indian and South Pacific Oceans has been developed using a new tropical cyclone (TC) archive for the Southern Hemisphere, and changes in geographical distribution of areas favourable for TC genesis related to changes in the El Niño-Southern Oscillation (ENSO) phases have been investigated. To explain these changes, large-scale environmental variables which influence TC genesis and development such as sea surface temperatures (SSTs), relative humidity in mid-troposphere, vertical wind shear and lower tropospheric vorticity have been examined. In the South Indian Ocean, reduction of TC genesis in the western part of the basin and its increase in the eastern part as well as displacement of the area favourable for TC genesis further away from the equator during La Niña events compared to El Niño events can be explained by changes in geographical distribution of relative humidity and vorticity across the basin as primary contributors; positive anomalies of SSTs observed during La Niña seasons in the eastern part of the basin additionally contribute to enhanced cyclogenesis near the Western Australia. In the South Pacific Ocean, changes in geographical distribution of relative humidity and vorticity appear to be the key large-scale environmental factors responsible for enhanced TC genesis in the eastern (western) part of the basin as well as for the northeast (southwest) shift of points of cyclogenesis during El Niño (La Niña) events, with vertical wind shear and SSTs as additional contributing large-scale environmental variables.
Highlights
Influence of the El Nino-Southern Oscillation (ENSO) phenomenon on tropical cyclone (TC) activity in the various regions of the Southern Hemisphere has been demonstrated in numerous studies (Nicholls, 1984; Hastings, 1990; Evans and Allen, 1992; Basher and Zheng, 1995; Nicholls et al, 1998; Camargo et al, 2007; Kuleshov et al, 2008)
This study aims to examine the TC genesis climatology over the South Indian Ocean and the South Pacific Ocean and to investigate changes in tropical cyclogenesis in different areas of the Southern Hemisphere in relation to changes of large-scale environmental factors associated with the ENSO phases
ENSO is a coupled ocean-atmosphere phenomenon and changes in oceanic conditions associated with El Nino and La Nina events are coupled with changes in atmospheric circulation
Summary
Influence of the El Nino-Southern Oscillation (ENSO) phenomenon on tropical cyclone (TC) activity in the various regions of the Southern Hemisphere has been demonstrated in numerous studies (Nicholls, 1984; Hastings, 1990; Evans and Allen, 1992; Basher and Zheng, 1995; Nicholls et al, 1998; Camargo et al, 2007; Kuleshov et al, 2008). Gray (1979) formulated six large-scale environmental parameters playing key roles in formation of TCs including the Coriolis parameter, low-level relative vorticity, relative humidity in the mid-troposphere, tropospheric vertical wind shear, ocean thermal energy, and difference in equivalent potential temperature between the surface and 500 hPa. Recently, Camargo et al (2007) used a genesis potential index which is similar to Gray’s seasonal genesis index (Gray, 1979) to diagnose ENSO effects on tropical cyclogenesis and concluded that in the basins of the Southern Hemisphere vertical wind shear and mid-tropospheric relative humidity are especially important, vorticity anomalies contribute most significantly in the central Pacific, and thermodynamic variable of the genesis potential index described in terms of potential intensity (Bister and Emanuel, 1998) plays a secondary role. Changes in sea surface temperature (SST) related to changes in ENSO phases are significant in the equatorial and near-equatorial regions of the Indian and Pacific Oceans, and it appears that the Published by Copernicus Publications on behalf of the European Geosciences Union
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