Abstract
Exploratory analysis using empirical orthogonal function revealed the presence of a stationary zonal wavenumber-4 (W4) pattern in the sea surface temperature (SST) anomaly in the southern subtropics (20°S–55°S). The signal over the Southern subtropics is seasonally phase-locked to the austral summer and persists up to mid-autumn. Thermodynamic coupling of atmosphere and the upper ocean helps in generating the W4 pattern, which later terminates due to the breaking of that coupled feedback. It is found that the presence of anomalous SST due to W4 mode in the surrounding of Australia affects the rainfall over the continent by modulating the local atmospheric circulation. During positive phase of W4 event, the presence of cold SST anomaly over the south-eastern and -western side of Australia creates an anomalous divergence circulation. This favours the moisture transport towards south-eastern Australia, resulting in more rainfall in February. The scenario reverses in case of a negative W4 event. There is also a difference of one month between the occurrence of positive and negative W4 peaks. This asymmetry seems to be responsible for the weak SST signal to the South of Australia. Correlation analysis suggests that the W4 pattern in SST is independent of other natural variabilities such as Southern Annular Mode, and Indian Ocean Dipole as well as a rather weak relationship with El Niño/Southern Oscillation.
Highlights
Exploratory analysis using empirical orthogonal function revealed the presence of a stationary zonal wavenumber-4 (W4) pattern in the sea surface temperature (SST) anomaly in the southern subtropics (20°S–55°S)
Weather and climate of the Southern Hemisphere are affected by different modes of climate variations such as El Niño/Southern Oscillation[11] (ENSO), Indian Ocean Dipole[12] (IOD), Atlantic Niño[13], Southern Annular Mode[14] (SAM), Antarctic Circumpolar W ave[15], Indian Ocean subtropical dipole[3] (IOSD), south Atlantic subtropical dipole[10,16] (SASD) and the South Pacific subtropical d ipole[17] (SPSD)
The presence of subtropical dipoles in the southern subtropical ocean basins have been reported in regional scale[3,10,39]
Summary
Exploratory analysis using empirical orthogonal function revealed the presence of a stationary zonal wavenumber-4 (W4) pattern in the sea surface temperature (SST) anomaly in the southern subtropics (20°S–55°S). The question arises whether W4 pattern is evident only in the atmospheric circulations as waves 1–318, or it is extended to the ocean like that of zonal wave number-326 To explore such a relationship and to extend the work of Y asunari[24], for the first time this study included the Pacific Ocean along with the Indo-Atlantic sector to analyze the inter-annual variation of SST over the southern subtropics on a global scale. Together with the generation mechanism, presence of an interesting global W4 mode in the subtropical SST and its impact on south-eastern Australian rainfall are discussed
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