Eddy Energy along the Tropical Storm Track in Association with ENSO

Abstract

The interaction between the seasonal mean circulation and the transient eddies over the western North Pacific (WNP) during El Niño-Southern Oscillation (ENSO) warm and cold years was investigated by the three-dimensional eddy kinetic energy (EKE) and eddy available potential energy (EAPE) budget equations for total eddy, high-frequency (< 10 days) and low-frequency (20-70 days) components. Composites of the energy results indicate that low-level anomalous cyclonic circulation, westerly jet and ascending motion associated with the eastward extension of warm SST during warm ENSO years are favorable for eddy barotropic energy conversion (CK) and eddy baroclinic energy conversions (CE). The enhancement of CK and CE might provide kinetic energy for the growth of high- and low-frequency transient eddies including tropical storms (TSs) from the Philippine Sea to the date line over the tropical WNP during warm ENSO years. In contrast, high- and low-frequency eddies convert EKE to seasonal mean circulation over the subtropical and mid-latitude WNP during warm years. Enhanced eddy baroclinic energy conversion plays an important role in the maintenance and enhancement of the subsequent development of transient eddies including TSs as they propagate northward.
The loss of EAPE to EKE due to the eddy baroclinic energy conversion is mainly supplemented by the generation of EAPE associated with eddy diabatic heating. However, the energy conversion from mean available potential energy (MAPE) to EAPE is also important due to the eddy vertical heat transport which is neglected in the two-dimensional EAPE budget equation. It is suggested that high- and low-frequency eddies including TSs may be self-development and intensify through their enhanced diabatic heating and vertical heat transport.