Annual and interannual variations of the Leeuwin Current at 32°S

Abstract

In this study, the annual and interannual variations of the Leeuwin Current at 32°S off the Western Australian coast are investigated. The mean annual cycle and the El Niño and La Niña composites of the Leeuwin Current temperature structures are obtained by linearly fitting a Taylor expansion to historical upper ocean temperature data. A temperature‐salinity relationship is used to derive the salinity field and geostrophy is assumed to calculate the current velocity. The downward tilting of the isotherms toward the coast and the strength of a near‐surface core of the low‐salinity water indicate the seasonal variation of the Leeuwin Current. Seasonally, the Leeuwin Current has the maximum poleward geostrophic transport of 5 Sv (10 6 m 3 s−1) during June–July. Interannually, the Leeuwin Current is distinctly stronger during a La Niña year and weaker during an El Niño year. The annual average poleward geostrophic transports in the mean, the El Niño and La Niña years are 3.4, 3.0, and 4.2 Sv respectively. Variations of the Leeuwin Current structure on annual and interannual timescales are coastally trapped. A linear relationship between the coastal sea level deviation at Fremantle and the Leeuwin Current transport is derived, which justifies and calibrates the usage of the Fremantle sea level as an index for the strength of the Leeuwin Current.