Interannual variations in the Hawaiian Lee Countercurrent
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The Hawaiian Lee Countercurrent (HLCC) is an eastward surface current flowing against the broad westward flow of the North Pacific subtropical circulation. Analyses of satellite altimeter data over 16 years revealed that the HLCC is characterized by strong interannual variations. The strength and meridional location of the HLCC axis varied significantly year by year. The eastward velocity of the HLCC was higher when the location of the axis was stable. Mechanisms for the interannual variations were explored by analyses of the altimeter data and results from a simple baroclinic model. The interannual variations in the strength of the HLCC did not correlate with those of the wind stress curl (WSC) dipole formed on the leeward side of the Hawaii Islands, although the WSC dipole has been recognized as the generation mechanism of the HLCC. Meridional gradients of the sea surface height anomaly (SSHA) across the HLCC generated by baroclinic Rossby waves propagating westward from the east of the Hawaii Islands were suggested as a possible mechanism for the interannual variations in the HLCC. The spatial patterns in the observed SSHAs were reproduced by a linear baroclinic Rossby wave model forced by wind fields from a numerical weather prediction model. Further analysis of the wind data suggested that positive and negative anomalies of WSC associated with changes in the trade winds in the area east of the Hawaii Islands are a major forcing for generating SSHAs that lead to the HLCC variations with a time lag of about 1 year.
KeywordsHawaiian Lee Countercurrent Interannual variation Satellite altimeter Baroclinic Rossby wave Wind stress curl Trade winds Reduced gravity model
We would like to thank the editor (Dr. Tomoki Tozuka), the reviewer (Dr. Hideharu Sasaki) and an anonymous reviewer, who helped improve the manuscript. We are grateful to the members of the Physical Oceanography Laboratory in Tohoku University for their fruitful discussions and valuable comments. This work was partly supported by the Sasagawa Scientific Research Grant, and the Global Center of Excellence Program, “Global Education and Research Center for Earth and Planetary Dynamics,” promoted by the Tohoku University. Dr. Eitarou Oka kindly provided the quality-controlled Argo float data. The altimeter data product was provided by the AVISO, the drifting buoy data by Marine Environmental Data Service, the surface wind stress data by the NOAA Coast Watch and NCEP/NCAR. We would like to thank all the data providers.
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