Aquarius reveals eddy stirring after a heavy precipitation event in the subtropical North Pacific

Abstract

This research presents a case study that examines an upper-ocean salinity response to intense rainfall using 4 years of observed salinity data from the Aquarius and Argo floats. The Aquarius sea surface salinity (SSS) in the subtropical region of the North Pacific reveals a notable event in which SSS is locally freshened by intense rainfall. Although the SSS pattern shortly after the rainfall reflects the atmospheric pattern, its final form reflects the ocean’s dynamic structure; low-salinity water distributes in the shape of an anticyclonic eddy. Observations using Argo profiling floats confirm that low-salinity water dominates the entire water column of the eddy above 70 m. We found that an eddy stirring effect would play an important role in forming the eddy-shaped low-salinity water. As a precondition, intense rainfall on the tropical side enhances the meridional gradient of background SSS with freshened low-salinity water to the south and saline water to the north. The anticyclonic eddy located at the enhanced SSS front stirs the water in a clockwise direction. Specifically, northward flow in the western side of the eddy transports fresh water, whereas southward flow in the eastern side transports saline water. As this northward–southward flow moves the eddy to the west where the transported fresh water is dominant as a baroclinic Rossby wave, this dynamic process would form the eddy-shaped low-salinity water. This clear view of such eddy stirring is due to the inherent nature of the subtropical region that is characterized by less frequent precipitation.

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Acknowledgements

The authors would like to thank the editor Dr. Tomoki Tozuka and three anonymous reviewers who provided valuable comments that contributed to the improvement of this manuscript. The Aquarius product version 4.5 was provided by the NASA/JPL PO.DAAC (ftp://podaac-ftp.jpl.nasa.gov/), the satellite altimeter data were provided by AVISO (http://www.aviso.altimetry.fr/en/data.html/), the precipitation data were provided by GPCP (ftp://meso.gsfc.nasa.gov/), and the wind data were provided by IFREMER (http://apdrc.soest.hawaii.edu/datadoc/ascat.php). We are grateful to Dr. Dudley B. Chelton for providing the eddy tracking dataset available at http://cioss.coas.oregonstate.edu/eddies/ as well as Dr. Eitarou Oka for providing quality-controlled Argo float data available at http://ocg.aori.u-tokyo.ac.jp/member/eoka/data/. Finally, the authors appreciate Enago (http://www.enago.jp) for the English language review. This work was supported by the Sasakawa Scientific Research Grant from the Japan Science Society and by Ministry of Education, Culture, Sports, Science and Technology (MEXT; KAKENHI Grant Number JP16H01584).

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Correspondence to Hiroto Abe.

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Abe, H., Ebuchi, N., Ueno, H. et al. Aquarius reveals eddy stirring after a heavy precipitation event in the subtropical North Pacific. J Oceanogr 75, 37–50 (2019). https://doi.org/10.1007/s10872-018-0482-0

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Keywords

  • Aquarius
  • Sea surface salinity
  • Precipitation
  • Eddy stirring
  • Baroclinic Rossby wave