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Interannual Variability in Sea Surface Height at Southern Midlatitudes of the Indian Ocean

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  • 1 Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan
  • | 2 National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington
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Abstract

This study examines interannual variability in sea surface height (SSH) at southern midlatitudes of the Indian Ocean (10°–35°S). Our focus is on the relative role of local wind forcing and remote forcing from the equatorial Pacific Ocean. We use satellite altimetry measurements, an atmospheric reanalysis, and a one-dimensional wave model tuned to simulate observed SSH anomalies. The model solution is decomposed into the part driven by local winds and that driven by SSH variability radiated from the western coast of Australia. Results show that variability radiated from the Australian coast is larger in amplitude than variability driven by local winds in the central and eastern parts of the south Indian Ocean at midlatitudes (between 19° and 33°S), whereas the influence from eastern boundary forcing is confined to the eastern basin at lower latitudes (10° and 17°S). The relative importance of eastern boundary forcing at midlatitudes is due to the weakness of wind stress curl anomalies in the interior of the south Indian Ocean. Our analysis further suggests that SSH variability along the west coast of Australia originates from remote wind forcing in the tropical Pacific, as is pointed out by previous studies. The zonal gradient of SSH between the western and eastern parts of the south Indian Ocean is also mostly controlled by variability radiated from the Australian coast, indicating that interannual variability in meridional geostrophic transport is driven principally by Pacific winds.

Significance Statement

A complete understanding of climate variability and change requires knowledge of the interactions between ocean basins on interannual to decadal time scales. In this study, we examined the cause of sea level variability in the south Indian Ocean and its connection with variability in the Pacific Ocean, using satellite observations and a one-dimensional wave model. We found that sea level variability at midlatitudes of the south Indian Ocean is mainly driven by El Niño–Southern Oscillation (ENSO). Surface wind anomalies associated with ENSO excite sea level variations in the Pacific Ocean, which propagates into the western coast of Australia through the Indonesian Archipelago and then into the interior of the south Indian Ocean. This study emphasizes the importance of connectivity via the oceanic route to understand midlatitude circulation variability in the south Indian Ocean.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Motoki Nagura, nagura@jamstec.go.jp

Abstract

This study examines interannual variability in sea surface height (SSH) at southern midlatitudes of the Indian Ocean (10°–35°S). Our focus is on the relative role of local wind forcing and remote forcing from the equatorial Pacific Ocean. We use satellite altimetry measurements, an atmospheric reanalysis, and a one-dimensional wave model tuned to simulate observed SSH anomalies. The model solution is decomposed into the part driven by local winds and that driven by SSH variability radiated from the western coast of Australia. Results show that variability radiated from the Australian coast is larger in amplitude than variability driven by local winds in the central and eastern parts of the south Indian Ocean at midlatitudes (between 19° and 33°S), whereas the influence from eastern boundary forcing is confined to the eastern basin at lower latitudes (10° and 17°S). The relative importance of eastern boundary forcing at midlatitudes is due to the weakness of wind stress curl anomalies in the interior of the south Indian Ocean. Our analysis further suggests that SSH variability along the west coast of Australia originates from remote wind forcing in the tropical Pacific, as is pointed out by previous studies. The zonal gradient of SSH between the western and eastern parts of the south Indian Ocean is also mostly controlled by variability radiated from the Australian coast, indicating that interannual variability in meridional geostrophic transport is driven principally by Pacific winds.

Significance Statement

A complete understanding of climate variability and change requires knowledge of the interactions between ocean basins on interannual to decadal time scales. In this study, we examined the cause of sea level variability in the south Indian Ocean and its connection with variability in the Pacific Ocean, using satellite observations and a one-dimensional wave model. We found that sea level variability at midlatitudes of the south Indian Ocean is mainly driven by El Niño–Southern Oscillation (ENSO). Surface wind anomalies associated with ENSO excite sea level variations in the Pacific Ocean, which propagates into the western coast of Australia through the Indonesian Archipelago and then into the interior of the south Indian Ocean. This study emphasizes the importance of connectivity via the oceanic route to understand midlatitude circulation variability in the south Indian Ocean.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Motoki Nagura, nagura@jamstec.go.jp
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