ENSO indices from sea surface salinity observed by Aquarius and Argo
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Analysis of the first 26 months of data from the Aquarius satellite confirms the existence of a sharp sea surface salinity (SSS) front along the equator in the western equatorial Pacific. Following several earlier studies, we use the longitudinal location of the 34.8-psu isohaline as an index, termed Niño-S34.8, to measure the zonal displacement of the SSS front and consequently the eastern edge of the western Pacific warm pool. The on-going collection of the Array for Real-time Geostrophic Oceanography (ARGO) program data shows high correlations between Niño-S34.8 and the existing indices of El Niño, suggesting its potential important role in ENSO evolution. Further analysis of the ARGO data reveals that SSS variability in the southeastern tropical Pacific is crucial to identify the type of El Niño. A new SSS index, termed the southeastern Pacific SSS index (SEPSI), is defined based on the SSS variability in the region (0°–10°S, 150°–90°W). The SEPSI is highly correlated with the El Niño Modoki index, as well as the Trans-Niño index, introduced by previous studies. It has large positive anomalies during central Pacific El Niño or El Niño Modoki events, as a result of enhanced zonal sea surface temperature gradients between the central and eastern tropical Pacific, and can be used to characterize the type of El Niño. The processes that possibly control these SSS indices are also discussed.
KeywordsSea surface salinity ENSO indices Equatorial Pacific Argo data Aquarius ocean salinity
T. Qu was supported by NASA as part of the Aquarius Science Team investigation through grant NNX12AG02G and by NSF through grant OCE11-30050. J.-Y. Yu was supported by NSF through grant AGS-1233542. The authors are grateful to N. Schneider, Y.T. Song, and C. Maes for many useful discussions on the topic and to T. Tozuka and three anonymous reviewers for the thoughtful comments and suggestions. School of Ocean and Earth Science and Technology contribution number 9123, and International Pacific Research Center contribution number IPRC-1058.
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