Abstract
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 Nino-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 Nino-S34.8 and the existing indices of El Nino, 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 Nino. 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 Nino Modoki index, as well as the Trans-Nino index, introduced by previous studies. It has large positive anomalies during central Pacific El Nino or El Nino 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 Nino. The processes that possibly control these SSS indices are also discussed.
Highlights
The El Nino and Southern Oscillation (ENSO), as one of the most prominent climate phenomena in the equatorial Pacific, has been extensively studied in the past decades (e.g., Suarez and Schopf 1988; Weisberg and Wang 1997; Jin 1997; Picaut et al 1997)
The sea surface salinity (SSS) front associated with the eastern edge of the western Pacific warm pool has been discussed by previous studies, and its link to ENSO has long been recognized (e.g., Delcroix 1998; Delcroix and Picaut 1998; Bosc et al 2009; Singh et al 2011; Qu et al 2014), The present study, based on newly available satellite (Aquarius) and in situ observations, confirms the high correlation of the SSS front with ENSO
As a proxy of the SSS front, we define the longitudinal location of the 34.8-psu isohaline as an index of El Nino, termed Nino-S34.8, which is slightly different from those introduced by previous studies (e.g., Delcroix 1998; Bosc et al 2009)
Summary
The El Nino and Southern Oscillation (ENSO), as one of the most prominent climate phenomena in the equatorial Pacific, has been extensively studied in the past decades (e.g., Suarez and Schopf 1988; Weisberg and Wang 1997; Jin 1997; Picaut et al 1997). We examine the zonal displacement of the SSS front along the equator in the western equatorial Pacific and its validity as an index of El Nino, using the first 26 months of Aquarius measurements combined with the on-going collection of the Argo data. The measurements extend from a typical top level around 5 m to about 2,000 m depth Based on these measurements, a temperature/salinity product of the global ocean was recently created at the Asian Pacific Data Research Center of the International Pacific Research Center, University of Hawaii. A temperature/salinity product of the global ocean was recently created at the Asian Pacific Data Research Center of the International Pacific Research Center, University of Hawaii This product provides near-real time temperature/ salinity maps at a spatial resolution of 1° every month. The smoothed weekly and monthly SSS data from all three beams for the period from August 2011 to September 2013 are used in the present analysis
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