Abstract
In this study, using the Bjerknes stability (BJ) index analysis, we estimate the overall linear El Nino-Southern Oscillation (ENSO) stability and the relative contribution of positive feedbacks and damping processes to the stability in historical simulations of Coupled Model Intercomparison Project Phase 5 (CMIP5) models. When compared with CMIP3 models, the ENSO amplitudes and the ENSO stability as estimated by the BJ index in the CMIP5 models are more converged around the observed, estimated from the atmosphere and ocean reanalysis data sets. The reduced diversity among models in the simulated ENSO stability can be partly attributed to the reduced spread of the thermocline feedback and Ekman feedback terms among the models. However, a systematic bias persists from CMIP3 to CMIP5. In other words, the majority of the CMIP5 models analyzed in this study still underestimate the zonal advective feedback, thermocline feedback and thermodynamic damping terms, when compared with those estimated from reanalysis. This discrepancy turns out to be related with a cold tongue bias in coupled models that causes a weaker atmospheric thermodynamical response to sea surface temperature changes and a weaker oceanic response (zonal currents and zonal thermocline slope) to wind changes.
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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