Abstract
This study investigates the interannual modes of the tropical Pacific using salinity from observations, ocean reanalysis output and CMIP6 products. Here we propose two indices of sea surface salinity (SSS), a monopole mode and a dipole mode, to identify the El Niño—South Oscillation (ENSO) and its diversity, respectively. The monopole mode is primarily controlled by atmospheric forcing, namely, the enhanced precipitation that induces negative SSS anomalies across nearly the entire tropical Pacific. The dipole mode is mainly forced by oceanic dynamics, with zonal current transporting fresh water from the western fresh pool into the western-central and salty water from the subtropics into the eastern tropical Pacific. Under a global warming condition, an increase in the monopole and dipole mode variance indicates an increase in both the central and eastern Pacific El Niño variability. The increase in central Pacific El Niño variability is largely due to enhanced vertical stratification during global warming in the upper layer, with intensified zonal advection. An eastern Pacific El Niño-like warming pattern contributes to the increase in eastern Pacific El Niño, with enhanced precipitation over the central-eastern tropical Pacific.
Highlights
The El Niño and Southern Oscillation (ENSO) is the most prominent climate mode in the tropical Pacific and significantly influences the world’s climate and ecosystem (Ropelewski and Halpert 1987; Vincent et al 2011; Cai et al 2014)
From the view of the atmosphere, the modulation of general circulation associated with ENSO is well represented by the difference in sea level pressure (SLP) between the western and eastern tropical Pacific (Ropelewski and Jones 1987; Ropelewski and Halpert 1987)
The western Pacific freshens significantly and high-salinity ridges emanating from the southern subtropics is still located at the central-eastern tropical Pacific in the future (Fig. 12f), which implies that the anomalous eastward currents would lead to a robust dipole mode
Summary
The El Niño and Southern Oscillation (ENSO) is the most prominent climate mode in the tropical Pacific and significantly influences the world’s climate and ecosystem (Ropelewski and Halpert 1987; Vincent et al 2011; Cai et al 2014). Modeling studies indicate that freshwater flux and ocean salinity play a role in SST variability and ENSO evolution through influencing surface stratification, equatorial thermocline and entrainment of subsurface water (Murtugudde and Busalacchi 1998; Yang et al 1999; Maes et al 2002; Zhang and Busalacchi 2009; Zhang and Busalacchi 2009; Qu et al 2014; Zheng et al 2014). We propose two SSS indices to explain the similarities and differences of the hydrological cycle between the CP- and EP-El Niño using results from an ocean state estimate of the consortium for Estimating the Circulation and Climate of the Ocean (ECCO) combined with the TAO/TRITON mooring observations.
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