News Release

Scientists construct new sea surface salinity indexes to distinguish two types of El Niño independently

Peer-Reviewed Publication

Science China Press


image: Sea surface temperature anomaly (a, b),sea surface salinity anomaly (d, f) and their corresponding indexes (e, f) for two types of El Niño events: EP (left) and CP El Niño events (right). view more 

Credit: ©Science China Press

As the strongest interannual variability in the tropics, El Niño-Southern Oscillation (ENSO) can affect the global climate. To date, two types of El Niño events (Eastern Pacific El Niño events and Central Pacific El Niño events, denoted as EP and CP El Niño events) have been identified according to their different spatial patterns and their impacts. In recent decades, associated with global warming, CP El Niño events have occurred with a higher frequency than EP El Niño events. Previous studies mainly employed sea surface temperature, i.e., SST-based indexes to distinguish the CP El Niño events and EP El Niño events. However, some special El Niño events, including the combination of CP and EP El Niño events cannot be adequately represented by these traditional SST-based indexes.


Ocean salinity (especially for SSS) is closed related with the atmospheric variations through surface evaporation and rainfall. Meanwhile, ocean salinity does not provide a direct feedback to the atmosphere. From this view, it is more convenient to trace the source of SSS than to trace that of SST. Several SSS indexes have been employed to identify El Niño events previously. In fact, these several SSS indexes are not independent because of their high correlations. Thus, the previous SSS indexes need to be employed together and then distinguish EP and CP El Niño events. 


This study aims to construct a pair of SSS indexes to distinguish CP and EP El Niño events conveniently. The SSS indexes can be independently to distinguish CP and EP El Niño events, that is, only one index can be adopted to adequately identify EP or CP El Niño events only. 


As published in the SCIENCE CHINA Earth Sciences, these six scholars revealed in their study, that the key areas exist for sea surface salinity variations to identify EP and CP El Niño events independently. The key areas are located over an arcuate area centered at (0°, 130°E) and in the equatorial central Pacific covering (5°S-5°N, 175°W-158°W) for EP El Niño events. For CP El Niño events, the key areas are located in the northeastern part of western Pacific covering (2°S-9°N, 142°E-170°E) and in the southeast Pacific covering (20°S-10°S, 135°W-95°W), respectively.


Based on the different key areas with SSS variation, sea surface salinity (SSS) indexes are constructed to identify EP and CP El Niño events independently. These SSS indexes are termed as the CP/EP El Niño SSS index (CSI/ESI). Employing CSI/ESI, the CP/EP El Niño events can be distinguished, respectively.   


Research aimed at obtaining an independent sea surface salinity index pair for CP and EP El Niño events whereas the SSS indexes proposed previously have highly correlation.


The SSS indexes can be adequately used to identify the two types of El Niño events and serve as another useful tool for monitoring ENSO. This may offer novel insight into how to represent the diversity of El Niño events.


“To our knowledge,” wrote the six researchers, “ The SSS indexes can be conveniently employed for identifying EP and CP El Niño events as those obtained using SST-based indexes”.


See the article: Zhi H, Lin P, Fang Z, Liu H, Zhang R-H, Bai W. 2021. Sea surface salinity-derived indexes for distinguishing two types of El Niño events in the tropical Pacific. Science China Earth Sciences, 64(8): 1267–1284,

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