Abstract
This study investigates correlations among interannual variabilities of sea surface wind, sea surface temperature (SST), and sea surface height anomaly (SSHA) in the tropical region from latitude 15°S to 15°N. Sea surface winds were derived from the European Space Agency (ESA)’s European Remote-Sensing Satellite (ERS)-1/2 scatterometer and the National Aeronautics and Space Administration (NASA)’s QuickSCAT observations; SST data were obtained from the National Oceanic and Atmospheric Administration (NOAA)’s Advanced Very-High-Resolution Radiometer (AVHRR) missions; and the SSHA data were acquired from the NASA TOPEX/Poseidon and Jason-1 altimeter measurements. All these datasets were resampled into 1° × 1° grids between 15°S and 15°N. The annual cycles were removed from all datasets and an empirical orthogonal function (EOF) analysis was applied to extract the major modes of spatial and temporal variability. The first EOF modes of the wind, SST, and SSHA revealed the interannual variability of each data source, reflecting spatio-temporal signatures related to El Nino Southern Oscillation (ENSO) events. The correlation results suggested that, during the strong El Nino period of 1997–1998, the wind variability led the variability of SST. A wind-forced delayed action oscillator (WDAO) system was proposed and analyzed using the ENSO modes of wind and SST data, covering the period from October 1995 to June 2002. The results show that the delayed SST mechanism is the strongest forcing factor in the WDAO system, and the wind forcing is the second strongest forcing factor. The correlations among SST change rate, the wind, and delayed/un-delayed SST also confirm the WDAO analysis’ results.
Highlights
Strong El Nino Southern Oscillation (ENSO) events are closely related to the interannual variability of tropical oceans
The wind can be a perturbation source during a developing period of El Nino events. This can be shown by the correlation analyses and the relevant forcing term in the wind-forced delayed action oscillator (WDAO) model. It is revealed from the WDAO equation that the wind forcing coefficient β is positive, suggesting that wind forcing is a positive feedback of a change in the sea surface temperature; the perturbed westerly wind can lead to an increase of SST in the eastern tropical Pacific and a decrease in the western tropical Pacific [23]
We utilized satellite observations of scatterometer-derived wind, Advanced Very-High-Resolution Radiometer (AVHRR) SST, and altimeter-derived sea surface height anomaly (SSHA) and extracted their interannual variability to represent the variability related to ENSO
Summary
Strong El Nino Southern Oscillation (ENSO) events are closely related to the interannual variability of tropical oceans. The interannual variability of the atmosphere and oceans may result from the self-sustained oscillation of the atmosphere–ocean coupling within a period of 3–5 years, as based on numerical simulations of tropical regions [1,2,3,4,5,6,7,8,9,10] This coupling system may be better explained as a delayed action oscillator (DAO) that combines positive and delayed negative feedback of the sea surface temperature (SST) in the tropical Pacific [6]. The upwelling Kelvin wave imposes a negative feedback on positive sea surface temperature anomalies via shallowing of the deepened thermocline caused by the downwelling Kelvin wave This delayed action of the upwelling Kelvin wave brings the warm event back to equilibrium and sometimes induces a cooling event (i.e., La Nina). This study can help increase our understanding of the atmosphere–ocean interaction in the worldwide tropical ocean and lead to an improved understanding of the interannual variability associated with ENSO events
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