Different features between high- and low-frequency equatorial Pacific zonal circulations and their possible mechanisms

The characteristics of high- (＜20 pentads) and low-frequency (＞60 pentads) sea level pressure (SLP) anomalies and their differences over the equatorial Pacific Ocean are investigated by using the European Center for Medium-Range Weather Forecasts reanalysis data and the National Oceanic and Atmospheric Administration observational data from 1982 to 2017. Specifically, the differences in zonal atmospheric circulation over the equatorial Pacific between high- and low-frequency are emphasized, and possible reasons for their differences are discussed by diagnosing the timescale of the equatorial Pacific ocean-atmosphere coupling. The results show that the Southern Oscillation only exists on the low-frequency timescale but is absent on the high-frequency timescale. On the low-frequency timescale, it is mainly modulated by ENSO signals through strong ocean-atmosphere interactions. In contrast, on the high-frequency timescale, there is no enough time for the ocean-atmosphere coupling. Correspondingly, SLP and wind anomaly fields display a basin-consistent pattern in the whole equatorial Pacific. The high-frequency zonal atmospheric feature is very similar to that of the Madden-Julian Oscillation (MJO), both having the eastward propagation speed of about 5 m/s, and 34% of the variance of high-frequency zonal atmospheric circulation can be linearly explained by MJO. The coherence spectrum analyses indicate that the ocean-atmosphere coupling is dependent on the timescale. The Southern Oscillation usually exists when the timescale is larger than 20 pentads, similar to the timescale for the equatorial oceanic Kelvin waves to traverse the equatorial Pacific.