Different Features Between High-and Low-frequency Equatorial Pacific Zonal Circulations And Their Corresponding Large-scale Circulation

The different features between high-?<20 pentads?and low-frequency?>60 pentads?equatorial Pacific zonal circulation and their possible mechanism are investigated by using European Centre for Medium-Range Weather Forecasts?ECMWF?reanalysis data and National Oceanic and Atmospheric Administration?NOAA?observational data.Then the large-scale circulation anomalies corresponding to the high-and low-frequency equatorial Pacific zonal circulation and the characteristics of high-frequency signals in the atmospheric field are further studied.Finally,the differences between high-and low-frequency equatorial zonal circulation are verified by CMIP5 model.The major conclusions can be summarized as follows:?1?Southern Oscillation only exists on the low-frequency timescale but absent on the high-frequency timescale.On the low-frequency timescale,it is mainly modulated by the ENSO signal through strong ocean-atmosphere interactions.In contrast,on the high-frequency timescale,there is no enough time for the ocean-atmosphere coupling.Correspondingly,wind and sea level pressure fields display basin consistent pattern in the whole equatorial Pacific.The coherence spectrum analyses indicate that the ocean-atmosphere coupling in the equatorial Pacific is dependent on the timescale.The Southern Oscillation usually exists when the timescale is larger than 20 pentads,similar to the time for that the equatorial oceanic kelvin wave traverses the equatorial Pacific.?2?The low-frequency equatorial Pacific zonal circulation corresponds to the ENSO-like tropical anomalous circulation and convection.During the evolution of the high-frequency zonal circulation,the wind and specific humidity field on the equatorial vertical profile show Madden-Julian Oscillation?MJO?-like propagation characteristics and vertical westward-tilted structure.The corresponding anomalous convective signal originates from the Indian Ocean and has MJO-like eastward propagation characteristics with a speed of about 5m/s.The high-frequency equatorial Pacific zonal circulation is closely related to MJO,and 34%of its variance can be explained by MJO linearly.The low-frequency equatorial Pacific zonal circulation mainly modulated by ENSO shows an obvious PNA-like anomalous atmospheric circulation at 500 h Pa.Significant differences exist in the extra-tropical anomalous circulation fields corresponding to the high-and low-frequency zonal circulation,which is caused by the different heat source positions corresponding to the high-and low-frequency signal.?3?Both the coupling model GFDL-CM3 and Had GEM2-CC can well simulate the spatial pattern of the Southern Oscillation,but the simulation of the key area in the eastern Pacific Ocean are slightly westward.Both models can simulate the east-west oscillation of sea level pressure anomaly?SLPA?in the equatorial Pacific on the low-frequency timescale,and the basin consistent pattern in the whole equatorial Pacific on the high-frequency timescale.Both models can qualitatively simulate the lead-lag relationship between the high-and low-frequency SLPA_EPAC and SLPA_INDO index.Only the model GFDL-CM3 can simulate the phenomenon that the high-frequency SLPA_INDO leads the SLPA_EPAC signal by one pentad.Although the model Had GEM2-CC can simulate the negative correlation between the low-frequency SLPA_EPAC and SLPA_INDO index,the deviation of the correlation and the lead-lag time corresponding to the maximum correlation coefficient is larger.Both models can well simulate the timescale dependence of ocean-atmosphere coupling in the equatorial Pacific,which means the ocean-atmosphere coupling in the equatorial Pacific requires 20pentads at least.