Characteristics And Dynamics Of Seasonal And Interannual Variabilities Of The North Equatorial Countercurrent Across The Pacific Basin

In this paper, the latest version of SODA (Simple Ocean Data Assimilation) data (0.5°×0.5°,1950.01-2008.12) of University of Maryland College of atmospheric and oceanic sciences is used for analysis and discussion of the main features of the North Equatorial countercurrent on seasonal and interannual scales across the pacific ocean (130.25°E-83.25°W), in terms of the current transport, the current width,the latitudinal position and the depth of the NECC core. By designing a variety of wind-forced 1.5-layer linear reduced gravity ocean model experimental programs, further explores of the formation mechanism of the seasonal variation of NECC are carried. It also reveals the impact of ENSO on the interannual variation of NECC. The Main conclusions are as follows:1. The phase of the seasonal variation of NECC transport of west of 140° E is just contrast to that of east of. In the West of 140° E, the NECC transport is abnormally strong from April to August, while it is abnormally weak from September to March of the following year. In the East of 140° E, during January to September period, the abnormal weakening signal of NECC transport is gradually spread from the eastern boundary to the West. During June to March of the following year period, the abnormal enhanced signal of NECC transport is gradually spread from the eastern boundary to the West. Its unusually southerly or northerly signal of the latitudinal position of NECC core, unusually deepening or shallowing signal of the depth of NECC core and the abnormal increases or decreases signal of NECC width also have westward properties.2. In the waters near the western boundary, local seasonal variations of sea surface wind stress plays a dominant role in NECC seasonal variations. While in the Central and Eastern Pacific, the seasonal variation of NECC is mainly affected by the westward Rossby waves. disturbance of the sea by the seasonal variation of sea surface wind stress in Eastern Ocean, by means of its own adjustment mechanism of disturbance (westward oceanic Rossby waves), control the seasonal variations of NECC in East of 140° E across the vast central and eastern basin-wide.3. The Interannual variabilities of Pacific North Equatorial countercurrent (NECC) transport are ahead of the development of the ENSO cycle. During the mature period of El Nino, the interannual anomalies of NECC transport gradually shift from increasing to decreasing. During the mature period of La Nina, the interannual anomalies of NECC transport gradually shift from decreasing to increasing. Interannual variabilities of the latitudinal position of NECC core synchronize with the ENSO cycle. During the mature period of El Nino, the interannual variability of the latitudinal position of NECC core is shifted to the South end. During the mature period of La Nina, the interannual variability of the latitudinal position of NECC core is shifted to the north end. Interannual anomalies of the depth of the Eastern and Western NECC core are highly correlated with ENSO cycle. During E1 Nino’s strongest period, interannual anomaly of the depth of eastern (western) part of NECC core reaches the deepest (shallowest) position.During La Nina’s strongest period, interannual anomaly of the depth of eastern (western) part of NECC core reaches the shallowest (deepest) position.4. The results of wind-forced 1.5-layer linear reduced gravity ocean model confirms that during El Nino and La Nina period, the anomalies of sea surface wind stress vorticity disturb the ocean, combined with marine’s own adjustment mechanism of disturbance (Westward Rossby wave, Coastal Kelvin wave, Eastward Equatorial Kelvin wave),causing anomalies of the displacement of thermocline, leading to an effect on the interannual variabilities of NECC.