9:30 AM - 9:45 AM
[AOS22-03] Interannual and inter-decadal variability of the North Equatorial Countercurrent in the Western Pacific
Keywords:Interannual and Inter-decadal Variability, Western Pacific, Northern Equatorial Countercurrent
Interannual and longer timescale variations of the North Equatorial Countercurrent (NECC) in the western Pacific are investigated using the multi-decade (1960-2014) hindcast by the Ocean general circulation model for the Earth Simulator (OFES). The OFES-simulated sea level and upper ocean circulation changes show favorable comparisons with available tide gauge data and repeat hydrographic surveys along the 137ºE meridian. An empirical orthogonal function (EOF) analysis reveals that the low-frequency NECC variability is dominated by two distinct modes. The first mode fluctuates interannually and shows strengthening and southward migration of the NECC concurrent with the development of El Niño events. Unlike the extra-tropical western Pacific Ocean circulation variability controlled by wind forcing west of the dateline, the interannual NECC variations are forced by equatorial wind forcing cumulative across the entire Pacific basin. The second mode of the NECC variability has a inter-decadal timescale and is characterized by NECC's progressive weakening in strength, migrating poleward, and broadening in width over the past 50 years. These long-term changes in NECC are caused by the corresponding changes in the trade wind system that weakened and expanded poleward in the past half a century across the Pacific basin.