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Zhaoyun Chen, Yuwu Jiang, Jia Wang, and Wenping Gong

Abstract

Satellite images show that the Pearl River plume is entrained into the upwelling front in the northeastern South China Sea. To understand the processes and extend to other coastal zones, an idealized numerical model is used to investigate the upwelling dynamics in response to the arrival of the river plume. Upon forcing by an upwelling-favorable wind, the model reproduces the upwelling frontal jet with a stratified water column, which takes the river plume far away from the mouth of the estuary. The river plume introduces additional upwelling and downwelling at its inshore and offshore sides (defined as plume-related secondary upwelling circulation), respectively. For the initially unstratified water column, the plume-related secondary upwelling circulation is stronger and extends to deeper water than for the stratified condition. The surface boundary layer thins and the offshore current intensifies in the river plume. The variations in wind-driven current over the deep-water shelf in different stratified conditions are modulated by the vertical profiles of the eddy viscosity, which are shown by a one-dimensional numerical model. Offshore transport is reinforced when the head of the river plume arrives. Thereafter, it is changed by the cross-shore baroclinic geostrophic component of velocity, due to alongshore density variation by the river plume. The horizontal gradient of stress on the two sides of the river plume is responsible for the plume-related secondary upwelling circulation owing to different stress decay scales inside and outside the river plume.

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Wenping Jiang, Gang Huang, Kaiming Hu, Renguang Wu, Hainan Gong, Xiaolong Chen, and Weichen Tao

Abstract

The impacts of El Niño–Southern Oscillation (ENSO) on the northwest Pacific (NWP) climate during ENSO decay summers are investigated based on the outputs of 37 coupled general circulation models (CGCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Large intermodel spread exists in the 37 state-of-the-art CGCMs in simulating the ENSO–NWP relationship. Eight high-skill and eight low-skill models are selected to explore how the bias arises. By comparing the results among high-skill models, low-skill models, and observations, the simulation skill of the ENSO–NWP relationship largely depends on whether the model can reasonably reproduce the ENSO decay pace. Warm SST anomaly bias in the equatorial western Pacific (EWP) is found to persist into the ENSO decay summer in the low-skill models, obstructing the formation of an anomalous anticyclone in the NWP. Further analysis shows that the warm EWP SST anomaly bias is possibly related to the excessive westward extension of cold tongue in these models, which increases climatological zonal SST gradient in the EWP. Under westerly wind anomalies, the larger climatological zonal SST gradient could lead to warmer zonal advections in the low-skill models than that in the high-skill models, which could lead to warm EWP SST anomaly bias in the low-skill models. And the warm EWP SST anomaly bias could strengthen westerly wind anomalies over the western Pacific by triggering convection and atmospheric Rossby waves, which, in turn, could maintain the warm SST anomaly bias in the EWP.

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