Diverse Relationship between ENSO and the Northwest Pacific Summer Climate among CMIP5 Models: Dependence on the ENSO Decay Pace

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.

The Changing Impact Mechanisms of a Diverse El Niño on the Western Pacific Subtropical High

“…Forcing from El Niño is a key source of WPSH variability. The El Niño impact on the WPSH has been intensively studied (e.g., He et al, 2015;Jiang et al, 2017Jiang et al, , 2018Kumar & Hoerling, 2003;Li et al, 2017;Paek et al, 2015Paek et al, , 2016Park et al, 2010;Sui et al, 2007;Wang et al, 2000;Wang & Zhang, 2002;Xie et al, 2009Xie et al, , 2016, and the prevailing view is that the WPSH typically intensifies during the decaying summer of El Niño (e.g., Wang et al, 2000;Xiang et al, 2013;Xie et al, 2009). A northwestern Pacific (NWP) local coupling mechanism (Wang et al, 2000) and an Indian Ocean Capacitor (IOC) mechanism (Xie et al, 2009) were proposed to explain how El Niño can still influence the WPSH intensity after its demise.…”

Impact of the MJO on the interannual variation of the Pacific–Japan mode of the East Asian summer monsoon

“…1a), there is an anticyclonic anomaly over the subtropical WNP and a cyclonic anomaly over mid-latitude East Asia. The anticyclonic anomaly, associated with suppressed precipitation anomalies over the tropical WNP, corresponds to a westward extended subtropical high that transports water vapor to East Asia along its northwest flank and results in enhanced rainfall along the East Asian rain band, as previous studies suggested (e.g., Lu 2001aLu , 2004Jiang et al 2017;Hu et al 2017;Li and Lu 2017). As a result, the rainfall anomalies are characterized by a seesaw pattern between the tropical WNP and the East Asian rain band.…”

Impact of differences in the decaying phase of El Niño on South and East Asia summer monsoon in CMIP5 models

“…This is mainly due to enhanced strength of WNP anticyclone and shift in position. Previous studies suggested that the underestimated El Niño-WNP summer monsoon relationship in CGCMs might be originated from the biases in TIO SST anomalies (Hu et al, 2014) or SST anomaly gradients between the TIO and equatorial western Pacific (Jiang et al, 2017) during El Niño decaying summers. Further, Li et al (2018) suggested that underestimation of WNP anticyclonic circulation during El Niño decay summers in CMIP5 models is highly associated with excessive equatorial Pacific cold tongue bias.…”

“…This is mainly due to enhanced strength of WNP anticyclone and shift in position. Previous studies suggested that the underestimated El Niño‐WNP summer monsoon relationship in CGCMs might be originated from the biases in TIO SST anomalies (Hu et al, ) or SST anomaly gradients between the TIO and equatorial western Pacific (Jiang et al, ) during El Niño decaying summers. Further, Li et al .…”