A Source of AGCM Bias in Simulating the Western Pacific Subtropical High: Different Sensitivities to the Two Types of ENSO

Abstract

This study reveals a possible cause of model bias in simulating the western Pacific subtropical high (WPSH) variability via an examination of an Atmospheric Model Intercomparison Project (AMIP) simulation produced by the atmospheric general circulation model (AGCM) developed at Taiwan's Central Weather Bureau (CWB). During boreal summer, the model overestimates the quasi-biennial (2-3 yr) band of WPSH variability but underestimates the low-frequency (3-5 yr) band of variability. The overestimation of the quasibiennial WPSH sensitivity is found to be due to the model's stronger sensitivity to the central Pacific El Niño-Southern Oscillation (CP ENSO) that has a leading periodicity in the quasi-biennial band. The model underestimates the low-frequency WPSH variability because of its weaker sensitivity to the eastern Pacific (EP) ENSO that has a leading periodicity in the 3-5-yr band. These different model sensitivities are shown to be related to the relative strengths of the mean Hadley and Walker circulations simulated in the model. An overly strong Hadley circulation causes the CWB AGCM to be overly sensitive to the CP ENSO, while an overly weak Walker circulation results in a weak sensitivity to the EP ENSO. The relative strengths of the simulated mean Hadley and Walker circulations are critical to a realistic simulation of the summer WPSH variability in AGCMs. This conclusion is further supported using AMIP simulations produced by three other AGCMs, including the CanAM4, GISS-E2-R, and IPSL-CM5A-MR models.

Contrasting two spring SST predictors for the number of western North Pacific tropical cyclones

“…The influences of CP El Niño on SSTG may occur through the pathway of modulating the WNPSH and AH. Previous studies have noted significant influences of CP El Niño on the WNPSH (Paek et al 2015) and Australian monsoon (Taschetto et al 2010). SSTs in the WWP and SWP could possibly be influenced by changing surface winds and evaporation associated with WNPSH and AH variations triggered by CP El Niño.…”

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

“…To represent the WPSH intensity, we define a WPSH index as the sea level pressure anomalies averaged over the NWP region (10°–30°N, 120°–160°E) following Paek et al (). We compare in Figure the evolution of this WPSH index composited for the three types of El Niño from their developing May to decaying August.…”

“…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.…”

“…These impact mechanisms were proposed before the diversity in El Niño types was fully recognized by the research community. Recent studies have examined whether and how the different types of El Niño impact the WPSH differently (e.g., Paek et al, , ) by separating the El Niño only into the EP and CP types. In this study, we show with observational analyses that CP El Niño impacts on the WPSH are different between the CP‐I and CP‐II subtypes.…”

Different Influences of Two El Niño Types on Low-Level Atmospheric Circulation over the Subtropical Western North Pacific

“…In this study, the name CP El Niño is used, and the conventional El Niño is referred to as eastern Pacific (EP) El Niño for comparison. Due to their distinct warming origins and patterns, the two types of El Niño can induce different atmospheric responses (Weng et al 2007(Weng et al , 2009Feng et al 2010Feng et al , 2011Feng et al , 2018Feng andHu et al 2012;Yu et al 2012Yu et al , 2015Yuan and Yang 2012;Karori et al 2013;Paek et al 2015Paek et al , 2016Xu et al 2017). The WNPAC occurs in the developing autumn of EP El Niño.…”