Using eddy geopotential height to measure the western North Pacific subtropical high in a warming climate

Chao He,Chunhui Li,Dejun Gu,Tianjun Zhou,Bin Zheng,Ailan Lin,Bo Wu

doi:10.1007/s00704-016-2001-9

Abstract

The western North Pacific subtropical high (WNPSH) is crucial to the East Asian summer climate, and geopotential height (H) is widely used to measure the WPNSH. However, a rapidly rising trend of H in the future is projected by the models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Diagnoses based on the hypsometric equation suggest that more than 80% of the rise in H are attributable to zonal uniform warming. Because circulation is determined by the gradient of H rather than its absolute magnitude, the spatially uniform rising trend of H gives rise to difficulties when measuring the WNPSH with H. These difficulties include an invalid western boundary of WNPSH in the future and spurious information regarding long-term trends and interannual variability of WNPSH. Using CMIP5 model simulations and reanalysis data, the applicability of a metric based on eddy geopotential height (He) to the warming climate is investigated. The results show that the He metric outperforms the H metric under warming climate conditions. First, the mean state rainfall-He relationship is more robust than the rainfall-H relationship. Second, the area, intensity, and western boundary indices of WNPSH can be effectively defined by the He = 0-m contour in future warming climate scenarios without spurious trends. Third, the interannual variability of East Asian summer rainfall is more closely related to the He-based WNPSH indices. We recommend that the He metric be adopted as an operational metric on the WNPSH under the current warming climate.

Highlights

The western North Pacific subtropical high (WNPSH) is the western extension of the North Pacific subtropical high (Yun et al 2015)
Based on the model projections of future climate from the Coupled Model Intercomparison Project Phase 5 (CMIP5) (Taylor et al 2012) and on the reanalysis data, the current study aims to address the following questions: can we still use the H metric in the warming climate? Is the He metric a good substitute for the H metric?
The western boundary of WNPSH may become invalid in a few decades if the current warming trend continues

Summary

Introduction

The western North Pacific subtropical high (WNPSH) is the western extension of the North Pacific subtropical high (Yun et al 2015). Recent studies have proposed a new metric of the WPNSH, i.e., eddy geopotential height (He), which is calculated by subtracting the simultaneous Hm (0°N–40°N, 180°W–180°E averaged H) from the original H (Zhou et al 2009; Huang et al 2015; Huang and Li 2015; Wu and Wang 2015) The He metric effectively captures the decadal change of the WNPSH seen in the late 1970s (Huang et al 2015; Huang and Li 2015; Wu and Wang 2015), but it is not clear whether the He metric is a good operational metric that well captures the mean state and interannual variability of the WNPSH in the warming climate. According to the four terms on the right-hand side of Eq (3), the response of H to global warming is determined by the following four factors: uniform warming, temperature pattern, surface pressure, and the cross term of the nonlinear effect

Difficulties with the H metric under the warming climate

A comparison between the H metric and the He metric under the warming climate

Findings

A comparison between the H metric and the He metric in model simulations