Impact of the Anomalous Latent Heat Flux Over the Kuroshio Extension on Western North American Rainfall in Spring: Interannual Variation and Mechanism

Jingchao Long,Chunlei Liu,Jianjun Xu,Zifeng Liu

doi:10.3389/feart.2020.609619

Abstract

The Kuroshio and its extension (KE) significantly influences regional climate through meridional heat transport from the tropical ocean. In this study, the observational and reanalysis datasets are used to investigate the impact of the latent heat flux (LHF) over the KE region on downstream rainfall and the underlying mechanism. The result shows a “seesaw” structure in rainfall anomaly, dominating the Western Canada and the southwestern North America with a correlation coefficient of 0.77 between the two modes. In strong LHF years, strengthened LHF favors to enhance precipitation in the Western Canada and reduce that in the southwestern North America. This is primarily associated with an anomalous cyclonic circulation over the KE region, which enhances southwesterly precipitation and latent heating in the middle troposphere. The heating excites an anomalous cyclonic circulation to its west and an anticyclonic circulation to its east, helping to reinforce the existing anomalous cyclonic circulation in turn and form a positive feedback. The conditions associated with La Niña events favor to above processes. To the upper troposphere, the deepened anomalous cyclonic circulation due to enhanced eddy activities and atmospheric baroclinic instability over the KE strengthens subtropical westerly jet stream and thereby extends eastward on the 200 hPa level. Correspondingly, an elongated zonally lower level cyclonic circulation anomaly across the North Pacific leads to a moisture convergence in the Western Canada, which is mainly resulted from the anomalous positive vorticity advection over the left side of the exit region of the jet stream. The opposite circumstance occurs in weak LHF years, presenting an opposed anomalous circulation and rainfall pattern.

Highlights

Sufficient water resources are extremely important for spring, the vegetation growing season (Choi and Kim, 2018), in the international agricultural center over western North America (WNA), especially the state of California (Swain et al, 2016)
We diagnose the relationship between LHF anomaly over the KE region and the western North American rainfall variability through correlation analysis and singular value decomposition (SVD)
Taking insight into the mechanism of this phenomenon, we found that in strong years, the strengthened LHF releases favors to enhance precipitation in the Western Canada and reduce that in the southwestern North America

Summary

Introduction

Sufficient water resources are extremely important for spring, the vegetation growing season (Choi and Kim, 2018), in the international agricultural center over western North America (WNA), especially the state of California (Swain et al, 2016). In order to gain insight of the rainfall fluctuation over the WNA, many investigations in cold season (e.g., winter and spring) and warm season (e.g., summer and autumn) have been conducted These results indicate that several atmospheric modes modulate spatial and temporal variability of precipitation from the synoptic scale to the decadal timescale (Leathers et al, 1991; Cayan et al, 1998; Dettinger et al, 1998; Seager and Hoerling, 2012; L’Heureux et al, 2015; Mundhenk et al, 2018; Guirguis et al, 2020)

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