Abstract
The effects of wintertime stratospheric polar vortex variation on the climate over the North Pacific Ocean during late winter and spring are analyzed using the National Centers for Environmental Predictions, version 2 (NCEP2) reanalysis dataset. The analysis revealed that, during weak polar vortex (WPV) events, there are noticeably lower geopotential height anomalies over the Bering Sea and greater height anomalies over the central part of the North Pacific Ocean than during strong polar vortex (SPV) events. The formation of the dipolar structure of the geopotential height anomalies is due to a weakened polar jet and a strengthened mid-latitude jet in the troposphere via geostrophic equilibrium. The mechanisms responsible for the changes in the tropospheric jet over the North Pacific Ocean are summarized as follows: when the stratospheric polar westerly is decelerated, the high-latitude eastward waves slow down, and the enhanced equatorward propagation of the eddy momentum flux throughout the troposphere at 60° N. Consequently, the eddy-driven jet over the North Pacific Ocean also shows a southward displacement, leading to a weaker polar jet but a stronger mid-latitude westerly compared with those during the SPV events. Furthermore, anomalous anti-cyclonic flows associated with the higher pressure over the North Pacific Ocean during WPV events induce a warming sea surface temperature (SST) over the western and central parts of the North Pacific Ocean and a cooling SST over the Bering Sea and along the west coast of North America. This SST pattern can last until May, which favors the persistence of the anti-cyclonic flows over the North Pacific Ocean during WPV events. A well-resolved stratosphere and coupled atmosphere-ocean model (CMCC-CMS) can basically reproduce the impacts of stratospheric polar vortex variations on the North Pacific climate as seen in NCEP2 data, although the simulated dipole of geopotential height anomalies is shifted more southward.
Highlights
Since the discovery of the downward propagation of the Northern Annular Mode (NAM) in the polar region [1,2], the importance of stratospheric processes has attracted increasing public attention for the improvement of the weather predictability and climate prediction [3,4,5,6]
The composite difference for a given field is calculated by averaging the differences in the detrended monthly mean field between anomalously weak polar vortex (WPV) events and strong polar vortex (SPV) events
5 includes the position of indicates the zonal mean wind each latitude. Meridionally beyond their critical line in which u = c [56]. This means that more transient momentum fluxes, of which the phase speed is larger than the reduced mean flow, are absorbed in the high latitudes, It should be noted that during WPV events, the critical line in the high latitudes is more significantly shifted toward a lower speed region than that during SPV events (Figure 5c), which is consistent with the weakened stratospheric polar jet in WPV events
Summary
Since the discovery of the downward propagation of the Northern Annular Mode (NAM) in the polar region [1,2], the importance of stratospheric processes has attracted increasing public attention for the improvement of the weather predictability and climate prediction [3,4,5,6]. Besides the cold temperature over the Eurasian continent caused by the polar vortex variation, the influence of the stratospheric polar vortex changes on the weather over North America has attracted a significant amount of attention [24]. Until now, there have been few studies to clarify why the Arctic stratospheric circulations can affect the surface climate over the North Pacific Ocean, which is one of the key processes involved in the connection between ASO and ENSO [28]. It is interesting to analyze the wave-mean flow interactions over the North Pacific associated with the stratospheric polar vortex variations Addressing these problems may help to improve the prediction of the tropospheric weather and climate over the North Pacific Ocean and even over the tropical climate. This article is structured as follows: Section 2 presents the data and methodology, Section 3 presents the main results, and Section 4 provides a summary of this work
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