Decadal Variation of Atmospheric Rivers in Relation to North Atlantic Tripole SST Mode

Jie Zhang,Yinglai Jia,Rui Ji,Yifei Wu

doi:10.3390/atmos12101252

Abstract

The North Atlantic tripole (NAT) is the leading mode of sea-surface temperature (SST) in the decadal time scale. Although the NAT is forced by North Atlantic oscillation (NAO), it also has an effect on the atmosphere; for example, the early winter tripole SST signal can influence storm tracks in March. As the NAT not only changes the baroclinicity of the lower layer but also modifies the moisture being released into the atmosphere, we surmise that the NAT has an impact on moisture transport and atmospheric rivers in the decadal time scale. Using ERA5 reanalysis data, the decadal variations in Atmospheric Rivers (ARs) in the North Atlantic in boral winter in relation to NAT phases were studied. During the positive NAT phase, the positive SST in the central and western North Atlantic increases the humidity and causes an anticyclonic wind response, which enhances the northeastward transport of moisture. As a result, ARs tend to be longer and transport more moisture toward northwestern Europe. This causes enhanced extreme rain in the UK and Norway. During the negative NAT phase, the positive SST anomalies in the south and east of the North Atlantic provide more moisture, induce a southward shift of the ARs and enhance extreme rain in the Iberian Peninsula. The Gulf Stream (GS) front is stronger during the negative NAT phase, increasing the frequency of the atmospheric front and enlarging the rain rate in ARs.

Highlights

Atmospheric rivers are long, narrow water vapor transport bands and they are a major means by which water vapor can be transported in mid-latitudes [1]
The North Atlantic oscillation (NAO) Index data were provided by the Climate Analysis Section, NCAR, Boulder, USA, using the definition developed by Hurrell et al [20] and the AMO Index data derived from HadISST using Trenberth and Shea’s definition [14]
The Atmospheric Rivers (ARs) occurrence frequency increased by about 6% in north Europe and decreased by more than 6% south of the Gulf Stream (GS) front, which is about one quarter of the total, suggesting a significant influence of the North Atlantic tripole (NAT) on ARs

Summary

Introduction

Atmospheric rivers are long, narrow water vapor transport bands and they are a major means by which water vapor can be transported in mid-latitudes [1] Due to their large amount of water vapor, ARs can cause precipitation and flooding when making landfall, especially in the mountainous regions [2]. In the interannual–decadal time scale, variation in ARs and extreme precipitation in western Europe are reported to be affected by the North Atlantic oscillation (NAO). As the direct response of the ocean mixed layer to NAO [10], the North Atlantic tripole (NAT) distribution is the leading mode of SSTs in the decadal time scale [11,12].

Data and Methods

AR Detection and Composition

Atmospheric Fronts Detection

Decadal Variation of ARs in Relation to the NAT

Findings

Conclusions and Discussions