High and low latitude types of the downstream influences of the North Atlantic Oscillation

Jie Song,Wen Zhou,Chongyin Li

doi:10.1007/s00382-013-1844-3

Abstract

Using reanalysis data, we find that the downstream-propagating quasi-stationary Rossby wave train associated with the North Atlantic Oscillation (NAO) generally propagates along a high (low)-latitude pathway during warm (cold) El Niño-Southern Oscillation (ENSO) boreal winters. Consistent with the different propagation directions of the NAO-related downstream wave train, during the warm (cold) ENSO winters, the NAO is associated with significant 300 hPa geopotential height anomalies over eastern Siberia (the Arabian Sea, the east coast of Asia at around 40°N, and the North Pacific), and the near-surface air temperature perturbations associated with the NAO over the high latitudes of Asia are relatively strong (weak). Based on these differences, we argue that the NAO has two distinct types of downstream influence: a high-latitude type and a low-latitude type. Furthermore, we argue that the two types of NAO’s downstream influence are modulated by the intensity of the subtropical potential vorticity (PV) meridional gradient over Africa. When this gradient is weak (strong), as in the warm (cold) ENSO winters, the NAO’s downstream influence tends to be of the high (low)-latitude type. These results are further supported by analysis of intraseasonal NAO events. We separate NAO events into two categories in terms of the intensity of the subtropical PV gradient over Africa. Composites of the NAO events accompanied by a weak (strong) subtropical PV gradient show that the NAO-related downstream wave train tends to propagate along a high (low)-latitude pathway.

Highlights

The North Atlantic Oscillation (NAO), which refers to a seesawing of atmospheric mass between the Arctic and the subtropical Atlantic, is recognized as the regionally dominant mode of atmospheric circulation variability over the North Atlantic (e.g., Van Loon and Rogers 1978; Barnston and Livezey 1987; Hurrell et al 2003)
Using reanalysis data, we find that the downstream-propagating quasi-stationary Rossby wave train associated with the North Atlantic Oscillation (NAO) generally propagates along a high-latitude pathway during warm El Nino-Southern Oscillation (ENSO) boreal winters
During the warm ENSO winters, the NAO-related wave train propagates downstream primarily along a high-latitude pathway, which might be interpreted as nonlinear wave reflection

Summary

Introduction

The North Atlantic Oscillation (NAO), which refers to a seesawing of atmospheric mass between the Arctic and the subtropical Atlantic, is recognized as the regionally dominant mode of atmospheric circulation variability over the North Atlantic (e.g., Van Loon and Rogers 1978; Barnston and Livezey 1987; Hurrell et al 2003) It has a dramatic influence on surface temperature variability over the entire Northern Hemisphere (NH), especially in boreal winters. Hurrell and Van Loon (1997) showed that the effect of anomalous transient eddies on the temperature perturbation is confined to the North Atlantic/ European sector and, more importantly, these eddies act to oppose the low-level temperature perturbation associated with the NAO Their results indicate that the zonal advection of the climatological temperature gradient by the anomalous westerly flow associated with the NAO induces the temperature perturbation over the downstream regions of the NAO (see Trigo et al 2002; in this study, the ‘‘downstream regions of the NAO’’ denotes the Eurasian continent, 0°–120°E). The NAO must work in combination with other processes to exert its influences on such regions

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