The Western Tibetan Vortex as an Emergent Feature of Near-Surface Temperature Variations.

Remco J De Kok,Walter W Immerzeel

doi:10.1029/2019gl085757

Abstract

Glaciers around the world are shrinking, yet in a region in northwestern High Mountain Asia (HMA), glaciers show growth. A proposed explanation for this anomalous behavior is related to the variability of the “Western Tibetan Vortex” (WTV), which correlates well with near‐surface temperatures in northwestern HMA. Using analytical formulations and ERA5 reanalysis data, we show that the WTV is the change of wind field resulting from changes in near‐surface temperature gradients in geostrophic flow and that it is not unique to northwestern HMA. Instead, we argue that net radiation is likely the main driver of near‐surface temperatures in Western HMA in summer and autumn. The decreasing strength of the WTV during summer in the twentieth century is thus likely the result of decreasing net radiation. We do argue that the WTV is a useful concept that could yield insights in other regions as well.

Highlights

Despite global warming, parts of High Mountain Asia (HMA), mainly the Western Kunlun Shan and Karakoram (WKSK), show growing glaciers, in contrast to most glaciers worldwide (e.g. Brun et al, 2017; Gardelle et al, 2012; Kääb et al, 2015)
Using analytical formulations and ERA5 reanalysis data, we show that the Western Tibetan Vortex” (WTV) is the change of wind field resulting from changes in near‐surface temperature gradients in geostrophic flow and that it is not unique to northwestern HMA
We argue that net radiation is likely the main driver of near‐surface temperatures in Western HMA in summer and autumn and that the WTV is the response of the atmosphere to changes in temperature

Summary

Introduction

Parts of High Mountain Asia (HMA), mainly the Western Kunlun Shan and Karakoram (WKSK), show growing glaciers, in contrast to most glaciers worldwide (e.g. Brun et al, 2017; Gardelle et al, 2012; Kääb et al, 2015). Possible positive trends in winter precipitation (Cannon et al, 2015; Kapnick et al, 2014; Norris et al, 2018), as well as an increase in irrigation‐induced summer precipitation, with corresponding increases in summer cloudiness (de Kok et al, 2018) in WKSK might favor glacier growth Another suggestion was put forward by Forsythe et al (2017), who found that temperatures in northwestern HMA highly correlate with the circulation around northwestern HMA at pressures between 500 and 200 hPa. Another suggestion was put forward by Forsythe et al (2017), who found that temperatures in northwestern HMA highly correlate with the circulation around northwestern HMA at pressures between 500 and 200 hPa This structure of correlated atmospheric variables was dubbed the “Karakoram Vortex,” or “Western Tibetan Vortex” (WTV) by Forsythe et al (2017) and following work by Li et al (2018, 2019), together referred to as FL17‐19 here. The causal relationship between temperatures and winds would be reversed compared with what was originally proposed

Data and Methods

Analytical Considerations

Winds and Vortices in Reanalysis Data

Drivers of Near‐Surface Temperatures in Northwestern HMA

Discussion and Conclusions