Poleward Shift of Northern Subtropics in Winter: Time of Emergence of Zonal Versus Regional Signals

Abstract

The Northern Hemisphere mid-latitudes will be exposed to hydroclimatic risk in next coming decades because the subtropical expansion. However, it is not clear when the anthropogenic signal will emerge from the internal climate variability. For this purpose, we investigate the time of emergence (ToE) of the hemispheric and regional shift of northern subtropical margins in the Max Planck Institute Grand Ensemble. For several indicators, the ToE of the poleward shift of Northern subtropical margin will not occur by the end of the 21st century, neither at regional nor at hemispheric scale. The exceptions are the Mediterranean/Middle East and, to a lesser degree, Western Pacific, where the ToE would occur earlier. According to our results, given the fundamental role played by internal variability, trends of Northern Hemisphere subtropical poleward shift that have been identified over last decades in reanalyses cannot be considered as robust signals of anthropogenic climate change. Plain Language Summary The projected poleward shift of subtropical margins will affect densely populated areas of the Northern Hemisphere that have limited or scarce water resources. Therefore, it is fundamental for societies in these areas to know when the effects of climate change on the regional environment will become evident, because this information can make the difference for effective adaptation and mitigation strategies. The position of the subtropical margins is intimately related to the zonal-mean Hadley Circulation, but is also affected by regional processes related to the distribution of continents, oceans and to their characteristics. In our study we show that the poleward shift of Northern subtropics has strong regional connotations and only in few areas, for example, the Mediterranean/Middle East and, to a lesser extent the Western Pacific, the anthropogenic climate change signal will emerge from the natural climate variability before the end of the 21st century. Recent literature emphasized how magnitude and significance of the HC poleward shift and tropical width expansion from 1979 onward disagree among studies: Rates are ranging from few tenths to several degrees latitude per decade, and they are not always significant (

Is Hadley Cell Expanding?

“…The poleward extension of the Hadley Cell may result in the extension of these arid regions to higher latitudes. For example, reduced precipitation and more frequent droughts have been observed in the subtropics (e.g., Southern Australia, the Mediterranean region, and Northern China), which are linked to the expansion of Hadley Cell for the past few decades, e.g., [1,2] and others. The frequent extreme events of heat waves occurring in Europe in recent years are attributed to the poleward extension of the subtropical high due to the Hadley Cell expansion and the northward shift of the westerly jet in summer.…”

“…In particular, the forced poleward shift of the storm track is most significant over the North Atlantic, which highlights the importance of a regional perspective when considering Hadley Cell width variations. More recent studies investigated the Hadley Cell edge based on the longitudinally varying methods and higher temporal resolution information [2,54]. For example, Martin et al [54] found significant contraction of the tropics over the eastern Pacific between 1981 and 2015, and slight but significant expansion in other region.…”

“…For example, Martin et al [54] found significant contraction of the tropics over the eastern Pacific between 1981 and 2015, and slight but significant expansion in other region. D'Agostino et al [2] found that the poleward shift of the Northern Hadley Cell edge potentially increases serious droughts in Mediterranean/Middle East and Western Pacific. Based on the analysis of the climate model output, Amaya et al [101] suggested that regional changes in hydrology largely follow the zonal mean in the Southern Hemisphere but exhibit strong regional differences in the Northern Hemisphere.…”

“…Previous studies found that the natural low-frequency variability associated with ocean dynamics has probably caused at least as much of the recent trend in Hadley Cell widening as human activity [82,101], which indicates that the Hadley Cell widening may not be predominantly human-induced. Using the largest ensemble available among the climate models Max Planck Institute-Grand Ensemble, the impacts of the internal climate variability and anthropogenic forcing have been disentangled by D'Agostino et al [2]. They suggested that the trends in a widening of Hadley Cell in reanalyses over last decades cannot be considered a robust signal of anthropogenic forcing, while the internal variability will prevail over the forced response until 2100.…”

“…The expansion of Hadley Cell with the poleward extension of the the subtropical dry zone may shift precipitation patterns and bring more frequent droughts to many densely populated regions that currently enjoy mild weather [161], which has potential to expand subtropical deserts and induce wildfires [1,162]. The Mediterranean/Middle East, Western Pacific, and Asian monsoon region will be exposed to more frequent droughts due to the poleward shift of Northern Hemisphere Cell [2,163]. Meanwhile, the subtropical highs over ocean coincide with regions of high salinity featuring low marine bioproductivity, and a widening Hadley Cell implies an expansion of these regions.…”