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

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. Key Points Recent trends of subtropical margins are not a robust signal of anthropogenic climate change in the Northern Hemisphere The internal climate variability prevails over the forced response until 2100 on Northern subtropical margin poleward shift at hemispheric scale The poleward shift of Northern subtropics will affect mainly Mediterranean/Middle East and Western Pacific sectors