[Nobel 2021] Dr. Žiga Zaplotnik (FMF): Recent changes of the tropical Hadley circulation: climate-change-induced trend or multidecadal variability?
Climate models are virtual laboratories that simulate coupled atmosphere-ocean-land processes. Built on the pioneering works of the 2021 Physics Nobel Prize winners Sykuro Manabe and Klaus Hasselmann and their collaborators, climate models have successfully predicted the rising global-mean surface temperatures. These have been unequivocally attributed to the increasing atmospheric greenhouse-gas concentrations. However, these models struggle to reproduce some observed features of the atmospheric circulation, suggesting a possible major role of internal climate variability or significant model biases. For example, they have predicted a weakening of the Hadley circulation (HC) in the recent decades, whereas observation-based reconstructions of the past weather (reanalyses) show a strengthening of HC. In our studies, we have attributed the long-term variability of the HC strength to the meridional gradient of adiabatic (latent) heating, which is related to precipitation gradients. However, the strengthening of the HC in the reanalyses is larger than the strengthening expected from the observed zonal-mean precipitation gradient (via Global Precipitation Climatology Project, GPCP). This suggests that the HC strength trends in the recent decades can be explained partly as an artifact of the misrepresentation of latent heating (model bias) and partly through (physical) long-term climate variability. To show that the latter is true, we compare the reanalysed data and the sea surface temperature (SST) observational data. Our analysis reveals that the changes in the HC strength can be a consequence of the Atlantic multidecadal oscillation (AMO) and related SST-induced diabatic and frictional processes which, in turn, drive global HC variability.
