Indication of a Possible Self-Regulation of Northern Hemisphere Mid-Tropospheric Temperatures and Its Connection to Upper-Level Winds in Reanalyses and Earth System Models

Previous studies proposed convective limits on Northern Hemisphere 500-hPa temperature to annual minimums of both the warmest temperature (Tmax) of ~-3°C in the tropics and the coldest temperature (Tmin) of ~-42°C in the Arctic. The previously found existence of convective heating over boreal winter oceans is further substantiated here in the strong temperature tendencies due to convection found in MERRA-2. We further substantiate the 500-hPa temperature limits using MERRA-2 and two other current generation reanalyses (ERA5 and JRA-55). In all three of these reanalyses, there are statistically significant trends in the annual maximums in Tmin in the Northern Hemipshere (from 0.40-0.66°C/decade); two have statistically significant trends in the annual minimums in Tmax (0.13 and 0.19°C/decade). As upper-level tropospheric winds are related to the meridional temperature gradient in the Northern Hemisphere through the thermal wind balance, we also analyze the trends in maximum zonal wind speed. There are very small trends in the yearly maximum in the highest 200-hPa zonal wind speeds in the Northern Hemisphere and a slight poleward movement in the latitude of the highest winds in the reanalyses. This does not point to the jet stream becoming wavier as has been hypothesized by others. The reanalysis climatology is then used to evaluate four current generation Earth system model simulations driven by observed sea surface temperature and sea ice. Such simulations generally have larger trends than in the reanalyses. They are all too cold when the warmest tropical temperatures are at their lowest in the mean annual cycle. Only one model produces the poleward movement of the latitude of highest winds. In this work, we demonstrate how the reanalysis trends can be used to assess which of the CMIP models are more reliable in the historic period and hence may provide more trustworthy future projections.