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 from a maximum of ~−3 °C in the tropics to a minimum of ~−42 °C in the Arctic. Here, we further explore this topic using three current generation reanalyses. All three reanalyses indicate that there has been statistically significant trends in the yearly maximums in the coldest temperatures in the Arctic at 500 hPa (from 0.40 to 0.66 °C decade⁻¹), while two have statistically significant trends in the yearly minimums in the warmest 500-hPa temperatures in the Northern Hemisphere (0.13 and 0.19 °C decade⁻¹). 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 was hypothesized by others. The reanalysis climatology is then used to evaluate four current generation Earth system models. These models driven by observed sea surface temperature and sea ice generally produce larger trends than represented by 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. The reanalysis trends presented here 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.