Northen hemisphere meridional and zonal temperature gradients and their relation to hydrologic extremes at mid-latitude: trends, variability and link to climate modes in observation and simulations

The jet stream dynamics and the associated mid-latitude storm track are modulated by large scale ocean-land boundary conditions, which depend on both the evolution of the known interannual and multi-decadal natural variability and on changes in meridional and zonal surface temperature gradients due to anthropogenic forcing. Here, within the framework of the Lorenz (1984) low-order atmospheric model, the Equator-to-Pole temperature Gradient (EPG) and the Ocean-Land temperature Contrast (OLC) are considered as drivers of mid-latitudinal circulation. The historical trends of the seasonal NH Equator-to-Pole temperature Gradient (EPG) and the Ocean-Land temperature Contrast (OLC) are explored, as well as their probability structure, and their potential relation to anthropogenic warming. The connection between dierent combinations of EPG and OLC and precipitation patterns at mid-latitudes are shown. Then, these variables and the relations described above are examined in two CGCM simulations of the 20th century. The results show that there exist systematic biases in the temporal simulation of the gradients at hand, which is likely propagated to the simulation of precipitation. However, we show that if a model is able to reasonably capture the observed relationship between the gradients and precipitation, then it produces large-scale spatial distributions of rainfall that are consistent with observed patterns. Therefore, an eort to address the biases in simulating EPG and OLC could lead to improved temporal and spatial simulations of precipitation in the models.