The Cyclic Climate Regime of the Recent Past

We apply a variant of Non-linear Principal Component Analysis to atmospheric observations and simulations from a Global Climate Model. Analysis of tropospheric geopotential height fields for the Northern Hemisphere shows that climate variability in the extra-tropics during the second half of the twentieth century is due to the existence of 3 main weather regimes. These regimes are similar to those identified recently by various groups. We show that by introducing a fourth weather regime, it is possible to build a climate cycle for the recent past. We will describe this cyclic climate regime, its relation to stratospheric variability and its link to the Arctic Oscillation. We will also describe the structure of the cyclic regime present in climate simulations conducted with a Global Climate Model.