Speaker
Dr
Alain Hauchecorne
(CNRS-LATMOS)
Description
The variability of the solar irradiance may have an impact of the Earth climate by changing the input of energy absorbed at the surface and in the atmosphere. However the amplitude of this impact is still very badly known due to the complexity of the processes involved in the solar forcing of the Earth atmosphere. Two main mechanisms have been proposed. The “bottom-up” forcing is the direct absorption of the visible and near IR solar irradiance by the surface. However the variability of the solar irradiance in these wavelengths bands is no more than 0.1% over an 11-year solar cycle and the impact on the global temperature at the surface is expected to be limited according to the last IPCC assessment. The “top-down” forcing is the related to the 5 to 10% change in the UV solar irradiance, which modifies the production of stratospheric ozone and the temperature of the middle atmosphere, inducing a dynamical coupling with the troposphere. In particular the rate of occurrence and the characteristics of polar Sudden Stratospheric Warmings (SSWs) can be affected. Recent studies have shown that temperature and pressure perturbations induced by SSWs can propagate down to the ground and can enter in resonance with modes of the tropospheric circulation like the Artic Oscillation/North Atlantic Oscillation (AO/NAO). The NAO is characterized by the pressure gradient between subtropical and Arctic latitudes over the North Atlantic. It is a dominant mode of the climate over Europe. A positive NAO phase will lead to weather warmer and wetter than normal in most of Europe and drier over the Mediterranean area. The reverse is observed with a negative NAO phase. Global climate modelling and observations have shown that a high solar activity favours the positive NAO phase and a low solar activity favours the negative NAO phase. This mechanism can explain part of the observed variability of the climate in the North hemisphere in correlation with the solar activity and is in agreement with a colder temperature above Europe during the Maunder minimum.
Primary author
Dr
Alain Hauchecorne
(CNRS-LATMOS)
