Processes shaping the spatial pattern and seasonality of surface air temperature changes in response to the anthropogenic forcing

Across the mid-1990s, the surface air temperature (SAT) warms more rapidly over some regions over the continent than global mean. To understand the physical processes, and the roles of the anthropogenic forcing, in the decadal SAT warming over different regions, we have performed a set of time-slice experiments using MetUM-GOML1: an atmospheric general circulation model coupled to an ocean mixed layer model.

The model simulated SAT changes in response to changes in greenhouse gas (GHG) concentrations and anthropogenic aerosol (AA) emissions bear great similarities in both spatial patterns and magnitudes to the observed ones, indicating a predominant role of changes in anthropogenic forcings in observed decadal SAT changes across the mid-1990s. Results from individual forcing experiments further indicate that the SAT warming is dominated by the increase in greenhouse gases (GHG), which leads to stronger land surface warming in northern Hemisphere in winter (DJF) than in summer (JJA). Changes in anthropogenic aerosol (AA) emission induce localized warming over Europe, Eurasian continent and Alaska. In Europe, SAT warming is stronger in JJA because the recent decadal decrease in AA emissions over Europe induces a substantial increase of clear sky short wave radiation, which warms local SAT strongly in JJA. Over Eurasia, the GHG induced warming is stronger in DJF because the SAT warming reduces the snow depth and decreases the surface albedo at mid-latitudes, which increases the net surface shortwave radiation and in turn amplifies the SAT warming. Over Alaska, both GHG and AA forcing changes lead to strong warming in DJF by changing large scale circulations. The GHG and AA forcing changes lead to an El Niño-like SST anomalies and enhance Aleutian low, which causes an increase in water vapor transport from North Pacific to Alaska and generates more cloud there. Thus, it is the enhanced local water vapor feedback and increased longwave cloud radiative effect (LW CRE) that amplify the SAT warming.