Abstract
State-of-the-art climate models predict an enhanced warming over global land areas in response to the greenhouse gas (GHG) forcing. In nature, however, the observed land surface warming is not spatially uniform in recent decades as GHG concentration has risen rapidly. There is no warming and even cooling trend in spring northwest North America (NWNA) surface temperature since 1980. Here, we link the observed multidecadal surface cooling trend in NWNA to the weakening of spring Aleutian low (AL), which is ecologically important during the growing season. The AL weakening leads to reductions of southwesterly wind and warm air transport from eastern North Pacific to NWNA. The spring AL weakening and associated NWNA surface cooling are reasonably reproduced by AMIP models, driven by the historical external forcing and imposed sea ice concentration (SIC) and sea surface temperature (SST) from observations. Further investigations into the roles of different forcing components suggest that the changes in the AL are most likely driven by the tropical SST changes in recent decades while the external forcing and SIC play minor roles. There is a zonal dipole pattern of trends in tropical SSTs since 1980, with central-eastern Pacific cooling and warming in other tropical basins, and the atmospheric responses to this SST change show a meridionally propagating Rossby wave train from tropics into subarctic North Pacific, which mediates the SST forcing and the AL changes.