Opposite CMIP3/CMIP5 trends in the wintertime Northern Annular Mode explained by combined local sea ice and remote tropical influences

A crucial challenge in climate studies is to determine how warming trends due to anthropogenic forcing may affect the natural modes of atmospheric variability. In the northern extratropics, the leading pattern of atmospheric dynamics is known as the Northern Annular Mode (NAM), often computed as the first empirical orthogonal function of sea level pressure (SLP) or geopotential height at 500 mbar (Z500). Here we compare wintertime NAM changes estimated from previous (third phase of the Coupled Model Intercomparison Project (CMIP3)) versus ongoing (fifth phase (CMIP5)) generations of multimodel projections for the 21st century, under similar emission scenarios (A2 scenario versus 8.5 W.m^-2Representative Concentration Pathway). CMIP3 projections exhibited a positive NAM trend, albeit this response differed between SLP and Z500, whereas CMIP5 projections rather reveal a negative trend, especially for Z500. We show that the CMIP3/CMIP5 discrepancies are mostly explained in early winter by the local consequence of faster Arctic sea ice loss in CMIP5 and in late winter by the remote influence through teleconnection of stronger warming in the western tropical Pacific. The attribution of CMIP3/CMIP5 discrepancies to the differences in emission scenarios is assessed by investigating NAM responses in common 1% CO₂idealized experiments.