On the Possible Link between Tropical Convection and the Northern Hemisphere Arctic Surface Air Temperature Change between 1958 and 2001

This study presents mechanisms for the polar amplification of surface air temperature that occurred in the Northern Hemisphere (NH) between the periods of 1958–77 (P1) and 1982–2001 (P2). Using European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) reanalysis data, it is found that over the ice-covered Arctic Ocean, the winter surface warming arises from dynamic warming (stationary eddy heat flux and adiabatic warming). Over the ice-free Arctic Ocean between the Greenland and the Barents Seas, downward infrared radiative (IR) flux is found to dominate the warming. To investigate whether the difference in the flow between P1 and P2 is due to changes in the frequency of occurrence of a small number of teleconnection patterns, a coupled self-organizing map (SOM) analysis of the 250-hPa streamfunction and tropical convective precipitation is performed. The latter field was specified to lead the former by 5 days. The results of the analysis showed that the P2 − P1 trend arises from a decrease in the frequency of negative phase PNA-like and circumglobal streamfunction patterns and a corresponding increase in the frequency of positive PNA-like and circumglobal streamfunction patterns. The occurrence of the two strong 1982–83 and 1997–98 El Niño events also contributes toward this trend. The corresponding trend in the convective precipitation is from below average to above average values in the tropical Indo-western Pacific region. Each of the above patterns was found to have an e-folding time scale from 6 to 8 days, which implies that the P2 − P1 trend can be understood as arising from the change in the frequency of occurrence of teleconnection patterns that fluctuate on intraseasonal time scales. The link between intraseasonal and interannual variability was further examined by linearly regressing various quantities against trend patterns with interannual variability subtracted. It was found that enhanced convective precipitation is followed 3–6 days later by the occurrence of the P2 − P1 circulation trend pattern, and then 1–2 days later by the corresponding trend pattern in the downward IR flux. This finding suggests that an increased frequency of the above sequence of events, which occurs on intraseasonal time scales, can account for the NH winter polar amplification of the surface air temperature via increased dynamic warming and downward IR flux.