ESRL/PSD Seminar Series

Arctic temperature has dramatically risen since 1979, with a common supposition being that sea ice decline has principally caused an Arctic Amplification of tropospheric warming (and 1000-500 hPa thickness increase) which in turn is hypothesized to have caused changes in lower latitude atmospheric circulations. Using climate simulations we first quantify the magnitude of Arctic (60°-90°N) warming resulting from various factors. It is shown that the Arctic deep troposphere warmed primarily due to remote, rather than in situ, forcings since 1979 with the main drivers being recent decadal fluctuations in sea surface temperatures (SSTs) and a long-term warming of SSTs, both occurring outside the polar cap. Arctic sea ice decline, though causing much of near-surface Arctic warming, contributed only about 20% to the magnitude of deep tropospheric warming while unforced decadal atmospheric variability may have accounted for up to 25%. We next diagnose the remote effects of Arctic sea ice loss on lower latitudes, and consistent with its weak contribution to an Arctic Amplification of column warming, found little impact on remote atmospheric circulations including the jet stream. Results further indicate that pronounced circulation changes that have occurred over the North Atlantic and North Pacific regions in recent decades were mainly a large amplitude expression of decadal atmosphere-ocean variability. Evidence is thus presented that these regional features of climate trends since 1979 were not caused by an Arctic amplification linked to sea ice loss. In consequence the Arctic troposphere has been mainly responding to rather than forcing mid-latitude weather and climate.