Midlatitude Moisture Contribution to Recent Arctic Tropospheric Summertime Variability

The dynamics of late summer Arctic tropospheric heat content variability is studied using reanalyses. In both trends and interannual variability, much of the August heat content variability in the Arctic mid-troposphere can be explained by the total-sensible plus latent-heat content variability at the midlatitude near surface in July. Climate models suggest that this connection is part of the global warming signal in September-November, but in reanalyses the connection is most strongly present in July-August variability and trends. It is argued that heat content signals are propagated from the midlatitude near surface to the Arctic midtroposphere approximately along climatological moist isentropes. High-frequency data reveal that the propagating signal is primarily driven by a few strong meridional heat flux events each summer season. Composite analysis on these events shows that August meridional heat fluxes into the Arctic midtroposphere are succeeded by positive heat content anomalies in the lower troposphere a few days later. This second connection between the Arctic midtroposphere and the Arctic lower troposphere could be sufficient to explain some of the recent Arctic 850-hPa temperature variability north of 75 degrees N.