Heat budget analysis of Northern Hemisphere high-latitude spring onset events

Abstract

Regional spring onset events are identified within four high-latitude sectors: the primary (critical) region over North Siberia (CR), Greenland-North America (G-NA), East Asia (EA), and Alaska (AL). To identify the primary forcing of the rapid temperature increases observed during spring onset, the contributions to the near-surface air temperature anomaly tendency are diagnosed within the thermodynamic equation for each of the four regional event categories. For each region, anomalous eddy heat flux convergence is the primary contributor to regional warming prior to, and during the early stages of, spring onset (through day +5). Thereafter, horizontal advection of the climatological-mean temperature by the large-scale circulation anomaly field emerges as the leading contributor to regional warming (during the later stages of spring onset). A parallel diagnostic of storm track strength (using the envelope function) reveals a systematic weakening of eddy activity within each region, leading to a reduction in the northward eddy heat flux out of the domain and an accumulation of heat within the region. For CR, G-NA, and EA events, an east-west dipole structure in the sea level pressure anomaly field, with lower (higher) pressure to the west (east), generates the anomalous southerly flow linked to late period linear warm advection. Our results indicate that anomalous dynamical processes associated with synoptic eddy activity and stationary wave patterns are the primary contributors to rapid temperature increase during Arctic spring onset events, with minimal contributions from anomalous diabatic processes.