A Study of the Extratropical Baroclinic Eddy Activity during Arctic Spring Onset

During spring, the Arctic atmospheric circulation experiences a dramatic transition, which is dominated by a shift from the winter pattern to the summer pattern in the middle and upper tropospheric circulation and a rapid surface warming north of 75N. We apply a new technique to identify the spring onset in terms of local rapid increase in the surface air temperature for 32 years from 1979 to 2010. The spring onset date exhibits strong interannual variability but no significant trend. While the whole Arctic region warms rapidly during spring onset, a critical region to the north of Siberia experiences the most dramatic warming. A composite synoptic analysis reveals a preferred circulation pattern with an anti-cyclonic circumpolar cell over the Arctic and a cyclonic cell over the east Eurasian continent, resulting in the transport of warm air into the critical warming region. A heat budget analysis shows that the warming is due to the combined effect of the large-scale circulation, baroclinic eddies, adiabatic and diabatic contributions. The baroclinic eddies play a fundamental role in this process: During the spring onset, a persistent weakening of eddy activity is observed in the North Atlantic storm track region while a packet of locally enhanced eddy activity is observed to propagate through the downstream region. As a consequence, anomalously high baroclinic eddy activity propagates eastward and, after spring onset, the eddy activity is enhanced within the north Siberian region, providing an enhanced northward heat transport into the critical region to sustain the rapid warming. To understand the detailed influence of this eddy activity, analyses of individual cyclone and anti-cyclone events will be investigated in individual case studies of spring onset to isolate different classes of spring onset events.