Observed winter cyclone tracks in the northern hemisphere in re‐analysed ECMWF data

Abstract The observed cyclone activity in the northern‐hemisphere winter (DJF) is investigated in 1979‐97 European Centre for Medium‐Range Weather Forecasts re‐analyses. The cyclone trajectories are determined automatically by a next‐neighbour search of minima identified in the 1000 hPa geopotential‐height field (z1000). These are compared with the traditional storm track, defined by the root mean square of the band‐pass filtered 500 hPa geopotential‐height anomaly. The analysis covers the North Atlantic and North Pacific basins. The trend of the cyclone density (distribution) is similar to that of the storm track in the North Atlantic, but opposite in the North Pacific. In the Atlantic, the total number of cyclones with low central z1000 increases, due to the northward shift towards lower regional climatological pressure, whereas the number of cyclones with large gradients decreases. In the Pacific, the number of intense (weak) cyclones with large (low) gradients increases (decreases). The cyclone‐track variability is investigated by a cluster analysts of the relative cyclone trajectories. The three centroids corresponding to north‐eastward and zonally propagating cyclones and nearly stationary ones are very similar in both ocean basins. The winter‐mean cluster occupation numbers of the two propagating cyclone clusters are positively correlated with one another in the Atlantic, and negatively in the Pacific. The three Atlantic cyclone clusters can be linked to particular aspects of the central European climate. The cyclonic activity can be related to teleconnection patterns which are dominant during northern hemisphere winter. During high North Atlantic Oscillation index winters (deeper Icelandic lows), the Atlantic cyclone density shifts northward and is associated with more stationary cyclones. During El Niño warm‐event winters, the Pacific cyclone density is shifted northward and their propagation is oriented more zonally as in high Pacific/North American index phases (deeper Aleutian lows). In the North Atlantic propagating cyclones occur less and stationary ones more frequently in El Nino winters. The correlations between the north‐east ward propagating cyclones in the Pacific and the north‐eastward and zonally propagating ones in the Atlantic suggest an interaction between the two storm tracks.