Cyclone activity

It should come as no surprise that the NAO/AO is allied with pronounced signals in cyclone frequency. Serreze et al. (1997a) inspected the NAO time series from 1966 through 1993. For each cold-season month (October through March), they extracted the seven years with the most positive and most negative NAO index values. For each month of the seven-year period, they extracted records of cyclone events over the North Atlantic, using an automated detection and tracking algorithm applied to SLP fields (see Chapter 4). The positive minus negative NAO difference field in cyclone counts from that study is reproduced in Figure 11.14. There is an obvious poleward (equatorward) shift in cyclone activity between the positive (negative) NAO phases, broadly consistent with the different patterns of SLP associated with NAO extremes. For the region corresponding to the climatologi-cal center of the Icelandic Low, cyclone events are more than twice as common under positive NAO extremes as compared to negative extremes. Systems found in this region during the positive phase are also significantly deeper than their low NAO counterparts. Similar results can be expected for composites based on the AO.

Figure 11.14 Positive minus negative NAO difference field of cyclone events for the cold season based on index extremes over the period1966-93.The contour interval is 20. Positive differences are shown by solid contours with negative differences shown by dash-dot contours (from Serreze et al., 1997a, by permission of AMS).

Figure 11.14 Positive minus negative NAO difference field of cyclone events for the cold season based on index extremes over the period1966-93.The contour interval is 20. Positive differences are shown by solid contours with negative differences shown by dash-dot contours (from Serreze et al., 1997a, by permission of AMS).

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