Long-Period Changes in the Frequency of Cyclones in the Northern Hemisphere Temperate Latitudes

Abstract

Fluctuations in the frequency of cyclones in various regions of the Northern Hemisphere (NH) temperate latitudes on time scales of the order of decades are analyzed in connection with changes in the indices of the leading modes of atmospheric circulation and changes in the zonal transport intensity in individual latitudinal zones. The possible manifestation in cyclone statistics of the well-known thesis about the displacement of storm tracks during warming in the direction of high latitudes is discussed. It is shown that, in general, for the NH temperate latitudes in winter, long-period changes in the frequency of cyclones are irregular fluctuations with scales of several decades, without a visible trend. In summer, the interdecade changes are weakly expressed, but there is a noticeable trend that is significant at the 5% level. In the northern and southern parts of the North Atlantic (NA) in winter, changes in frequency contain significant antiphase components with a period of about 10 years, which correlate well with changes in the North Atlantic Oscillation (NAO) index (the correlation is positive in the northern half; the coefficients are significant at the 0.1% level). Long-period changes in the frequency of cyclones in the North Pacific are generally similar to (but in the opposite phase of) changes in the North Pacific Index by Trenberth and Hurrell. Based on the analysis of a linear regression model, it was found that a significant contribution to changes in the frequency of cyclones in the regions of northern Europe–Western Siberia and the north of ER (ER) in the winter season was made by the circulation modes of the Atlantic–European sector: SCAND, NAO, East Atlantic mode EAM, EAWR (but the EAWR mode contribution is insignificant for the north of Europe–Western Siberia). In summer, for the north of ER and Western Siberia, a significant contribution was made by the SCAND and EAWR circulation modes. An analysis of concomitant changes in zonal wind speed at 700 hPa in the area of the main storm tracks in winter revealed that, for the hemisphere as a whole (0°–360°) in the latitude zone 45°–55° N, as well as in the zone 55°–65° N, changes in zonal wind are determined mainly by changes in the frequency of cyclones in the northern part of the NA and closely follow changes in the NAO. However, in more southern latitudes (35°–45° N), changes in the hemispheric zonal wind are observed, similar to long-period changes in the North Pacific Index in antiphase, the nature of which is unclear (since they do not appear in the Pacific sector itself). The shift of storm tracks to higher latitudes, expected with warming, is observed only for the northern branch of the Atlantic storm track during periods of NAO growth between 1960 and the mid-1990s and after 2010. In general, for the period since 1976, there has been an insignificant trend of about 0.07° latitude per decade.