Transformation of tropical cyclones into extratropical: The tendencies of 1970–2012

Abstract

The review of the global state of the climate for 2012, issued by the World Meteorological Organiza� tion, has emphasized the record number of tropical and extratropical cyclones in the Russian Far East. In 2012, six tropical cyclones were reported in this region, while the previous record (since 1970) was five cyclones, in 1981 and 2011 (http://library.wmo. int/pmb_ged/wmo_1108_en.pdf). Intensive atmo� spheric eddies, which are tropical cyclones, and espe� cially the most powerful of them (typhoons, or hurri� canes), are associated with severe weather and climatic anomalies and with the social–economic conse� quences as well [1–5]. Many tropical cyclones trans� form into extratropical [6–8]. The question is in to degree this is manifested in changes of cyclonic activ� ity in the extratropical atmosphere with respect to glo� bal climate changes [9–11]. The recent data indicate the frequent occurrence of intensive cyclones, including those transformed from tropical ones into the North Atlantic and northwestern Pacific oceans in extratropical latitudes. Under the tendency towards global warming, we can expect an increase in the number of intensive cyclones in the warmer and more humid troposphere. In the present work, we estimate the tendencies of changes in pro� cesses of the transformation of tropical cyclones into extratropical ones for various oceanic basins in the Northern and Southern hemispheres, based on the data of 1970–2012 [12]. During this analysis, we also used specified and supplemented data from [13]. Table 1 presents the average annual numbers of tropical cyclones Ntc, those transformed into extratro� pical ones Netc, and the ratio Netc/Ntc, based on the data of 1970–2012, for various oceanic basins. In the Northern Hemisphere these are the NWPO (north� western Pacific Ocean), NEPO (northeastern Pacific Ocean), NAO (northern Atlantic Ocean), and NIO (northern Indian Ocean); in the Southern hemisphere these are the SIO (southern Indian Ocean), SPO (southern Pacific Ocean), and SAO (southern Atlantic Ocean). The table also contains the average values for hemispheres and the world. The average mean square interannual deviations from average values are given in brackets. The highest frequency of tropical cyclone transfor� mation into extratropical ones is typical for the NWPO region: more than ten per year on average, with the maximal value of 17 (in 2004) and a minimum of 2 (in 1973). In comparison to the NWPO region, the average frequency of the respective events in the NAO, SIO, and SPO regions is less by more than twice (5, lesser than 5, and lesser than 5, respectively). In particular years, the number of tropical cyclones that transform into extratropical ones can reach in the NAO region 8–10, up to the record value of 12 in 2012, when almost 2/3 of 19 tropical cyclones in the Atlantic Ocean transformed into extratropical ones.