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Extra-tropical cyclone climatology and shifts in climate regime in the Northern Hemisphere Grant, Andrew P.

Abstract

Extra-tropical cyclones are an important feature of mid-latitude weather and climate. The interdecadal variability of extra-tropical cyclones is assessed here. This is undertaken with respect to changes in Northern Hemisphere atmospheric circulation associated with a shift in climate regime around 1976/77. Data from the Goddard Institute of Space Studies (GISS) Atlas of Extra-tropical Cyclones and National Snow and Ice Data Centre (NSIDC) Arctic Cyclone Track Data Set are used to construct a climatology of cyclones and intense cyclones (those reaching a minimum sea level pressure less than 980hPa) in the Northern Hemisphere. Time series of cyclone data in various regions are analysed to assess relationships with indices representing the 1976/77 shift and changes in trend over the period investigated. The possibility of a second shift around 1989 is also investigated. Indices used are based on phases of the Pacific Decadal Oscillation (PDO). Emperical Orthogonal Function (EOF) analyses are also used to examine possible relationships with other climate indices such as the Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and Southern Oscillation Index (SOI). Analyses limited to specific regions are also undertaken to assess reliability of the data and the importance of specific regions to cyclone occurrence in the North Pacific. Shifts in climate regime are also assessed with respect to changes in atmospheric circulation in the Northern Hemisphere. This is undertaken by analysing variability in a major feature of the circulation, the Siberian High. The nature of the Siberian High is observed to change around the 1976/77 shift. Similar changes are also observed in cyclone frequency between different climate regime phases. Additionally, significant trends over the time period investigated are observed in cyclone and intense cyclone frequency in most regions of the Northern Hemisphere. These trends are most prevalent in intense cyclone frequency. It is concluded that changes in atmospheric circulation associated with the 1976/77 regime shift affected the frequency of cyclones and intense cyclones in the Northern Hemisphere. Evidence suggests that this occurs on a hemispheric scale, though it is likely that effects of the 1989 event are limited to the Pacific region.

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