A Climatology And Variation Research Of Extratropical Cyclones In The North Hemisphere

Extratropical cyclone (EC) is one of the important synoptic systems which affect weathervariability over a wide area of mid and high latitudes, and it can cause significant or intenseweather phenomena and disasters. As a result, it has the scientific and practical significanceto do some research on climate characteristics, activity law and variability trends ofextratropical cyclones (ECs) for understanding weather and climate change and extremeclimate events. Based on an objective detecting and tracking method of ECs, EuropeanCenter for Medium-Range Weather Forecasts (ECMRWF) reanalysis data ERA40are appliedto statistically analyze climatology and variations of ECs over the North Hemisphere andEast Asia, as well as storm tracks over the North Hemisphere. The possible reasons aboutvariations of EC and storm track during the last44years in20th century were discussed interms of large scale circulation index and atmospheric baroclinicity. After that, the simulatedcapability of6global climate system models (BCC-CSM1.1, CanESM2, GFDL-ESM2M,HadGEM2-CC, MPI-ESM-LR and NorESM1-M), released latterly by Coupled ModelIntercomparison Project Phase5(CMIP5), about ECs over the North Hemisphere was tested.At last, the future possible variation tends of ECs and storm track over the North Hemisphereunder RCP4.5were estimated by using these6climate system models. The main conclusionsummed up as follows:(1) Based on sea level pressure field, cyclone movement extrapolation and local guideairflow, the objective method for detecting and tracking ECs can track more than80%realcyclone processes, and successfully represent the spatial and frequency distribution ofcyclones' climatological features. A clustering analysis produced6major cyclone trajectories,among which4trajectories would have an affect on China, and have important connectionswith severe weather/climate phenomena such as spring sand storms, winter blizzard, heavyrainfall and offshore wind.(2) A linear increase trend of EC process numbers in the north and decrease in south partof North Hemisphere for the second half of20th century was revealed, which reflected thepoleward shift of the storm track in the North Hemisphere. The cyclone process number had anegative and positive correlation with Arctic Oscillation index (AO) in the lower and higherlatitude region respectively, indicating that ECs of the North Hemisphere migrate to the pole when polar vortex strengthens and shrinks, while ECs migrate to the lower latitudes whenpolar vortex weakens and moves southward. It was found that the interdecadal variations ofstorm tracks over North Pacific and North Atlantic had a close relationship with their ECactivities. Storm track over North Atlantic had a poleward shift, which was consistent withthe counterparts of the North Hemisphere, while storm track over North Pacific had anequatorward shift. The same phase variations of atmospheric baroclinity on400hPa levelmight be one reason for the southward migration and enhancement of ECs and storm trackover North Pacific; northward shift and enhancement over North Atlantic.(3) Accompanied by a northward shift of cyclone source region, the north cyclonenumber in East Asia indicated obvious decrease trend after the mid1980s. This is becauseatmospheric baroclinicity weakened in this region, and the strong baroclinicity zone moved tohigher latitudes. There was a linear increased trend of the south cyclone number during thelast44years of20th century, mainly because North Pacific storm track, located in40-55°N,had a shift trend to lower latitude, resulting in the southward migration of the NorthwestPacific cyclone formation region.(4) The simulation capability for space and frequency distribution of cyclones'climatological features was better than that for time series by6global climate system modelsof CMIP5. For simulations, Cyclone processes were fewer, life spans were longer, anddeepen rates were slower. The models of BCC-CSM1.1can represent northward shift of ECsactivities, and at least4models could reflect linear change trends of ECs' pressure over theNorth Hemisphere(5) By using the six global climate system models for assessment, it was discovered thatthe changes of ECs' features under RCP4.5during2053-2099period would be moreremarkable than that during2006-2052period compared with20th century. The models'simulation results had some differences, however, we found an agreement of reducedcyclogenesis and cyclone activity over large North Hemisphere region at the end of21thcentury, especially in the lower latitudes. Moreover, more than half of the models showed thatcyclone center pressure over the North Hemisphere would decline in the future. Most modelsestimated that North Atlantic storm track would keep for moving poleward, but withweakened strength; more than half of the models forecasted that North Pacific storm trackwould also move northward with inconsistent strength changes in different seasons. Changesof East Asia cyclones were similar and consistent with that in the North Hemisphere. Nomatter north or south cyclones, the formation and activity frequency would decrease, andcyclone pressure would decline significantly.(6) It was found in all6models that the baroclinic zone on middle-troposphere wouldexpand to higher level and higher latitudes, and jet stream axis on top-troposphere would move to higher level in the future under RCP4.5. These changes would be more notable in theSouth Hemisphere. The changes of baroclinic zone can reflects the changes of storm track, sothis supports the above conclusion of possible poleward shift of two ocean storm paths in thefuture. In the future North Hemisphere, the northward movements of baroclinic zone andstorm track would offset the reduction of the ground cyclone number in the higher latitudes,which explained the reason why cyclones in the lower latitudes would decrease moresignificantly than that in the higher latitudes.