The Weakening Trends And Interannual Variations Of Tropospheric Enstrophy In The Northern Hemisphere In The Past Three Decades

Using the NCEP/NCAR monthly mean reanalysis from1980to2009, the authors have investigated the weakening trends in enstrophy in troposphere of the northern hemisphere under background of global warming. The diagnosis on the interannual variation of enstrophy in troposphere of the northern hemisphere shows some interesting features. The results of the present work are helpful for our better understanding the circulation changes and the influences of global warning. The results are summarized as follows:(1) The distribution of enstrophy in the northern hemisphere has significant spatial and temporal changed. Largest values of enstrophy are found in the northwest Pacific. In northern Africa including the Northeast of U.S. and eastern Canada, and in the North Atlantic region the relatively larger values of enstrophy are found.(2) A weakening trend of enstrophy in the northern hemisphere during1980-2009are discovered, especially in the Pacific and polar regions. The zone12.5°N～50。N is the key area where the changes of enstrophy dominate the variations of enstrophy in the northen hemiphere. Changes of the subtropical westerly jet in association with the meridional gradient of air temperature show that the jet intensity has a weakening trend, which is in accordance with the decaying of enstrophy. A vector E is employed to diagnose the Rossby wave propagations in the atmosphere, showing that the disturbances in the atmosphere elongate more meridional in the northern hemisphere except western Europe. The meridional eddy heat fluxes in the northen hemiphere have also been weakening in the past30years.(3) The change of enstrophy in the northern hemisphere shows a biennial tendency in the atmosphere in the northern hemisphere. Diagnosis by the tendency equation of enstrophy shown that the main changes of enstrophy in the northern hemisphere are due to the tilt and residual terms. The divergence term also play a certain role in the enstrophy changes. Changes due to tilting are found to be and northern hemisphere negatively correlated with the enstrophy changes. The enstrophy variations in the northern hemisphere correlate closely with the temperature, precipitation, vorticity and other elements in global domain especially in the north Pacific.(4) Enstrophy changes in different season have different distribution characteristics in the northern hemisphere. The biggest changes of enstrophy are found in the boreal winter other than seasons. The weakest are found in boreal summer. The weakening trends of enstrophy are observed in all seasons especially in spring and winter. Correlation of the changes of enstrophy in different seasons with those of annual mean shows that the main reason of the weakening of enstrophy in th northern Pacific is due to the weakening trends of enstrophy in spring and autumn, and enstrophy weakening in winter cause the weakening trends in polar regions. Enstrophy of positive vorticity and negative vorticity in the northern hemisphere also show trends of weakening in all seasons, especially in the northern and western Pacific in winter.