Climatology Of Interactions Between The Northern And Southern Hemispheric Atmospheres And Cross-equatorial Flow

Cross equatorial flow (CEF) is an important section of tropic atmospheric circulation, and its distribution has close relationship with global general circulation, in other words, CEF is the tache of interaction between the southern and northern hemispheres. More creditable reanalysis is adopted to study the characteristics and position of CEF and the atmospheric mass flux which induced by CEF. Moreover, based on the Newton's second law, the efficiency ratio of the first to second forces is educed by the results of mass flux. Finally, the relationship between CEF and the autumn precipitation in Southeastern China is discussed. The main results of this study are as follows:(1) The comparison result among NCEP/NCAR, NCEP-DOE AMIP-2, ERA40 and JRA25 reanalysis in this paper shows clear evidence that there are significant distinctions in the atmospheric mass flux between NH and SH among these four datasets. The constraint relationship between the atmospheric mass transfer and surface pressure is used in the analyses. Overall, when the mass flux is northerly (southerly) over the equator, the mean surface pressure in NH increases (decreases) in both ERA40 and JRA25, satisfying the constraint above. However, the trends of the surface pressure and mass flux in NCEP 1 might conflict with each other to a great extent. Furthermore, not only at the equator, the research is also expanded to the latitudes from 60°S to 60°N. The differences among the four datasets are also notable. The output of ERA-40 might be more rational. The result of JRA-25 also reflects the verdict better. The spurious trends in the meridional wind might exist in ERA-40, NCEP 1, NCEP 2 and JRA-25, especially in NCEP 1, and comparing with NCEP 1 the trends are improved in NCEP 2 to a certain extent. The causes for the discrepancy among these four reanalyses might be the different assimilation and parameterization procedures they adopt.(2) By regarding ERA40 reanalysis which is more authentic as observation, the mass transport of the model output of eight AGCMs of AMIP in IPCC 4th assessment has been evaluated and compared. When considering the zonal mean meridional atmospheric mass flux at the equator, the results of NCAR_CCSM3₀, MPIJECHAM5 and UKMO _HADGEM1 are close to that of the observation. MIROC3_2MEDRES and IAP_FGOALS1_0G require further betterment and improvement. Based on the vertical profile of v-wind at the equator, dividing the atmosphere into four layers (let 700 hPa, 300 hPa and 70 hPa be the division), and calculating the cross-latitude mass transport. The eight models have better simulation abilities of the layers of 700 hPa-ps and 70-300 hPa uniformity, nevertheless there are more difference between the eight models in the layer of 300-700 hPa, in details, UKMO _HADGEM1 model approach the observation most, and IAP_FGOALS1_0G, CNRM_CM3 and MIROC3_2MEDRES need to improve. Except MIROC3_2MEDRES, the other seven models have the ability of simulating the annual cycle of mass transport in 10-70 hPa on the whole. Generally, UKMO _HADGEM1 simulates the cross equatorial atmospheric mass flux best, MPI_ECHAM5 better, but NCAR_CCSM3₀, GISS_MODEL_ER and GFDL_CM2₁ have lower capabilities in simulating the mass transport of whole level and 300-700 hPa layer, but deep capabilities in 700 hPa-p_s and 70-300 hPa. Otherwise, IAP_FGOALS1_0G and CNRM_CM3 present some insufficiencies when cross equatorial atmospheric mass flux is discussed.(3) The atmospheric mass transport concentrates at 700 hPa-p_s and 70-300 hPa cross the equator, and it is stronger in monsoon region E than that of zonal mean and nonmonsoon region W. It is deduced that mean forcing can be express by the change rate of mass flux. [ F₁ ] and [ F₃ ] arealways much greater than [F₂] and [ F₄ ] whether in region E, W or in zonal mean regime. Theforcing in region E has greater power than zonal mean forcing, but region W is on the opposite whether in summer and winter or in lower and upper level. The efficiency ratio of the first to second forces is 4:1 in summer and 3:1 in winter at lower troposphere. The ratio is 2.5:1 in upper troposphere. On the average, the efficiency ratio of the first to second forces is 3:1.(4) The distribution and climatology variations of CEF are studied based on ERA40 reanalysis which is more credible. The representative levels are ascertained on 925 hPa and 200 hPa for lower and upper troposphere, respectively. Moreover, the situations of CEF are confirmed by the relationship between the grid on which the meridional wind is stronger and the other grids. There are eight (nine) CEF in summer (winter) lower troposphere, and three (nine) CEF in summer (winter) upper troposphere. The series of every CEF and zonal mean meridional wind anomaly are examined, and their climatology variations are studied carefully. The results show that every CEF has its special long-term trend and inter-decadal variations, so could not express without exception. Furthermore, the periods of every CEF are examined by wave-let analysis. In general, the CEF in summer lower troposphere except 70°W all have the period of 2-3 years ever in some certain years.(5) The vertical spread of zonal mean meridional wind has two modes. One is upward, the other is downward. Their corresponding time coefficients have the period of 2 years. The up mode plays a dominant role, and the down one exists, too. The speed of downwards is faster than that of upwards. The results of every CEF are as follows, the four CEF of 42.5°E, 105°E, 127.5°E and 150ooooooooE have the similar characteristic of vertical spread. They have two modes. One is upward propagating, the other is downward. Their corresponding time coefficients have the period of 2 years. The up mode plays a dominant role, and the down one exists, too. The speed of downward propagating is faster than that of upward one. The 87.5°E CEF has not only single upward, but also upward and downward coexisting propagating. The vertical propagating of 70°W CEF anomaly presents two modes. One is upward propagating as key fashion associated with downward; the other is single downward propagating. The 102.5°W and 12.5°W CEF anomaly have downward and upward propagating synchronously, and the downward one in the highest flight.(6) It is found that there is a significantly positive correlation between the summer 105°E CEF and autumn precipitation in Southeastern China. However, why for a season long that this CEF leading precipitation? It reveals that the correlation between the autumn 105°E CEF and precipitation in Southeastern China is more significant. The summer 105°E CEF has a high correlation coefficient with the later autumn CEF. It declares that the anomaly of 105°E CEF prefers persisting over the later autumn. There is a significantly positive correlation between the autumn 105°E CEF and precipitation also. Therefore, it presents a significant correlation between the summer 105°E CEF and autumn precipitation in Southeastern China.