The Variability Of The Atmosphere Circulation Cell And Its Relationship With ENSO

This study defines a new vector index to characterize the overturning circulations, in longitudinal direction which is good at not only describing the center position and intensity of three-cell circulation, but also showing the spatial distribution of the meridional circulations. On the other hand, this new index also shows the latitudinal circulation in the globe especially in the Pacific, the Indian and Atlantic Ocean. Furthermore, this study discusses the enhancing of the boreal winter Hadley circulation and its relationship with ENSO. The results of this paper can be summarized in the flowing aspects:1. The geographical distribution of zonal and meridional component of the new index shows the the closed circulation in corresponding direciton using NCEP/NCAR reanalysis monthly data from 1961 to 2000. The analysis shows that the center position and intensity of the zonal mean meridional (MMC) and the relationship between the intensity of them are well characterized by the new index. And it shows the spatial distribution of MMC in every season and the reverse of circulation in the monsoon regions from winter to summer. Moreover, there are two apparent meridional circulation zones, one is along east coast of Eurasia and the other locates over eastern bank of Northern America. They connect the high- and low-latitude together and can be further confirmed by the vector consisted of vertical and meridional wind in the meridional-vertical plane in detail (not shown). These two zones with more than 30°width are not consist with the conventional three-cell concept, that is, the poleward motion from equator could only reach about 30°N (S). On the other hand, it shows the zonal component of the index mainly distributes in low latitudes of both hemisphere, and the most obvious shift with seasons happens in the India Ocean. Moreover, there is another zonal circulation north of 30°N which is the weakest in summer and has a nearly fixed position over the Aleutian and Alaska all the year. Finally, we use the vector index to characterize the circulation cell in real atmosphere and the change of its direction in the process of motion. The results shows the overturning circulations mainly distribute in the low-latitude, which come from the heat source regions.2. The annual cycle of the intensity of every MMC is discussed by the latitudinal component of the vector index, the results shows: the intensity and center position of both hemisphere Hadley cells have significant annual cycle, and the trace of their intensity varying are symmetric about the mouth of July, that is to say, before (after) July, the intensity of the boreal (austral) Hadley cell is increasing (decreasing). The trace of variability of center position is also similar to that of intensity, it is stepping northward during the month of 1～8, then go back. Comparing the two Hadley cell in both hemispheres, the southern one is more symmetric in the trace of the variability of its center, which move northward from January to June and fixed during the period of June-August, then go back. Furthermore, we employ EOF analysis to determine the principle modes of the annual cycle of the latitudinal index, the results show it is dominated by the equatorially asymmetric and symmetric modes which are reverse in variability of their intensity, suggesting there is a trade-off between them and the intensity of each MMC in every mouth is determined by them.3. The annual cycle of intensity of the closed zonal circulation cells over the Pacific, the Indian and Atlantic Ocean is discussed using the longitudinal component of vector index. There is a similar trace of variability of the zonal overturning circulation intensity in the pacific and north Atlantic, which is reversed with that of the India Ocean. Comparing their intensity, the biggest one is in Indian, then in the Atlantic and Pacific. Furthermore, there are two independent zonal circulations in the northern and southern Atlantic Ocean, they are completely differ in the trace of intensity varying. The former (latter) looks like the square wave (parabola). The cells in region of the India and northern Atlantic are strengthen, whereas that in pacific are weaken and that in the southern Alantic has no change during the warm year of ENSO. The zonal index has the same modes with the latitudinal one, that is, the equatorially asymmetric and symmetric modes. the latter explains 70.2% of total variance, and the time series of its intensity has a significant period about one year, whereas the period of the latter is only half year.4. The intensity index of every MMC computed from the index and mass stream function using the NCEP/NCAR and ERA40 reanalysis monthly data. The result congruously shows the significant enhancing trend exits in the boreal winter and spring Hadley, the boreal spring Ferrel, the austral summer Ferrel and the austral winter Polor circulation. Moreover, the regions with the significant upward trend mainly distribute in the low latitudes of the African and Indian Ocean, including South China sea and the eastern Asian. The equatorially symmetric and asymmetric modes are also dominated the variability of the latitudinal index filed for every season, and the former mode represents the variability of the index mainly on the interannual time scale and has closed correlation with ENSO, while the latter is similar to the distribution of significant upward trend on the decadal time scale, suggesting these significant regions may be controlled by the same system. A point should be mentioned is that the symmetric mode is almost same in very season.5. The year-to-year variability of the boreal winter mass stream function of the mean meridional circulation (MMC) is dominated by the equatorially asymmetric mode and symmetric mode. The former (latter) is linked with the boreal Hadley cell mainly on the decadal (interannual) time-scale. The asymmetric mode index (AMI) shows a clear upward trend during last decades. The strong SST warming in the tropical Indo-west Pacific warm-pool might be an important factor that contributes to the intensification of the asymmetric mode, and the SST warming in the southern tropical Atlantic Ocean and southeast Pacific also contributes partially to this intensification. The symmetric mode index (SMI) shows a significant interannual variability with very robust and stable correlation with ENSO, implying the variability of the Hadley cell is mainly associated with ENSO on the interannual time-scale. Besides, the symmetric mode variability is also connected with the SST in the tropical Indo-west Pacific warm-pool.