Tibetan Plateau vortices (TPVs) are major rain-producing systems over the Tibetan Plateau. Some TPVs can move off the plateau under certain conditions and impact rainfall over Eastern China. Accordingly, the eastward propagation distances of the TPVs moving off the plateau (EPDs) are closely related to the areas of rainfall associated with TPVs. In this study, the moving-off TPVs during May-August of 1998–2015 are classified into two groups according to their EPDs, and the circulations and heating fields at the times when the TPVs move off the plateau (i.e. moving-off times) are investigated based on reanalysis data. The dynamic and thermodynamic conditions to the east of the Tibetan Plateau are found to significantly impact the EPDs. In the middle and lower troposphere, the zonal ranges of negative geopotential height anomalies to the east of the Tibetan Plateau are in accordance with the EPDs of the TPVs, indicating that anomalous lows play a favourable role in the eastward movement of TPVs. In addition, the anomalous highs to the northeast of the Tibetan Plateau and over Southeastern China also benefit the maintenance of cyclonic circulation to the east of the plateau. Meanwhile, in the upper troposphere, the jet stream over Northeast Asia is beneficial for divergence at 200 hPa. Accordingly, ascending motion associated with the upper-level divergence and lower-level convergence is observed, with the zonal extent corresponding well to the EPDs in the two situations. The atmospheric thermodynamic factors also show a remarkable effect on the EPDs. The TPVs move farther away when the unstable stratification and water vapour convergence extend further eastward. The heating ranges above 500 hPa coincide with the EPDs of TPVs, implying a close relationship between the heating fields and the EPDs. These results benefit prediction on EPDs and further on rainfall to the east of the Tibetan Plateau.
Abstract The structure and evolution features of the quasi-biweekly (10–20 day) oscillation (QBWO) in boreal spring over the tropical Indian Ocean (IO) are investigated using 27-yr daily outgoing longwave radiation (OLR) and the National Centers for Environment Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data. It is found that a convective disturbance is initiated over the western IO and moves slowly eastward. After passing the central IO, it abruptly jumps into the eastern IO. Meanwhile, the preexisting suppressed convective anomaly in the eastern IO moves poleward in the form of double-cell Rossby gyres. The analysis of vertical circulation shows that a few days prior to the onset of local convection in the eastern equatorial IO an ascending motion appears in the boundary layer. Based on the diagnosis of the zonal momentum equation, a possible boundary layer–triggering mechanism over the eastern equatorial IO is proposed. The cause of the boundary layer convergence and vertical motion is attributed to the free-atmospheric divergence in association with the development of the barotropic wind. It is the downward transport of the background mean easterly momentum by perturbation vertical motion during the suppressed convective phase of the QBWO that leads to the generation of a barotropic easterly—the latter of which further causes the free-atmospheric divergence and, thus, the boundary layer convergence. The result suggests that the local process, rather than the eastward propagation of the disturbance from the western IO, is essential for the phase transition of the QBWO convection over the eastern equatorial IO.
Based on the Lagrangian particle dispersion model, HYSPLIT 4.9, this study analyzed the summertime atmospheric moisture sources and transportation pathways affecting six subregions across China. The sources were: Midlatitude Westerly (MLW), Siberian-Arctic regions (SibArc), Okhotsk Sea (OKS), Indian Ocean (IO), South China Sea (SCS), Pacific Ocean (PO), and China Mainland (CN). Furthermore, the relative contributions of these seven moisture sources to summertime precipitation in China were quantitatively assessed. Results showed that the CN precipitation source dominates the interannual and interdecadal variation of precipitation in most subregions, except Southwest and South China. The Northeast China vortex and Pacific–Japan (PJ) teleconnection, which transport water vapor from the MLW, OKS and PO sources, are crucial atmospheric systems and patterns for the variation of precipitation in Northeast China. The interannual variation of precipitation in Northwest and North China is mainly dominated by mid–high-latitude Eurasian wave trains, which provide the necessary dynamical conditions and associated moisture transport from the MLW and SibArc sources. In addition, an enhanced western North Pacific subtropical high (WNPSH) accompanied by the East Asian–western North Pacific summer monsoon and PJ teleconnection, transports extra moisture to North China from the SCS and PO sources, as well to the Yangtze River Valley and South China. The Indian summer monsoon (ISM) is also critically important for the interdecadal change in precipitation over the Yangtze River Valley and South China, via the southwesterly branch of moisture transport from the IO source. The interdecadal changes in precipitation over Southwest China are determined by the IO and SCS sources, via enhanced WNPSH coupling with a weakened ISM. These results suggest that the interdecadal and interannual variations of moisture sources contribute to the attendant variation of summertime precipitation in China via large-scale circulation regimes in both the mid–high and lower latitudes.
The relationship between interannual variations of the seasonal mean monsoon and intraseasonal oscillation (ISO) during boreal summer over the Indian monsoon region is investigated. The result shows a negative correlation between all‐India summer monsoon rainfall and the ISO intensity. It is argued that the negative correlation is primarily attributed to the impact of the mean state of Indian summer monsoon on ISO. A strong Indian monsoon leads to the weakening of convection over the equatorial eastern Indian Ocean. The latter may further suppress the eastward and northward propagating ISO variances over the Indian Ocean and lead to a weakened intraseasonal activity over the Indian monsoon region. The ISO may feed back to the Indian summer mean monsoon through nonlinear eddy momentum transport. In addition, the ISO intensity over India increases (decreases) during the ENSO developing (decaying) summers.
A new coupled climate system model (CSM) has been developed at the Chinese Academy of Meteorological Sciences (CAMS) by employing several state-of-the-art component models. The coupled CAMS-CSM consists of the modified atmospheric model [ECmwf-HAMburg (ECHAM5)], ocean model [Modular Ocean Model (MOM4)], sea ice model [Sea Ice Simulator (SIS)], and land surface model [Common Land Model (CoLM)]. A detailed model description is presented and both the pre-industrial and "historical" simulations are preliminarily evaluated in this study. The model can reproduce the climatological mean states and seasonal cycles of the major climate system quantities, including the sea surface temperature, precipitation, sea ice extent, and the equatorial thermocline. The major climate variability modes are also reasonably captured by the CAMS-CSM, such as the Madden–Julian Oscillation (MJO), El Niño–Southern Oscillation (ENSO), East Asian Summer Monsoon (EASM), and Pacific Decadal Oscillation (PDO). The model shows a promising ability to simulate the EASM variability and the ENSO–EASM relationship. Some biases still exist, such as the false double-intertropical convergence zone (ITCZ) in the annual mean precipitation field, the overestimated ENSO amplitude, and the weakened Bjerknes feedback associated with ENSO; and thus the CAMS-CSM needs further improvements.
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