Lu Riyu

Interannual meridional displacement of the East Asian upper-tropospheric jet stream in summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS’s meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June–July–August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.

Indices of the summertime western North Pacific subtropical high

By averaging June-July-August (JJA) mean geopotential height anomalies at 850 hPa over the specified areas, the author proposes two innovative and succinct parameters to objectively define the zonal and meridional displacements of the western North Pacific subtropical high (WNPSH) in summer, respectively. Thus, these two indices and the present results may provide a basis for validating atmospheric general circulation models simulating the WNPSH. For the zonal index, the specified area is the west edge (110°–150°E, 10°–30°N) of the WNPSH. For the meridional index, the specified area is the northwest edge (120°–150°E, 30°–40°N) of the WNPSH. The interannual variations of these two indices are found to be independent. The results from a composite analysis based on the meridional index are in good agreement with previous studies based on case analyses.The two indices are compared with the existing indices announced by the National Climate Center (NCC) in China, on the interannual timescale. Despite slight differences, the interannual variations of the presented indices are basically similar to those of the NCC indices, and thus the circulation and precipitation associated with the present indices exhibit similar features to those associated with the NCC indices. Furthermore, an analysis of the differences between the associations of the present indices and the NCC indices shows that the presented indices are better than the NCC indices. An important result is that the zonal index is related to a more outstanding anomaly of precipitation, especially in East Asia and the Philippine Sea, both based on the presented indices and the NCC indices.The two indices can also be used to describe the seasonal march of the WNPSH during summer, namely, the poleward and eastward shifts. It is found that climatologically, the WNPSH shifts poleward and eastward rapidly in middle July, but the amplitudes of the poleward and eastward shifts are more remarkable in the summers when the WNPSH is located poleward and eastward in average.

Atmospheric circulations and sea surface temperatures related to the convection over the western pacific warm pool on the interannual scale

The difference is examined in atmospheric circulation and Sea Surface Temperatures (SSTs) in the trop-ics and subtropics between weak and strong convection over the tropical western Pacific warm pool (signified as WPWP). The WPWP is chosen as the region (110–160°E, 10–20°N), where the Outgoing Longwave Radiation (OLR) shows a great year-to-year variance. A composite study was carried out to examine the differences in atmospheric circulation and SSTs between weak and strong convection over WPWP. First, NCEP/NCAR re-analysis data and satellite-observed OLR data are used to examine the differences. ERA data, in which the OLR data are calculated, are then used for re-examination.The composite results show that the differences are remarkably similar in these two sets of data. The difference in circulations between weak and strong convection over WPWP is significantly associated with westward extension of the North Pacific subtropical anticyclone and stronger westerlies at the northwestern edge of the subtropical anticyclone. It also corresponds with the significant easterly anomaly and the descent anomaly in situ, i.e., over the WPWP. The most prominent characteristics of the difference of SSTs between weak and strong convection over the WPWP are the significant positive SST anomalies in the Indian Ocean, the Bay of Bengal and the South China Sea. In WPWP, however, there are only weak negative SST anomalies. Thus, the anomaly of OLR over WPWP is weakly associated with the SST anomalies in situ, while closely associated with the SST anomalies west of WPWP.

Anomalies in the Tropics Associated with the Heavy Rainfall in East Asia during the Summer of 1998

The summer of 1998 was characterised by a severe flood in East Asia. The possible linkages were examined among the anomalies in the tropics that may be associated with the severe flood. The anomalies of 1998 are obtained by removing the climatology, which is the average of the ECMWF (European Centre for Medium Range Weather Forecasts) Re—Analysis (ERA) data over 15 years from 1979 to 1993, from the corresponding fields of 1998, which are obtained from the ECMWF operational analyses.In comparison to the results of Nitta (1990), it was found that there are considerable similarities in the atmospheric circulation anomalies between the summers of 1998 and 1988, in the tropics as well as in middle—high latitudes. It was shown that the atmospheric convection is slightly suppressed over the tropical western Pacific. In general, the suppressed convection corresponds to a negative anomaly of SST in the warm pool region. In the summers of 1998 and 1988, however, there are positive anomalies of SSTs in the tropical western Pacific, corresponding to the suppressed convection over there. This slightly suppressed convection may not provide a viable forcing mechanism for the severe flood in East Asia. It was postulated that the zonal wind anomalies in the tropics, in addition to the atmospheric convection over the tropical western Pacific, influence the position and intensity of the North Pacific subtropical high.In both summers of 1998 and 1988, while the stronger convection occurs over the warmer tropical Indian Ocean, the suppressed convection corresponds to the positive anomalies of SSTs in the tropical western Pacific. A possible explanation was given for the broken relationship between SSTs and OLR, (Outgoing Longwave Radiation) by analyzing the large—scale atmospheric circulation anomalies in the tropics.The heat fluxes at the surface in the warm pool of the tropical western Pacific and tropical Indian Ocean were also examined by using the ERA–15 data. To avoid the inconsistency between the ERA–15 and the operational analyses, the anomalies of the heat fluxes at the surface in the warm pool region in the summer of 1988, instead of the summer of 1998, were examined. The anomalous latent heat flux and the net solar radiation flux are the main reason for the positive anomalies of SSTs in the tropical Indian Ocean and in the tropical western Pacific, respectively. The suppressed convection over the tropical western Pacific allows more solar radiation fluxes downward at the surface, which would increase the SSTs.

Dominant patterns of summer rainfall anomalies in East China during 1951–2006

The dominant patterns of summer rainfall anomalies in East China were studied using Empirical Orthogonal Function (EOF) analysis. The results indicate that after the late 1970s, the first and second dominant patterns switched. During the period before the late 1970s, the spatial pattern of the first (second) dominant mode was the “Yangtze River pattern” (the “South China pattern”), but this changed to the “South China pattern” (the “Yangtze River pattern”) after the late 1970s. This decadal change in the dominant patterns resulted from a significant decadal change in summer rainfall over South China after the late 1970s, i.e., a negative phase during 1978–1992 and a positive phase during 1993–2006. When the decadal variation of rainfall in East China is omitted from the analysis, the first and second dominant patterns represent the “Yangtze River pattern” and the “South China pattern”, respectively. These results suggest that when decadal variation is included, the rainfall in China may be dominated by one mode during certain periods and by another in other periods. For the interannual variability when decadal variation is excluded, however, the first and second modes can be easily distinguished, and their order has been stable since at least 1951.

A decadal shift of summer surface air temperature over Northeast Asia around the mid-1990s

This study identifies a decadal shift of summer surface air temperature (SAT) over Northeast Asia, including southeastern parts of Russia, Mongolia and northern China, around the mid-1990s. The results suggest that the SAT over the Northeast Asia experienced a significant warming after 1994 relative to that before 1993. This decadal shift also extends to northern China, and leads to a warmer summer over Northeast China and North China after the mid-1990s. The decadal warming over Northeast Asia is found to concur with the enhancement of South China rainfall around the mid-1990s. On the one hand, both the Northeast Asian SAT and South China rainfall exhibit this mid-1990s decadal shift only in summer, but not in other seasons. On the other hand, both the Northeast Asian SAT and South China rainfall exhibit this mid-1990s decadal shift not only in the summer seasonal mean, but also in each month of summer (June, July and August). Furthermore, the decadal warming is found to result from an anticyclonic anomaly over Northeast Asia, which can be interpreted as the response to the increased precipitation over South China, according to previous numerical results. Thus, we conclude that the warming shift of summer Northeast Asian SAT around the mid-1990s was a remote response to the increased precipitation over South China.

A possible linkage in the interdecadal variability of rainfall over North China and the Sahel

The instrumental records of precipitation, including some historical documentary evidence, show that the rainfall in North China during the rainy season (July and August) exhibits an interdecadal variability similar to the Sahelian rainfall. Both these areas exhibited a weak interdecadal rainfall variability prior to the 1950s, and experienced a long-lasting drought since the 1960s, with two rainfall decreasing transitions, one around the year 1965 and another in the late 1970s. NCEP/NCAR reanalysis data are used to analyze the associated changes in atmospheric circulation during the second decrease transition. The changes of local atmospheric circulation at the end of the 1970s, at both lower and upper levels, contribute to the less precipitation in North China and the Sahel.

The Coupled Model Predictability of the Western North Pacific Summer Monsoon with Different Leading Times

Abstract Leading time length is an important issue for modeling seasonal forecasts. In this study, a comparison of the interannual predictability of the Western North Pacific (WNP) summer monsoon between different leading months was performed by using one-, four-, and seven-month lead retrospective forecasts (hindcasts) of four coupled models from Ensembles-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES) for the period of 1960-2005. It is found that the WNP summer anomalies, including lower-tropospheric circulation and precipitation anomalies, can be well predicted for all these leading months. The accuracy of the four-month lead prediction is only slightly weaker than that of the one-month lead prediction, although the skill decreases with the increase of leading months.

Decreasing Trend in Summer Precipitation over the Western Sichuan Basin since the 1950s

Abstract Changing precipitation in the densely populated Sichuan basin may have a great impact on human life. This study analyzes the change in summer precipitation since 1951 over the western Sichuan basin, one of the regions of the heaviest rainfall in China, by using two datasets provided by the Chinese Meteorological Data Center. The results indicate that summer (from June to September) precipitation over the western Sichuan basin shows a significantly decreasing trend. The summer precipitation over this region has decreased by about 20% since the 1950s, with a rate of decrease of about 40 mm per decade.

Numerical Simulation of the Effect of the SST Anomalies in the Tropical Western Pacific on the Blocking Highs over the Northeastern Asia

The effects of the sea surface temperature (SST) anomalies in the tropical western Pacific on the atmospheric circulation anomalies over East Asia are simulated by the IAP-GCM with an observed and idealized distributions of the SST anomalies in the tropical western Pacific, respectively. Firstly, the atmospheric circulation anomalies during July and August, 1980 are simulated by three anomalous experiments including the global SST anomaly experiment, the tropical SST anomaly experiment and the extratropical SST anomaly experiment, using the observed SST anomalies in 1980. It is shown that the SST anomalies in the tropical ocean greatly influence the formation and maintenance of the blocking high over the northeastern Asia, and may play a more important role than the SST anomalies in the extratropical ocean in the influence on the atmospheric circulation anomalies.Secondly, the effects of the SST anomalies in the tropical western Pacific on the atmospheric circulation anomalies over East Asia are also simulated with an idealized distribution of the SST anomalies in the tropical western Pacific. The simulated results show that the negative anomalies of SST in the tropical western Pacific have a significant effect on the formation and maintenance of the blocking high over the northeastern Asia.