Caiming Shen

A Pacific Decadal Oscillation record since 1470 AD reconstructed from proxy data of summer rainfall over eastern China

[1] Recent studies indicated that the spatial pattern and temporal variability of summer rainfall over eastern China are well correlated with the Pacific Decadal Oscillation (PDO). Here we used a data set of the drought/flood index (a proxy of summer rainfall) since 1470 AD to reconstruct the annual PDO index. The reconstruction indicates that the PDO is a robust feature of North Pacific climate variability throughout the study period, however, the major modes of oscillation providing the basic PDO regime timescale have not been persistent over the last 530 years. The quasi-centennial (75-115-yr) and pentadecadal (50-70-yr) oscillations dominated the periods before and after 1850, respectively. Our analysis suggest that solar forcing fluctuation on quasi-centennial time scale (Gleissberg cycle) could be the pace-maker of the PDO before 1850, and the PDO behavior after 1850 could be due, in part, to the global warming.

Response of Summer Precipitation over Eastern China to Large Volcanic Eruptions

Studies of the effects of large volcanic eruptions on regional climate so far have focused mostly on temperature responses. Previous studies using proxy data suggested that coherent droughts over eastern China are associated with explosive low-latitude volcanic eruptions. Here, the authors present an investigation of the responses of summer precipitation over eastern China to large volcanic eruptions through analyzing a 1000-yr global climate model simulation driven by natural and anthropogenic forcing. Superposed epoch analyses of 18 cases of large volcanic eruption indicate that summer precipitation over eastern China significantly decreases in the eruption year and the year after. Model simulation suggests that this reduction of summer precipitation over eastern China can be attributed to a weakening of summer monsoon and a decrease of moisture vapor over tropical oceans caused by large volcanic eruptions.

Association of the Rainy Season Precipitation with Low-Level Meridional Wind in the Yangtze River Valley and North China

AbstractThis study first used measurements to establish the association between the rainy season precipitation in the Yangtze River valley (YRV) and north China (NC) and the 850-hPa meridional wind, and then evaluated the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models’ simulations of both the associations and precipitation amount. It is shown that there exists a statistically significant positive correlation in the June–July precipitation and wind gradient over the YRV, and in the July–August precipitation and wind over NC. These associations are robust at daily, monthly, and interannual scales. Although many models are found to be capable of simulating the associations, the precipitation amount is still quite inadequate when compared with observations, thus raising the issue of the importance of lower-level wind simulations.

Simulation of the Interdecadal Pacific Oscillation and its impacts on the climate over eastern China during the last millennium

The Interdecadal Pacific Oscillation (IPO) and its impacts on precipitation over eastern China during the last millennium are investigated through analyzing two 1000 year global climate model simulations. Results show that the model does not simulate a prolonged period of negative IPO before A.D. 1300 suggested by the proxy reconstruction, although it does simulate centennial periods of negative and positive IPO. The simulated IPO exhibits several low-frequency oscillations, including 10–23, 30–33, 35–70, and 85–110 years. However, it remains an open question whether simulated IPO oscillations longer than bidecadal mode can be or not be considered as essentially internal modes of variability. The simulations indicate that precipitation over eastern China is associated with the IPO. When the strongest IPO signal of 53–77 year oscillation occurs, the summer precipitation goes of opposite sign over North China (NC) and the middle and lower Yangtze River Vally (MLYRV), i.e., dipole mode such as the “flood-in-south and drought-in-north” pattern that occurs during the positive IPO episodes and vice versa. While coherent mode is dominant when the relatively weak IPO signal occurs, and warm phases of the IPO coincide with coherent drought and vice versa. The association between 53–77 year oscillation of the IPO and precipitation in NC is more marked than that in the MLYRV. Our results suggest that the internal variability of climate system like the IPO may play an important role in precipitation over eastern China, at least on the 53–77 year oscillation.

Summer precipitation changes over the Yangtze River Valley and North China: Simulations from CMIP3 models

We present a study of summer precipitation changes over the Yangtze River Valley (YRV) and North China (NC) simulated from 20 models of the CMIP3 (phase 3 of the Coupled Model Intercomparison Project). It is found that the LASG-FGOALS-g1.0 (fgoals) model has the highest ability in simulating both the interannual variability of individual regions and the seesaw pattern of the two regions observed during the past few decades. Analyses of atmospheric circulations indicate that the variability in precipitation is closely associated with the 850 hPa meridional winds over the two regions. Wetness in the YRV and dryness in NC are corresponding to strong meridional wind gradient and weak meridional wind over these two regions, respectively. The ability of a coupled general circulation model (CGCM) to simulate precipitation changes in the YRV and NC depends on how well the model reproduces both observed associations of precipitation with overlying meridional winds and observed meridional wind features in summer. Analysis of future precipitation changes over the two regions projected by the fgoals model under the IPCC scenarios B1 and A1B suggests a significant increase of 7–15% for NC after 2040s due to the strengthened meridional winds, and a slight increase over the YRV due to less significant intensification of the Mei-yu front.

Past Millennium Contrasting Hydroclimate Patterns Between Monsoonal Northern China and Arid Central Asia: a Modeling Study

Widely distributed proxy records show that there were out-of-phase behaviors of moisture change between arid central Asia (ACA) and monsoonal northern China during the Little Ice Age (LIA) and Medieval Climate Anomaly (MCA). We examined spatial pattern differences between the MCA and LIA to identify contrasting patterns of summer precipitation variability, and to diagnose explanatory mechanisms through the analysis of a 1000-year global climate model simulation driven by natural and anthropogenic forcing. The results show that the model was able to roughly produce the general features of MCA-LIA hydroclimatic spatial differences between monsoonal northern China and ACA, with a relatively wet MCA found in monsoonal northern China and a relatively dry MCA found in ACA. A further analysis of associated circulations shows that increased summer precipitation in monsoonal northern China was caused by the strengthening of summer monsoon, while the decline in summer precipitation in ACA was caused by an anomalous northward displacement of the subtropical westerly jet stream. Our analyses suggest that both effective solar forcing and El-Niño Southern Oscillation (ENSO) may produce these contrasting patterns of precipitation between monsoonal northern China and ACA. Due to a change in the probability of ENSO phases at the centennial time scale found in our experiments may be attributed to solar irradiances, higher effective solar irradiances during the MCA relative to those of the LIA may have been the ultimate forcing mechanism for the simulated precipitation differences between the MCA and LIA.