Wang Huijun

The weakening of the Asian monsoon circulation after the end of 1970's

The transition of the global atmospheric circulation in the end of 1970s can clearly be detected in the atmospheric temperature, wind velocity, and so on. Wavelet analysis reveals that the temporal scale of this change is larger than 20 years. Studies in this work indicate that the trend of the transition over the mid-latitude Asia is opposite to that of global average for some variables at the middle troposphere. Another finding of this research is that the African-Asian monsoon circulation is weaker and the trade wind over the tropical eastern Pacific is weaker as well after this transition. Such a signal may be found in the summer precipitation over China as well.

A Quick Report on a Dynamical Downscaling Simulation over China Using the Nested Model

Abstract This paper describes a dynamical downscaling simulation over China using the nested model system, which consists of the modified Weather Research and Forecasting Model (WRF) nested with the NCAR Community Atmosphere Model (CAM). Results show that dynamical downscaling is of great value in improving the model simulation of regional climatic characteristics. WRF simulates regional detailed temperature features better than CAM. With the spatial correlation coefficient between the observation and the simulation increasing from 0.54 for CAM to 0.79 for WRF, the improvement in precipitation simulation is more perceptible with WRF. Furthermore, the WRF simulation corrects the spatial bias of the precipitation in the CAM simulation.

Evaluation of East Asian climatology as simulated by seven coupled models

Using observation and reanalysis data throughout 1961–1990, the East Asian surface air temperature, precipitation and sea level pressure climatology as simulated by seven fully coupled atmosphere-ocean models, namely CCSR/NIES, CGCM2, CSIRO-Mk2, ECHAM4/OPYC3, GFDL-R30, HadCM3, and NCAR-PCM, are systematically evaluated in this study. It is indicated that the above models can successfully reproduce the annual and seasonal surface air temperature and precipitation climatology in East Asia, with relatively good performance for boreal autumn and annual mean. The models’ ability to simulate surface air temperature is more reliable than precipitation. In addition, the models can dependably capture the geographical distribution pattern of annual, boreal winter, spring and autumn sea level pressure in East Asia. In contrast, relatively large simulation errors are displayed when simulated boreal summer sea level pressure is compared with reanalysis data in East Asia. It is revealed that the simulation errors for surface air temperature, precipitation and sea level pressure are generally large over and around the Tibetan Plateau. No individual model is best in every aspect. As a whole, the ECHAM4/OPYC3 and HadCM3 performances are much better, whereas the CGCM2 is relatively poorer in East Asia. Additionally, the seven-model ensemble mean usually shows a relatively high reliability.

Relationships between the North Pacific Oscillation and the typhoon/hurricane frequencies

Relationships between the North Pacific Oscillation (NPO) and the typhoon as well as hurricane frequencies are documented. The correlation between NPO index in June–July–August–September and the annual typhoon number in the western North Pacific is 0.37 for the period of 1949–1998. The NPO is correlated with the annual hurricane number in the tropical Atlantic at −0.28 for the same period. The variability of NPO is found to be concurrent with the changes of the magnitude of vertical zonal wind shear, sea-level pressure patterns, as well as the sea surface temperature, which are physically associated with the typhoons and hurricanes genesis. The NPO associated atmospheric circulation variability is analyzed to explain how NPO is linked with variability of the tropical atmospheric circulation in the western Pacific and the tropical Atlantic, via the atmospheric teleconnection.

Relationship and its instability of ENSO — Chinese variations in droughts and wet spells

Monthly data of Self-Calibrated Palmer Drought Severity Index (PDSI) from 1951 to 2000 are calculated using historical precipitation and temperature data for Chinese 160 stations. Temporal and spatial patterns of the first empirical orthogonal function (EOF) of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature, which is similar to the linear PDSI trend during 1951–2000 calculated using all monthly data. The EOF analysis also reveals that the leading mode correlates significantly with ENSO events in time and space. The ENSO EOF shows that during the typical warm phase of ENSO, surface conditions are drier in most regions of China, especially North China, but wetter than normal in the southern regions of Changjiang River, and Northwest China. During the typical cold phase of ENSO, these anomalies reverse sign. From 1951 to 2000, there are large multi-year to decadal variations in droughts and wet spells over China, which are all closely related to strong El Niño events. In other words, when one strong El Niño event happens, there is a possible big variability in droughts and wet spells over China on the multi-year or decadal scale. Studies also suggest that during the last 2–3 decades climate changes over China, especially North China’s drying and northwest China’s wetting, are closely related to the shift in ENSO towards warmer events and global warming since the late 1970s. The instability of the relationship is also studied. It is revealed that there is a good correlation between ENSO and Chinese variations in droughts and wet spells in the 3–8-year band, but the correlation between ENSO and Chinese variations in droughts and wet spells is instable. Studies suggest that there are decadal changes in the correlation: the wavelet coherency between ENSO and Chinese variations in droughts and wet spells is high during 1951–1962 and 1976–1991, but low during 1963–1975 and 1992–2000.

Natural interdecadal weakening of East Asian summer monsoon in the late 20th century

Based on the reanalysis data throughout 1948–2002 ] as derived from the United States National Centers for Environmental Prediction and National Center for Atmospheric Research, it is revealed that East Asian summer monsoon (EASM) intensity weakens on an interdecadal timescale since the mid-1960s, and twice interdecadal jumps are recorded in the EASM intensity index series in the late 20th century, respectively occurring in the mid-1960s and midto late 1970s. Six globally coupled atmosphere-ocean models’ outputs under the SRES A2 greenhouse gas and aerosol emission scenario, provided by the IPCC Data Distribution Center and the Hadley Center for Climate Prediction and Research, are then systematically examined. It follows that the above EASM weakening is not closely related to synchronizing anthropogenic global warming, and, therefore, it should be qualitatively natural change process. Over the 21st century, the EASM intensity is likely increased slightly by continually intensified greenhouse effect relative to the late 20th century.

Relationship between Arctic Oscillation and Pacific Decadal Oscillation on decadal timescale

The relationship between the Pacific Decadal Oscillation (PDO) and the Arctic Oscillation (AO) on decadal timescale in the extended winter (November–March) is investigated in this study. The results indicate that AO plays an important role in the low frequency variability of PDO. When AO leads PDO by 7–8 years, the lagging correlation between them becomes the strongest with correlation coefficient 0.77. The leading decadal variability of AO pro-vides a valuably precursory signal for predicting the variability of PDO. The results of regression and lagging correlation reveal the possible mechanism for the AO-PDO coupling: A strong AO would lead to an enhanced Aleutian Low that is linked to PDO by ocean-atmosphere interaction in the North Pacific, and vice versa.

Interdecadal Variability of the East Asian Summer Monsoon in an AGCM

It is well known that significant interdecadal variation of the East Asian summer monsoon (EASM) occurred around the end of the 1970s. Whether these variations can be attributed to the evolution of global sea surface temperature (SST) and sea ice concentration distribution is investigated with an atmospheric general circulation model (AGCM). The model is forced with observed monthly global SST and sea ice evolution through 1958–1999. A total of four integrations starting from different initial conditions are carried out. It is found that only one of these reproduces the observed interdecadal changes of the EASM after the 1970s, including weakened low-level meridional wind, decreased surface air temperature and increased sea level pressure in central China, as well as the southwestward shift of the western Pacific subtropical high ridge and the strengthened 200-hPa westerlies. This discrepancy among these simulated results suggests that the interdecadal variation of the EASM cannot be accounted for by historical global SST and sea ice evolution. Thus, the possibility that the interdecadal timescale change of monsoon is a natural variability of the coupled climate system evolution cannot be excluded.

Analysis on the decadal scale variation of the dust storm in North China

In this paper, the temporal variation characteristics of the dust storm in North China are investigated. Based on power spectrum analysis and wavelet analysis, 1956-1970 and 1985-1999 are chosen as the high-frequency and low-frequency dust storm decades respectively. Analysis results clearly show that the spring and wintertime anomalous atmospheric circulation between these two decades are significantly different. Compared with the former decade, there are a strengthened polar vortex, enhanced westerlies near 50°N, and a weak East Asian major though in the winter of the latter decade. The north and center parts of the Siberian high and the Aleutian low become weak. The southerly and easterly wind anomalies appear over the north and east parts of China, which implies the weakening of East Asian winter monsoon. Meanwhile, northern China experiences warmer winters and wetter springs, which are in favor of the weakening of dust storm intensity in North China. There are significant out-of-phase relationships between dust frequency and wintertime westerly intensity, as well as between dust frequency and Arctic Oscillation. It is also found that the frequency of dust weather is strongly related to winter-springtime East Asian monsoon intensity.

Statistical Downscaling Prediction of Summer Precipitation in Southeastern China

Abstract A statistical downscaling approach based on multiple-linear-regression (MLR) for the prediction of summer precipitation anomaly in southeastern China was established, which was based on the outputs of seven operational dynamical models of Development of a European Multi-model Ensemble System for Seasonal to Interannual Prediction (DEMETER) and observed data. It was found that the anomaly correlation coefficients (ACCs) spatial pattern of June-July-August (JJA) precipitation over southeastern China between the seven models and the observation were increased significantly; especially in the central and the northeastern areas, the ACCs were all larger than 0.42 (above 95% level) and 0.53 (above 99% level). Meanwhile, the root-mean-square errors (RMSE) were reduced in each model along with the multi-model ensemble (MME) for some of the stations in the northeastern area; additionally, the value of RMSE difference between before and after downscaling at some stations were larger than 1 mm d–1. Regionally averaged JJA rainfall anomaly temporal series of the downscaling scheme can capture the main characteristics of observation, while the correlation coefficients (CCs) between the temporal variations of the observation and downscaling results varied from 0.52 to 0.69 with corresponding variations from −0.27 to 0.22 for CCs between the observation and outputs of the models.