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Wang, H, Xie SP, Kosaka Y, Liu QY, Du Y.  2019.  Dynamics of Asian summer monsoon response to anthropogenic aerosol forcing. Journal of Climate. 32:843-858.   10.1175/jcli-d-18-0386.1   AbstractWebsite

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20 degrees and 40 degrees N and weakens the EASM north of 20 degrees N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north-south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean-atmosphere coupling.

Li, JB, Xie SP, Cook ER, Chen FH, Shi JF, Zhang DD, Fang KY, Gou XH, Li T, Peng JF, Shi SY, Zhao YS.  2019.  Deciphering human contributions to Yellow River flow reductions and downstream drying using centuries-long tree ring records. Geophysical Research Letters. 46:898-905.   10.1029/2018gl081090   AbstractWebsite

The Yellow River flow has decreased substantially in recent decades, and the river often dried up in the lower reach and failed to reach the sea. Climate change and human disruption have been suggested as major causes of the flow reduction, but quantification of their relative contribution is challenging due to limited instrumental records and disturbance by dams. Here we use a basin-wide tree ring network to reconstruct the Yellow River flow for the past 1,200 years and show that the flow exhibits marked amplitude variations that are closely coupled to the hydrological mean state swings at multidecadal to centennial timescales. Recent flow should have increased to the highest level of the past 1,200 years if there were no human disruption. However, human activities have caused a loss of nearly half of natural flow since the late 1960s and are the main culprit for recent downstream flow reduction. Plain Language Summary Recent Yellow River flow reductions have had major impacts on China's economy and water policy. The short and heavily human-modified gauge records are unable to reveal natural flow variability now and in the past. Here we use tree rings to reconstruct long-term Yellow River flow, which enables an assessment of natural flow variability and the detection of human contributions to recent flow reductions. Our 1,200-year reconstruction reveals that under natural conditions the Yellow River flow should have increased markedly since the early twentieth century. However, the observed flow decreased since the late 1960s and such a decrease must be predominately caused by human interventions instead of climate change.

Lin, L, Xu YY, Wang ZL, Diao CR, Dong WJ, Xie SP.  2018.  Changes in extreme rainfall over India and China attributed to regional aerosol-cloud interaction during the late 20th century rapid industrialization. Geophysical Research Letters. 45:7857-7865.   10.1029/2018gl078308   AbstractWebsite

Both mean and extreme rainfall decreased over India and Northern China during 1979-2005 at a rate of 0.2%/decade. The aerosol dampening effects on rainfall has also been suggested as a main driver of mean rainfall shift in India and China. Conflicting views, however, exist on whether aerosols enhance or suppress hazardous extreme heavy rainfall. Using Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel ensemble, here we show that only a subset of models realistically reproduces the late-20th-century trend of extreme rainfall for the three major regions in Asia: drying in India and Northern China and wetting in Southern China, all consistent with mean rainfall change. As a common feature, this subset of models includes an explicit treatment of the complex physical processes of aerosol-cloud interaction (i.e., both cloud-albedo and cloud-lifetime effects), while simulation performance deteriorates in models that include only aerosol direct effect or cloud-albedo effect. The enhanced aerosol pollution during this rapid industrialization era is the leading cause of the spatially heterogeneous extreme rainfall change by dimming surface solar radiation, cooling adjacent ocean water, and weakening moisture transport into the continental region, while GHG warming or natural variability alone cannot explain the observed changes. Our results indicate that the projected intensification of regional extreme rainfall during the early-to-mid 21st-century, in response to the anticipated aerosol reduction, may be underestimated in global climate models without detailed treatment of complex aerosol-cloud interaction. Plain Language Summary Over Asia, a robust pattern of drying-wetting-drying trend over three most populated regions (India, South China, and North China, respectively) have been observed in the past few decades. Yet the cause of the 30-year trend is rather unclear, with conflicting arguments on the importance of natural variability, the greenhouse gas, land cover, and aerosols. Most of the previous studies, however, fail to provide a holistic explanation for all three major regions simultaneously. The aerosol-cloud interaction-induced oceanic cooling, as we show here, provides a critical piece in reproducing the past trend. Only a fraction of climate models with complex treatment of aerosol-cloud interaction capture the observed pattern; thus, unconstrained model data set provides biased outlook of extreme rainfall in this region.

Siler, N, Kosaka Y, Xie SP, Li XC.  2017.  Tropical ocean contributions to California's surprisingly dry El Nino of 2015/16. Journal of Climate. 30:10067-10079.   10.1175/jcli-d-17-0177.1   AbstractWebsite

The major El Nino of 2015/16 brought significantly less precipitation to California than previous events of comparable strength, much to the disappointment of residents suffering through the state's fourth consecutive year of severe drought. Here, California's weak precipitation in 2015/16 relative to previous major El Nino events is investigated within a 40-member ensemble of atmosphere-only simulations run with historical sea surface temperatures (SSTs) and constant radiative forcing. The simulations reveal significant differences in both California precipitation and the large-scale atmospheric circulation between 2015/16 and previous strong El Nino events, which are similar to (albeit weaker than) the differences found in observations. Principal component analysis indicates that these ensemble-mean differences were likely related to a pattern of tropical SST variability with a strong signal in the Indian Ocean and western Pacific and a weaker signal in the eastern equatorial Pacific and subtropical North Atlantic. This SST pattern was missed by the majority of forecast models, which could partly explain their erroneous predictions of above-average precipitation in California in 2015/16.

Liu, L, Xie SP, Zheng XT, Li T, Du Y, Huang G, Yu WD.  2014.  Indian Ocean variability in the CMIP5 multi-model ensemble: the zonal dipole mode. Climate Dynamics. 43:1715-1730.   10.1007/s00382-013-2000-9   AbstractWebsite

The performance of 21 Coupled Model Intercomparison Project Phase 5 (CMIP5) models in the simulation of the Indian Ocean Dipole (IOD) mode is evaluated. Compared to CMIP3, CMIP5 models exhibit a similar spread in IOD intensity. A detailed diagnosis was carried out to understand whether CMIP5 models have shown improvement in their representation of the important dynamical and thermodynamical feedbacks in the tropical Indian Ocean. These include the Bjerknes dynamic air-sea feedback, which includes the equatorial zonal wind response to sea surface temperature (SST) anomaly, the thermocline response to equatorial zonal wind forcing, the ocean subsurface temperature response to the thermocline variations, and the thermodynamic air-sea coupling that includes the wind-evaporation-SST and cloud-radiation-SST feedback. Compared to CMIP3, the CMIP5 ensemble produces a more realistic positive wind-evaporation-SST feedback during the IOD developing phase, while the simulation of Bjerknes dynamic feedback is more unrealistic especially with regard to the wind response to SST forcing and the thermocline response to surface wind forcing. The overall CMIP5 performance in the IOD simulation does not show remarkable improvements compared to CMIP3. It is further noted that the El Nio-Southern Oscillation (ENSO) and IOD amplitudes are closely related, if a model generates a strong ENSO, it is likely that this model also simulates a strong IOD.