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Chao He, Tim Li, and Wen Zhou

Abstract

Summer monsoon rainfall supplies over 55% of annual precipitation to global monsoon regions. As shown by more than 70% of models, including 30 models from CMIP5 and 30 models from CMIP6 under high-emission scenarios, North American (NAM) monsoon rainfall decreases in a warmer climate, in sharp contrast to the robust increase in Asian–African monsoon rainfall. A hierarchy of model experiments is analyzed to understand the mechanism for the reduced NAM monsoon rainfall in this study. Modeling evidence shows that the reduction of NAM monsoon rainfall is related to both direct radiative forcing of increased CO2 concentration and SST warming, manifested as fast and slow responses to abrupt CO2 quadrupling in coupled GCMs. A cyclone anomaly forms over the Eurasian–African continental area due to enhanced land–sea thermal contrast under increased CO2 concentration, and this leads to a subsidence anomaly on its western flank, suppressing the NAM monsoon rainfall. The SST warming acts to further reduce the rainfall over the NAM monsoon region, and the El Niño–like SST warming pattern with enhanced SST warming over the equatorial Pacific plays a key role in suppressing NAM rainfall, whereas relative cooling over the subtropical North Atlantic has no contribution. A positive feedback between monsoon precipitation and atmospheric circulation helps to amplify the responses of monsoon rainfall.

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Siyan Dong, Ying Sun, and Chao Li

Abstract

This paper examines the possible influence of external forcings on observed changes in precipitation extremes in the mid-to-high latitudes of Asia during 1958–2012 and attempts to identify particular extreme precipitation indices on which there are better chances to detect the influence of external forcings. We compare a recently compiled dataset of observed extreme indices with those from phase 5 of the Coupled Model Intercomparison Project (CMIP5) simulations using an optimal fingerprinting method. We consider six indices that characterize different aspects of extreme precipitation, including annual maximum amount of precipitation falling in 1 day (Rx1day) or 5 days (Rx5day), the total amount of precipitation from the top 5% or top 1% daily amount on wet days, and the fraction of the annual total precipitation from these events. For single-signal analysis, the fingerprints of external forcings including anthropogenic agents are robustly detected in most studied extreme indices over all Asia and for midlatitude Asia but not for high-latitude Asia. For two-signal analysis, anthropogenic influence is detectable in these indices over Asia at 5% or slightly less than 5% significance level, whereas natural influence is not detectable. In high-latitude Asia, anthropogenic influence is detected only in a fractional index, representing a stark contrast to the midlatitude and full Asia results. We find relatively smaller internal variability and thus higher signal-to-noise ratio in the fractional indices when compared with the other ones. Our results point to the need for studying precipitation extreme indices that are less affected by internal variability while still representing the relevant nature of precipitation extremes to improve the possibility of detecting a forced signal if one is present in the data.

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Chao He, Yuhao Wang, and Tim Li

Abstract

El Niño induces an anomalous easterly wind along the equator and a pair of anomalous anticyclones straddling the equator over the tropical Indian Ocean (TIO) during the autumn of its developing phase. Based on 30 coupled models participating in CMIP5, these atmospheric circulation anomalies over TIO are substantially weakened by about 12%–13% K−1 under global warming scenarios, associated with a weakened zonal gradient of the sea surface temperature (SST) anomaly. The mechanism for the response is investigated based on a hierarchy of model experiments. Based on stand-alone atmospheric model experiments under uniform and patterned mean-state SST warming, the atmospheric circulation anomaly over TIO during the autumn of the developing El Niño is also substantially weakened by about 8% K−1 even if the interannual variability of SST remains exactly unchanged, suggesting that the primary cause resides in the atmosphere rather than the SST anomaly. The tropospheric static stability is robustly enhanced under global warming, and experiments performed by a linear baroclinic model show that a much weaker atmospheric circulation anomaly over TIO is stimulated by an unchanged diabatic heating anomaly under a more stable atmosphere. The weakened atmospheric circulation anomaly due to enhanced static stability weakens the zonal gradient of the SST anomaly within TIO through local air–sea interaction, and it acts to further weaken the atmospheric circulation anomaly. The enhanced static stability of the troposphere is probably the primary cause and the air–sea interaction within TIO is a secondary cause for the weakened impact of the developing El Niño on atmospheric circulation variability over TIO.

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Yuhao Wang, Chao He, and Tim Li

Abstract

El Niño stimulates an anomalous cyclone over the North Pacific during its developing phase. Using 30 CGCMs and 11 AGCMs from CMIP5, we find a weakly strengthened anomalous North Pacific cyclone (NPC) in a warmer climate in CGCMs, and intermodel uncertainty exists. A similar change of the anomalous NPC is found in AGCMs with increased mean state SST but with a stronger amplitude of enhancement. Based on a simple Gill model, the diabatic heating anomaly, mean state static stability, and meridional gradient of relative vorticity are identified to be responsible for the change of the anomalous NPC. Analyses of the CMIP5 models suggest that the change of the anomalous NPC is largely determined by the competition between the enhanced diabatic heating anomaly and the enhanced mean state static stability. The amplitude of enhancement of the anomalous NPC is strongly modulated by the change of precipitation anomaly over the equatorial central-eastern Pacific, which depends on the changes of mean state SST and the El Niño–related SST anomaly. Compared with a uniform warming, an El Niño–like mean state SST warming favors a much stronger enhancement of the anomalous NPC, by enhancing the mean state precipitation and latent heating anomaly associated with the precipitation anomaly over the equatorial Pacific. However, the air–sea coupling acts to weaken the SST anomaly associated with El Niño in the CGCMs, which further reduces the enhancement of the anomalous NPC.

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Chao He, Tianjun Zhou, and Tim Li

Abstract

The western North Pacific subtropical anticyclone (WNPAC) is the most prominent atmospheric circulation anomaly over the subtropical Northern Hemisphere during the decaying summer of an El Niño event. Based on a comparison between the RCP8.5 and the historical experiments of 30 coupled models from the CMIP5, we show evidence that the anomalous WNPAC during the El Niño–decaying summer is weaker in a warmer climate although the amplitude of the El Niño remains generally unchanged. The weakened impact of the sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) on the atmosphere is essential for the weakened anomalous WNPAC. In a warmer climate, the warm tropospheric temperature (TT) anomaly in the tropical free troposphere stimulated by the El Niño–related SSTA is enhanced through stronger moist adiabatic adjustment in a warmer mean state, even if the SSTA of El Niño is unchanged. But the amplitude of the warm SSTA over TIO remains generally unchanged in an El Niño–decaying summer, the static stability of the boundary layer over TIO is increased, and the positive rainfall anomaly over TIO is weakened. As a result, the warm Kelvin wave emanating from TIO is weakened because of a weaker latent heating anomaly over TIO, which is responsible for the weakened WNPAC anomaly. Numerical experiments support the weakened sensitivity of precipitation anomaly over TIO to local SSTA under an increase of mean-state SST and its essential role in the weakened anomalous WNPAC, independent of any change in the SSTA.

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Chao Li, Dirk Notz, Steffen Tietsche, and Jochem Marotzke

Abstract

To examine the long-term stability of Arctic and Antarctic sea ice, idealized simulations are carried out with the climate model ECHAM5/Max Planck Institute Ocean Model (MPI-OM). Atmospheric CO2 concentration is increased over 2000 years from preindustrial levels to quadrupling, is then kept constant for 5940 years, is afterward decreased over 2000 years to preindustrial levels, and is finally kept constant for 3940 years.

Despite these very slow changes, the sea ice response significantly lags behind the CO2 concentration change. This lag, which is caused by the ocean's thermal inertia, implies that the sea ice equilibrium response to increasing CO2 concentration is substantially underestimated by transient simulations. The sea ice response to CO2 concentration change is not truly hysteretic and is in principle reversible.

The authors find no lag in the evolution of Arctic sea ice relative to changes in annual-mean Northern Hemisphere surface temperature. The summer sea ice cover changes linearly with respect to both CO2 concentration and temperature, while the Arctic winter sea ice cover shows a rapid transition to a very low sea ice coverage. This rapid transition of winter sea ice is associated with a sharply enhanced ice–albedo feedback and a sudden onset of convective-cloud feedback in the Arctic.

The Antarctic sea ice cover retreats continuously without any rapid transition during the warming. Compared to Arctic sea ice, Antarctic sea ice shows a much more strongly lagged response to changes in CO2 concentration. It even lags behind the surface temperature change, which is caused by a different response of ocean deep convection during the warming and the cooling periods.

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Yaheng Tan, Francis Zwiers, Song Yang, Chao Li, and Kaiqiang Deng

Abstract

Performance in simulating atmospheric rivers (ARs) over western North America based on AR frequency and landfall latitude is evaluated for 10 models from phase 5 of the Coupled Model Intercomparison Project among which the CanESM2 model performs well. ARs are classified into southern, northern, and middle types using self-organizing maps in the ERA-Interim reanalysis and CanESM2. The southern type is associated with the development and eastward movement of anomalous lower pressure over the subtropical eastern Pacific, while the northern type is linked with the eastward movement of anomalous cyclonic circulation stimulated by warm sea surface temperatures over the subtropical western Pacific. The middle type is connected with the negative phase of North Pacific Oscillation–west Pacific teleconnection pattern. CanESM2 is further used to investigate projected AR changes at the end of the twenty-first century under the representative concentration pathway 8.5 scenario. AR definitions usually reference fixed integrated water vapor or integrated water vapor transport thresholds. AR changes under such definitions reflect both thermodynamic and dynamic influences. We therefore also use a modified AR definition that isolates change from dynamic influences only. The total AR frequency doubles compared to the historical period, with the middle AR type contributing the largest increases along the coasts of Vancouver Island and California. Atmospheric circulation (dynamic) changes decrease northern AR type frequency while increasing middle AR type frequency, indicating that future changes of circulation patterns modify the direct effect of warming on AR frequency, which would increase ARs (relative to fixed thresholds) almost everywhere along the North American coastline.

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Xianjin Li, Yi Chao, James C. McWilliams, and Lee-Lueng Fu

Abstract

The upper Pacific Ocean Current and temperature have been simulated by a three-dimensional ocean general circulation model (OGCM) with two different vertical-mixing schemes. One corresponds to the modified Richardson number–dependent scheme of Pacanowski and Philander (PP); the other is adapted from the newly developed K-Profile Parameterization (KPP) scheme. The performance of both schemes in a Pacific OGCM is evaluated under the same model configuration and boundary conditions. Model and data comparisons are made for the mean state, annual cycle, and interannual-to-interdecadal variability. In the Tropics, both the PP and KPP schemes produce reasonably realistic tropical thermal and current structures; however, KPP is better than PP in several important aspects. For example, the KPP scheme simulates a more realistic thermocline and significantly reduces the cold surface temperature bias in the eastern equatorial Pacific. The depth of the maximum Equatorial Undercurrent simulated by the KPP scheme is much closer to the observation. In the extratropics the KPP scheme is significantly better than the PP scheme in simulating the thermal and current structures, including the annual mean, annual cycle, and interannual-to-interdecadal variability.

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Chao He, Ziqian Wang, Tianjun Zhou, and Tim Li

Abstract

Coupled climate system models consistently show that the low-level southerly wind associated with the East Asian summer monsoon (EASM) is enhanced under anthropogenic greenhouse gas forcing, and the enhanced EASM was attributed to the enhanced land–sea thermal contrast by previous studies. Based on a comparison of the global warming scenarios with the present-day climate in an ensemble of 30 coupled models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), we show evidence that changes in land–sea thermal contrast cannot explain the enhanced EASM circulation in terms of the seasonality. Indeed, the enhanced low-level southerly wind over East Asia is associated with a large-scale anomalous cyclone around the Tibetan Plateau (TP), and numerical simulation by the Linear Baroclinic Model suggests that the enhanced latent heating over the TP associated with enhanced precipitation is responsible for this low-level cyclone anomaly and the enhanced EASM circulation projected by the coupled models. Moisture budget analysis shows that enhanced hydrological recycling and enhanced vertical moisture advection due to increased specific humidity have the largest contribution to the increased precipitation over the TP, and more than half of the intermodel uncertainty in the projected change of EASM circulation is associated with the uncertainty in the changes of precipitation over the TP. Therefore, the TP plays an essential role in enhancing the EASM circulation under global warming through enhanced latent heating over the TP.

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Chao Li, Francis Zwiers, Xuebin Zhang, Guilong Li, Ying Sun, and Michael Wehner

Abstract:

This study presents an analysis of daily temperature and precipitation extremes with return periods ranging from 2 to 50 years in the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble of simulations. Judged by similarity with reanalyses, the new-generation models simulate the present-day temperature and precipitation extremes reasonably well. In line with previous CMIP simulations, the new simulations continue to project a large-scale picture of more frequent and more intense hot temperature extremes and precipitation extremes and vanishing cold extremes under continued global warming. Changes in temperature extremes outpace changes in global annual mean surface air temperature (GSAT) over most land masses, while changes in precipitation extremes follow changes in GSAT globally at roughly the Clausius-Clapeyron rate of ∼7%/°C. Changes in temperature and precipitation extremes normalized with respect to GSAT do not depend strongly on the choice of forcing scenario or model climate sensitivity, and do not vary strongly over time, but with notable regional variations. Over the majority of land regions, the projected intensity increases and relative frequency increases tend to be larger for more extreme hot temperature and precipitation events than for weaker events. To obtain robust estimates of these changes at local scales, large initial-condition ensemble simulations are needed. Appropriate spatial pooling of data from neighboring grid cells within individual simulations can, to some extent, reduce the needed ensemble size.

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