How much of the inter-decadal climate transition over eastern China is attributable to internal variability and how much is due to external forcing?

A recent study quantified the relative contributions of internal climate variability and external forcing to the inter-decadal climate transition over eastern China during 1959–2001.
Published in Earth & Environment
How much of the inter-decadal climate transition over eastern China is attributable to internal variability and how much is due to external forcing?
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The observed annual precipitation showed a clear inter-decadal transition over eastern China during the second half of the 20th century, characterized by a long-term drying trend in North China and a wetting trend in South China. The inter-decadal transition of the dipole pattern commonly referred to as the “south flood–north drought” (SFND) pattern and has major impacts on water resources, agriculture, ecosystems and human society in eastern China. 

Many studies have investigated the impacts of internal climate variability (e.g., Pacific Decadal Oscillation, Atlantic Multi-decadal Oscillation) and external forcing (e.g., greenhouse gases, aerosols, volcanic activity, land use and land cover change) on the SFND pattern. However, the relative contribution of internal variability and external forcing to this inter-decadal transition of precipitation/temperature pattern remains unclear. The attribution of the phase transition in inter-decadal variations of regional climate is hindered by the inability of coupled atmosphere-ocean models to capture the observed temporal evolution of climate.

We tried to separate internal variability from the externally forced climate response through a set of dynamical downscaling simulations with lateral boundary conditions derived from reanalysis data and a large ensemble mean of the CMIP5 historical simulations. In doing so, we successfully isolated the response of the regional climate to the global internal climate variability and external forcing.

The regional model simulation driven by ERA40 reanalysis dataset was able to capture the observed dipole pattern of the precipitation and the corresponding temporal variation (Fig. 1). The inter-decadal transition of the SFND pattern is dominated by the internal climate variability. In contrast, external forcing tends to cause a drying trend over eastern China which can be further attributed to the increasing anthropogenic aerosol.

Fig. 1 Linear trends in the annual mean precipitation. Linear trends (mm day−1 decade−1) in a observational data (CN05.1 and APHRO), b the WRF historical (HIST) run, c the response to external forcing (EF) and d the WRF internal variability (IV) run during the time period 1959–2001. The hatched areas and asterisks denote a significance level of 0.05. The two boxes in a indicating the location of North China (35–43°N, 110–122°E) and South China (24°–32°N, 110°–122°E). The two gray lines on the map show the location of the Yellow River in North China and the Yangtze River in central China. The time series of regional mean precipitation anomalies (shaded areas; mm day−1) and the linear trends (lines; mm day−1 yr−1) over e North China and f South China. The correlation coefficients between HIST and observation are shown in the upper right corner of each panel. 

We further quantified the relative contributions of internal and external forcing to the dipole climate pattern using a relative weight analysis method (Fig. 2). The internal climate variability accounted for about 65% (55%) of the inter-decadal transition in anomalous precipitation in South (North) China during the second half of the 20th century. The internal climate variability clearly dominates the inter-decadal transition in precipitation over South China. However, both the internal climate variability and external forcing have important roles in determining drying tendency in North China. In terms of the changes in the annual mean surface air temperature, external forcing accounts for 70% of the warming trend over North China. By contrast, the internal variability dominates the cooling trend of the summer surface air temperature over the Yangtze River basin.

Fig. 2 Relative weight analysis of the annual mean precipitation and temperature. Relative weight analysis was applied to quantify the contributions of internal variability (IV) and external forcing (EF) to various timescales of the annual mean precipitation/temperature at 2 m over the grids in North China (NC: 35–43°N, 110–122°E; gold), South China (SC: 24–32°N, 110–122°E; green) and eastern China (EC: 22–43°N, 110–122°E; gray), respectively. Different combinations of intrinsic mode functions (IMFs) from EEMD represent the interannual variability (IMF1+2), decadal variability (IMF3+4) and nonlinear trend (IMF5), respectively. The nonlinear trend corresponds to the inter-decadal transition of a the annual precipitation pattern or b the warming trend. Bootstrap sampling with replacement was performed 1000 times on the spatial fields of the combinations of IMFs to estimate the uncertainty range of the relative weights of IV and EF. Box-plot elements: center line, median; dots, mean; box limits, upper (75th) and lower (25th) percentiles; whiskers, 1.5 times the interquartile range.

This study proposed an approach combining a large ensemble mean of global climate models and regional model simulations to separate the response of the regional climate to internal climate variability from external forcing. It provides a novel framework for the attribution of regional climate change that could also be applied to other regions, such as North America and northeastern Australia. 

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