The Contribution of Internal Variability to Asian Mid-Latitude Warming

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  • 1 Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China
  • 2 Department of Atmospheric and Oceanic Sciences and Institute of Atmospheric Sciences, Fudan University, Shanghai, China
  • 3 Innovation Center of Ocean and Atmosphere System, Zhuhai Fudan Innovation Research Institute, Zhuhai, China
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Abstract

The tropospheric warming in the Northern Hemisphere (NH) mid-latitudes has been an important factor in regulating weather and climate since the 20th century. Apart from anthropogenic forcing leading to the mid-latitude warming, this study investigates the possible contribution of internal variability to Asian mid-latitude warming and its role in East Asian circulation changes in boreal summer, using four reanalysis datasets in the past century and a set of 1800-year preindustrial control simulations of the Community Earth System Model version1 large ensemble (CESM-LE).

The surface and tropospheric warming in the Asian mid-latitudes is associated with a strong upper-level geopotential height rise north of the Tibetan Plateau (TP). Linear trends of 200-hPa geopotential height (Z200) confirm a dipole of an anomalous high north of the TP and an anomalous low over the Iranian Plateau in 1958-2017. The leading internal circulation mode bears a striking resemblance to the Z200 trend in the past 60 and 100 years, indicating that the long-term trend may be partially of internal origin. The Asian mid-latitude warming is also found in preindustrial simulations of CESM-LE, further suggesting that internal variability explains at least part of the temperature change in the Asian mid-latitudes, that is in a chain of wave trains along the NH mid-latitudes. The Asian warming decreases meridional gradient of geopotential height, resulting in the weakening of westerly winds over the TP and the TP thermal forcing. Thus, it is essential to consider the role of internal variability in shaping East Asian surface temperature and East Asian summer monsoon changes in the past decades.

Corresponding author address: Prof. Liguang Wu Department of Atmospheric and Oceanic Sciences & Institute of Atmospheric Sciences, Fudan University, Shanghai 200438, China. E-mail: liguangwu@fudan.edu.cn

Abstract

The tropospheric warming in the Northern Hemisphere (NH) mid-latitudes has been an important factor in regulating weather and climate since the 20th century. Apart from anthropogenic forcing leading to the mid-latitude warming, this study investigates the possible contribution of internal variability to Asian mid-latitude warming and its role in East Asian circulation changes in boreal summer, using four reanalysis datasets in the past century and a set of 1800-year preindustrial control simulations of the Community Earth System Model version1 large ensemble (CESM-LE).

The surface and tropospheric warming in the Asian mid-latitudes is associated with a strong upper-level geopotential height rise north of the Tibetan Plateau (TP). Linear trends of 200-hPa geopotential height (Z200) confirm a dipole of an anomalous high north of the TP and an anomalous low over the Iranian Plateau in 1958-2017. The leading internal circulation mode bears a striking resemblance to the Z200 trend in the past 60 and 100 years, indicating that the long-term trend may be partially of internal origin. The Asian mid-latitude warming is also found in preindustrial simulations of CESM-LE, further suggesting that internal variability explains at least part of the temperature change in the Asian mid-latitudes, that is in a chain of wave trains along the NH mid-latitudes. The Asian warming decreases meridional gradient of geopotential height, resulting in the weakening of westerly winds over the TP and the TP thermal forcing. Thus, it is essential to consider the role of internal variability in shaping East Asian surface temperature and East Asian summer monsoon changes in the past decades.

Corresponding author address: Prof. Liguang Wu Department of Atmospheric and Oceanic Sciences & Institute of Atmospheric Sciences, Fudan University, Shanghai 200438, China. E-mail: liguangwu@fudan.edu.cn
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