Title: Synoptic-scale characteristics, large-scale controls, trends, and future projections of summer high temperature extremes in China
Authors: Wang, Weiwen (王偉文)
Abstract: Summer heat waves with persistent extreme high temperatures have been occurring with increasing frequency in recent decades. These extreme events have disastrous consequences for human health, economies, and ecosystems. This study investigates the synoptic-scale characteristics, large-scale controls on interannual variability, secular changes, and future projections of extreme hot events in China.
First of all, three recent summers with intense and protracted heat waves are investigated: the summers of 2003, 2006, and 2013, with high temperatures located mainly in southeastern, eastern, and southwestern China, respectively. It is found that the 2003 heat wave can be divided into two phases. In the first phase, a heat center was located in the southeast coastal provinces during the first 20 days of July. The maximum southward displacement of the East Asian jet stream (EAJS) in this phase induced anticyclonic anomalies to the south, associated with southwestward intensification of the western North Pacific subtropical high (WNPSH), and high temperatures were blocked to the south of the Yangtze River. In the second phase, the heat wave extended northward to cover a large area of eastern China. A poleward displacement of the EAJS and an enhanced WNPSH over the midlatitudes of eastern China resulted in a "heat dome" over the region. This atmospheric anomaly in the second phase of 2003 occurred in the East China heat wave of 2013 as well. But the area of high temperatures extended even farther north in August 2013. In the case of 2006, the extreme poleward displacement of the EAJS, coupled with the extraordinary westward extension of the WNSPH, blocked the moisture supply from the southwest monsoon. The positive feedback between the dry land and the atmosphere produced a low-level heat low in the troposphere and may have reinforced this extreme hot event.
For interannual variability, we focus on Southeast China (south of 35°N and east of 105°E). High temperature extremes (HTEs) are defined as days with daily maximum temperature above the 90th percentile. Two key domains in the upper level that are associated with HTE variation, the "exit" and the "tail" of the EAJS, are identified. Poleward displacement of the exit is associated with warming tropospheric temperatures over East Asia and tends to be linked with high HTE frequency, while enhancement of the tail is associated with cooling tropospheric temperatures in the northern Pacific and tends to be linked with low HTE frequency. Furthermore, these two domains are in essence two sectors of the phase-locked circumglobal teleconnection (CGT) pattern in the Northern Hemisphere. Linkages are found between HTEs in Southeast China and precipitation anomalies in the Indian summer monsoon region, and also in extratropical regions such as northeastern Europe. These teleconnections are set up through the CGT pattern associated with the westerly jet in the midlatitudes. Furthermore, an interdecadal change in HTEs occurred in the late 1980s. Before this interdecadal shift, the interannual variability of HTEs was linked mainly to temperature adjustments associated with the meridional displacement of the EAJS, while after the shift, HTEs were found to follow the El Niño-Southern Oscillation (ENSO) cycle, which may be due to intensified and persistent ENSO activities, tropical Indian Ocean warming, and changes in atmospheric teleconnections. Impacts of the EAJS, the South Asian high (SAH), and the WNPSH on HTEs are further investigated based on EOF analysis. It is found that mainly the first leading EOF mode with a homogeneous spatial pattern shows dominance before the interdecadal shift, while both of the first two leading EOF modes show dominance after the interdecadal shift. A possible mechanism for the bonding of HTEs in Southeast China with the EAJS, the SAH, and the WNPSH in the ENSO-monsoon coupled system is proposed.
Finally, statistical modeling of summer hot spells in China during 1960–2005 and their simulations in the historical experiment of the Coupled Model Intercomparison Project Phase 5 (CMIP5) are investigated. A technique called the hot spell model (HSM), introduced by Furrer et al. (2010) for modeling hot spells by extending the point process approach to extreme value theory, is applied. Specifically, the frequency of summer hot spells is modeled by a Poisson distribution, their intensity is modeled by a generalized Pareto distribution, and their duration is modeled by a geometric distribution. Results show that the HSM permits realistic modeling of summer hot spells in China. Trends in the frequency, duration, and intensity of hot spells were estimated based on the HSM for the observed period from 1960 to 2005. Furthermore, the performance of the CMIP5 historical run models in simulating hot spell characteristics and trends was assessed based on the HSM. Climate models with good performance were selected to conduct an ensemble projection of hot spell intensity, frequency, and duration and their trends in future decades.
Notes: CityU Call Number: QC906 .W36 2014; xx, 171 p. : ill. (some col.) 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2014.; Includes bibliographical references (p. 146-171)
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