Relationship between ENSO and Winter Synoptic Temperature Variability over Asian-Pacific-American Region under Global Warming

Abstract

In this study, the long-term variability of the relationship between El Niño–Southern Oscillation (ENSO) and winter synoptic temperature variability (STV) is investigated based on observations and CMIP5/6 model outputs, and analyzed an extreme temperature case during winter 2018/2019.

Firstly, the interdecadal variability of the relationship between ENSO and winter STV over the Asian-Pacific-American region is investigated based on observational data from 1951 to 2018. An interdecadal shift in the ENSO-STV relationship occurred in the 1980s over Eastern China, changing from significant in Period 1 (P1, 1951-1987) to insignificant in Period 2 (P2, 1988-2018). But the ENSO-STV relationship is significantly stable over North America for the whole period. In addition, a possible reason for this interdecadal shift in the ENSO-STV relationship over Eastern China is also investigated. During P1, the ENSO pattern is significantly correlated to the temperature gradient over Northeast Asia, which is the key region influencing the intensification of extratropical eddies. The intensification of extratropical eddies over Northeast Asia is directly associated with the magnitude of STV over Eastern China. But in P2, the ENSO pattern is not related to the temperature over Northeast Asia. Therefore, the change in the ENSO pattern from P1 to P2 contributes to the interdecadal shift in the ENSO-STV relationship in the 1980s over Eastern China by influencing the temperature gradient over Northeast Asia, while ENSO can influence the temperature gradient over North America for the whole period. Furthermore, the possible role of the ENSO patterns in P1 and P2 is also examined by using an atmospheric general circulation model, highlighting that the pattern of SST variation is a determining factor in regulating STV in different regions.

Secondly, the relationship between ENSO and winter STV over the Asian-Pacific-American region is examined in 26 CMIP5/6 model outputs. Compared to observations, most models fail to simulate the correct ENSO-STV relationship in historical simulations. To investigate the possible bias in the ENSO-STV simulation, two possible processes for the connection between ENSO and winter STV are examined in high pattern score (HPS) models and low pattern score (LPS) models, respectively. On the one hand, both HPS and LPS models can overall reproduce a reasonable relationship between STV and the mean-flow conditions supporting of extratropical eddies development in the mid-latitudes. On the other hand, only HPS models can well capture the relationship between ENSO and the intensification of extratropical eddies development, while LPS models fail to simulate this feature, indicating that the bias in the simulated ENSO-STV relationship among CMIP5/6 models can be traced back to ENSO simulation. Furthermore, the bias of the ENSO simulation is characterized by an unreasonable SST pattern bias, with an excessive westward extension of warm SST anomalies over the western Pacific and weak warm SST anomalies over the equatorial central-eastern Pacific, resulting in the underestimation of the zonal SST anomaly gradient among models. Therefore, the ENSO pattern bias induces an unrealistic circulation and temperature gradient over the Asian-Pacific-American region, affecting the simulations of the ENSO-STV connection. In addition, the ENSO-STV relationship over the Asian-Pacific-American region is still robust in future projections based on HPS models, providing implications for the selection of future climate predictors.

Finally, in winter 2018/2019, southeast coastal China experienced extreme warm temperatures due to a weak East Asian winter monsoon. Based on observations from 10 meteorological stations and reanalysis data, the large-scale circulation patterns associated with this extreme warm winter and the possible driving mechanism of its related sea surface temperature (SST) anomalies are investigated in this study. During this winter, many places in this region reached their highest winter mean temperature record and had more extreme warm days and fewer extreme cold days compared to climatology. According to the circulation patterns during winter 2018/2019, several large-scale circulation conditions associated mainly with the weak East Asian winter monsoon are identified: the eastward shift of the Siberian high and a shallower East Asian trough, which is related to the low blocking frequency over the Aleutian region, are both unfavorable for cold air intrusion southward. Meanwhile, strong low-level southerly wind anomalies over southeast China are related mainly to the 2018/2019 El Niño event. Furthermore, the possible role of SST anomalies over the North Atlantic and tropical western Pacific is examined by using an atmospheric general circulation model, suggesting that both the “tripole pattern” of North Atlantic SST and tropical western Pacific SST anomalies in winter 2018/2019 played a role in influencing the East Asian trough. The combined effect of all these factors seems responsible for this extreme warm winter over southeast coastal China.

Date of Award	25 Aug 2021
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wen ZHOU (Supervisor)