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Haijun Yang and Lu Wang

Abstract

The tropical oceanic response to the extratropical thermal forcing is quantitatively estimated in a coupled climate model. This work focuses on comparison of the responses between the tropical Atlantic and Pacific. Under the same extratropical forcing, the tropical sea surface temperature responses are comparable. However, the responses in the tropical subsurface in the two oceans are distinct. The tropical subsurface response in the Atlantic can be twice of that in the Pacific. The maximum subsurface temperature change in the tropical Pacific occurs in the eastern lower thermocline, while that in the tropical Atlantic occurs in the west and well below the lower thermocline. The different responses in the tropical Atlantic and Pacific are closely related to the different changes in the meridional overturning circulations. The Pacific shallow overturning circulation, or the subtropical cell, tends to slow down (speed up) in response to the extratropical warming (cooling) forcing. The changes in the upwelling in the eastern equatorial Pacific as well as the shallow subduction from the extratropical southern Pacific along the eastern boundary are accountable for the eastern Pacific temperature change. The Atlantic overturning circulation consists of the shallow subtropical cell and the deep thermohaline circulation. A weakened thermohaline circulation will result in a strengthened northern subtropical cell, in which the change in the lower branch, or the low-latitude North Brazil Current, can cause strong response below the western tropical thermocline. Here the coastal Kelvin wave along the western boundary on the intermediate isopycnal level also plays an important role in the equatorward conveying of the climate anomalies in the mid-to-high-latitude Atlantic, particularly during the initial stage of the extratropical forcing.

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Er Lu, Jiawei Hao, and Kexin Yang

Abstract

The temporal–spatial variations of the static stability of dry air and the relative importance of their influencing quantities are explored. Derivation shows that while it links to the vertical difference of temperature, static stability also relates to the temperature itself. The static stability is expressed as a nonlinear function of temperature and the vertical difference of temperature. The relative importance of the two influencing quantities is assessed with linear regression. Tests show that the linear fitting method is robust. The results of the dominance rely on the data examined, which include an interannual variation, a seasonal variation, and a spatial variation that consists of the grid points over the globe. It is revealed that in the lower troposphere, while the temporal variations of static stability are dominated by the vertical difference of temperature, the temperature itself may also have considerable influence, especially over the high latitudes of the two hemispheres. In the stratosphere, temperature tends to have more contributions. Over the Antarctic, temperature dominates the seasonal and interannual variations of the static stability. The spatial variation of the static stability of July is influenced by both temperature and its vertical difference before 1980, but after that it is dominated by temperature.

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Yang Gao, Jian Lu, and L. Ruby Leung

Abstract

This study investigates the North Atlantic atmospheric rivers (ARs) making landfall over western Europe in the present and future climate from the multimodel ensemble of phase 5 of the Coupled Model Intercomparison Project (CMIP5). Overall, CMIP5 captures the seasonal and spatial variations of historical landfalling AR days, with the large intermodel variability strongly correlated with the intermodel spread of historical near-surface westerly jet position. Under representative concentration pathway 8.5 (RCP8.5), AR frequency is projected to increase significantly by the end of this century, with 127%–275% increase at peak AR frequency regions (45°–55°N). While thermodynamics plays a dominant role in the future increase of ARs, wind changes associated with the midlatitude jet shifts also significantly contribute to AR changes, resulting in dipole change patterns in all seasons. In the North Atlantic, the model-projected jet shifts are strongly correlated with the simulated historical jet position. As models exhibit predominantly equatorward biases in the historical jet position, the large poleward jet shifts reduce AR days south of the historical mean jet position through the dynamical connections between the jet positions and AR days. Using the observed historical jet position as an emergent constraint, dynamical effects further increase future AR days over the equatorward flank above the increases from thermodynamical effects. Compared to the present, both total and extreme precipitation induced by ARs in the future contribute more to the seasonal mean and extreme precipitation, primarily because of the increase in AR frequency. While AR precipitation intensity generally increases more relative to the increase in integrated vapor transport, AR extreme precipitation intensity increases much less.

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Shan He, Song Yang, Mengmeng Lu, and Zhenning Li

Abstract

The Afro-Eurasian intermediate-frequency atmospheric teleconnection conveys meteorological signals zonally, leads to various atmospheric variations, and causes extreme events along its path. This study, aimed at demonstrating the characteristics of the teleconnection, reveals that the teleconnection accounts for nearly half of the atmospheric variability and significantly influences different meteorological fields. With the propagation of signals associated with the teleconnection, local weather varies from prolonged dry and warm days to extended wet and cold days. El Niño–Southern Oscillation (ENSO) modulates the interannual variation of the teleconnection: it becomes more active and its downstream pattern shifts southward during El Niño events. Two responsible mechanisms are proposed for the ENSO modulation: the eddy-to-eddy interaction that leads to the change in the activeness of the teleconnection and the waveguide effect that accounts for the shift of the teleconnection. First, the El Niño–related Atlantic anomalies of the Rossby wave train and storm track amplify the Atlantic disturbances of the intermediate frequency and thus the activeness of the teleconnection. Second, during El Niño years, the East Asian jet stream shifts southward, resulting in the southward shifts of the downstream waveguide effect and thus the downstream pattern. This study also demonstrates that when investigating an atmospheric mode or its impacts, the signals of different time scales should be separated and the cross-frequency interactive systems necessitate examinations.

Open access
Mengmeng Lu, Zhiming Kuang, Song Yang, Zhenning Li, and Hanjie Fan

Abstract

Eurasian snow, one of the most important factors that influence the Asian monsoons, has long been viewed as a useful predictor for seasonal monsoon prediction. In this study, observations and model simulations are used to demonstrate a bridging role of the winter snow anomaly over northern China and southern Mongolia (NCSM) in the relationship between the East Asian winter monsoon (EAWM) and the East Asian summer monsoon (EASM). Enhanced snow in NCSM results in local surface and tropospheric cooling, strengthening the EAWM through cold-air intrusion induced by northerly wind anomalies. In turn, the stronger EAWM provides a favorable condition for enhanced snowfall over East Asia to the south, indicating an active snow–EAWM interaction. The continental cooling could be maintained until summer due to the memory effect of snowmelt and moistening as well as the snow–monsoon interaction in the spring, causing changes in the meridional temperature gradient and associated upper-level westerlies in the summer. The interaction between the strengthened westerlies over the northern Tibetan Plateau and the topography of the plateau could lead to anomalous downstream convergence and compensating divergence to the south. Therefore, anomalous cyclonic circulation and increased rainfall occur over northeastern China and the Korean Peninsula, but anticyclonic circulation and decreased rainfall appear over the subtropical East Asia–Pacific region. Moreover, limited analysis shows that, compared to sea surface temperature feedback, the direct impact of snow anomaly on the EAWM–EASM connection seems more important.

Open access
Kaiqiang Deng, Song Yang, Mingfang Ting, Chundi Hu, and Mengmeng Lu

Abstract

The mid-Pacific trough (MPT), occurring in the upper troposphere during boreal summer, acts as an atmospheric bridge connecting the climate variations over Asia, the Pacific, and North America. The first (second) mode of empirical orthogonal function analysis of the MPT, which accounts for 20.3% (13.4%) of the total variance, reflects a change in its intensity on the southwestern (northeastern) portion of the trough. Both modes are significantly correlated with the variability of tropical Pacific sea surface temperature (SST). Moreover, the first mode is affected by Atlantic SST via planetary waves that originate from the North Atlantic and propagate eastward across the Eurasian continent, and the second mode is influenced by the Arctic sea ice near the Bering Strait by triggering an equatorward wave train over the northeast Pacific.

A stronger MPT shown in the first mode is significantly linked to drier and warmer conditions in the Yangtze River basin, southern Japan, and the northern United States and wetter conditions in South Asia and northern China, while a stronger MPT shown in the second mode is associated with a drier and warmer southwestern United States. In addition, an intensified MPT (no matter whether in the southwestern or the northeastern portion) corresponds to more tropical cyclones (TCs) over the western North Pacific (WNP) and fewer TCs over the eastern Pacific (EP) in summer, which is associated with the MPT-induced ascending and descending motions over the WNP and the EP, respectively.

Open access
Qin Wen, Kristofer Döös, Zhengyao Lu, Zixuan Han, and Haijun Yang

ABSTRACT

The role of the Tibetan Plateau (TP) in El Niño–Southern Oscillation (ENSO) variability is investigated using coupled model experiments with different topography setups. Removing the TP results in weakened trade winds in the tropical Pacific, an eastward shift of atmospheric convection center, a shallower mixed layer in the equatorial Pacific, and a flattened equatorial thermocline, which leads to an El Niño–like sea surface temperature (SST) response. In association with these mean climate changes in the tropical atmosphere–ocean system, the ENSO variability exhibits a much stronger amplitude in the world without the TP. Detailed diagnoses reveal that in the absence of the TP, both thermocline feedback in the eastern equatorial Pacific and Ekman pumping feedback in the central-eastern equatorial Pacific are enhanced substantially, leading to stronger ENSO variability. The changes of these two feedbacks are caused by the eastward shift of the atmospheric convection center and enhanced ocean sensitivity; the latter is due to the shallower mixed layer and flattened thermocline. This study suggests that the presence of the TP may be of fundamental importance for modern-day tropical climate variability; namely, the TP may have played a role in suppressing ENSO variability.

Open access
Mengmeng Lu, Song Yang, Junbin Wang, Yuting Wu, and Xiaolong Jia

Abstract

The thermal effect of the entire Tibetan Plateau (TP) tends to strengthen the South Asian summer monsoon (SASM); however, how does this monsoon component respond to the thermal conditions of different TP domains? How do the thermal conditions of the entire TP influence other monsoons, including the East Asian summer monsoon (EASM) and the Southeast Asian summer monsoon (SEASM)? These questions are addressed by conducting an experiment with the CESM, which is forced by reducing the surface albedo over the plateau by half, from a TP-averaged 0.20 to 0.10, from May to September, and similar experiments for different TP domains. Both observational and model results show that the entire TP heating intensifies the large-scale Asian monsoon, the SASM, and the EASM but surprisingly weakens the SEASM. It is also surprising that the TP heating exerts a stronger effect on the EASM than on the SASM. The southern TP (south of 35°N) does not show the strongest impact on the SASM in comparison with other TP domains, and it exerts the weakest impact on the EASM, which is most strongly influenced by the thermal effect of the eastern (east of 90°E) and northern TP. The western TP weakens the SEASM (as do the other domains), and it strengthens other monsoon components. The thermal conditions of the southern and eastern TP are accompanied by signals of tropical atmospheric response at relatively broader spatial scales, whereas those of the northern TP more apparently lead to a significant wave train extending eastward from the TP to western Eurasia over the higher latitudes.

Open access
Sijia Lou, Yang Yang, Hailong Wang, Jian Lu, Steven J. Smith, Fukai Liu, and Philip J. Rasch

ABSTRACT

El Niño–Southern Oscillation (ENSO) is the leading mode of Earth’s climate variability at interannual time scales with profound ecological and societal impacts, and it is projected to intensify in many climate models as the climate warms under the forcing of increasing CO2 concentration. Since the preindustrial era, black carbon (BC) emissions have substantially increased in the Northern Hemisphere. But how BC aerosol forcing may influence the occurrence of the extreme ENSO events has rarely been investigated. In this study, using simulations of a global climate model, we show that increases in BC emissions from both the midlatitudes and Arctic weaken latitudinal temperature gradients and northward heat transport, decrease tropical energy divergence, and increase sea surface temperature over the tropical oceans, with a surprising consequential increase in the frequency of extreme ENSO events. A corollary of this study is that reducing BC emissions might serve to mitigate the possible increasing frequency of extreme ENSO events under greenhouse warming, if the modeling result can be translated into the climate in reality.

Open access
Robert Lund, Xiaolan L. Wang, Qi Qi Lu, Jaxk Reeves, Colin Gallagher, and Yang Feng

Abstract

Undocumented changepoints (inhomogeneities) are ubiquitous features of climatic time series. Level shifts in time series caused by changepoints confound many inference problems and are very important data features. Tests for undocumented changepoints from models that have independent and identically distributed errors are by now well understood. However, most climate series exhibit serial autocorrelation. Monthly, daily, or hourly series may also have periodic mean structures. This article develops a test for undocumented changepoints for periodic and autocorrelated time series. Classical changepoint tests based on sums of squared errors are modified to take into account series autocorrelations and periodicities. The methods are applied in the analyses of two climate series.

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