ALBERT

Description: It has been argued that fewer cold extremes will be expected to occur over most midlatitude areas, because of anthropogenic-induced global warming. However, East Asia repeatedly suffered from unexpected cold spells during the winter of 2015/16, and the low surface air temperature (SAT) during 21–25 January 2016 broke the previous calendar record from 1961. We hypothesize that cold extremes such as these occur because of Arctic amplification (AA) of global warming. To test this hypothesis, we analyzed the changes of SAT variability in the winter season over East Asia. Our results show that the SAT variability (measured by the standard deviation of the winter season daily mean SAT) over East Asia has significantly increased in the era of AA during 1988/89–2015/16 and exhibits a polarization between warm and cold extremes, popularly dubbed as “weather whiplash.” This phenomenon is driven by both the thermodynamic effects of global warming and the dynamic effects of AA. Global warming favors a rising SAT and more frequent warm extremes. The AA phenomenon strengthens the wavy components of midlatitude circulation, leading to more frequent blockings over the Ural region and a stronger Siberian high in north Asia. This dynamic effect of AA enhances the intrusion of cold air from Siberia into East Asia and causes cold extremes. Because there is a comparable increase of frequency of both warm and cold extremes, the SAT variability significantly increases in unison with AA, but little change is observed in the seasonal mean SAT of East Asia. This implies increased risks of both cold and warm extremes over East Asia exist even during global warming.

Description: In this study, it was found that the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are shown to be the two major drivers of the interdecadal variability of summer rainfall over East Asia. The first leading mode (PC1) of this interdecadal variability—associated with an in-phase variation of rainfall anomalies along the Yangtze River valley and Huanghe–Huaihe River valley in China—is attributed to the PDO, while the second leading mode (PC2)—associated with seesawlike rainfall anomalies between the Yangtze River valley and Huanghe–Huaihe River valley—is attributed to the AMO. The AMO teleconnects its influence to the East Asian region, and beyond, through a circumglobal stationary baroclinic wave train extending from the Atlantic Ocean, through the Eurasian continent, and extending to North America. The AMO also altered the nature of the PDO through this atmospheric teleconnection, resulting in the occurrence of a different PDO pattern (“pseudo-PDO”) between the 1960s and 2010s. The pseudo-PDO has a different anomalous SST pattern in both the tropical and midlatitude Pacific compared to the conventional PDO. The pseudo-PDO causes a distinct atmospheric response in East Asia leading to an opposite relationship with the PC1 compared to the conventional PDO, thus leading to a change in the direction of the influence of the PDO on PC1 between the 1880s–1950s and the 1960s–2010s.

Description: Defining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979–2006 period. The existing indices can be classified into five categories: the east–west thermal contrast, north–south thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Niño, while the first category reflects the second mode that corresponds to the developing El Niño. The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring. The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon.

Description: In this paper, radiosonde temperature time series (RTT) from 1958 to 2005 collected by the 116-station Chinese radiosonde network are examined. Quality control and homogenization are used to obtain a reliable RTT. The homogenization results revealed significant discontinuities in the RTT. Analysis suggested that 70% data availability is the minimum data requirement (MDR) for these RTTs. A new dataset is built by meeting this MDR, which reduced the number of potential stations from 116 to 92. Analysis on this dataset reveals that warming trends in the troposphere and cooling trends in the stratosphere were weakened by reducing the stations. Averaged RTT trends for China were generally consistent with those of global scale, but with some discrepancies. During 1958–2005, averaged temperatures in China tended to decrease in the lower stratosphere and upper troposphere, in contrast to warming trends in the mid- and lower troposphere. The trends varied with two different subperiods. For 1958–78, cooling trends in the entire atmosphere were similar to trends at the global scale. For 1979–2005, warming occurred in the lower troposphere, with the amplitude of the warming tending to weaken with increases in altitude and shifting to a cooling trend above 400 hPa. Seasonal trend structures suggest that warming in the lower troposphere is attributable to temperature increases in December–February (DJF); cooling in the upper troposphere and stratosphere was found mainly in June–August (JJA). Unlike with results of a larger spatial scale, a robust cooling layer was found around 300 hPa.

Description: Resolution of a complex Hermitian matrix derived from monthly mean 850-hPa wind fields during the summer season (June–August) from 1968 to 2004 revealed four different modes of East Asian summer monsoon (EASM) variability. The leading EASM mode, accounting for 19.6% of the variance, is characterized by two different modes (M11 and M12) or their combination. Both portray a closed cyclonic or anticyclonic circulation anomaly over the western North Pacific (WNP), South China Sea (SCS), and southeastern China; corresponding anomalous geopotential height fields show a wave train structure from the WNP across Japan, the Okhotsk Sea, and Alaska to North America. Thus, the leading EASM mode characterizes the teleconnection pattern of the WNP-EASM. The correlation between M11 (M12) and the dynamic index for the WNP-EASM is 0.85 (0.51). M11 has leading spectral peaks at 15 and 3 yr, whereas M12 displays a predominant peak at 2 yr. It is found that M11 has interdecadal variations, with the transition years being circa 1973 and 1989, respectively. M11 is closely related to air–sea interactions in the SCS and the northwestern Pacific, and its association with the convective heat source over the northwestern Pacific is secondary. In contrast, M12 is closely related to the tropical convective heat source rather than tropical western Pacific sea surface temperature (SST). The second EASM mode, accounting for 12.8% of the variance, is identified and characterized by two distinct and alternating modes or their linear combination (M21 and M22). One mode (M21) closely relates to the dual blocking high pattern detected in anomalous sea level pressure (SLP) and 500-hPa geopotential heights over the Ural Mountains and the Okhotsk Sea. The other (M22) corresponds to a dipole blocking anomaly in anomalous SLP and geopotential heights, with opposing anomalous centers in the south of Japan and the Korean peninsula, and the area between Lake Baikal and the Okhotsk Sea. M22 shows significant correlations with summer mean rainfall in southern and southeastern China. Thus, a single index of EASM is inappropriate for investigating and predicting the EASM.

Description: Observational evidence indicates that the correlation between Tibetan Plateau (TP) winter snow and East Asian (EA) summer precipitation changed in the late 1990s. During the period 1979–99, the positive correlation between the TP winter snow and the summer precipitation along the Yangtze River valley (YRV) and southern Japan was disrupted by the decadal climate shift. In contrast, the summer precipitation over the Huaihe River valley (HRV) and the Korean Peninsula showed a strong positive correlation with the preceding winter snow over the TP during the period 2000–11. The radiosonde temperature measurements over the TP show a pronounced warming since the late 1990s. This warming is associated with the significant increase in surface sensible heat flux and longwave radiation into atmosphere. The latter is closely related to the decrease of surface albedo and the soil hydrological effect of melting snow due to the decadal decrease in the preceding winter and spring snow over the TP. The TP warming induced by the decrease in winter snow, together with the cooling of the sea surface temperature in the tropical central and eastern Pacific, intensifies the land–sea thermal contrast in the subsequent spring and summer over EA, thus causing a northward advance of the EA summer monsoon. Accompanying the northward migration of the summer monsoon, the summer precipitation belt over EA shifts northward. Consequently, the high summer precipitation region over EA correlating with the preceding winter snow over the TP has shifted northward from the YRV and southern Japan to the HRV and the Korean Peninsula since the late 1990s.

Description: Studies on the nonlinear natures of the spatiotemporal structure of the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM) in the context of nonlinear interaction among the North Atlantic Oscillation (NAO), the Pacific–North American pattern (PNA), and the stratospheric polar vortex (SPV) are performed. The non-Gaussianity of the multivariate probability density function (PDF) in the phase space spanned by their indices is examined first. Five local maxima potentially related to circulation regimes are identified from the so-called angular PDF. One opposite pair of these regimes is found to correspond to the positive and negative phases of the AO/NAM. Since the authors are not sure that, due to uncertainty as suggested by statistical tests, some of the above regimes are non-Gaussian, the nonlinearity of phase-space tendency is employed as an assistant measure to identify them as nonlinear modes. It seems phase-space tendency traditionally estimated from time difference failed to be effective because of its dependence on Δt. To overcome this drawback a low-order stochastic dynamical model is established empirically from the indices. The investigation on the basic deterministic dynamics of this model suggests that the existence of regimes, such as those associated with the AO/NAM, can primarily be explained by its nonlinear deterministic part. However, two problems still remain unsolved: 1) one of the local maxima was almost not identified and 2) life cycles of the basic deterministic dynamics are too long to be related to the low-frequency variability. By introducing a multilevel approach of modeling, further insight into the residual noise of the above stochastic model can address these two issues quite well.

Description: The Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM) is attributed to wave–mean flow interaction over the extratropical region of the Northern Hemisphere. This wave–mean flow interaction is closely related to three atmospheric centers of action, corresponding to three regional oscillations: the NAO, the PNA, and the stratosphere polar vortex (SPV), respectively. It is then natural to infer that local wave–mean flow interactions at these three centers of action are dynamically coupled to each other and can thus explain the main aspects of the three-dimensional coherent structure of the annular mode, which also provides a possible way to understand how the local NAO–PNA–SPV perspective and the hemispheric AO/NAM perspective are interrelated. By using a linear stochastic model of coupled oscillators, this study suggests that two coupling modes among the PNA, NAO, and SPV are related to the two-dimensional pattern in sea level pressure of the AO. Although both of them may contribute to the AO/NAM, only one is related to the three-dimensional equivalent barotropic structure of the NAM, while the other one is mainly restricted to the troposphere. So the equivalent barotropic structure of the NAM, as usually revealed by the regression of the zonal wind against the AO index, is the manifestation of just one coupling mode. Another coupled mode is a baroclinic mode that resembles the NAM only in the troposphere. However, this similarity in spatial structures does not imply that the total variability of the AO/NAM index can be explained by those of the NAO–PNA–SPV or their coupling modes, because of the existence of the variability that may contribute to the AO/NAM, produced outside of these three regions. It is estimated that the coupling modes can jointly explain 44% of the variance of the AO/NAM index.