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Jinqing Zuo, Hong-Li Ren, and Weijing Li

Abstract

In the boreal winter, the Arctic Oscillation (AO) evidently acts to influence surface air temperature (SAT) anomalies in China. This study reveals a large intraseasonal variation in the relationship between the winter AO and southern China SAT anomalies. Specifically, a weak in-phase relationship occurs in December, but a significant out-of-phase relationship occurs in January and February. The authors show that the linkage between the AO and southern China SAT anomalies strongly depends on the AO-associated changes in the Middle East jet stream (MEJS) and that such an AO–MEJS relationship is characterized by a significant difference between early and middle-to-late winter. In middle-to-late winter, the Azores center of high pressure anomalies in the positive AO phase usually extends eastward and yields a significantly anomalous upper-level convergence over the Mediterranean Sea, which can excite a Rossby wave train spanning the Arabian Sea and intensify the MEJS. In early winter, however, the Azores center of the AO is apparently shifted westward and is mainly confined to the Atlantic Ocean; in this case, the associated change in the MEJS is relatively weak. Both observational diagnoses and experiments based on a linearized barotropic model suggest that the MEJS is closely linked to the AO only when the latter generates considerable upper-level convergence anomalies over the Mediterranean Sea. Therefore, the different impacts of the AO on the MEJS and the southern China SAT anomalies between early and middle-to-late winter are primarily attributed to the large intraseasonal zonal migrations of the Azores center of the AO.

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Chi-Cherng Hong and Tim Li

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A record-breaking, long-persisting extreme cold anomaly (ECA) over Southeast Asia, accompanied by an intraseasonal convection over the Maritime Continent, is identified during the La Niña mature phase in February 2008. The cause of the ECA, in particular the role of the intraseasonal oscillation (ISO) and El Niño–Southern Oscillation (ENSO) on the ECA, is investigated by diagnosing observations and conducting numerical experiments.

The ECA is associated with an enhanced prolonged Siberian high (SH) and a persistent northerly anomaly over Southeast Asia. In contrast to conventional cold surges, which are characterized by a synoptic time scale (less than 10 days), the northerly anomaly associated with the ECA persisted for a month or so. The onset of the northerly anomaly is concurrent with a phase change of an ISO over Sumatra. Unlike the normal ISO that continues its eastward journey, the convection associated with this ISO stationed there during all of February 2008. Numerical experiments with an anomaly atmospheric GCM suggest that the ISO heating over the Maritime Continent is responsible for initiating and maintaining the northerly anomaly.

The westward progression of the La Niña is crucial for blocking the ISO. The circulation and SST anomalies associated with the La Niña moved westward at a speed of about 15° longitude per month. By early February, the suppressed convective anomaly had moved to the far western Pacific. The westward shift of the cold episode prevented the ISO from moving farther eastward. In addition to its blocking effect, the La Niña also enhanced the heating over the Maritime Continent through the anomalous Walker circulation. Therefore, it is the combined effect of the ISO and ENSO that maintained a prolonged positive heating anomaly, which resulted in a persistent northerly anomaly and thus the ECA.

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Run Wang and Hong-Li Ren

Abstract

El Niño–Southern Oscillation (ENSO) events exhibit a diversity of amplitudes, spatial patterns, and life cycles, with the main ENSO periods concentrated in the 3–7-yr [low-frequency (LF)] and 2–3-yr [quasi-biennial (QB)] bands. In this study, the spatiotemporal diversity of ENSO is quantitatively examined by extracting the two ENSO modes, namely, the LF and QB components of ENSO, from the traditional Niño-3.4 index and connecting them with the spatial types of ENSO. El Niño events can be regrouped as the QB-dominated central-Pacific ENSO-like (QB-CP), LF-dominated eastern-Pacific ENSO-like (LF-EP), and LF-dominated mixing (LF-mixing) types. La Niña events with vague spatial patterns can also have the same categorization. The QB-CP and LF-EP El Niño types both have a high-amplitude QB component. Meanwhile, the former is less affected by its powerless LF component, but the latter is controlled by its strong LF component. Ocean dynamics of the two El Niño types are distinct from each other. The thermocline feedback dominates the growth of the two El Niño types and contributes to the phase transition of the LF-EP type, while the zonal advective feedback is of increasing importance in the QB-CP El Niño and mainly contributes to the phase transitions of the two El Niño types. Additionally, the LF-mixing type with ambiguous spatial features and complex life cycles is distinguished from the other two types. These results indicate that the two ENSO modes coexist in the tropical Pacific air–sea system, and their combination with changing amplitude is the key to explaining the spatiotemporal diversity of ENSO.

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Yuntao Wei and Hong-Li Ren

Abstract

This study investigates modulation of El Niño–Southern Oscillation (ENSO) on the Madden–Julian oscillation (MJO) propagation during boreal winter. Results show that the spatiotemporal evolution of MJO manifests as a fast equatorially symmetric propagation from the Indian Ocean to the equatorial western Pacific (EWP) during El Niño, whereas the MJO during La Niña is very slow and tends to frequently “detour” via the southern Maritime Continent (MC). The westward group velocity of the MJO is also more significant during El Niño. Based on the dynamics-oriented diagnostics, it is found that, during El Niño, the much stronger leading suppressed convection over the EWP excites a significant front Walker cell, which further triggers a larger Kelvin wave easterly wind anomaly and premoistening and heating effects to the east. However, the equatorial Rossby wave to the west tends to decouple with the MJO convection. Both effects can result in fast MJO propagation. The opposite holds during La Niña. A column-integrated moisture budget analysis reveals that the sea surface temperature anomaly driving both the eastward and equatorward gradients of the low-frequency moisture anomaly during El Niño, as opposed to the westward and poleward gradients during La Niña, induces moist advection over the equatorial eastern MC–EWP region due to the intraseasonal wind anomaly and therefore enhances the zonal asymmetry of the moisture tendency, supporting fast propagation. The role of nonlinear advection by synoptic-scale Kelvin waves is also nonnegligible in distinguishing fast and slow MJO modes. This study emphasizes the crucial roles of dynamical wave feedback and moisture–convection feedback in modulating the MJO propagation by ENSO.

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Yu Nie, Hong-Li Ren, and Yang Zhang

Abstract

Considerable progress has been made in understanding the internal eddy–mean flow feedback in the subseasonal variability of the North Atlantic Oscillation (NAO) during winter. Using daily atmospheric and oceanic reanalysis data, this study highlights the role of extratropical air–sea interaction in the NAO variability during autumn when the daily sea surface temperature (SST) variability is more active and eddy–mean flow interactions are still relevant. Our analysis shows that a horseshoe-like SST tripolar pattern in the North Atlantic Ocean, marked by a cold anomaly in the Gulf Stream and two warm anomalies to the south of the Gulf Stream and off the western coast of northern Europe, can induce a quasi-barotropic NAO-like atmospheric response through eddy-mediated processes. An initial southwest–northeast tripolar geopotential anomaly in the North Atlantic forces this horseshoe-like SST anomaly tripole. Then the SST anomalies, through surface heat flux exchange, alter the spatial patterns of the lower-tropospheric temperature and thus baroclinicity anomalies, which are manifested as the midlatitude baroclinicity shifted poleward and reduced baroclinicity poleward of 70°N. In response to such changes of the lower-level baroclinicity, anomalous synoptic eddy generation, eddy kinetic energy, and eddy momentum forcing in the midlatitudes all shift poleward. Meanwhile, the 10–30-day low-frequency anticyclonic wave activities in the high latitudes decrease significantly. We illustrate that both the latitudinal displacement of midlatitude synoptic eddy activities and intensity variation of high-latitude low-frequency wave activities contribute to inducing the NAO-like anomalies.

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Hong-Li Ren and Fei-Fei Jin

Abstract

The El Niño–Southern Oscillation (ENSO) tends to behave arguably as two different “types” or “flavors” in recent decades. One is the canonical cold-tongue-type ENSO with major sea surface temperature anomalies (SSTA) positioned over the eastern Pacific. The other is a warm-pool-type ENSO with SSTA centered in the central Pacific near the edge of the warm pool. In this study, the basic features and main feedback processes of these two types of ENSO are examined. It is shown that the interannual variability of upper-ocean heat content exhibits recharge–discharge processes throughout the life cycles of both the cold tongue (CT) and warm pool (WP) ENSO types. Through a heat budget analysis with focus on the interannual frequency band, the authors further demonstrate that the thermocline feedback plays a dominant role in contributing to the growth and phase transitions of both ENSO types, whereas the zonal advective feedback contributes mainly to their phase transitions. The westward shift of the SSTA center of the WP ENSO and the presence of significant surface easterly wind anomalies over the far eastern equatorial Pacific during its mature warm phase are the two main factors that lead to a reduced positive feedback for the eastern Pacific SSTA. Nevertheless, both the WP and CT ENSO can be understood to a large extent by the recharge oscillator mechanism.

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Samantha Ferrett, Matthew Collins, and Hong-Li Ren

Abstract

The rate of damping of tropical Pacific sea surface temperature anomalies (SSTAs) associated with El Niño events by surface shortwave heat fluxes has significant biases in current coupled climate models [phase 5 of the Coupled Model Intercomparison Project (CMIP5)]. Of 33 CMIP5 models, 16 have shortwave feedbacks that are weakly negative in comparison to observations, or even positive, resulting in a tendency of amplification of SSTAs. Two biases in the cloud response to El Niño SSTAs are identified and linked to significant mean state biases in CMIP5 models. First, cool mean SST and reduced precipitation are linked to comparatively less cloud formation in the eastern equatorial Pacific during El Niño events, driven by a weakened atmospheric ascent response. Second, a spurious reduction of cloud driven by anomalous surface relative humidity during El Niño events is present in models with more stable eastern Pacific mean atmospheric conditions and more low cloud in the mean state. Both cloud response biases contribute to a weak negative shortwave feedback or a positive shortwave feedback that amplifies El Niño SSTAs. Differences between shortwave feedback in the coupled models and the corresponding atmosphere-only models (AMIP) are also linked to mean state differences, consistent with the biases found between different coupled models. Shortwave feedback bias can still persist in AMIP, as a result of persisting weak shortwave responses to anomalous cloud and weak cloud responses to atmospheric ascent. This indicates the importance of bias in the atmosphere component to coupled model feedback and mean state biases.

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Xiaowei Hong, Riyu Lu, and Shuanglin Li

Abstract

The meridional displacements of the upper-tropospheric westerly jet streams over both West Asia and East Asia (WJMD and EJMD, respectively) in summer manifest as the leading pattern of the zonal wind anomalies over their local domains on the interannual time scale. This study identifies a significant interannual relationship between these two leading patterns, tending to displace in the same meridional direction, either northward or southward. It is shown that the Silk Road pattern, which is a zonal teleconnection pattern along the Asian jet, is an important modulator for the WJMD–EJMD relationship. Another factor is the precipitation anomalies over a broad tropical region, including the Indian subcontinent, the northern Indian Ocean, and the western North Pacific Ocean. Enhanced or suppressed tropical precipitation respectively induces a northward or southward displacement of the primary body of the jet stream from West Asia to East Asia, which contributes to the in-phase WJMD–EJMD relationship, as suggested by both the observations and simple model results. The SST anomalies associated with the tropical precipitation and in-phase WJMD–EJMD relationship are also discussed. The WJMD–EJMD relationship may have an important implication for identifying and explaining the teleconnections of rainfall and temperature over the broad area from West Asia to East Asia, and from the tropics to the extratropics.

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Hong-Chang Ren, Jinqing Zuo, and Weijing Li

Abstract

The interannual variability of boreal summer sea surface temperature (SST) in the tropical Atlantic displays two dominant modes, the Atlantic zonal mode highlighting SST variations in the equatorial–southern tropical Atlantic (ESTA) region and the northern tropical Atlantic (NTA) mode focusing on SST fluctuations in the NTA region except in the Gulf of Guinea. Observational evidence indicates that both the boreal summer ESTA and NTA warming are accompanied by a pair of anomalous low-level anticyclones over the western tropical Pacific, and the NTA-related anticyclone is more obvious than the ESTA-related one. Both atmosphere-only and partially coupled experiments conducted with the Community Earth System Model version 1.2 support the observed NTA–Pacific teleconnection. In contrast, the ESTA-induced atmospheric circulation response is negligible over the tropical Pacific in the atmosphere-only experiments, and although the response becomes stronger in the partially coupled experiments, obvious differences still exist between the simulations and observation. The ESTA-induced atmospheric circulation response features an anomalous low-level cyclone over the western tropical Pacific in the partially coupled experiments, opposite to its observed counterpart. It is found that the ESTA warming coincides with significantly La Niña–like SST anomalies in the central–eastern equatorial Pacific, the influence of which on the tropical atmospheric circulation is opposite to that of the ESTA warming, and therefore contributes to difference between the ESTA-related simulations and observation. Moreover, the cold climatological mean SST in the ESTA region is unfavorable to enhancing the ESTA–Pacific teleconnection during boreal summer.

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Xiaowei Hong, Riyu Lu, and Shuanglin Li

Abstract

The Silk Road Pattern (SRP) is an upper-tropospheric teleconnection pattern along the Asian westerly jet in summer on the interannual time scale, and it exerts great influences on the climate of the Eurasian continent. Results in the present study indicate that the SRP exhibits considerable distinctions between early and late summers (i.e., 1 June–9 July and 10 July–31 August, respectively). The SRP is stronger and more geographically fixed in late summer in comparison with its counterpart in early summer. Furthermore, the SRP is closely connected with the summer North Atlantic Oscillation (SNAO) in late summer, but not in early summer. This closer connection in late summer is manifested clearly in the leading mode of upper-tropospheric meridional wind anomalies over the North Atlantic–Eurasian continent domain. The intensified SNAO–SRP relationship in late summer can be explained by the subseasonal change of the SNAO: albeit being a seesaw pattern common in both early and late summers, there is a shift of this pattern toward the northwest–southeast one in late summer from a north–south one in early summer. The southeastern pole of SNAO in late summer extends into the Eurasian continent, and efficiently triggers the SRP to propagate along the Asian jet. By contrast, the south pole of SNAO in early summer is confined over the North Atlantic and is thus less effective to trigger the SRP propagation.

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