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Renguang Wu

Abstract

Analysis of observations shows that in-phase transitions from the Indian summer monsoon (ISM) to the Australian summer monsoon (ASM) have occurred both in El Niño–Southern Oscillation (ENSO) and non-ENSO years. The present study investigates possible roles of the Indian Ocean in the in-phase ISM-to-ASM transitions. It is shown that an anomalous ISM leads to sea surface temperature (SST) anomalies in the tropical Indian Ocean through wind–evaporation effects. The resultant Indian Ocean SST anomalies induce an anomalous ASM of the same sign as the ISM through an anomalous east–west circulation over the eastern Indian Ocean and the Maritime Continent–northern Australia. The results indicate that the in-phase ISM-to-ASM transitions in non-ENSO years can be accomplished through monsoon–Indian Ocean interactions. The results of observational analysis are confirmed with numerical model experiments.

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Renguang Wu

Abstract

The present study investigates processes for out-of-phase transitions from the Australian summer monsoon (ASM) to the Indian summer monsoon (ISM). Two types of out-of-phase ASM-to-ISM transitions have been identified, depending on the evolution of the Pacific El Niño–Southern Oscillation (ENSO) events. The first type of transition is accompanied by a phase switch of ENSO in boreal spring to early summer. In the second type of transition, ENSO maintains its phase through boreal summer. The direct ENSO forcing plays a primary role for the first type of out-of-phase ASM-to-ISM transition, with complementary roles from the north Indian Ocean sea surface temperature (SST) anomalies that are partly induced by ENSO. The second type of out-of-phase ASM-to-ISM transition involves air–sea interaction processes in the tropical Indian Ocean that generate the north Indian Ocean SST anomalies and contribute to the monsoon transition. The initiation of tropical Indian Ocean air–sea interaction is closely related to ENSO in observations, but could also occur without ENSO according to a coupled general circulation model simulation. Results of numerical simulations substantiate the role of the Indian Ocean air–sea interaction in the out-of-phase ASM-to-ISM transition.

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Shangfeng Chen and Renguang Wu

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This study investigates interdecadal changes in the relationship between interannual variations of boreal spring sea surface temperature (SST) in the North Atlantic and surface air temperature (SAT) over the mid-to-high latitudes of Eurasia during 1948–2014. Analyses show that the connection between the spring North Atlantic tripole SST anomaly pattern and the Eurasian SAT anomalies has experienced marked interdecadal shifts around the early 1970s and mid-1990s. The connection is strong during 1954–72 and 1996–2014 but weak during 1973–91. A diagnosis indicates that interdecadal changes in the connection between the North Atlantic SST and Eurasian SAT variations are associated with changes in atmospheric circulation anomalies over Eurasia induced by the North Atlantic tripole SST anomaly pattern. Further analyses suggest that changes in atmospheric circulation anomalies over Eurasia are related to changes in the position of atmospheric heating anomalies over the North Atlantic, which may be due to the change in mean SST. Marked atmospheric heating anomalies appear over the tropical western North Atlantic during 1954–72 and 1996–2014 but over the subtropical central-eastern North Atlantic during 1973–91. Barotropic model experiments confirm that different background flows may also contribute to changes in anomalous atmospheric circulation over Eurasia.

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Renguang Wu and Bin Wang

Abstract

Using station rainfall data and the NCEP–NCAR reanalysis, the authors investigate changes in the interannual relationship between the east Asian summer monsoon (EASM) and El Niño–Southern Oscillation (ENSO) in the late 1970s, concurrent with the Pacific climate shift. The present study focuses on decaying phases of ENSO because changes in developing phases of ENSO are less significant. Remarkable changes are found in the summer rainfall anomaly in northern China and Japan. From pre- to postshift period, the summer rainfall anomaly in eastern north China during decaying phases of El Niño changed from above to below normal, whereas that in central Japan changed from negative to normal. Consistent with this, the barotropic anticyclonic anomaly over the Japan Sea changed to cyclonic; the associated anomalous winds changed from southerly to northerly over the Yellow Sea–northeastern China and from northeasterly to northwesterly over central Japan.

The change in the ENSO–related east Asian summer circulation anomaly is attributed to changes in the location and intensity of anomalous convection over the western North Pacific (WNP) and India. After the late 1970s, the WNP convection anomaly is enhanced and shifted to higher latitudes due to increased summer mean SST in the Philippine Sea. This induces an eastward shift of an anomalous low pressure from east Asia to the North Pacific along 30°–45°N during decaying phases of El Niño. Thus, anomalous winds over northeastern China and Korea switch from southeasterly to northeasterly. Before the late 1970s, an anomalous barotropic anticyclone develops over east Asia and anomalous southerlies prevail over northeastern China during decaying phases of El Niño. This may relate to anomalous Indian convection through a zonal wave pattern along 30°–50°N. After the late 1970s, anomalous Indian convection weakens, which reduces the impact of the Indian convection on the EASM.

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Bangliang Yan and Renguang Wu

Abstract

The mature phases of El Niño events show a strong tendency of locking to the end of the calendar year. The roles of seasonal variations of the basic state and the relative contributions of individual components of the basic state in this phase locking are investigated using the Zebiak–Cane model. It is shown that seasonal variations of the mean state from July to November have a positive contribution and those from December to June have a negative contribution to preexisting warm SST anomalies. Among the basic-state parameters, the sea surface temperature (SST) is a major factor for the locking of the El Niño mature phase through the anomalous advection of the mean SST gradient. This result differs from previous studies that attribute the El Niño phase locking mainly to seasonal changes in the mean wind divergence. The present result indicates the importance of a proper simulation of mean SST and its seasonal evolution for the simulation of El Niño phase locking in coupled models. Further experiments show that the phase locking of the El Niño mature phase cannot be explained by a balance between the warming trend due to downwelling Kelvin waves and the cooling trend due to upwelling Rossby waves.

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Renguang Wu and Shangfeng Chen

Abstract

Surface air temperature (SAT) anomalies tend to persist from winter to the following spring over the mid- to high latitudes of Eurasia. The present study compares two distinct cases of Eurasian SAT anomaly evolution and investigates the reasons for the persistence of continental-scale mid- to high-latitude Eurasian SAT anomalies from winter to following spring (termed persistent cases). The persisting SAT anomalies are closely associated with the sustenance of large-scale atmospheric circulation anomaly pattern over the North Atlantic and Eurasia, featuring a combination of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and the Scandinavian pattern, from winter to spring. The combined circulation anomalies result in SAT warming over most of mid- to high-latitude Eurasia via anomalous wind-induced temperature advection. The sustenance of atmospheric circulation anomaly pattern is related to the maintenance of the North Atlantic triple sea surface temperature (SST) anomaly pattern due to air–sea interaction processes. The Barents Sea ice anomalies, which form in winter and increase in spring, also partly contribute to the sustenance of atmospheric circulation anomalies via modulating thermal state of the lower troposphere. In the cases that notable SAT warming (cooling) in winter is replaced by pronounced SAT cooling (warming) in the subsequent spring—termed reverse cases—the North Atlantic SST anomalies become small and the Greenland Sea ice change is a response to atmospheric change in spring. Without the support of lower boundary forcing, the atmospheric circulation anomaly pattern experiences a reverse in the spatial distribution from winter to spring likely due to internal atmospheric processes.

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Renguang Wu and Wenting Hu

Abstract

The period from April to June is the time of transition from spring to summer over the north Indian Ocean and the South China Sea. Analysis shows that precipitation anomaly changes from April to June may indicate summer (June–August) mean precipitation anomalies over the South China Sea and the Arabian Sea. This study documents and compares the evolution of precipitation, surface wind, and sea surface temperature (SST) anomalies during the spring to summer transition corresponding to April–June precipitation anomaly changes and April–June mean precipitation anomalies over the South China Sea and the Arabian Sea. Over the South China Sea, a clear signal of local air–sea interaction is identified corresponding to the precipitation anomaly change, as indicated by a sequence of less precipitation, higher SST, more precipitation, and lower SST. In contrast, the mean precipitation anomaly features a response to remote SST forcing and a local forcing of atmosphere on the ocean. The evolution of surface heat flux anomalies supports the air–sea interaction over the South China Sea during the transition season. Over the Arabian Sea, local SST forcing contributes to both precipitation anomaly changes and mean precipitation anomalies through modulating atmospheric stability. A local negative feedback of atmosphere on SST is observed in the Arabian Sea as in the South China Sea. The surface heat fluxes make a large contribution to local SST change before May in the South China Sea but a small one in the Arabian Sea. Surface heat fluxes are important for local SST change after May in both the South China Sea and the Arabian Sea.

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Zhuoqi He and Renguang Wu

Abstract

The observations show that the covariability between the western North Pacific (WNP) and the South China Sea (SCS) summer rainfall has experienced an obvious weakening since the early 2000s. During the period 1982–2003, the combined north Indian Ocean (NIO), central North Pacific (CNP), and central equatorial Pacific (CEP) sea surface temperature (SST) forcing results in a high coherence between the WNP and SCS summer rainfall variations via a zonally elongated anomalous lower-level cyclone over the western Pacific. During the period 2004–16, the Indian Ocean SST contribution is largely weakened, and the WNP rainfall variability is dominated by the enhanced Pacific SST forcing with an eastward retreated lower-level wind and rainfall anomalies, whereas the SCS rainfall variability is mainly associated with local air–sea interaction processes. The results obtained from observational analysis are supported by numerical experiments with atmospheric and coupled general circulation models. The change in the coherence of interannual summer rainfall variability over the WNP and SCS has important implications for regional climate prediction in South and East Asia.

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Lei Song and Renguang Wu

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A strong cold event hit eastern China around 24 January 2016 with surface air temperature reaching more than 10°C below the climatological mean in most regions of eastern China south of 40°N. A total of 37 strong cold events similar to the January 2016 event with temperature anomalies over eastern China exceeding −5°C have been identified during the winters from 1979/80 to 2015/16. A comparative analysis of events with surface temperature anomalies of the same intensity but limited to north of 40°N indicates that the southward invasion of cold air to eastern China south of 40°N is related to two factors. One is the latitudinal location of the upper-level wave train, the surface Siberian high, and the midtropospheric East Asian trough over the mid- to high-latitude Eurasian continent. The other is a subtropical upper-level wave train emanating from the midlatitude North Atlantic. The emergence of the subtropical wave train is related to the positive phase of the North Atlantic Oscillation (NAO). When the mid- to high-latitude wave train is located too far northward and the subtropical wave train induces an anomalous midtropospheric high over southern China, the East Asian trough does not extend southwestward and the Siberian high does not expand southeastward. In such a case, the cold air mainly affects northeastern China and northern Japan.

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Wenting Hu and Renguang Wu

Abstract

The study analyzes precipitation variability and related air–sea interaction processes over the South China Sea (SCS) and tropical Indian Ocean (TIO) during the spring-to-summer transition season. It is found that physical processes are very different for the variations of seasonal mean and the monthly departures from the seasonal mean. Corresponding to the seasonal mean anomaly, remote forcing from the equatorial Pacific is a major factor for the precipitation variability with a prominent negative feedback of the atmosphere on the ocean. However, from the viewpoint of the monthly anomaly departure from the seasonal mean, a pronounced local coupled air–sea interaction is detected in both the SCS and TIO that features a sequential process of less rainfall, higher sea surface temperature (SST), more rainfall, and lower SST. The evolution of the SST tendency is well coordinated with that of net surface heat flux in the SCS and TIO. During the transition season, shortwave radiation is a dominant term for the SST change in the SCS, whereas both shortwave radiation and latent heat flux are responsible for the SST change in the TIO. The local air–sea relationship shows an obvious spatiotemporal variation during the transition season. Furthermore, the SST anomaly departure in the TIO (SCS) in April (May) could be considered as an indicator for local precipitation anomaly departure in May (June).

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