Renguang Wu

The present study documents a pronounced interdecadal change in summer rainfall over southern China around 1992/93 and explores the plausible reasons for this change. The summer rainfall is persistently below normal during 1980–92 and above normal during 1993–2002. Coherent changes in atmospheric circulation are identified over East Asia and the South China Sea (SCS)–western North Pacific (WNP). The increase in rainfall is accompanied by an increase in lower-level convergence, midtropospheric ascent, and upper-level divergence over southern China. The changes in lower-level winds feature two anomalous anticyclones: one over the SCS–subtropical WNP, and the other over north China–Mongolia. The outflows from the two anomalous anticyclones converge over southern China, leading to anomalous moisture convergence, enhanced ascent, and increased rainfall. The development of the northern anticyclone is related to an increase in the Tibetan Plateau snow cover in the preceding winter–spring that leads to a contrast in temperature change between the plateau and the surrounding regions. The relatively small temperature change over the plateau, coupled with increases in temperature to the west and the east, leads to an increase in surface pressure extending northward from the plateau. The development of the southern anticyclone is related to an increase in sea surface temperature in the equatorial Indian Ocean that enhances lower-level convergence and ascent. The accompanying upper-level divergent flows from the tropical Indian Ocean to the SCS–WNP lead to the development of anomalous descent and lower-level anomalous anticyclone over the SCS–WNP.

Observations indicate that equatorial South American rainfall anomalies in boreal winter are preceded by the same sign Central American rainfall anomalies in boreal summer and tend to be succeeded by opposite sign Central American rainfall anomalies in the following summer. The in‐phase relationship from summer to winter rainfall is attributed to direct effects of El Niño‐Southern Oscillation (ENSO). Anomalous convection associated with warm ENSO events induces anomalous descent and suppresses precipitation along the convection region that migrates southeastward from Central America in boreal summer to tropical South America in boreal winter. The out‐of‐phase relationship from winter to summer rainfall is related to tropical North Atlantic sea surface temperature (SST) anomalies induced by wind‐evaporation and cloud‐radiation changes in response to ENSO‐generated anomalous convection over tropical South America. In years when ENSO switches its phase from winter to summer, the direct effects of ENSO can also lead to the out‐of‐phase relationship.

The present study performs a mixed‐layer heat budget analysis and a hybrid coupled model experiment to understand the roles of surface heat fluxes and oceanic processes in the tropical Indian Ocean asymmetric mode in boreal spring. The heat budget analysis shows that the surface heat fluxes play a dominant role in sea surface temperature (SST) changes in most of the tropical Indian Ocean. In the southwest tropical Indian Ocean, the SST warming during October–November that initiates the cross‐equatorial SST gradient for the development of the asymmetric mode is primarily due to surface latent heat flux, with secondary contribution from meridional and vertical advection. The SST warming around April that sustain the cross‐equatorial SST gradient is also mainly due to latent heat flux. The ocean downwelling is large during late winter and boreal spring, which prevents the weakening of the SST anomalies. The results of a numerical experiment with a hybrid coupled model in which an atmospheric general circulation model is coupled to a slab ocean model in the tropical Indian Ocean demonstrates that the asymmetric mode can develop without ocean dynamics. Thus, the thermodynamic air‐sea interactions play an essential role for the development of the tropical Indian Ocean asymmetric mode.

The present study documents the relationship between the surface heat flux and sea surface temperature (SST) and estimates the contribution of different processes to SST changes in the midlatitude North Pacific. It is found that the atmosphere‐ocean relationship displays pronounced differences between winter and summer and between the western and central basins of the North Pacific. In the western basin, the shortwave radiation change associated with a positive cloud‐SST feedback is a dominant factor for the development of SST anomalies in boreal summer. In boreal winter, surface latent heat flux anomalies develop in response to SST and in turn damp the SST anomalies. The meridional advection induced by anomalous geostrophic currents near the coast and by anomalous Ekman currents away from the coast has important contributions to the SST changes. In the central basin, the latent heat flux change is a dominant factor for the development of SST anomalies, and the shortwave radiation mainly responds passively to SST anomalies in boreal summer. Both latent heat flux and meridional advection contribute to the SST changes in boreal winter. The wind changes over the North Pacific play an important role for the SST changes both in winter and summer through their impacts on cloud, surface wind speed, advection of atmospheric boundary layer moisture, and Ekman transport.

This study documents the variability of noise in the equatorial Pacific associated with the El Niño and La Niña events based on the ensemble retrospective forecasts of the Climate Forecast System. It is found that the noise (measured by the ensemble spread) in the western equatorial Pacific zonal wind stress is enhanced around and before the peak of El Niño events and weakened around 2–3 months after the peak of El Niño events. The change in the wind stress noise is communicated to the thermocline depth in the eastern equatorial Pacific with about 1 month time lag. The eastern equatorial Pacific SST noise, however, decreases during the decay stages of El Niño events while the corresponding noise in the surface zonal wind and heat flux increases. This decrease in the SST noise is related to the weakening of the SST front and the suppression of the tropical instability waves along the flanks of the equatorial Pacific cold tongue that is associated with the SST warming due to El Niño events. Similar relations are seen during La Niña events except that the changes are in opposite sense. As such, the signal‐to‐noise ratio and, thus, the predictability for the eastern equatorial Pacific SST is relatively high during warm events compared to cold events.

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.

The present study documents the so-called spring prediction and persistence barriers in association with El Niño–Southern Oscillation (ENSO) in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) retrospective forecasts. It is found that the spring prediction and persistence barriers in the eastern equatorial Pacific sea surface temperature (SST) are preceded by a boreal winter barrier in the western equatorial Pacific zonal wind stress. The time of the persistence barrier is closely related to the time of the ENSO phase transition, but may differ from the time of the lowest variance. The seasonal change of the signal-to-noise ratio cannot explain the persistence barrier. While the noise may lead to a drop of skill around boreal spring in the western equatorial Pacific zonal wind stress, its impacts on the skill of eastern equatorial Pacific SST is small. The equatorial Pacific zonal winds display an excessive response to ENSO-related SST anomalies, which leads ... Abstract The present study documents the so-called spring prediction and persistence barriers in association with El Niño–Southern Oscillation (ENSO) in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) retrospective forecasts. It is found that the spring prediction and persistence barriers in the eastern equatorial Pacific sea surface temperature (SST) are preceded by a boreal winter barrier in the western equatorial Pacific zonal wind stress. The time of the persistence barrier is closely related to the time of the ENSO phase transition, but may differ from the time of the lowest variance. The seasonal change of the signal-to-noise ratio cannot explain the persistence barrier. While the noise may lead to a drop of skill around boreal spring in the western equatorial Pacific zonal wind stress, its impacts on the skill of eastern equatorial Pacific SST is small. The equatorial Pacific zonal winds display an excessive response to ENSO-related SST anomalies, which leads ...