Ruidan Chen

This study reveals that the summer marine heat waves (MHWs) in the South China Sea (SCS) could be classified into two spatial types, with warming centers over the central and northern SCS (denoted as C-MHW and N-MHW), respectively. The formation of C-MHW is related to a vigorous low-level anticyclonic anomaly centered in the northern SCS. Locally, the anticyclone is accompanied with anomalous sinking motion and northeasterly anomaly over the central SCS, which enhance insolation heating and inhibit evaporative cooling associated with the southwesterly monsoon. Meanwhile, the accompanied anticyclonic wind stress anomaly and reduced wind stress induce anomalous Ekman downwelling and weaken the climatological upwelling over the central SCS, contributing to the central warming center second to the surface heat flux. From a large-scale view, the SCS anticyclone is associated with the transition from El Niño to La Niña. In summer, the SCS anticyclone is favored by the zonal SSTA gradient over the Maritime Continent to the central Pacific via triggering anomalous easterlies extending to the central SCS, by the warming around the Maritime Continent via meridional circulation and by the Indian Ocean warming via Kelvin wave response and zonal circulation. Compared with C-MHW, N-MHW results from a weaker and smaller anticyclonic anomaly associated with weaker air–sea interaction. The anticyclone favors the northern warming center mainly via insolation heating. As for the tropical SSTA, persistent warming occurs around the Maritime Continent and the Indian Ocean but with a smaller amplitude compared with C-MHW; thus, the relevant air–sea interaction processes and the resultant SCS anticyclone are weaker.

This paper assesses the extended‐range forecast skills of the extreme heat events (EHEs) over South China based on three subseasonal‐to‐seasonal models (European Centre for Medium‐Range Weather Forecasts [ECMWF], National Centers for Environmental Prediction [NCEP], and China Meteorological Administration [CMA]). Overall, ECMWF has the best skill, NCEP the second and CMA the poorest. The predicting skills of EHEs depend on the predicting skills of relevant circulation. Cases studies (June 4–6, 1999, August 19–29, 2009, and August 3–5, 2010) show that the three models generally predict circulation anomalies weaker than observation, leading to the misses of some extreme heat days (EHDs). In these cases, ECMWF is able to well predict the influence of tropical circulation, capture the major characteristics of mid‐latitude circulation but with a slower propagating speed. NCEP could capture the main signals of tropical (mid‐latitude) circulation, but with slower propagating speed (slower propagating speed, deviated direction or more northward location). CMA might produce some EHDs but is derived from the circulation anomaly with the wrong origin or location. Therefore, ECMWF could predict the EHEs most accurately, NCEP could reasonably predict the formation of EHEs and tend to have more delayed predictions, while CMA has the poorest skill due to the false origins of anomalies. These results suggest potential ways to improve the current models' extended‐range forecast skills.

The summer daily maximum temperature (T_max) over South China features obvious quasi‐biweekly oscillation (QBWO) and its QBWO sources mainly include three types. The first type is influenced by tropical and mid–high‐latitude antiphase coupling QBWO. Successive suppressed and enhanced convection, accompanying with anticyclonic and cyclonic anomalies, propagate northward from the northwest of the Philippines to South China. At the peak warm phase, an anomalous high dominates over South China and causes significant warming. Meanwhile, an anomalous low associated with a southeastward‐propagating wave train occurs to the north and leading to abnormal cooling in situ. The second type originates from the mid–high latitudes. Before the warm phase, an anomalous high around the Ural Mountains triggers two wave trains propagating southeastward along the great circle route and eastward along the westerly jet, respectively, favouring an anomalous high over North–Northeast China. At the peak warm phase, the anomalous high extends southward, causing significant warming over South–Central China. The third type is influenced by tropical and mid–high‐latitude in‐phase coupling QBWO. The tropical anomalies are similar to the first type but the convection anomalies are centred to the northeast of the Philippines and propagate northwestward. The mid–high‐latitude wave trains are similar to the second type but with relatively weaker southeastward‐propagating wave train. At the peak warm phase, the anomalous highs from the Tropics and mid–high latitudes together dominate a large meridional scope and induce wide‐range warming over eastern China. Intrinsically, the QBWO of the convection north of the Philippines and mid–high‐latitude wave trains are important signals of the atmospheric variability and might be applied to the forecast of South China T_max.

This study reveals that the interannual variability of the western edge of the western North Pacific (WNP) subtropical high (WNPSH) in early summer experienced an interdecadal decrease around 1990. Correspondingly, the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP (WNPHA) in abnormal years possess smaller ranges after 1990. The different influences of the tropical SSTAs are important for this interdecadal change, which exhibit slow El Niño decaying pattern before 1990 while rapid transformation from El Niño to La Niña after 1990. The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences. Before 1990, the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation, respectively. Meanwhile, the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response, which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific. After 1990, the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation, respectively. Overall, the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990. Consequently, the WNPHA features narrower zonal range and less eastward extension after 1990, corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH. On the other hand, the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990, contributing to the change of the El Niño SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.

This study reveals the interdecadal changes in the interannual variability of the summer temperature over Northeast Asia (NEA), which presents an enhancement around the early 1990s and a reduction after the mid-2000s. The stronger NEA temperature variability after the early 1990s is favored by the enhanced influence of the Pacific–Japan (PJ) teleconnection, which is remotely modulated by the southeastern tropical Indian Ocean (SETIO). After the early 1990s, the mean state over the SETIO presents relatively warmer SST and ascending motion, favoring a good relationship between the local SST and convection. Therefore, the SETIO SST could prominently influence the local convection and subsequently modulate the convection over the western North Pacific (WNP) via a cross-equatorial overturning circulation. The abnormal convection over the WNP further triggers the PJ teleconnection to influence NEA. However, these ocean–atmosphere processes disappear before the early 1990s. In this period, the mean state over the SETIO features relatively colder SST and subsiding motion, accompanied by a poor relationship between the local SST and convection. Therefore, the variability of convection over the SETIO is weak, thus the atmospheric variability over the WNP is also weakened and the PJ teleconnection presents a different distribution that could not influence NEA. The reduced variability of NEA temperature after the mid-2000s is related to the feeble influence of the PJ teleconnection and the reduced variability of the SETIO SST, which is modulated by the SST over the tropical central–eastern Pacific during the preceding winter to spring.