ENSO induces coherent climate anomalies over the Indo-western Pacific, but these anomalies outlast SST anomalies of the equatorial Pacific by a season, with major effects on the Asian summer monsoon. This review provides historical accounts of major milestones and synthesizes recent advances in the endeavor to understand summer variability over the Indo-Northwest Pacific region. Specifically, a large-scale anomalous anticyclone (AAC) is a recurrent pattern in post-El Niño summers, spanning the tropical Northwest Pacific and North Indian oceans. Regarding the ocean memory that anchors the summer AAC, competing hypotheses emphasize either SST cooling in the easterly trade wind regime of the Northwest Pacific or SST warming in the westerly monsoon regime of the North Indian Ocean. Our synthesis reveals a coupled oceanatmosphere mode that builds on both mechanisms in a two-stage evolution. In spring, when the northeast trades prevail, the AAC and Northwest Pacific cooling are coupled via wind-evaporation-SST feedback. The Northwest Pacific cooling persists to trigger a summer feedback that arises from the interaction of the AAC and North Indian Ocean warming, enabled by the westerly monsoon wind regime. This Indo-western Pacific ocean capacitor (IPOC) effect explains why El Niño stages its last act over the monsoonal Indo-Northwest Pacific and casts the Indian Ocean warming and AAC in leading roles. The IPOC displays interdecadal modulations by the ENSO variance cycle, significantly correlated with ENSO at the turn of the 20th century and after the 1970s, but not in between. Outstanding issues, including future climate projections, are also discussed.

Predictability of summer climate anomalies over East Asia and the northwestern Pacific is investigated using observations and a multimodel hindcast ensemble initialized on 1 May for the recent 20-30 yr. Summertime East Asia is under the influence of the northwestern Pacific subtropical high (PASH). The Pacific-Japan (PJ) teleconnection pattern, a meridional dipole of sea level pressure variability, affects the northwestern PASH. The forecast models generally capture the association of the PJ pattern with the El Niñ o-Southern Oscillation (ENSO).The Silk Road pattern, a wave train along the summer Asian jet, is another dominant teleconnection that influences the northwestern PASH and East Asia. In contrast to the PJ pattern, observational analysis reveals a lack of correlations between the Silk Road pattern and ENSO. Coupled models cannot predict the temporal phase of the Silk Road pattern, despite their ability to reproduce its spatial structure as the leading mode of atmospheric internal variability. Thus, the pattern is rather unpredictable at monthly to seasonal lead, limiting the seasonal predictability for summer in East Asia.The anomalous summer of 2010 in East Asia is a case in point, illustrating the interference by the Silk Road pattern. Canonical anomalies associated with a decayed El Niñ o and developing La Niñ a would have the PJ pattern bring a cold summer to East Asia in 2010. In reality, the Silk Road pattern overwhelmed this tendency, bringing a record-breaking hot summer instead. A dynamical model experiment indicates that European blocking was instrumental in triggering the Silk Road pattern in the 2010 summer.

Slow modulation of interannual variability and its relationship to El Niñ o-Southern Oscillation (ENSO) isinvestigated for the period of 1870-2007 using shipboard surface meteorological observations along a frequently traveled track across the north Indian Ocean (NIO; from the Gulf of Aden through Malacca Strait) and the South China Sea (to Luzon Strait). During the decades in the late nineteenth-early twentieth century and in the late twentieth century, the El Niñ o-induced NIO warming persists longer than during the 1910s-mid-1970s, well into the summer following the peak of El Niñ o. During the epochs of the prolonged NIO warming, rainfall drops and sea level pressure rises over the tropical northwest Pacific in summer following El Niñ o. Conversely, during the period when the NIO warming dissipates earlier, these atmospheric anomalies are not well developed. This supports the Indian Ocean capacitor concept as a mechanism prolonging El Niñ o influence into summer through the persistent Indian Ocean warming after El Niñ o itself has dissipated.The above centennial modulation of ENSO teleconnection to the Indo-northwest Pacific region is reproduced in an atmospheric general circulation model forced by observed SST. The modulation is correlated not with the Pacific decadal oscillation but rather with the ENSO variance itself. When ENSO is strong, its effect in the Indo-northwest Pacific strengthens and vice versa. The fact that enhanced ENSO teleconnections occurred 100 years ago during the late nineteenth-early twentieth century indicates that the recent strengthening of the ENSO correlation over the Indo-western Pacific may not entirely be due to global warming but reflect natural variability.

[1] The skills of 11 coupled ocean-atmosphere general circulation models (CGCMs) are investigated in the prediction of seasonal rainfall and circulation anomalies over the northwest (NW) Pacific for the period 1980-2001, with a focus on the summer following the mature phase of El Niño (hereafter JJA(1)). It is shown that the first empirical orthogonal function (EOF) mode of sea level pressure is closely tied to the second EOF mode of rainfall variability over the NW Pacific during JJA(1), indicative of strong feedback between circulation and convection. Most coupled models and the associated multimodel ensemble well predict these EOF modes and their relationship with high fidelity. Coupled models are capable of predicting suppressed rainfall over the NW Pacific in JJA(1). A few models fail to predict the concurrent weak negative sea surface temperature (SST) anomalies on the southeastern flank of the anomalous anticyclone. This suggests that remote forcing via teleconnections is important for NW Pacific rainfall prediction in those models. In some models, local air-sea interactions seem also to play a role. Specifically, remote forcing by tropical Indian Ocean (TIO) SST variability is identified as influential on NW Pacific climate during JJA(1). TIO SST affects the atmosphere over the NW Pacific by two mechanisms, via the equatorial Kelvin wave and the intensification of the subtropical westerly jet. Overall, models are successful in predicting the antisymmetric patterns of precipitation and winds over TIO during spring, which are critical in sustaining the TIO warming through the subsequent summer.