The El Niño-Southern Oscillation (ENSO) significantly affects the weather and climate over the East Asian region. During the boreal winter season, when El Niño events mature, a strong anomalous anticyclone prevails over the western North Pacific (WNP) due to the thermodynamic and dynamical forcing of the tropical convection change, which brings abundant warm and moist air to the East Asian continent, resulting in abnormally wet and warm climatic conditions and a weakened East Asian winter monsoon (EAWM). When La Niña events occur, a roughly opposite picture is observed.
However, recent studies have revealed that this ENSO influence is unstable on subseasonal timescales. Compared to the late winter (January-February) EAWM, the early winter (November-December) EAWM has been shown to be more responsive to ENSO and thus more predictable in operational climate models. Despite this, our understanding of when and how the ENSO-EAWM relationship begins to weaken is still immature. Based on high temporal resolution observations and reanalysis datasets, this study aims to reveal the evolutionary character of the EAWM responses to ENSO throughout the entire boreal winter season.
We find that, although the weakened EAWM can be stably detected throughout almost all the wintertime days, an exception occurs during midwinter, January 14-23, when the EAWM is slightly strengthened (note that a negative EAWM index represents anomalous northerly winds and thus a strengthened EAWM), suggesting a breakdown of the ENSO-EAWM teleconnection. The East Asian precipitation and surface air temperature responses exhibit similar characteristics, i.e., the enhanced precipitation response to El Niño cannot be detected during midwinter (Figure 1a). Meanwhile, the ENSO-regressed atmospheric circulation anomaly also shows unique features in midwinter. Different from that in early (P1) and late (P3) winter, the northern East Asia is controlled by an anomalous cyclone rather than an anticyclone in midwinter (P2). Evident northwesterly rather than southerly wind anomalies prevail over the East Asian region, which leads to a strengthened EAWM (Figure 1b-d). From a large-scale perspective, this northwesterly wind anomaly is associated with a strengthened Siberian high and a deepened East Asian trough, which is embedded in the Rossby wave train originating from the negative phase of the North Atlantic oscillation (NAO) pattern. However, in P1, a positive NAO pattern is observed. We examine the temporal evolution of the NAO response to ENSO. Consistent with our previous study, the NAO abruptly reverses its phase from positive to negative in early January, just about a week before the breakdown of the EAWM response. This abrupt NAO phase reversal is suggested to be triggered by a climatological weakening of the Atlantic jet meridional shear in early January, which shifts the propagation direction of the ENSO-induced low-frequency Rossby waves before they enter the North Atlantic region.
This lead-lag relationship indicates a possible role of the NAO phase transition in the breakdown of the EAWM response. When the NAO turns to negative phase, a strong wave flux emanates from the NAO anomaly center over the North Atlantic, propagates eastward across Siberia, and turns progressively southeastward to influence the East Asian atmospheric circulation. A few days later, we can see a strengthened Siberian high and a deepened East Asian trough. These suddenly appearing atmospheric circulation systems exert strong northerly wind anomalies, which strengthen the EAWM and thus lead to the breakdown of the conventional ENSO-EAWM teleconnection mediated by the anomalous WNP anticyclone. This lead-lag physical relationship can be further consolidated by the NAO and EAWM responses during the two super El Niño winters, namely the winters of 1997-98 and 2015-16 (Figure 2).
Our finding provides important insights into the non-stationarity of the ENSO effects on the EAWM and also the understanding of the sub-seasonal predictability of the East Asian winter climate. The ENSO influence on the East Asian winter climate is thought to be mainly through the modulation of the anomalous WNP anticyclone. We highlight here that this ENSO-NAO-EAWM pathway also plays an important role in mediating the ENSO footprint in this region, which should be taken into account in future studies.
Reference:
- Geng, X., Noh, KM., Kim, K. & Kug, JS. Midwinter breakdown of ENSO climate impacts in East Asia. npj Clim Atmos Sci 6, 155 (2023). https://doi.org/10.1038/s41612-023-00474-4.
- Geng, X., Zhao, J. & Kug, JS. ENSO-driven abrupt phase shift in North Atlantic oscillation in early January. npj Clim Atmos Sci 6, 80 (2023). https://doi.org/10.1038/s41612-023-00414-2.