Role of Abnormally Enhanced MJO over the Western Pacific in the Formation and Subseasonal Predictability of the Record-Breaking Northeast Asian Heatwave in the Summer of 2018

Abstract

In the summer of 2018, Northeast Asia experienced a heatwave event that broke the existing high-temperature records in several locations in Japan, the Korean Peninsula, and northeastern China. At the same time, an unusually strong Madden–Julian oscillation (MJO) was observed to stay over the western Pacific warm pool. Based on reanalysis diagnosis, numerical experiments, and assessments of real-time forecast data from two subseasonal-to-seasonal (S2S) models, we discovered the importance of the western Pacific MJO in the generation of this heatwave event, as well as its predictability at the subseasonal time scale. During the prolonged extreme heat period (11 July–14 August), a high pressure anomaly with variability at the intraseasonal (30–90 days) time scale appeared over Northeast Asia, causing persistent adiabatic heating and clear skies in this region. As shown in the composites of MJO-related convection and circulation anomalies, the occurrence of this 30–90-day high anomaly over Northeast Asia was linked with an anomalous wave train induced by tropical heating associated with the western tropical Pacific MJO. The impact of the MJO on the heatwave was further confirmed by sensitivity experiments with a coupled GCM. As the western Pacific MJO-related components were removed by nudging prognostic variables over the tropics toward their annual cycle and longer time scales (>90 days) in the coupled GCM, the anomalous wave train along the East Asian coast disappeared and the surface air temperature in Northeast Asia lowered. The MJO over the western Pacific warm pool also influenced the predictability of the extratropical heatwave. Our assessments of two S2S models’ real-time forecasts suggest that the extremity of this Northeast Asian heatwave can be better predicted 1–4 weeks in advance if the enhancement of MJO convection over the western Pacific warm pool is predicted well.

Atmospheric tropical modes are important drivers of Sahelian springtime heatwaves

“…In the southeastern part of Australia, Parker et al (2014) showed that warm conditions are significantly associated with the phases 3-6 of the MJO. More recently, the summer 2018 long lasting heatwave over Northeast Asia was found by Hsu et al (2020) to have been favoured by an unusually strong MJO over the western Pacific warm pool.…”

Boreal summer intraseasonal oscillation in a superparameterized general circulation model: effects of air–sea coupling and ocean mean state

“…In our study, air-sea coupling has a similar effect on the simulated BSISO under different ocean background states, suggesting that the role of coupling may largely depend on the phase relationship between the convection and SST anomalies. Compared to daily coupling frequency between the atmosphere and ocean components of CGCMs, improved fidelity for the BSISO simulation is shown in CGCMs with sub-daily coupling frequency (Woolnough et al, 2007;Klingaman et al, 2011;Hu et al, 2015). In our study, the model ocean tends to respond quickly to the atmospheric convection, which may be associated with the sensitive mixed layer depth due to the absent ocean dynamics in KPP.…”

Exceptionally Persistent Madden‐Julian Oscillation Activity Contributes to the Extreme 2020 East Asian Summer Monsoon Rainfall

“…Superimposed on the large‐scale SST warming in the IO, an exceptionally persistent quasistationary MJO activity was observed during June–July 2020, very different from the eastward‐propagating features seen typically for MJO events. Tropical MJO‐associated convection activity has been long recognized as an important factor contributing to extreme temperature and rainfall anomalies over East Asia (e.g., Y. Chen & Zhai, 2017; Hong & Li, 2009; Hsu et al., 2020). During 2020, the MJO remained in Phases 1–3 (the tropical IO) throughout June–July with 59 active days during these 2 months (Figure 4a), far beyond the climatology (Figure 4c).…”