A record-breaking heat wave struck western North America (WNA) from late June through early July in 2021, which affected several large cities that otherwise rarely experience extreme heat waves, such as Portland, Oregon, and Seattle, Washington in the United States, and Vancouver in Canada. An extreme maximum temperature of 49.6°C-the highest ever recorded in Canada-was measured on 29 June in the village of Lytton (Schiermeier, 2021). There were more than 700 deaths-the majority believed to have been linked to the extreme heat (Murphy, 2021)-and 180 wildfires in the British Columbia, Canada (Schiermeier, 2021).According to the observed daily surface air maximum temperature (T max ) data from the U.S. Climate Prediction Center (CPC/NOAA, 2021), extreme heat at 8°C higher than the climatological mean occurred over southwestern North America during 16-20 June 2021 (Figure 1a). Then, the high-temperature center moved toward northwestern North America, with the T max reaching 41.3°C (about +16°C relative to the climatology) in the Canadian provinces of British Columbia and Alberta during 26-30 June (Figure 1c). In early July, although the heat wave weakened slightly over WNA (blue boxes), the T max there still exceeded the 90th percentile (white cross-hatched areas in Figures 1d-1f). The area-averaged T max over WNA showed "heat spike" days in early June, followed by exceptionally hot days during late June (Figure 1g). The peak temperature of 35.3°C was recorded on 28 June, which was far above the climatology (+13.1°C). The heat wave then dissipated around mid-July. Similar results were also observed in the ERA5 reanalysis data (not shown
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.
The 2018 tropical cyclone (TC) season in the North Pacific was very active, with 39 tropical storms including eight typhoons/hurricanes. This activity was successfully predicted up to 5 months in advance by the Geophysical Fluid Dynamics Laboratory Forecast‐Oriented Low Ocean Resolution (FLOR) global coupled model. In this work, a suite of idealized experiments with three dynamical global models (FLOR, Nonhydrostatic Icosahedral Atmospheric Model, and Meteorological Research Institute Atmospheric General Circulation Model) was used to show that the active 2018 TC season was primarily caused by warming in the subtropical Pacific and secondarily by warming in the tropical Pacific. Furthermore, the potential effect of anthropogenic forcing on the active 2018 TC season was investigated using two of the global models (FLOR and Meteorological Research Institute Atmospheric General Circulation Model). The models projected opposite signs for the changes in TC frequency in the North Pacific by an increase in anthropogenic forcing, thereby highlighting the substantial uncertainty and model dependence in the possible impact of anthropogenic forcing on Pacific TC activity.
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