Unprecedented Heatwave in Western North America during Late June of 2021: Roles of Atmospheric Circulation and Global Warming

An extraordinary and unprecedented heatwave swept across western North America(i.e.,the Pacific Northwest)in late June of 2021,resulting in hundreds of deaths,a massive die-off of sea creatures off the coast,and horrific wildfires.Here,we use observational data to find the atmospheric circulation variabilities of the North Pacific and Arctic-Pacific-Canada patterns that co-occurred with the development and mature phases of the heatwave,as well as the North America pattern,which coincided with the decaying and eastward movement of the heatwave.Climate models from the Coupled Model Intercomparison Project(Phase 6)are not designed to simulate a particular heatwave event like this one.Still,models show that greenhouse gases are the main reason for the long-term increase of average daily maximum temperature in western North America in the past and future.

Causes of the Extreme Hot Midsummer in Central and South China during 2017:Role of the Western Tropical Pacific Warming

This study investigates why an extreme hot midsummer occurred in Central and South China(CSC) during 2017. It is shown that the western North Pacific subtropical high(WNPSH) was abnormally intensified and westward-extending,resulting in anomalous high pressure and consequent extreme heat over CSC. The abnormal WNPSH was favored by the warming of the western tropical Pacific(WTP), which was unrelated to ENSO and manifested its own individual effect.The WTP warming enhanced the convection in-situ and led to anomalous high pressure over CSC via a local meridional circulation. The influence of the WTP was confirmed by CAM4 model experiments. A comparison between the 2017 midsummer and 2010 midsummer(with a stronger WNPSH but weaker extreme heat) indicated that the influence of the WNPSH on extreme heat can be modulated by the associated precipitation in the northwestern flank.The role of the WTP was verified by regression analyses on the interannual variation of the WTP sea surface temperature anomaly(SSTA). On the other hand, the WTP has undergone prominent warming during the past few decades, resulting from decadal to long-term changes and favoring extreme warm conditions. Through a mechanism similar to the interannual variation, the decadal to long-term changes have reinforced the influence of WTP warming on the temperature over CSC,contributing to the more frequent hot midsummers recently. It is estimated that more than 50% of the temperature anomaly over CSC in the 2017 midsummer was due to the WTP warming, and 40% was related to the decadal to long-term changes of the WTP SSTA.

The 2020 Summer Floods and 2020/21 Winter Extreme Cold Surges in China and the 2020 Typhoon Season in the Western North Pacific

China experienced significant flooding in the summer of 2020 and multiple extreme cold surges during the winter of 2020/21.Additionally,the 2020 typhoon season had below average activity with especially quiet activity during the first half of the season in the western North Pacific(WNP).Sea surface temperature changes in the Pacific,Indian,and Atlantic Oceans all contributed to the heavy rainfall in China,but the Atlantic and Indian Oceans seem to have played dominant roles.Enhancement and movement of the Siberian High caused a wavier pattern in the jet stream that allowed cold polar air to reach southward,inducing cold surges in China.Large vertical wind shear and low humidity in the WNP were responsible for fewer typhoons in the first half of the typhoon season.Although it is known that global warming can increase the frequency of extreme weather and climate events,its influences on individual events still need to be quantified.Additionally,the extreme cold surge during 16–18 February 2021 in the United States shares similar mechanisms with the winter 2020/21 extreme cold surges in China.

Modulation of Tropical Cyclogenesis in the Western North Pacific by the Quasi-Biweekly Oscillation

The quasi-biweekly oscillation （QBWO） is the second most dominant intraseasonal mode over the westem North Pacific （WNP） during boreal summer. In this study, the modulation of WNP tropical cyclogenesis （TCG） by the QBWO and its association with large-scale patterns are investigated. A strong modulation of WNP TCG events by the QBWO is found. More TCG events occur during the QBWO＇s convectively active phase. Based on the genesis potential index （GPI）, we further evaluate the role of environmental factors in affecting WNP TCG. The positive GPI anomalies associated with the QBWO correspond well with TCG counts and locations. A large positive GPI anomaly is spatially correlated with WNP TCG events during a life cycle of the QBWO. The low-level relative vorticity and mid-level relative humidity appear to be two dominant contributors to the QBWO-composited GPI anomalies during the QBWO＇s active phase, followed by the nonlinear and potential intensity terms. These positive contributions to the GPI anomalies are partly offset by the negative contribution from the vertical wind shear. During the QBWO＇s inactive phase, the mid-level relative humidity appears to be the largest contributor, while weak contributions are also made by the nonlinear and low-level relative vorticity terms. Meanwhile, these positive contributions are partly cancelled out by the negative contribution from the potential intensity. The contributions of these environmental factors to the GPI anomalies associated with the QBWO are similar in all five flow patterns--the monsoon shear line, monsoon confluence region, monsoon gyre, easterly wave, and Rossby wave energy dispersion associated with a preexisting TC. Further analyses show that the QBWO strongly modulates the synoptic-scale wave trains （SSWs） over the WNP, with larger amplitude SSWs during the QBWO＇s active phase. This implies a possible enhanced （weakened） relationship between TCG and SSWs during the active （inactive） phase. This study improves our understanding of the modulation of WNP TCG by the QBWO and thus helps with efforts to improve the intraseasonal prediction of WNP TCG.

On the ENSO Mechanisms

The El Nino-Southern Oscillation (ENSO) is an interannual phenomenon involved in the tropical Paci- fic Ocean-atmosphere interactions. The oscillatory nature of ENSO requires both positive and negative ocean-atmosphere feedbacks. The positive feedback is dated back to Bjerknes' hypothesis in the 1960s, and different negative feedbacks have been proposed since the 1980s associated with the delayed oscillator, the western Pacific oscillator, the recharge-discharge oscillator, and the advective-reflective oscillator. The de- layed oscillator assumes that wave reflection at the western boundary provides a negative feedback for the coupled system to oscillate. The western Pacific oscillator emphasizes equatorial wind in the western Pacific that provides a negative feedback for the coupled system. The recharge-discharge oscillator argues that discharge and recharge of equatorial heat content cause the coupled system to oscillate. The advective-re- flective oscillator emphasizes the importance of zonal advection associated with wave reflection at both the western and eastern boundaries. All of these physics are summarized in a unificd ENSO oscillator. The de- layed oscillator, the western Pacific oscillator, the recharge-discharge oscillator, and the advective-reflec- tive oscillator can be extracted as special cases of the unified oscillator. As suggested by this unified oscillator, all of the previnus ENSO oscillator mechanisms may be operating in nature.

Physical mechanism of the rapid increase in intense and long-lived extreme heatwaves in the Northern Hemisphere since 1980

Since 1980,both the intensity and duration of summer heatwaves in the middle and high latitudes of the Northern Hemisphere have significantly increased,leading this region to become a critical area for a significant increase in the frequency of intense and long-lived extreme heatwaves.We found that stronger and more persistent high-pressure systems and lower soil moisture before the events were the main drivers of intense and long-lived extreme heatwaves in western Europe and the middle and high latitudes of North America.However,in eastern Europe and Siberia,lower cloud cover before events is also a main driver of this type of extreme heatwave,in addition to the above drivers.These factors are coupled with each other and can change heatwave intensity and duration by influencing surface radiation processes during events.Using the self-organizing map classification method,we found that 6 weather patterns with increased frequency,intensity,and duration were the main dynamic reasons leading to the increase in intense and long-lived extreme heatwaves after 1980.In addition,the decrease in summer average soil moisture in most areas of the mid-high latitudes of the Northern Hemisphere and the decrease in average cloud cover in eastern Europe and Siberia are found to be the main thermodynamic reasons leading to the increase in these extreme heatwaves.

Influences of three oceans on record-breaking rainfall over the Yangtze River Valley in June 2020

The rainfall over the Yangtze River Valley(YRV)in June 2020 broke the record since 1979.Here we show that all three oceans of the Pacific,Indian and Atlantic Oceans contribute to the YRV rainfall in June 2020,but the Atlantic plays a dominant role.The sea surface temperature(SST)anomalies in three oceans are associated with the two vorticity anomalies:negative 200-hPa relative vorticity anomalies over North China(NC)and negative 850-hPa relative vorticity anomalies in the South China Sea(SCS).The rainfall anomalies in the YRV are mainly controlled by atmospheric process associated with the NC vorticity.The positive SST anomalies in May over the western North Atlantic induce positive geopotential height anomalies in June over the mid-latitude North Atlantic,which affect the rainfall anomalies in the YRV by changing the NC vorticity via Atlantic-induced atmospheric wave train across Europe.The Indian Ocean and tropical North Atlantic,as capacitors of Pacific El Niño events in the preceding winter,affect the SCS vorticity associated with the anomalous anticyclone over the SCS and also facilitate the YRV rainfall by providing favorable moisture conditions.This study suggests that the May SST over the western North Atlantic is a good predictor of June rainfall anomalies in the YRV and highlights the important impacts of three-ocean SSTs on extreme weather and climate events in China.