The 2021 Pacific Northwest Heat Wave and Associated Blocking: Meteorology and the Role of an Upstream Cyclone as a Diabatic Source of Wave Activity

Abstract

The heat wave that enveloped the Pacific Northwest from late June through early July 2021 delivered unprecedented temperatures to the normally cool region -108°F (42°C) in Seattle, 116°F (47°C) in Portland -and claimed over 1,000 lives mostly in British Columbia (AON, 2021). One preliminary study puts it in a 1-in-1,000 years event category (Philip et al., 2021). As with most heat waves in the midlatitudes (Fang & Lu, 2020;Pfahl & Wernli, 2012), the event was associated with an anomalous behavior of the jet stream (atmospheric blocking).In this study, we address (a) the dynamics that led to the formation of an unusually strong blocking anticyclone and (b) how the blocking anticyclone drove extreme surface temperatures during this particular event. Both topics have been studied extensively in a broader context. For example, backward trajectory studies (e.g., Zschenderlein et al., 2019) identified subsidence in the free troposphere, as well as the prolonged downward solar radiation, as a key ingredient for extreme surface heat inside a persistent blocking anticyclone. Soil moisture feedback was also a significant contributor to past major heat events in Europe (Black et al., 2004;Whan et al., 2015). We will show that during the Pacific Northwest event of 2021 the formation of an upper-level blocking preceded the extreme surface temperatures by 2-3 days, demonstrating a top-down thermodynamic control of blocking on the surface temperatures. The warm air mass inside the block was created upstream and its arrival capped convection over land and raised surface temperatures to an unusual level.The mechanism of block formation is a major topic in its own right. Synoptic eddies migrating along the jet stream have long been thought of as feeding and maintenance mechanisms of blocks (Luo,

Response of moist and dry processes in atmospheric blocking to climate change

“…Fitting a Theil-Sen regression model, we demonstrate that the changes in the size and intensity of blocks scale well with the increase in latent heating, especially for winter blocks over the oceans. This agrees with previous work that demonstrated the causal effect of latent heating on block intensity and size in numerical case study experiments (Croci-Maspoli et al 2007, Grams and Archambault 2016, Maddison et al 2020, Steinfeld et al 2020, Neal et al 2022.…”

Drivers and Mechanisms of the 2021 Pacific Northwest Heatwave

“…Then, in a seamless transition as the air became engulfed in the anticyclone at a high altitude, subsidence and the associated adiabatic heating enabled a continuous conversion of potential into internal energy. Our findings confirm and expand on Neal et al (2022) who focused on the dynamical implications of upwind latent heating but suggested that the unusual tropospheric warmth in late June 2021 in the PNW originated in lower latitudes. The authors also reasoned that heat released from condensation may have contributed to the elevated temperatures in the troposphere, but did not calculate backward trajectories or perform any other detailed analyses.…”

“…Even though hot events are exclusively diagnosed based on temperature here, many of them are associated with anticyclonic circulation aloft, which implies adiabatic warming due to subsidence particularly in the last 72 hr (Zschenderlein et al, 2020). While this adiabatic heating-converting potential into internal energy-is often considered to be a key driver of high near-surface temperatures in the midlatitudes (e.g., Bieli et al, 2015), it cannot explain the presence of high potential temperatures in the upper troposphere such as witnessed during the 2021 PNW heatwave (Neal et al, 2022). Therefore, to investigate the unprecedented free tropospheric heat during the 2021 PNW event, we consider the initial state of air 15 days prior to arriving, and processes occurring en route that shape the final state above the respective hot extreme.…”

How Unexpected Was the 2021 Pacific Northwest Heatwave?

“…This perturbation seeded a Rossby wave train, which propagated eastward along a midlatitude wave guide, and modified the upper tropospheric winds associated with the wave guide as it progressed. By June 25th, an omega-block had developed over the PNW, which progressed eastward and intensified over the course of the heatwave (Neal et al, 2022;Philip et al, 2021). A cross-Pacific atmospheric river also transported latent heat into the region (Mo et al, 2022).…”