The 3D Structure of Northern Hemisphere Blocking Events: Climatology, Role of Moisture, and Response to Climate Change

Abstract

We present a comprehensive composite analysis of the 3D structure of blocks and its response to future climate change over North Pacific, North Atlantic, and Russia in summers and winters. Using reanalysis and two large-ensemble datasets from CESM1 and GFDL-CM3, we investigate the following: The climatology of 3D structure, physical processes governing the temperature anomaly associated with blocks, ability of GCMs to reproduce the 3D structure, and its response to RCP8.5. In reanalysis, over both ocean and land, the anomalous winds are equivalent-barotropic in the troposphere and stratosphere, and temperature anomalies are positive throughout the troposphere and negative in the lower stratosphere. The main seasonal and regional differences are that blocks are larger/stronger in winters, and over oceans, the temperature anomaly is shifted westward due to latent heating. Analyzing the temperature tendency equation shows that in all three sectors, adiabatic warming due to subsidence is the main driver of the positive temperature anomaly; however, depending on season and region, meridional thermal advection and latent heating might have leading-order contributions too. Both GCMs are found to reproduce the climatological 3D structure remarkably well, but sometimes disagree on future changes. Overall, the future summertime response is weakening of all fields (except for specific humidity), although the impact on near-surface temperature is not necessarily weakened; e.g., the blocking-driven near-surface warming over Russia intensifies. The wintertime response is strengthening of all fields, except for temperature in some cases. Responses of geopotential height and temperature are shifted westward in winters, most likely due to latent heating.

The Summertime Pacific‐North American Weather Regimes and Their Predictability

“…To identify summertime weather regimes in the reanalysis data, we have applied SOM clustering analysis (Kohonen, 2012) to the daily Z500 anomalies over the North Pacific‐North American sector (0°–90°N and 150°E−60°W). Anomalies are calculated by removing a 31‐day running mean centered around that calendar date of summer and averaged over all 40 yr of each reanalysis data set (Nabizadeh et al., 2019, 2021). Following Bao and Wallace (2015) and Gervais et al.…”

The role of atmospheric dynamics and large‐scale topography in driving heatwaves

“…High‐pressure anticyclones can develop into atmospheric blocking due to wave breaking and can persist for several days. The effect of blocking on near‐surface temperature varies with the seasonal cycle (e.g., Sousa et al ., 2018; Nabizadeh et al ., 2021). During summer, large‐scale subsidence and increased solar radiation associated with clear‐sky conditions within the anticyclone lead to strong near‐surface warming, which in turn increases the likelihood of a heatwave (Pfahl and Wernli, 2012; Schaller et al ., 2018; Zschenderlein et al ., 2019; 2020; Li et al ., 2020; Xu et al ., 2020).…”

“…Summer heatwaves are therefore also often associated with weak pressure gradients, that is, a weak large‐scale circulation (Spensberger et al ., 2020). In winter, enhanced long‐wave radiative cooling during the longer nights and decreased solar radiation within blocking anticyclones lead to near‐surface vertical temperature inversions, separating the cold boundary layer from the free atmosphere, which is heated by large‐scale subsidence (Nabizadeh et al ., 2021). Thus, in contrast to summer, winter blocking situations are not necessarily associated with extreme warm surface temperatures (e.g., Sousa et al ., 2018).…”

Increasing Health Risks During Outdoor Sports Due To Climate Change in Texas: Projections Versus Attitudes

“…(2019) reported an increase in the size of the blocking events, which to some extent explains these projected increases in the size of heat waves. Temperature anomalies caused by blocking events are shown to intensify over land under climate change (Nabizadeh et al., 2021).…”