Optimal North Pacific Blocking Precursors and Their Deterministic Subseasonal Evolution during Boreal Winter

Abstract

Atmospheric blocking is associated with sensible weather impacts such as anomalous precipitation and flooding, cold air outbreaks, and heat waves. Given the asymmetry in the persistence characteristics of anticyclones and cyclones, many studies have emphasized the role of nonlinearities in blocking onset and maintenance. However, previous studies have demonstrated that both linear and nonlinear dynamics can amplify blocks. In this paper the structure and evolution of North Pacific blocking on weekly time scales is investigated using two methods: statistical analysis not requiring linearity, and a linear inverse model (LIM) composed of tropical outgoing longwave radiation and extratropical streamfunction, which relies on purely linear (and linearly parameterized) dynamics. Both approaches produce a similar evolution of North Pacific blocking. Using the LIM, the optimal precursors to blocking are determined, which at a 14-day lead time include an upper-level east Pacific anticyclone and suppressed convection over the central tropical Pacific. The tropics and extratropics both contribute to the deterministic evolution of blocking, with the tropics acting on longer time scales but imposing a weaker response than that contributed by the extratropics. The tropical contribution was driven by La Niña–like conditions that produce a hemispheric anticyclonic anomaly, while the extratropical initial conditions produce an equivalent barotropic, wavelike pattern. The LIM’s ability to reproduce the observed blocking evolution suggests the predictable evolution of blocking on weekly time scales can be modeled in a linear framework, and that subseasonal forecasting of North Pacific blocking needs to consider both tropical and extratropical conditions.

The Critical Role of Euro‐Atlantic Blocking in Promoting Snowfall in Central Greenland

“…The warm temperature anomalies associated with the SON events (Figure 5a) are located northwest of the blocking anticyclone in the North Sea (Figure 4), which is consistent with the poleward advection of warm air masses on the western flanks of atmospheric blocks (Breeden et al., 2020; Masato et al., 2014). Additionally, the 500 hPa geopotential height and wind anomalies during the SON events (Figure 4c) are similar to circulation patterns corresponding to positive surface temperature anomalies observed in central and southeast Greenland (Gallagher et al., 2020).…”

“…The positive temperature anomalies associated with the JJA snowfall events first develop west of the blocking anticyclone (Figures 2b, 2c and 3a), which is consistent with the poleward advection of warm air masses on the western flanks of atmospheric blocking anticyclones (Breeden et al., 2020; Masato et al., 2014). Additionally, when assessing extreme warming events at Summit Station during JJA in 2012, Neff et al.…”

“…The positive temperature anomalies associated with the JJA snowfall events first develop west of the blocking anticyclone (Figures 2b,2c and 3a), which is consistent with the poleward advection of warm air masses on the western flanks of atmospheric blocking anticyclones (Breeden et al, 2020;Masato et al, 2014). Additionally, when assessing extreme warming events at Summit Station during JJA in 2012, Neff et al (2014) found that ridging to the southeast of the GrIS favored the southerly transport of warm air along the western coast of Greenland, similar to the location of the anomalous blocking frequency associated with JJA snowfall events.…”

“…Atmospheric blocks are persistent anticyclones that can significantly disrupt the mean westerly flow both upstream and downstream of the block (Berggren et al., 1949; Rex, 1950), modifying regional precipitation, temperature, and moisture patterns (Henderson et al., 2016; Lenggenhager & Martius, 2019; Masato et al., 2013; Pelly & Hoskins, 2003). Poleward anomalous warm air advection and moisture transport have been found to occur on the western flank of blocking anticyclones (Breeden et al., 2020; Masato et al., 2014; Mundhenk et al., 2016). Additionally, blocks can shift storm tracks latitudinally and steer cyclones poleward (Benedict et al., 2019; Booth et al., 2017; Henderson et al., 2021; Henderson & Maloney, 2018; Pasquier et al., 2019).…”

Subseasonal Prediction of the State and Evolution of the North Pacific Jet Stream

“…Forecasts verifying during jet retractions and equatorward shifts also exhibit larger errors during the first 2 weeks of the forecast period compared to forecasts verifying during jet extensions and poleward shifts. Notably, both jet retractions and equatorward shifts are associated with the development of an upper‐tropospheric North Pacific ridge, which can be strongly influenced by diabatic processes that occur within midlatitude cyclones along the Pacific storm track or in conjunction with tropical convection (e.g., Bosart et al., 2017; Breeden et al., 2020; Martinez‐Alvarado et al., 2016; Teubler & Riemer, 2016; Torn & Hakim, 2015). The inability for models to represent the extent, magnitude, and cumulative influence of these diabatic processes on the upper‐tropospheric flow pattern is hypothesized to contribute to the larger‐than‐average forecast errors associated with jet retractions and equatorward shifts during the first 2 weeks of the forecast period.…”