Tropical precipitation influencing boreal winter midlatitude blocking

Abstract

Recent studies using reanalysis data and complex models suggest that the Tropics influence midlatitude blocking. Here, the influence of tropical precipitation anomalies is investigated further using a dry dynamical model driven by specified diabatic heating anomalies. The model uses a quasi-realistic setup based on idealized orography and an idealized representation of the land-ocean thermal contrast. Results concerning the El Niño Southern Oscillation and the Madden-Julian Oscillation are mostly consistent with previous studies and emphasize the importance of tropical dynamics for driving the variability of blocking at midlatitudes. It is also shown that a common bias in models of the Coupled Model Intercomparison Project Phase 5 (CMIP5), namely, excessive tropical precipitation, leads to an underestimation of midlatitude blocking in our model, also a common bias in the CMIP5 models. The strongest blocking anomalies associated with the tropical precipitation bias are found over Europe, where the underestimation of blocking in CMIP5 models is also particularly strong. K E Y W O R D S blocking bias, CMIP5, dry atmospheric general circulation model, ENSO, Midlatitude blocking, MJO, precipitation bias

Circumglobal Rossby wave patterns during boreal winter highlighted by wavenumber/phase speed spectral analysis

“…On the other hand, many largescale flow patterns feature a coordination between the two storm tracks. There is robust evidence that low-frequency tropical forcing, as the El Nino-Southern Oscillation (ENSO) or the Madden-Julian Oscillation (MJO), manifests itself first in anomalies over the North Pacific storm track that are subsequently propagated, via the forcing of transient Rossby waves, to the North Atlantic storm track (Cassou, 2008;Henderson et al, 2016;Zheng et al, 2018;Gollan et al, 2019).…”

Sensitivity of a simple atmospheric model to changing surface friction with implications for seasonal prediction

“…This study considers a scenario where all predictability arises from a stationary wave train caused by a tropical diabatic heating anomaly, and a long model run with constant boundary conditions is regarded as an ensemble of seasonal forecasts. Therefore, the experiments outlined above are repeated with a modified restoration temperature field which applies a diabatic heating anomaly following the approach by Gollan et al (2019). Comparing the ensemble-mean and ensemble-spread response to changing surface friction enables deductions about the signal-to-noise ratio.…”

“…Comparing the ensemble-mean and ensemble-spread response to changing surface friction enables deductions about the signal-to-noise ratio. Gollan et al (2019) make use of PUMA to investigate the influence of a stationary Rossby wave train produced by a localized heating anomaly on atmospheric blocking in the Extratropics. The three-dimensionally Gaussian-shaped anomaly with an amplitude of 1.5 K⋅day −1 centred in the equatorial mid-troposphere is designed to be weak El Niño-like, although, in contrast to realistic El Niño, the anomaly does not form a dipole and the longitudinal position is arbitrary in an otherwise zonally symmetric set-up.…”

Circumglobal Rossby wave patterns during boreal winter highlighted by space–time spectral analysis

“…On the other hand, several hemispheric mechanisms connecting the two storm tracks have been identified. There is robust evidence that low-frequency tropical forcing, as the El Niño-Southern Oscillation (ENSO) or the Madden-Julian Oscillation (MJO), manifests itself first in anomalies over the North Pacific storm track that are subsequently propagated, via the forcing of transient Rossby waves, to the North Atlantic storm track (Cassou, 2008;Henderson et al, 2016;Zheng et al, 2018;Gollan et al, 2019). However, interaction between the two storm tracks can in principle result from purely extratropical forcing, for instance in the form of baroclinically induced wave resonance (Yang et al, 1997;Franzke et al, 2006).…”