Tim Woollings

Abstract:

The winter North Atlantic Oscillation (NAO) has varied on interannual and decadal timescales over the last century, associated with variations in the speed and latitude of the eddy driven jet stream. This paper uses hindcasts from two operational seasonal forecast sys tems (the European Centre for Medium-range Weather Forecasts (ECMWF)’s seasonal forecast system, and the UK Met Office global seasonal forecast system) and a century long atmosphere-only experiment (using the ECMWF’s Integrated Forecasting System model) to relate seasonal prediction skill in the NAO to these aspects of jet variability. This shows that the NAO skill realised so far arises from interannual variations in the jet, largely associated with its latitude rather than speed. There likely remains further potential for predictability on longer, decadal timescales. In the small sample of mod els analysed here, improved representation of the structure of jet variability does not trans late to enhanced seasonal forecast skill.

Abstract:

Many general circulation models (GCMs) fail to capture the observed frequency of atmospheric blocking events in the Northern Hemisphere, however few studies have examined models in the Southern Hemisphere (SH) and those studies that have, have often been based on only a few models. To provide a comprehensive view of how the current generation of coupled GCMs perform in the SH and how blocking frequency changes under enhanced greenhouse gas forcing, we examine the output of 23 models from the Coupled Model Intercomparison Project Phase 5. We find that models have differing biases during winter, when blocking occurrence is highest, though models underestimate blocking frequency south of Australia during summer. We show that models generally have a reduction in blocking frequency with future anthropogenic forcing, particularly in the Australia‐New Zealand sector with the number of winter blocked days reduced by about one third by the end of the 21st century.

Abstract:

The North Atlantic Oscillation (NAO) and eddy-driven jet contain a forced component arising from sea surface temperature (SST) variations. Due to large amounts of internal variability, it is not trivial to determine where and to what extent SSTs force the NAO and jet. A linear statistical-dynamic method is employed with a large climate ensemble to compute the sensitivities of the winter and summer NAO and jet speed and latitude to the SSTs. Key regions of sensitivity are identified in the Indian and Pacific basins, and the North Atlantic tripole. Using the sensitivity maps and a long observational SST dataset, skilful reconstructions of the NAO and jet time series are made. The ability to skilfully forecast both the winter and summer NAO using only SST anomalies is also demonstrated. The linear approach used here allows precise attribution of model forecast signals to SSTs in particular regions. Skill comes from the Atlantic and Pacific basins on short lead times, whilst the Indian Ocean SSTs may contribute to the longer term NAO trend. However, despite the region of high sensitivity in the Indian Ocean, SSTs here do not provide significant skill on interannual timescales which highlights the limitations of the imposed SST approach. Given the impact of the NAO and jet on Northern Hemisphere weather and climate, these results provide useful information that could be used for improved attribution and forecasting.

Abstract:

We investigate the opposing effects of direct radiative forcing and sea surface warming on the atmospheric circulation using a hierarchy of models. In large ensembles of three general circulation models, direct CO2 forcing produces a wavenumber 5 stationary wave over the Northern Hemisphere in summer. Sea surface warming produces a similar wave, but with the opposite sign. The waves are also present in the Coupled Model Intercomparison Project phase 5 ensemble with opposite signs due to direct CO2 and sea surface warming. Analyses of tropical precipitation changes and equivalent potential temperature changes and the results from a simple barotropic model show that the wave is forced from the tropics. Key forcing locations are the Western Atlantic, Eastern Atlantic and in the Indian Ocean just off the east coast of Africa. The stationary wave has a significant impact on regional temperature anomalies in the Northern Hemisphere summer, explaining some of the direct effect that CO2 concentration has on temperature extremes. Ultimately, the climate sensitivity and future changes in the land–sea temperature contrast will dictate the balance between the opposing effects on regional changes in mean and extreme temperature and precipitation under climate change.

Abstract:

Simulations using a dry, idealized general circulation model (GCM) are conducted to systematically investigate the eddy-driven jet’s sensitivity to the location of boundary-layer drag. Perturbations of boundary-layer drag solely within the baroclinic zone reproduce the eddy-driven jet responses to global drag variations. The implications for current theories of eddy-driven jet shifts are discussed. Hemispherically-asymmetric drag simulations in equinoctial and solstitial thermal conditions show that perturbations of surface drag in one hemisphere have negligible effects on the strength and latitude of the eddy-driven jet in the opposite hemisphere. Jet speed exhibits larger sensitivities to surface drag in perpetual winter simulations, while sensitivities in jet latitude are larger in perpetual summer simulations. Near-surface drag simulations with an Earth-like continental profile show how surface drag may facilitate tropical-extratropical teleconnections by modifying waveguides through changes in jet latitude. Longitudinally confined drag simulations demonstrate a novel mechanism for localizing storm tracks. A theoretical analysis is used to show that asymmetries in the Bernoulli function within the baroclinic zone are important for the eddy-driven jet latitude responses because they directly modulate the sensitivity of the zonal-mean zonal wind to drag in the boundarylayer momentum balance. The simulations contained herein provide a rich array of case studies against which to test current theories of eddy-driven jet and storm-track shifts; and the results affirm the importance of correct, well-constrained locations and intensities of boundary-layer drag in order to reduce jet and storm-track biases in climate and forecast models.