The linear sensitivity of the North Atlantic Oscillation and eddy-driven jet to SSTs
Journal of Climate American Meteorological Society 32:19 (2019) 6491-6511
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.Forced summer stationary waves: the opposing effects of direct radiative forcing and sea surface warming
Climate Dynamics Springer Nature 53:7-8 (2019) 4291-4309
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.The eddy-driven jet and storm-track responses to boundary-layer drag: insights from an idealized dry GCM study
Journal of the Atmospheric Sciences American Meteorological Society 76:4 (2019) 1055-1076
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.Contrasting mechanisms of summer blocking over western Eurasia
Geophysical Research Letters Wiley 45:21 (2018) 12,040-12,048
Abstract:
The formation of summer blocking events appears to have been mostly studied for a few individual events often associated with heat waves. Here we investigate summer blocking event dynamics in three areas over western Eurasia in order to draw some more general conclusions, mostly in terms of high‐ and low‐frequency processes. A 2‐D blocking event detection algorithm is applied to the 500‐hPa‐geopotential field from the ERA‐40 and ERA‐Interim reanalyses over the 1958–2017 period. It is shown that both high‐ and low‐frequency processes are important to initiate blocking events over southern central Europe. Blocking events over western Russia are preceded by a significant low‐frequency large‐scale wave train, and their formation and maintenance are dominated by low‐frequency processes only. Finally, it is shown that the risk of extreme seasons such as summer 2010 cannot be accurately estimated from the Poisson statistics of past events.Ensemble sensitivity analysis of Greenland blocking in medium‐range forecasts
Quarterly Journal of the Royal Meteorological Society Wiley 144:716 (2018) 2358-2379