Tim Woollings

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.

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.

Abstract:

The North Atlantic Oscillation (NAO) is the leading mode of variability in the large scale circulation over the North Atlantic in winter, and strongly influences the weather and climate of Europe. On synoptic timescales, the negative phase of the NAO often corresponds to the occurrence of a blocking episode over Greenland. Hence, the dynamics and predictability of these blocking events is of interest for the prediction of the NAO and its related impacts over a wide region. Ensemble sensitivity analysis utilises the information contained in probabilistic forecast ensembles to calculate a statistical relationship between a forecast metric and some precursor condition. Here the method is applied to 15‐day forecasts of a set of 26 Greenland blocking events using the state‐of‐the‐art European Centre for Medium‐Range Weather Forecasts (ECMWF) forecasting system. The ensemble sensitivity analysis shows that Greenland blocking does not develop in isolation in these forecasts, but instead the blocking is sensitive to remote precursors, such as 500 hPa and 50 hPa geopotential height, particularly in the low‐frequency flow. In general, there are more significant sensitivities to anomalies in the tropics than in the polar regions. Stratospheric sensitivities tend to emerge at later lead times than tropospheric sensitivities. The strongest and most robust sensitivities correspond to a Rossby wave precursor reaching from the Pacific basin across North America.

Abstract:

This study investigates the influence of atmospheric initial conditions on winter seasonal forecasts of the North Atlantic Oscillation (NAO). Hindcast (or reforecast) experiments – which differ only in their initial conditions – are performed over the period 1960–2009, using prescribed sea surface temperature (SST) and sea‐ice boundary conditions. The first experiment (“ERA‐40/Int IC”) is initialized using the ERA‐40 and ERA‐Interim reanalysis datasets, which assimilate upper‐air, satellite and surface observations; the second experiment (“ERA‐20C IC”) is initialized using the ERA‐20C reanalysis dataset, which assimilates only surface observations. The ensemble mean NAO skill is largest in ERA‐40/Int IC (r = 0.54), which is initialized with the superior reanalysis data. Moreover, ERA‐20C IC did not exhibit significantly more NAO hindcast skill (r = 0.38) than in a third experiment, which was initialized with incorrect (shuffled) initial conditions. The ERA‐40/Interim and ERA‐20C initial conditions differ substantially in the tropical stratosphere, where the quasi‐biennial oscillation (QBO) of zonal winds is not present in ERA‐20C. The QBO hindcasts are highly skilful in ERA‐40/Int IC – albeit with a somewhat weaker equatorial zonal wind amplitude in the lower stratosphere – but are incorrect in ERA‐20C IC, indicating that the QBO is responsible for the additional NAO hindcast skill; this is despite the model exhibiting a relatively weak teleconnection between the QBO and NAO. The influence of the QBO is further demonstrated by regressing out the QBO influence from each of the hindcast experiments, after which the difference in NAO hindcast skill between the experiments is negligible. Whilst ERA‐40/Int IC demonstrates a more skilful NAO hindcast, it appears to have a relatively weak predictable signal; this is the so‐called “signal‐to‐noise paradox” identified in previous studies. Diagnostically amplifying the (weak) QBO–NAO teleconnection increases the ensemble‐mean NAO signal with negligible impact on the NAO hindcast skill, after which the signal‐to‐noise problem seemingly disappears.