Tim Woollings

Abstract:

The occurrence of destructive mesoscale 'polar low' cyclones in the subpolar North Atlantic is projected to decline under anthropogenic change, due to an increase in atmospheric static stability. This letter reports on the role of changes in ocean circulation in shaping the atmospheric stability. In particular, the Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in response to anthropogenic forcing, leading to a local minimum in warming in this region. The reduced warming is restricted to the lower troposphere, hence contributing to the increase in static stability. Linear correlation analysis of the CMIP3 climate model ensemble suggests that around half of the model uncertainty in the projected stability response arises from the varied response of the AMOC between models. © 2012. American Geophysical Union. All Rights Reserved.

Abstract:

Projected changes in the extra-tropical wintertime storm tracks are investigated using the multi-model ensembles from both the third and fifth phases of the World Climate Research Programme's Coupled Model Intercomparison Project (CMIP3 and CMIP5). The aim is to characterize the magnitude of the storm track responses relative to their present-day year-to-year variability. For the experiments considered, the 'middle-of-the-road' scenarios in each CMIP, there are regions of the Northern Hemisphere where the responses of up to 40% of the models exceed half of the inter-annual variability, and for the SouthernHemisphere there are regions where up to 60% of the model responses exceed half of the inter-annual variability. Citation: Harvey, B. J., L. C. Shaffrey, T. J. Woollings, G. Zappa, and K. I. Hodges (2012), How large are projected 21st century storm track changes?, Geophys. Res. Lett., 39, L18707,. © 2012. American Geophysical Union.

Abstract:

In this article, Northern Hemisphere winter midlatitude blocking is analysed through its wave-breaking characteristics. Rossby wave breaking is identified as a key process in blocking occurrence, as it provides the mechanism for the meridional reversal pattern typical of blocking. Two indices are designed to detect the major properties of wave breaking, i.e. the orientation (cyclonic/anticyclonic-direction of breaking or DB index) and the relative contribution of air masses (warm/cold-relative intensity or RI index). The use of the DB index differentiates between the anticyclonic cases over Europe and Asia and the cyclonic events over the oceanic basins. One of the three regions displaying cyclonic type was found over the Atlantic Ocean, the other two being over the Pacific Ocean. The first of these is located over the western side of the Pacific and is dominated by warm air extrusions, whereas the second is placed northward of the exit region of the jet stream, where the meridional θ gradient is much weaker. Two European blocking types have been detected using the RI index, which separates out the cases dominated by warm and cold air masses. The latter cases in particular exhibited a well-structured dipole, with associated strong anomalies in both temperature and precipitation. © 2011 Royal Meteorological Society.

Abstract:

There is urgent need for a consistent blocking identification method that can be used and compared across reanalyses, models and climate scenarios. We present such a method and diagnose daily blocking frequency in 43 years (1958-2000) of ERA-40 Reanalysis for indices defined on both the commonly used geopotential height and potential temperature fields as well as a zonal wind index. Applications of various blocking indices to the same data highlights the importance of a consistent methodology for comparison and a method that identifies blocks along a path that varies with the latitude of the storm track. Since the method accommodates blocking detection using 500 mb zonal-wind which is readily available in climate model output, we diagnose blocking in 14 CMIP3 models under two different greenhouse gas scenarios. Blocking duration remains nearly constant among the scenarios, but a robust reduction in blocking frequency with global warming is demonstrated. © 2011 Springer-Verlag.

Abstract:

A poleward shift of the mid-latitude storm tracks in response to anthropogenic greenhouse-gas forcing has been diagnosed in climate model simulations. Explanations of this effect have focused on atmospheric dynamics. However, in contrast to storm tracks in other regions, the North Atlantic storm track responds by strengthening and extending farther east, in particular on its southern flank. These adjustments are associated with an intensification and extension of the eddy-driven jet towards western Europe and are expected to have considerable societal impacts related to a rise in storminess in Europe. Here, we apply a regression analysis to an ensemble of coupled climate model simulations to show that the coupling between ocean and atmosphere shapes the distinct storm-track response to greenhouse-gas forcing in the North Atlantic region. In the ensemble of simulations we analyse, at least half of the differences between the storm-track responses of different models are associated with uncertainties in ocean circulation changes. We compare the fully coupled simulations with both the associated slab model simulations and an ocean-forced experiment with one climate model to establish causality. We conclude that uncertainties in the response of the North Atlantic storm track to anthropogenic emissions could be reduced through tighter constraints on the future ocean circulation. © 2012 Macmillan Publishers Limited. All rights reserved.