Climate dynamics

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:

The primary objective of the United Nations Framework Convention on Climate Change is to stabilize greenhouse gas concentrations at a level that will avoid dangerous climate impacts. However, greenhouse gas concentration stabilization is an awkward framework within which to assess dangerous climate change on account of the significant lag between a given concentration level and the eventual equilibrium temperature change. By contrast, recent research has shown that global temperature change can be well described by a given cumulative carbon emissions budget. Here, we propose that cumulative carbon emissions represent an alternative framework that is applicable both as a tool for climate mitigation as well as for the assessment of potential climate impacts. We show first that both atmospheric CO(2) concentration at a given year and the associated temperature change are generally associated with a unique cumulative carbon emissions budget that is largely independent of the emissions scenario. The rate of global temperature change can therefore be related to first order to the rate of increase of cumulative carbon emissions. However, transient warming over the next century will also be strongly affected by emissions of shorter lived forcing agents such as aerosols and methane. Non-CO(2) emissions therefore contribute to uncertainty in the cumulative carbon budget associated with near-term temperature targets, and may suggest the need for a mitigation approach that considers separately short- and long-lived gas emissions. By contrast, long-term temperature change remains primarily associated with total cumulative carbon emissions owing to the much longer atmospheric residence time of CO(2) relative to other major climate forcing agents.

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.