Search Results

You are looking at 1 - 3 of 3 items for

  • Author or Editor: M. Kidston x
  • All content x
Clear All Modify Search
Joseph Kidston, G. K. Vallis, S. M. Dean, and J. A. Renwick

Abstract

The question of whether an increase in the atmospheric eddy length scale may cause a poleward shift of the midlatitude jet streams is addressed. An increase in the length scale of the eddy reduces its zonal phase speed and so causes eddies to dissipate farther from the jet core. If the eddy dissipation region on the poleward flank of the jet overlaps with the eddy source latitudes, shifting this dissipation to higher latitudes will alter which latitudes are a net source of baroclinic eddies, and hence the eddy-driven jet stream may shift poleward. This behavior does not affect the equatorward flank of the jet in the same way because the dissipation region on the equatorward flank is well separated from the source latitudes. An experiment with a barotropic model is presented in which an increase in the length scale of a midlatitude perturbation results in a poleward shift in the acceleration of the zonal flow. Initial investigations indicate that this behavior is also important in both observational data and the output of comprehensive general circulation models (GCMs). A simplified GCM is used to show that the latitude of the eddy-driven jet is well correlated with the eddy length scale. It is argued that the increase in the eddy length scale causes the poleward shift of the jet in these experiments, rather than vice versa.

Full access
Joseph Kidston, D. M. W. Frierson, J. A. Renwick, and G. K. Vallis

Abstract

The characteristics of the dominant pattern of extratropical variability (the so-called annular modes) are examined in the context of the theory that eddy-driven jets are self-maintaining. It is shown that there is genuine hemispheric symmetry in the variation of the zonal wind in the Southern Hemisphere but not the Northern Hemisphere. The annular mode is shown to be baroclinic in nature; it is associated with changes in the baroclinic eddy source latitude, and the latitude of the eddy source region is organized by the mean flow. This behavior is expected if there is a baroclinic feedback that encourages the maximum baroclinic instability to be coincident with the maximum zonal wind speed, and discourages the meridional vacillation of the eddy-driven jet stream. It is shown that the strength of the thermally indirect circulation that gives rise to the baroclinic feedback appears to influence the time scale of the annular mode. When the thermally indirect circulation is stronger the annular mode has a longer e-folding time in a simplified GCM. Preliminary results indicate that the same dynamics are important in the real atmosphere.

Full access
A. Anav, P. Friedlingstein, M. Kidston, L. Bopp, P. Ciais, P. Cox, C. Jones, M. Jung, R. Myneni, and Z. Zhu

Abstract

The authors assess the ability of 18 Earth system models to simulate the land and ocean carbon cycle for the present climate. These models will be used in the next Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) for climate projections, and such evaluation allows identification of the strengths and weaknesses of individual coupled carbon–climate models as well as identification of systematic biases of the models. Results show that models correctly reproduce the main climatic variables controlling the spatial and temporal characteristics of the carbon cycle. The seasonal evolution of the variables under examination is well captured. However, weaknesses appear when reproducing specific fields: in particular, considering the land carbon cycle, a general overestimation of photosynthesis and leaf area index is found for most of the models, while the ocean evaluation shows that quite a few models underestimate the primary production.The authors also propose climate and carbon cycle performance metrics in order to assess whether there is a set of consistently better models for reproducing the carbon cycle. Averaged seasonal cycles and probability density functions (PDFs) calculated from model simulations are compared with the corresponding seasonal cycles and PDFs from different observed datasets. Although the metrics used in this study allow identification of some models as better or worse than the average, the ranking of this study is partially subjective because of the choice of the variables under examination and also can be sensitive to the choice of reference data. In addition, it was found that the model performances show significant regional variations.

Full access