17.1 An investigation of storm track response to climate change using an idealized dry GCM

Friday, 21 June 2013: 8:45 AM
Viking Salons ABC (The Hotel Viking)
Cheikh O. Mbengue, California Institute of Technology, Pasadena, CA; and T. Schneider

The midlatitude storm tracks, where the most intense extratropical cyclonesare found, are important fixtures in the general circulation. They are instrumental, for example, in balancing heat, momentum, and moisture budgets and are therefore responsible for the weather and climate patterns over large areas. Climate models, supported by some observational data, have demonstrated that the storm tracks migrate poleward in global warming scenarios; however, the dynamical mechanisms responsible for this migration remain unclear. Our work seeks to deepen understanding of the dynamical mechanisms that are responsible for storm track migration.

Competing mechanisms present in the comprehensive climate models often used to study storm track dynamics make it difficult to determine the primary mechanisms responsible for a given response. Thus, we study storm track dynamics in a simplified and idealized framework, which enables the decoupling of key parameters that have already been identified in the literature as important to storm track dynamics. Using a statistically zonally symmetric, dry general circulation model (GCM), we conduct a series of numerical simulations to help understand storm track response to global mean temperatures and to the tropical convective static stability, which we can vary independently. We define the storm tracks to be regions of zonally and temporally averaged maxima of barotropic eddy kinetic energy (EKE). This storm track definition facilitates the use of previously established scalings between the magnitude of bulk measures of mean available potential energy (MAPE) and EKE. We decompose MAPE changes into individual factors (temperature gradient changes, static stability changes, tropopause height changes) to obtain a mechanistic understanding of the storm track response in our simulations.

The simulations provide several insights, which enable us to test and extend existing theories of the mechanisms driving the poleward migration of the storm tracks. We demonstrate that dry dynamics account for part of the storm track response by showing a poleward migration in a dry atmosphere with fixed pole-equator temperature contrast and increasing radiative equilibrium mean temperature, without changes in convective static stability. We also show scalings between the location of maxima of surface MAPE and of barotropic EKE. In the simulations where we independently vary the tropical convective static stability, we find a marked poleward migration of the storm tracks. However, our decomposition shows that meridional temperature gradients, and not static stability, determine the location and the intensity of the storm tracks. This suggests that although the storm tracks are sensitive to the tropical convective static stability, it influences them indirectly. Therefore, we seek mechanisms that could provide the tropical-extratropical communication necessary to generate the observed response to tropical convective stability changes; one possibility is the expanding Hadley cell since our simulations show that the storm tracks generally migrate in tandem with the terminus of the Hadley cell.

Finally, preliminary results reveal that the supercriticality remains relatively constant along the storm tracks as they migrate. Here, the supercriticality is defined as the ratio of the depth over which eddies act to stabilize the thermal stratification in the troposphere to the total depth of the troposphere. Therefore, we investigate the possibility of predicting the magnitude of storm track response to warming scenarios through a decomposition of a supercriticality measure.

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