205 research outputs found

    Geostrophic vortex dynamics

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    Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1988By generalizing the method of contour dynamics to the quasigeostrophic two layer model, we have proposed and solved a number of fundamental problems in the dynamics of rotating and stratified vorticity fields. A variety of rotating and translating potential vorticity equilibria (V-states) in one and two layers have been obtained, shedding new light on potential vorticity dynamics in the geostrophic context. In particular,the equivalent barotropic model is shown to be a singular limit of the two-layer model for scales large compared to the radius of deformation. The question of coalescence of two vortices in the same layer (merger) and· in different layers (alignment) is studied in detail. Critical initial separation distances for coalescence are numerically established as functions of the radius of deformation and the relative thickness of the layers at rest. The connection between coalescence and the existence of stable rotating doubly-connected V-states is shown to be an illuminating generalization of the Euler results. The question of filamentation of two-dimensional vorticity interfaces is addressed from a new geometrical perspective. The analysis of the topology of the streamfunction in a frame of reference rotating with the instantaneous angular velocity of the vorticity distribution (the corotating frame) is shown to yield new powerful insights on the nonlinear evolution of the vorticity field. In particular, the presence of hyperbolic (critical) points of the corotating streamfunction that come in contact with the vorticity interface is found to be directly responsible for the generation of filaments. The importance ofthe position of the critical points of the comoving streamfunction is found to generalize to the two-layer quasigeostrophic context. They are shown to play the crucial role in determining the limits, in parameter space, on the existence of a number of two-layer rotating and translating potential vorticity equilibria

    Double Tropopause Formation in Idealized Baroclinic Life Cycles: The Key Role of an Initial Tropopause Inversion Layer

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    Recent studies have shown that double tropopauses exist in all seasons, and at all longitudes, in the midlatitudes. As of yet, the key mechanism responsible for their formation is not known. In this study, we explore the connection between double tropopauses and midlatitude baroclinic eddies. This is investigated in the context of idealized life cycle experiments. The key finding of this study is that large areas of double tropopauses form spontaneously at the nonlinear stage of the life cycle evolution, provided an extratropical tropopause inversion layer is present in the balanced initial temperature profile. We also show that the areas covered with double tropopauses grow as the strength of the initial tropopause inversion layer is increased. Without such a layer, as in canonical examples of baroclinic life cycles much studied in the literature, no double tropopause formation occurs. In agreement with observations, double tropopauses in our life cycle experiments form predominantly in areas of cyclonic flow at upper levels. However, the air masses that end up between the two tropopauses are found to originate from high latitudes. This appears to differ from a recently published case study, where the air between double tropopauses was shown to originate partly from low latitudes. Such a discrepancy suggests that more than one pathway may exist to advect air masses between the two tropopauses

    Anthropogenic impact on Antarctic surface mass balance, currently masked by natural variability, to emerge by mid-century

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    Global and regional climate models robustly simulate increases in Antarctic surface mass balance (SMB) during the twentieth and twenty-first centuries in response to anthropogenic global warming. Despite these robust model projections, however, observations indicate that there has been no significant change in Antarctic SMB in recent decades. We show that this apparent discrepancy between models and observations can be explained by the fact that the anthropogenic climate change signal during the second half of the twentieth century is small compared to the noise associated with natural climate variability. Using an ensemble of 35 global coupled climate models to separate signal and noise, we find that the forced SMB increase due to global warming in recent decades is unlikely to be detectable as a result of large natural SMB variability. However, our analysis reveals that the anthropogenic impact on Antarctic SMB is very likely to emerge from natural variability by the middle of the current century, thus mitigating future increases in global sea level

    Antarctic Climate Response to Stratospheric Ozone Depletion in a Fine Resolution Ocean Climate Model

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    We investigate the impact of stratospheric ozone depletion on Antarctic climate, paying particular attention to the question of whether eddy parameterizations in the ocean fundamentally alter the results. This is accomplished by contrasting two versions of the Community Climate System Model (version 3.5), one at 0.1° ocean and sea ice resolution and the other at 1° with parameterized ocean eddies. At both resolutions, pairs of integrations are performed: one with high (1960) and one with low (2000) ozone levels. We find that the effect of ozone depletion is to warm the surface and the ocean to a depth of 1000 m and to significantly reduce the sea ice extent. While the ocean warming is somewhat weaker when the eddies are resolved, the total loss of sea ice area is roughly the same in the fine and coarse resolution cases
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