Geostrophic vortex dynamics

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

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

Lifetime Dependent Flux into the Lowermost Stratosphere for Idealized Trace Gases of Surface Origin

The flux of idealized trace gases across the thermal tropopause is quantified as a function of their chemical lifetime using the Model of Atmospheric Transport and Chemistry (MATCH) driven by National Centers for Environmental Prediction (NCEP) reanalyses. The flux is computed in the limit of instant stratospheric chemical loss, and tropospheric chemistry is idealized as decay with a constant lifetime, τc. Emissions are idealized as time independent, with either a generic anthropogenic pattern or a uniform ocean source. We find that the globally averaged flux into the stratosphere normalized by surface emissions is ∼1% for τc= 8 days and ∼30% for τc ∼ 140 days, slowly approaching the long-lived limit of balance between stratospheric sinks and surface sources. The qualitative τc dependence of the globally averaged flux is captured by a simple one-dimensional model. The flux patterns computed with MATCH for the NCEP reanalyses are insensitive to τc and reveal preferred pathways into the stratosphere: The divergent circulation feeding isentropic cross-tropopause transport, storm tracks in the winter hemisphere, and isentropic transport to high latitudes

Chaotic Lagrangian Trajectories around an Elliptical Vortex Patch Embedded in a Constant and Uniform Background Shear Flow

The Lagrangian flow around a Kida vortex [J. Phys. Soc. Jpn. 5 0, 3517 (1981)], an elliptical two‐dimensional vortex patch embedded in a uniform and constant background shear, is described by a nonintegrable two‐degree‐of‐freedom Hamiltonian. For small values of shear, there exist large chaotic zones surrounding the vortex, often much larger than the vortex itself and extremely close to its boundary. Motion within the vortex is integrable. Implications for two‐dimensional turbulence are discussed

Contrasting short and long term projections of the hydrological cycle in the Southern extratropics

Analysis of model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that, in the zonal mean, the near-term projections of summertime changes of precipitation in the Southern Hemisphere (SH) subtropics are very widely scattered among the models. As a consequence, over the next 50 years, the CMIP5 multimodel mean projects no statistically significant trends in the SH subtropics in summer. This appears to be at odds with the widely reported, and robust, poleward expansion of the subtropical dry zones by the end of the twenty-first century. This discrepancy between the shorter- and longer-term projections in SH summer, as shown here, rests in the recovery of the ozone hole in the coming decades, as a consequence of the Montreal Protocol. This is explicitly demonstrated by analyzing model experiments with the Whole Atmosphere Community Climate Model, version 4 (WACCM4), a high-top model with interactive stratospheric chemistry, and coupled to land, ocean, and sea ice components. Contrasting WACCM4 integrations of the representative concentration pathway 4.5 with and without trends in surface concentrations of ozone-depleting substances allows for demonstrating that stratospheric ozone recovery will largely offset the induced “wet gets wetter and dry gets drier” projections and the accompanying poleward expansion of the subtropical dry zone in the SH. The lack of near-term statistically significant zonal-mean changes in the SH hydrological cycle during summer is of obvious practical importance for many parts of the world, and it might also have implications for the Southern Ocean and the Antarctic continent

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

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

Southern Hemisphere Cloud–Dynamics Biases in CMIP5 Models and Their Implications for Climate Projections

This study quantifies cloud–radiative anomalies associated with interannual variability in the latitude of the Southern Hemisphere (SH) midlatitude eddy-driven jet, in 20 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two distinct model types are found. In the first class of models (type I models), total cloud fraction is reduced at SH midlatitudes as the jet moves poleward, contributing to enhanced shortwave radiative warming. In the second class of models (type II models), this dynamically induced cloud radiative warming effect is largely absent. Type I and type II models have distinct deficiencies in their representation of observed Southern Ocean clouds, but comparison with two independent satellite datasets indicates that the cloud–dynamics behavior of type II models is more realistic. Because the SH midlatitude jet shifts poleward in response to CO₂ forcing, the cloud–dynamics biases uncovered from interannual variability are directly relevant for climate change projections. In CMIP5 model experiments with abruptly quadrupled atmospheric CO₂ concentrations, the global-mean surface temperature initially warms more in type I models, even though their equilibrium climate sensitivity is not significantly larger. In type I models, this larger initial warming is linked to the rapid adjustment of the circulation and clouds to CO₂ forcing in the SH, where a nearly instantaneous poleward shift of the midlatitude jet is accompanied by a reduction in the reflection of solar radiation by clouds. In type II models, the SH jet also shifts rapidly poleward with CO₂ quadrupling, but it is not accompanied by cloud radiative warming anomalies, resulting in a smaller initial global-mean surface temperature warming

Contrasting short and long term projections of the hydrological cycle in the Southern extratropics

Analysis of model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that, in the zonal mean, the near-term projections of summertime changes of precipitation in the Southern Hemisphere (SH) subtropics are very widely scattered among the models. As a consequence, over the next 50 years, the CMIP5 multimodel mean projects no statistically significant trends in the SH subtropics in summer. This appears to be at odds with the widely reported, and robust, poleward expansion of the subtropical dry zones by the end of the twenty-first century. This discrepancy between the shorter- and longer-term projections in SH summer, as shown here, rests in the recovery of the ozone hole in the coming decades, as a consequence of the Montreal Protocol. This is explicitly demonstrated by analyzing model experiments with the Whole Atmosphere Community Climate Model, version 4 (WACCM4), a high-top model with interactive stratospheric chemistry, and coupled to land, ocean, and sea ice components. Contrasting WACCM4 integrations of the representative concentration pathway 4.5 with and without trends in surface concentrations of ozone-depleting substances allows for demonstrating that stratospheric ozone recovery will largely offset the induced “wet gets wetter and dry gets drier” projections and the accompanying poleward expansion of the subtropical dry zone in the SH. The lack of near-term statistically significant zonal-mean changes in the SH hydrological cycle during summer is of obvious practical importance for many parts of the world, and it might also have implications for the Southern Ocean and the Antarctic continent

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

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

Chaotic Lagrangian Trajectories around an Elliptical Vortex Patch Embedded in a Constant and Uniform Background Shear Flow

The Lagrangian flow around a Kida vortex [J. Phys. Soc. Jpn. 5 0, 3517 (1981)], an elliptical two‐dimensional vortex patch embedded in a uniform and constant background shear, is described by a nonintegrable two‐degree‐of‐freedom Hamiltonian. For small values of shear, there exist large chaotic zones surrounding the vortex, often much larger than the vortex itself and extremely close to its boundary. Motion within the vortex is integrable. Implications for two‐dimensional turbulence are discussed