Abstract

Suarez and Duffy have noted an interesting bifurcation in a two-level gridpoint general circulation model when strong tropical heating is imposed. This bifurcation results in a model climatology with strong upper-level westerlies in the tropics. In this paper, it is argued that this bifurcation is essentially due to the dominant role played by extratropical baroclinic transients in the tropical angular momentum budget. A series of numerical experiments is analyzed with a global two-level primitive equation model, using spectral truncation in the horizontal. The model climatologies in these experiments fall into two categories: 1) conventional, that is, weak upper-level easterlies/westerlies in the tropics; and 2) superrotating, that is, strong upper-level westerlies in the tropics. An attempt is made to explain the maintenance of the general circulation in these two radically different climatologies by studying the properties of unstable normal modes for the two different time-mean states. The spectral characteristics of angular momentum transport due to transient eddies in these two climatologies are also discussed. To understand the meridional propagation of transient eddies, the notion of a “modal” refractive index in the quasigeostrophic approximation is introduced. From this analysis it is concluded that the conventional climatology is stable to weak perturbations, with the “restoring” force being provided primarily by extratropical baroclinic eddies. Strong perturbations completely change the propagation characteristics of these eddies, leading to a bifurcation of the general circulation. This has interesting implications for the range of validity of two-level models and the transitivity of tropospheric general circulation.

Abstract

A simple one-dimensional model of the quasi-biennial oscillation is discussed. Our model is essentially a generalization of the Holton–Lindzen models. We consider a large number of vertically propagating internal waves interacting simultaneously with the mean flow. Effects of both wave damping and critical level absorption are included, but wave–wave interaction is neglected. The effects of momentum advection due to the Hadley circulation are also parameterized. This model is used to study how the mean flow in the equatorial lower stratosphere would respond to forcing by a tropospheric wave spectrum with a significant amount of momentum flux at slow horizontal phase speeds. We find that a “continuous” wave spectrum forces mean flow oscillations in a manner quite similar to a “discrete” two-wave spectrum. But the factors that control the period and amplitude of the oscillations, in the case of a continuous spectrum, seem to be quite different. Our results also suggest that mean rising motion in the tropics may play an important role in determining the vertical structure of the QBO near the tropopause.

Abstract

The characteristics of atmospheric low-frequency variability and midlatitude SST variability as simulated by the National Center for Atmospheric Research’s Climate System Model are analyzed in the vicinity of the North Pacific and North Atlantic basins. The simulated spatial patterns of variability correspond quite well to those seen in observational datasets, although there are some differences in the amplitudes of variability. Companion uncoupled integrations using the atmospheric component of the coupled model are also analyzed to identify the mechanisms of midlatitude SST variability on interannual timescales. These integrations are subject to a hierarchy of SST boundary conditions, ranging from the climatological annual cycle to global monthly mean observed SST. Even uncoupled atmospheric model integrations forced by climatological SST boundary conditions are capable of simulating the spatial patterns of atmospheric variability fairly well, although coupling to an interactive ocean does produce some improvements in the spatial patterns. However, the presence of realistic SST variability, especially in the Tropics, is necessary to obtain the right variance amplitudes for the different modes of variability. It appears that coupling to an interactive ocean essentially reorders, rather than reshapes, the dominant modes of atmospheric low-frequency variability. The results indicate that the dominant modes of SST variability in each ocean basin are forced by the respective dominant modes of atmospheric low-frequency variability in the vicinity of the ocean basin. The relationship between atmospheric variability and the surface heat flux is also analyzed. Evidence is found for a local thermal feedback in the coupled integration, associated with the finite heat capacity of the ocean, that acts to damp surface heat flux variability. It is also shown that the relationship between midlatitude SST anomalies and the surface heat flux in the Atmospheric Model Intercomparison Project–type of atmospheric model integrations is quite different from that in the coupled model integration.

Abstract

The interaction between tropical Atlantic variability and El Niño–Southern Oscillation (ENSO) is investigated using three ensembles of atmospheric general circulation model integrations. The integrations are forced by specifying observed sea surface temperature (SST) variability over a forcing domain. The forcing domain is the global ocean for the first ensemble, limited to the tropical ocean for the second ensemble, and further limited to the tropical Atlantic region for the third ensemble. The ensemble integrations show that extratropical SST anomalies have little impact on tropical variability, but the effect of ENSO is pervasive in the Tropics. Consistent with previous studies, the most significant influence of ENSO is found during the boreal spring season and is associated with an anomalous Walker circulation. Two important aspects of ENSO’s influence on tropical Atlantic variability are noted. First, the ENSO signal contributes significantly to the “dipole” correlation structure between tropical Atlantic SST and rainfall in the Nordeste Brazil region. In the absence of the ENSO signal, the correlations are dominated by SST variability in the southern tropical Atlantic, resulting in less of a dipole structure. Second, the remote influence of ENSO also contributes to positive correlations between SST anomalies and downward surface heat flux in the tropical Atlantic during the boreal spring season. However, even when ENSO forcing is absent, the model integrations provide evidence for a positive surface heat flux feedback in the deep Tropics, which is analyzed in a companion study by Chang et al. The analysis of model simulations shows that interannual atmospheric variability in the tropical Pacific–Atlantic system is dominated by the interaction between two distinct sources of tropical heating: (i) an equatorial heat source in the eastern Pacific associated with ENSO and (ii) an off-equatorial heat source associated with SST anomalies near the Caribbean. Modeling this Caribbean heat source accurately could be very important for seasonal forecasting in the Central American–Caribbean region.

Abstract

The performance of thermal surface boundary conditions based on energy balance models for the atmosphere is tested using a two-dimensional (meridional plane) ocean model. The results are compared to those from an idealized ocean – atmosphere coupled system. The latter consists of a two-dimensional Boussinesq ocean model coupled to a two-layer global atmospheric model. The various thermal boundary conditions are applied to the same ocean model used in the coupled system, and their ability to capture the essential atmospheric feedbacks is investigated. Some of the effects associated with the atmospheric eddy moisture transport are also incorporated by empirically relating variations in the surface freshwater flux to variations in the surface heat flux based on the coupled model results. Comparisons with the coupled results show a considerable improvement in the characteristics of the equilibria of the ocean thermohaline circulation when the alternative thermohaline boundary conditions are used instead of the so-called “mixed boundary conditions” commonly used in ocean-only integrations. Furthermore, the response of the pole-to-pole equilibrium to a freshening of the high northern latitudes is in remarkably good agreement with the one observed in the coupled model. However, a tendency for the “energy balance” boundary conditions to overstabilize the circulation is detected, and limitations in the present treatment of the eddy moisture transport effects are found, especially in the presence of convective adjustment.

Abstract

The three-dimensional nature of breaking Rossby waves in the polar wintertime stratosphere is studied using an idealized global primitive equation model. The model is initialized with a well-formed polar vortex, characterized by a latitudinal band of steep potential vorticity (PV) gradients. Planetary-scale Rossby waves are generated by varying the topography of the bottom boundary, corresponding to undulations of the tropopause. Such topographically forced Rossby waves then propagate up the edge of the vortex, and their amplification with height leads to irreversible wave breaking.

These numerical experiments highlight several nonlinear aspects of stratospheric dynamics that are beyond the reach of both isentropic two-dimensional models and fully realistic GCM simulations. They also show that the polar vortex is contorted by the breaking Rossby waves in a surprisingly wide range of shapes.

With zonal wavenumber-1 forcing, wave breaking usually initiates as a deep helical tongue of PV that is extruded from the polar vortex. This tongue is often observed to roll up into deep isolated columns, which, in turn, may be stretched and tilted by horizontal and vertical shears. The wave amplitude directly controls the depth of the wave breaking region and the amount of vortex erosion. At large forcing amplitudes, the wave breaking in the middle/lower portions of the vortex destroys the PV gradients essential for vertical propagation, thus shielding the top of the vortex from further wave breaking.

The initial vertical structure of the polar vortex is shown to play an important role in determining the characteristics of the wave breaking. Perhaps surprisingly, initially steeper PV gradients allow for stronger vertical wave propagation and thus lead to stronger erosion. Vertical wind shear has the notable effect of tilting and stretching PV structures, and thus dramatically accelerating the downscale stirring. An initial decrease in vortex area with increasing height (i.e., a conical shape) leads to focusing of wave activity, which amplifies the wave breaking. This effect provides a geometric interpretation of the “preconditioning” that often precedes a stratospheric sudden warming event. The implications for stratospheric dynamics of these and other three-dimensional vortex properties are discussed.

Abstract

An idealized coupled ocean–atmosphere is constructed to study climatic equilibria and variability. The model focuses on the role of large-scale fluid motions in the climate system. The atmospheric component is an eddy-resolving two-level global primitive equation model with simplified physical parameterizations. The oceanic component is a zonally averaged sector model of the thermohaline circulation. The two components exchange heat and freshwater fluxes synchronously. Coupled integrations are carried out over periods of several centuries to identify the equilibrium states of the ocean–atmosphere system. It is shown that there exist at least three types of equilibria, which are distinguished by whether they have upwelling or downwelling in the polar regions. Each of the coupled equilibria has a close analog in the ocean-only model with mixed boundary conditions. The oceanic circulation in the coupled model exhibits natural variability on interdecadal and longer timescales. The dominant interdecadal mode of variability is associated with the advection of oceanic temperature anomalies in the sinking regions. The sensitivity of the coupled model to climatic perturbations is studied. A rapid increase in the greenhouse gas concentrations leads to a collapse of the meridional overturning in the ocean. Introduction of a large positive surface freshwater anomaly in the high latitudes leads to a temporary suppression of the sinking motion, followed by a rapid recovery, due primarily to the high latitude cooling associated with the reduction of oceanic heat transport. In this evolution, the secondary roles played by the atmospheric heat transport and moisture transport in destabilizing the thermohaline circulation are compared, and the former is found to be dominant.

Abstract

Vertical wind shear over the tropical Atlantic Ocean plays an important role in mediating hurricane activity. The vertical shear variability over the main development region for Atlantic hurricanes is affected by local factors as well as by the remote influence of the El Niño–Southern Oscillation (ENSO) phenomenon, as indicated by observational and climate modeling analyses. Climate model simulations of the ENSO–shear relationship are compared with observations. It is shown that there is a strong influence of background mean flow on the ENSO–shear relationship, because of the inherently nonlinear nature of vertical wind shear. In particular, the simulation of zonal flow over the tropical Atlantic is shown to play a critical role in how the remote influence of ENSO modulates the shear. Even with realistic simulations of the ENSO-induced westerly anomaly in the upper troposphere, overestimated easterly background flow in the model simulations can alter the relationship between ENSO and vertical wind shear, resulting in decreased vertical wind shear during warm ENSO events. This nonlinear superposition has important implications for the assessment of trends in large-scale environmental factors affecting tropical cyclogenesis in climate change simulations.

Abstract

The role of the wind–evaporation–sea surface temperature (WES) feedback in the low-frequency natural variability of the tropical Atlantic is studied using an atmospheric global climate model—the NCAR Community Climate Model, version 3 (CCM3)—thermodynamically coupled to a slab ocean model (SOM). The coupled model is modified to suppress the WES feedback and is compared to a control run. Singular value decomposition (SVD) analysis over the tropical Atlantic reveals that the coupled meridional mode of the Atlantic Ocean is amplified in the presence of the WES feedback. In its absence, the meridional mode still exists, but with a weaker amplitude. A feedback mechanism that involves the near-surface specific humidity is proposed to sustain the weaker Atlantic meridional mode in the absence of the WES feedback. Similar analysis of coupled model integrations when forced with an artificial El Niño–Southern Oscillation (ENSO)-like SST cycle in the Pacific reveals that in the presence of the WES feedback, the meridional mode is the preferred mode of response of the tropical Atlantic to ENSO forcing. In the absence of the WES feedback, the tropical Atlantic response is unlike the meridional mode and the effects of tropospheric warming and subsidence dominate. Regression analysis over the tropical Atlantic reveals that the meridional mode response to ENSO peaks in the spring and begins to decay in the fall in the coupled model in the presence of the WES feedback. The WES feedback also appears to be responsible for the northward migration of the ITCZ during ENSO events.

Abstract

The role of the wind–evaporation–sea surface temperature (WES) feedback in the propagation of the high-latitude cooling signal to the tropical oceans using the NCAR atmospheric Community Climate Model (CCM3) coupled thermodynamically to a slab-ocean model (SOM) is studied. Abruptly imposed additional Northern Hemispheric sea ice cover equivalent to the Last Glacial Maximum (LGM; 18 kyr BP) in the model causes a Northern Hemisphere–wide cooling, as well as the generation and amplification of an anomalous cross-equatorial meridional SST dipole associated with a southward migration of the intertropical convergence zone (ITCZ) stabilizing within a period of 5 yr. In experiments where the WES feedback is switched off explicitly by modifying the sensible and latent heat flux bulk aerodynamic formulations over the oceans in CCM3, imposed Northern Hemispheric sea ice also results in widespread northern cooling at the same rate as the unmodified run, suggesting that the WES feedback is not essential in the propagation of the high-latitude cooling signal to the deep tropics. However, the WES-off experiment generates a weaker cross-equatorial SST dipole with a modest southward movement of the ITCZ, suggesting that the WES feedback is responsible for amplifying SST and atmospheric anomalies in the deep tropics during their transition to the new equilibrium state. The propagation of high-latitude cooling to the deep tropics is proposed to be caused by the decrease of near-surface specific humidity in the northern tropics.