Multiple equilibria and oscillatory modes in a mid-latitude ocean-forced atmospheric model (original) (raw)
Related papers
Atmospheric Dynamics Triggered by an Oceanic SST Front in a Moist Quasigeostrophic Model
Journal of the Atmospheric Sciences, 2012
To understand the atmospheric response to a midlatitude oceanic front, this paper uses a quasigeostrophic (QG) model with moist processes. A well-known, three-level QG model on the sphere has been modified to include such processes in an aquaplanet setting. Its response is analyzed in terms of the upper-level atmospheric jet for sea surface temperature (SST) fronts of different profiles and located at different latitudes. When the SST front is sufficiently strong, it tends to anchor the mean atmospheric jet, suggesting that the jet’s spatial location and pattern are mainly affected by the latitude of the SST front. Changes in the jet’s pattern are studied, focusing on surface sensible heat flux and on moisture effects through latent heat release. It is found that latent heat release due to moist processes is modified when the SST front is changed, and this is responsible for the meridional displacement of the jet. Moreover, both latent heat release and surface sensible heat flux con...
Atmospheric dynamics triggered by an oceanic SST front in
ABSTRACT To understand the atmospheric response to a mid-latitude oceanic front, this paper uses a quasi-geostrophic (QG) model with moist processes. A well-known, three-level QG model on the sphere has been modified to include such processes. Its response is analyzed in terms of the upper-level atmospheric jet for sea surface temperature (SST) fronts of different profiles and located at different latitudes.
Coupled Ocean-Atmosphere Dynamics in a Simple Midlatitude Climate Model
Journal of Climate, 2001
Midlatitude air-sea interactions are investigated by coupling a stochastically forced two-layer quasigeostrophic channel atmosphere to a simple ocean model. The stochastic forcing has a large-scale standing pattern to simulate the main modes of low-frequency atmospheric variability. When the atmosphere interacts with an oceanic mixed layer via surface heat exchanges, the white noise forcing generates an approximately red noise sea surface temperature (SST) response. As the SST adjusts to the air temperature changes at low frequency, thus decreasing the heat flux damping, the atmospheric spectra are slightly reddened, the power enhancement increasing with the zonal scale because of atmospheric dynamics. Decadal variability is enhanced by considering a first baroclinic oceanic mode that is forced by Ekman pumping and modulates the SST by entrainment and horizontal advection. The ocean interior is bounded at its eastern edge, and a radiation condition is used in the west. Primarily in wintertime conditions, a positive feedback takes place between the atmosphere and the ocean when the atmospheric response to the SST is equivalent barotropic. Then, the ocean interior modulates the SST in a way that leads to a reinforcement of its forcing by the wind stress, although the heat flux feedback is negative. The coupled mode propagates slowly westward with exponentially increasing amplitude, and it is fetch limited. The atmospheric and SST spectral power increase at all periods longer than 10 yr when the coupling with the ocean interior occurs by entrainment. When it occurs by advection, the power increase is primarily found at neardecadal periods, resulting in a slightly oscillatory behavior of the coupled system. Ocean dynamics thus leads to a small, but significant, long-term climate predictability, up to about 6 yr in advance in the entrainment case.
A sustained oscillation in a toy-model of the coupled atmosphere-ocean system
Arxiv preprint arXiv:1106.1779, 2011
Interaction between atmospheric mid-latitude flow and wind-driven ocean circulation is studied coupling two idealized low-order spectral models. The barotropic Charney-DeVore model with three components simulates a bimodal mid-latitude atmospheric circulation in a channel with two stable flow patterns induced by topography. The wind-driven ocean double gyre circulation in a square basin (of half the channel length) is modeled by an equivalent barotropic formulation of the Veronis model with 21 components, which captures Rossby-wave dynamics and nonlinear decadal variability. When coupled, the atmosphere forces the ocean by wind-stress while, simultaneously, the ocean affects the atmosphere by thermal forcing in terms of a vorticity source. Coupled atmosphere-ocean simulations show two stable flow patterns associated with the topographically induced atmospheric bimodality and a sustained oscillation due to interaction between atmospheric bimodality and oceanic Rossby dynamics. The oscillation is of inter-annual to inter-decadal periodicity and occurs in a reasonably wide parameter domain.
An ocean model's response to North Atlantic Oscillation-like wind forcing
Geophysical Research Letters, 1998
The response of the Atlantic Ocean to North Atlantic Oscillation (NAO)-like wind forcing has been investigated using an ocean-only general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the interannual to multi-decadal frequency response of the ocean to a winter wind anomaly pattern. South of 30 N, the sea surface temperature (SST) response of the model was almost exactly in phase with the forcing and largely independent of the forcing frequency suggesting that the subtropical ocean response to the overlying atmosphere is fast and direct. Poleward of 30 N and in particular in the Gulf stream extension region strong SST anomalies lagged the forcing by several years. They were sustained by deep reaching temperature anomalies which were then re-exposed to the atmosphere during the winter season. Overall the strength of the SST response increased slightly with longer forcing periods. In the subpolar gyre, however, the model showed a broad response maximum in the decadal band (12-16 years). The implications for the existence of a decadal coupled mode are discussed.
On the origin of interdecadal oscillations in a coupled ocean–atmosphere model
Tellus A, 2007
A B S T R A C T Interdecadal oscillations are analysed in a coupled ocean-atmosphere model made of a planetary geostrophic ocean model within an idealized geometry, coupled to a zonally-averaged tropospheric atmosphere model. The interdecadal variability that arises spontaneously in this coupled system is caused by intrinsic ocean dynamics, the coupled air-sea feedbacks being not essential. The spatial pattern of the variability bears some resemblance with observations and results obtained with atmosphere-ocean general circulation models (AOGCMs) as well as simpler climate models: large and quasi-stationary upper ocean temperature-dominated density anomalies are found in the north-western part of the ocean basin along with weaker, westward propagating anomalies in the remaining interior. The basic physical mechanism that lies at the heart of the existence of the interdecadal mode is a large-scale baroclinic instability of the oceanic mean flow in the vicinity of the western boundary, characteristic of ocean models forced by constant surface fluxes. Freshwater feedbacks associated with the hydrological cycle are found to have only a modest influence on the interdecadal mode. The presence of a periodic channel mimicking the Antarctic Circumpolar Current at high southern latitudes prevents the oceanic baroclinic instability to occur in the Southern Hemisphere.
Arxiv preprint physics/0511208, 2005
A quasi-geostrophic intermediate complexity model is considered, providing a schematic representation of the baroclinic conversion processes which characterize the physics of the mid-latitudes atmospheric circulation. The model is relaxed towards a given latitudinal temperature profile, which acts as baroclinic forcing, controlled by a parameter T E determining the forced equator-topole temperature gradient. As T E increases, a transition takes place from a stationary regime to a periodic regime, and eventually to an earth-like chaotic regime where evolution takes place on a strange attractor. The dependence of the attractor dimension, metric entropy, and bounding box volume in phase space is studied by varying both T E and model resolution. The statistical properties of observables having physical relevance, namely the total energy of the system and the latitudinally averaged zonal wind, are also examined. It is emphasized that while the attractor's properties are quite sensitive to model resolution, the global physical observables depend less critically on it. For more detailed physical observables, such as the latitudinal profiles of the zonal wind, model resolution again may be critical: the effectiveness of the zonal wind convergence, acting as barotropic stabilization of the baroclinic waves, heavily relies on the details of the latitudinal structure of the fields. The necessity and complementarity of both the dynamical systems and physical approach is underlined.
Monthly Weather Review
The middle-latitude standing wave problem is investigated by means of a quasi-geostrophic, linear, steadJ;-state model in which the zonal current is perturbed by the lower boundary topography and by a distribution of heat sources and sinks. All the perturbations are assumed to have a single meridional wavelength and the dissipation is considered to take place in the surface boundary layer using, as a first approach, a horizontally uniform drag coefficient. After investigating some basic properties of the model atmosphere, some computations are made to determine its response to the combined forcing by topography and by diabatic heating for January 1962. The resulting perturbations are found to be in rather good agreement with the observed standing waves. The results also indicate that the standing waves forced by the topography are in about the same position as those forced by the diabatic heating and that the former have somewhat larger amplitudes than the latter. The effect of allowing the drag coefficient to have one constant value over the continents and a smaller constant value over the oceans is examined and found to be quite important when the ratio of the two values is 6, but small (yet such as t o bring the computed and observed eddies into closer agreement than in the case of a uniform drag coefficient) for a ratio of 2.
Dynamics of Atmospheres and Oceans, 2003
A simplified coupled ocean-atmosphere model, consisting of a one-layer bidimensional ocean model and a one-layer unidimensional energy balance atmospheric model [J. Clim. 13 (2000) 232] is used to study the unstable interactions between zonal winds and ocean gyres. In a specific range of parameters, decadal variability is found. Anomalies, quite homogeneous zonally, show small-scale wavelength in latitude: perturbations emerge and grow at the southern limb of the intergyre boundary and propagate southward before decaying. The wind stress anomalies are proportional to the meridional gradient of the atmospheric temperature anomalies: this ratio acts as a positive amplification factor, as confirmed by a parameter sensitivity analysis. Assuming zonally-averaged anomalies harmonic in the meridional direction, a very simple analytical model for the perturbations is derived, based on forced Rossby wave adjustment of the western boundary current and its associated anomalous heat transport: it accounts for the scale selection, the growth and the southward propagation of sea surface temperature anomalies in the subtropical gyre. The latter is not only due to the slow advection by the mean current, but to a prevailing mechanism of self-advecting coupled oceanic and atmospheric waves, out of phase in latitude. Relevance to the observational record is discussed.
The nonlinear response of an equatorial ocean to oscillatory forcing
Journal of Marine Research, 1993
The effect of upper ocean variability on deep equatorial flow is simulated by prescribing a forcing wave field near the eastern boundary in a one and a half layer shallow water model. Equatorially trapped long Rossby waves are generated by the forcing, have approximately linear dynamics, and propagate westward. Near the western boundary the dynamics are nonlinear, and there is a large mean flow. With forcing symmetric about the equator, the nonlinear response has two distinct phases. When the interior flow in the vicinity of the equator is westward near the western boundary, a poleward flowing western boundary current forms. This flow separates from the boundary several deformation radii away from the equator. When the interior flow is eastward, a recirculation gyre sets up. This gyre has dynamics similar to the mid-latitude recirculation of the Gulf Stream. The zonal scale of the gyre depends not only on the amplitude of the interior wave field, but also on the period of oscillation and the magnitude of the viscosity. The meridional structure and amplitude of the zonal flow can be understood using a model of a constant potential vorticity zonally elongated gyre. The net Lagrangian circulation resulting from the combination of the interior wave field and the nonlinear flow near the western boundary is found by tracking floats in the model. In the interior, fluid parcels move westward along the equator in the Stokes drift of the Rossby waves. The potential vorticity of fluid parcels is altered near the western boundary so that the floats are returned to the interior poleward of the equator. Significant mixing of fluid parcels occurs near the western boundary.