Abstract

The distorted physics (DP) technique of Bryan and Lewis is applied to an ocean GCM to allow longer time steps to be taken. The model includes a hybrid vertical mixing scheme for tracers, consisting of a Kraus–Turner mixed layer model and a Richardson number–dependent diffusion term. The model also includes rotation of the diffusive tracer fluxes along the isopycnal surfaces, according to the scheme of Redi. The focus of this paper is on the interaction of DP with this mixing scheme, rather than on the physical performance of the scheme itself. With standard parameter settings as used in a recent climate change study, the equilibrium solutions produced in a long integration using DP and a time step of 1 day are not robust and drift on switching back to the standard time step of 1 h (without DP). Three regions are affected: the Antarctic Circumpolar Current, the Greenland–Iceland–Norwegian (GIN) Sea, and the base of the tropical mixed layer. The drifts are found to be due both to the DP technique itself and to the time step sensitivities in the model that are independent of DP.

Modifications to the isopycnal diffusion scheme are presented, which eliminate the drifts in the Antarctic Circumpolar Current and GIN Sea. However, the drift at the base of the tropical mixed layer is found to be due to an interaction between the mixed layer model, the Richardson number–dependent diffusion, and the implicit time stepping used for the diffusion equation. This is demonstrated in an idealized one-dimensional model. In the current coarse-resolution GCM configuration, the Richardson number dependence has little effect on the model's equilibrium solution (in the small time step limit), so the time step dependence can be reduced by removing the Richardson number scheme. However, in models with finer (horizontal) resolution, such as those currently used for studies of tropical variability and the next generation of global climate models, this is unlikely to be acceptable. Integrated diffusion-only mixing schemes may be the most practical long-term solution. Meanwhile, care is needed when using hybrid mixing schemes as in the current study to ensure that time truncation errors are at an acceptable level.

Abstract

A primitive equation numerical model is used to study the large amplitude behavior of unstable waves on an oceanic density front, concentrating on a single wave mode corresponding to the fastest growing linear solution. At Small amplitude the model results agree well with linear theory, and at large amplitude “backward breaking” occurs and cortex pairs are formed, as have been observed in laboratory experiments. Vortex stretching due to advection across layer depth contours favors formation of the vortex pairs, with the result that the β effect is not necessary for vortex detachment, as it was in a previous quasigeostrophic study by Ikeda.

Examination of the energetics allows a life cycle to be identified for the waves, and shows that kinetic energy is fed into the mean flow through Reynolds stress. It is shown that the β effect is important in determining the precise form of the mean flow generated, and this is interpreted in terms of the deep potential vorticity fluxes. For realistic parameters the mean flows generated agree well with observations of deep mean flows near the Gulf Stream; in particular there is a counterflow (westward) directly below the original position of the front and a positive (eastward) flow displaced to the south. This pattern is not found in the results of eddy-resolving general circulation models and is qualitatively different from the three-jet structure found in Ikeda's study of a

symmetric, quasi-geostrophic jet.

Abstract

No Abstract Available.

Abstract

Previous discussions of the “bright band,” a radar phenomenon associated with the freezing level, have treated it as related to a “normal” lapse rate—a progressive decrease of temperature with height, with surface temperatures above freezing. This paper deals with bright bands observed when surface temperatures were at, or below, freezing. Three cases are examined, all snowstorms of the 1960–61 season. In each case inspection of the available upper-air soundings confirmed the existence of prominent inversions aloft associated with the observed bright bands.

Abstract

The consistency between observed changes in Subantarctic Mode Water (SAMW) properties at 32°S in the Indian Ocean and model simulations is explored using the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3). Hydrographic data collected in 2002 show that the water mass is warmer and saltier on isopycnals than in 1987, in contrast to the isopycnal freshening observed between 1962 and 1987. The response of HadCM3 under a range of forcing scenarios is explored and the observed freshening is only seen in experiments that include greenhouse gas forcing; however, there is no subsequent return to more saline conditions in 2002. The response of the model to greenhouse gas forcing is dominated by a persistent freshening trend, the simulated water mass variability agrees well with that suggested by the limited observations. Comparing model isopycnal changes from the forced experiments with a control run shows that the changes from the 1960s to 2002 are best explained by internal variability. This is in contrast to earlier work, which attributed the observed isopycnal freshening to anthropogenic forcing. Although the model shows that at present an anthropogenic climate change signal is not detectable in SAMW, the model water mass freshens on isopycnals during the twenty-first century under increased greenhouse gas forcing. This is consistent with recent heat content observations, which suggest that the salting is unlikely to persist. In HadCM3, this freshening is due to an increasing surface heat flux and Ekman heat and freshwater flux into the water mass formation region. This paper emphasizes the importance of higher-frequency observations of SAMW if detection and attribution statements are to be made.

Abstract

In an experiment with the latest version of the Hadley Centre climate model the model response has been analyzed after the thermohaline circulation (THC) in the Atlantic Ocean has been suppressed. The suppression is induced by a strong initial perturbation to the salinity distribution in the upper layer of the northern North Atlantic. The model is then allowed to adjust freely. Salinity gradually increases and deep water formation in the Greenland and Norwegian Seas restarts, later also in the Labrador Sea. The meridional overturning recovers after about 120 yr. In the first few decades when the overturning is very weak surface air temperature is dominated by cooling of much of the Northern Hemisphere and weak warming of the Southern Hemisphere, leading to maximum global cooling of 0.9°C. The disruption to the atmosphere's radiation balance results in a downward flux anomaly at the top of the atmosphere, maximally 0.55 W m^–2 in the first decade then decreasing with the THC recovery.

The processes responsible for the recovery of the THC is examined in detail. In future model development this will help to reduce uncertainty in modeling THC stability. The recovery is driven by coupled ocean–atmosphere response. Northward salt transport by the subtropical gyre is crucial to the recovery of salinity in the North Atlantic. A southward shift of the ITCZ creates positive salinity anomalies in the tropical North Atlantic. This supports the northward salt transport by the subtropical gyre that helps to restart deep water formation and the THC.

Abstract

This paper describes a model intercomparison between a Bryan-Cox-type ocean model and an isopycnic-coordinate ocean model. The two models are integrated for 30 years on a domain of the North Atlantic stretching from 20°S to 82°N. The main purpose of this work is to illuminate aspects of the respective models that give a realistic representation of the North Atlantic circulation and the physical processes that occur therein, and to identify those which need to be improved. To this end, the same forcing fields and as many of the same parameter settings as possible were chosen so that differences between the models would be due to distinct model features rather than choice of parameters. Where we felt that using the same setup between the models was inappropriate or impossible, we examined the possible difference this could make to the simulators.

In the isopycnic model, the path of the North Atlantic Current after separation is simulated quite realistically, whereas in the Bryan-Cox model it becomes much too zonal in the central North Atlantic. This difference has an impact on the simulation in the subpolar gyre and in the Greenland-Iceland-Norway basin.

The representation of dense overflows across the Greenland-Iceland-Scotland ridge is found to be the main underlying difference in the way the simulators develop in the two models. The isopycnic model has specified isopycnic and diapycnic mixing, and its deep, dense flows over the ridge system retain their water properties. This is not the case in the Bryan-Cox model, in which the quasi-isopycnal mixing tensor includes both explicit background horizontal mixing (which could have a diapycnic component) and implicit diapycnic mixing arising from a limitation on the allowable slope of the mixing tensor. It is found that in this model the dense overflows mix vigorously with the surrounding warmer, saltier water as they flow over the ridge so that their water properties change relatively quickly as they travel downstream.

The formation of subpolar mode waters occurs primarily in the Irminger Basin in both models, and in the isopycnic model this mode water has reasonable characteristics. In the Bryan-Cox model the processes down-stream of the dense overflows are degraded due to the development of a homogeneous water mass below the surface, originating from the mixed overflow water. This water mass prevents the mixed layer from deepening substantially in the subpolar gyre, and so prevents the formation of realistic amounts of mode water. The growth of this homogeneous water mass may also be at least partly responsible for the zonality of the North Atlantic Current in the Bryan-Cox model.

The results provide guidance on the future development of both types of model.

Abstract

In a companion paper, two ocean general circulation models were implemented in order to simulate and intercompare the main features of the North Atlantic circulation: the Atlantic Isopycnic Model (AIM) and the Hadley Centre Bryan-Cox-type ocean model (HC). Starting from the same initial state and using the same mechanical and thermohaline forcing datasets, both models were spun up from rest for 30 years. This paper examines the western boundary currents, meridional heat transport and subtropical gyre ventilation.

AIM transports more heat poleward in the subtropics (with peak annual-mean meridional heat transport of 0.63 PW) than HC (which transports up to 0.48 PW), a difference that arises primarily due to surface-poleward and deep-equatorward flows, which are stronger, and at warmer and colder extremes, than in HC. However, HC displays stronger heat transport across the subpolar gyre (with a secondary maximum of 0.36 PW compared to 0.24 PW in AIM), consistent with stronger subpolar gyre heat gain (due to a more zonal North Atlantic Current path, leading to larger relaxation surface heat fluxes).

To quantify the effect of diapycnic mixing and bathymetry two separate 30-year integrations of the isopycnic model, without diapycnal mixing and with the same bathymetry as HC, were undertaken. The isopycnic model is relatively insensitive to these two aspects of model setup on the 30-year timescale.

Both models develop subtropical gyres of annual mean strength ∼45 Sv (Sv ≡ 10⁶ m³ s⁻¹ (due to essentially identical Sverdrup responses), although AIM displays stronger seasonal cycles of Gulf Stream transport than HC (probably due to differences in topographic responses). At subtropical latitudes deep western boundary currents are weaker in AIM (∼5 Sv) than in HC (∼10 Sv). although in HC them is an approximate halving in strength of the DWBC as it progresses south of Florida, due to abyssal recirculation and upwelling.

In the subtropical gyro AIM displays a clear pattern of ventilation, and potential vorticity is, to a large degree, conserved along particle trajectories inside the thermocline. Ventilation pathways are less sharply defined in HC and, compared to AIM, horizontal mixing of temperature and salinity more strongly limits the degree to which water properties (including potential vorticity) are conserved along isopycnals. Both models annually renew realistic quantities of subtropical mode water, AIM forming 15 Sv compared to 20 Sv in HC. Subsurface isopycnal warming in AIM is related to 30-year trends of surface cooling with little corresponding change in salinity. Subsurface isopycnal cooling in HC is due to surface cooling and freshening.

Abstract

A substantial fraction of the deep ocean is ventilated in the high-latitude North Atlantic. Consequently, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. However, owing to the Lagrangian nature of the process, many aspects of deep Atlantic Ocean ventilation and its representation in climate simulations remain obscure. We investigate the nature of ventilation in the high-latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a comprehensive set of Lagrangian trajectory experiments. Backward-in-time trajectories from a model-defined North Atlantic Deep Water (NADW) reveal the locations of subduction from the surface mixed layer at high spatial resolution. The major fraction of NADW ventilation results from subduction in the Labrador Sea, predominantly within the boundary current (~60% of ventilated NADW volume) and a smaller fraction arising from open ocean deep convection (~25%). Subsurface transformations—due in part to the model’s parameterization of bottom-intensified mixing—facilitate NADW ventilation, such that water subducted in the boundary current ventilates all of NADW, not just the lighter density classes. There is a notable absence of ventilation arising from subduction in the Greenland–Iceland–Norwegian Seas, due to the re-entrainment of those waters as they move southward. Taken together, our results emphasize an important distinction between ventilation and dense water formation in terms of the location where each takes place, and their concurrent sensitivities. These features of NADW ventilation are explored to understand how the representation of high-latitude processes impacts properties of the deep ocean in a state-of-the-science numerical simulation.

Abstract

The North Atlantic Ocean subpolar gyre (NA SPG) is an important region for initializing decadal climate forecasts. Climate model simulations and paleoclimate reconstructions have indicated that this region could also exhibit large, internally generated variability on decadal time scales. Understanding these modes of variability, their consistency across models, and the conditions in which they exist is clearly important for improving the skill of decadal predictions—particularly when these predictions are made with the same underlying climate models. This study describes and analyzes a mode of internal variability in the NA SPG in a state-of-the-art, high-resolution, coupled climate model. This mode has a period of 17 yr and explains 15%–30% of the annual variance in related ocean indices. It arises because of the advection of heat content anomalies around the NA SPG. Anomalous circulation drives the variability in the southern half of the NA SPG, while mean circulation and anomalous temperatures are important in the northern half. A negative feedback between Labrador Sea temperatures/densities and those in the North Atlantic Current (NAC) is identified, which allows for the phase reversal. The atmosphere is found to act as a positive feedback on this mode via the North Atlantic Oscillation (NAO), which itself exhibits a spectral peak at 17 yr. Decadal ocean density changes associated with this mode are driven by variations in temperature rather than salinity—a point which models often disagree on and which may affect the veracity of the underlying assumptions of anomaly-assimilating decadal prediction methodologies.