Surface Freshwater Flux Research Articles

Lagrangian mixed layer modeling of the western equatorial Pacific

Processes that control the upper ocean thermohaline structure in the western equatorial Pacific are examined using a Lagrangian mixed layer model. The one‐dimensional bulk mixed layer model of Garwood (1977) is integrated along the trajectories derived from a nonlinear 1½ layer reduced gravity model forced with actual wind fields. The Global Precipitation Climatology Project (GPCP) data are used to estimate surface freshwater fluxes for the mixed layer model. The wind stress data which forced the 1½ layer model are used for the mixed layer model. The model was run for the period 1987–1988. This simple model is able to simulate the isothermal layer below the mixed layer in the western Pacific warm pool and its variation. The subduction mechanism hypothesized by Lukas and Lindstrom (1991) is evident in the model results. During periods of strong South Equatorial Current, the warm and salty mixed layer waters in the central Pacific are subducted below the fresh shallow mixed layer in the western Pacific. However, this subduction mechanism is not evident when upwelling Rossby waves reach the western equatorial Pacific or when a prominent deepening of the mixed layer occurs in the western equatorial Pacific due to episodes of strong wind and light precipitation associated with the El Niño‐Southern Oscillation. Comparison of the results between the Lagrangian mixed layer model and a locally forced Eulerian mixed layer model indicated that horizontal advection of salty waters from the central Pacific strongly affects the upper ocean salinity variation in the western Pacific, and that this advection is necessary to maintain the upper ocean thermohaline structure in this region.

A 1-D atmospheric energy balance model developed for ocean modelling

We present a simple, deterministic energy balance model. The model is designed to represent the atmospheric component of the coupled atmosphere-ocean system. It is a one dimensional, global model with time and space resolutions of one year and 10° of latitude respectively. The model predicts the surface air temperature and estimates the surface freshwater flux diagnostically. The coupling between the atmospheric model and an ocean model is accomplished by heat and freshwater fluxes at their interface. The heat flux is calculated according to the difference in the surface air temperature and ocean surface temperature, while the freshwater flux is estimated from the latent heat transport in the atmosphere by a diagnostic equation. Two parameterizations for the latent heat transport are proposed, which distinguishes the two versions of the model.

Thermohaline circulation: Energetics and variability in a single‐hemisphere basin model

A series of numerical experiments have been carried out to explore the variability of the thermohaline circulation and the energetics of halocline catastrophe. It is found that when the amplitude of surface freshwater flux is smaller than a critical value, the thermohaline circulation is in a thermal mode, with deep water formed in the north. When the freshwater flux amplitude is supercritical, the thermohaline circulation does not reach a single steady state. Instead, the model ocean is in a continuous transition between a slow, quasi‐steady saline mode and an energetic, unsteady thermal mode. For cases with no wind stress, the saline mode is characterized by sinking along the equator. For cases with wind stress, the saline mode is characterized by intermediate water formation at midlatitude. During the saline mode phase, the deep water gradually becomes warm and salty. Thus at the end of the saline mode phase, within the northern basin there is cold and relatively fresh water lying on top of warm and salty water. Such a vertical structure is potentially very unstable because small perturbations can grow, supported by the release of potential energy during strong cooling. A quantity called the diabatic available potential energy index is introduced as an indicator for such convective instability. Thus a final equilibrium in the saline mode cannot be reached; instead, the model ocean flips to a very energetic thermal mode in which violent overturning destroys the vertical stratification. After the energy is released, the saline cell supported by precipitation in the subpolar basin advances southward and the model ocean returns to the saline mode. The whole cycle will be repeated.

The Response of a Global Ocean General Circulation Model to Climatological Surface Boundary Conditions for Temperature and Salinity

Abstract The response of a global ocean general circulation model (OGCM) to different surface heat and freshwater fluxes is examined. The aim is to determine the most appropriate way to spin up an OGCM prior to coupling it with an atmospheric GCM. Attention is focused on surface boundary conditions for salinity since other studies suggest that so-called “mixed” boundary conditions (i.e., a restoring condition on sea surface temperature and flux condition on sea surface salinity) can yield multiple equilibrium states in ocean models. Under the influence of restoring surface boundary conditions, the rate of overturning in the model North Atlantic is in good agreement with observed estimates. Under the influence of mixed surface boundary conditions, however, the rate of deep-water formation in the model North Atlantic is far in excess of the observed estimates. If the equilibrium solution obtained under restoring boundary conditions is subjected to an equivalent set of mixed boundary conditions, the presence...

Comments on “Two Stable Equilibria of a Coupled Ocean-Atmosphere Model”

Manabe and Stouffer (1988) previously have presented the results of experiments with a global coupled ocean-atmosphere climate model, which suggests that the climate of the earth may be bimodal in character. They couple a general circulation model of the world oceans (OGCM) to a general circulation models of the atmosphere (AGCM) by imposing continuity of heat and freshwater fluxes and stresses at the ocean-atmosphere interface. They attribute the large difference between the results and the modern ocean to a systematic bias in the model surface ocean freshwater flux. The authors of this comment attribute the bias in the MS OAGCM and the necessity of the freshwater flux correction to a failure of the model to adequately simulate surface freshwater fluxes, in particular for the GIN sea. The authors feel that if the GIN Sea is indeed critical, a possible solution may be to use a nested ocean-atmosphere grid in this region similar to that implemented for ocean eddies by Spall and Holland (1991).

The influence of high‐latitude surface forcing on the global thermohaline circulation

The basin‐averaged, latitude‐depth ocean model of Wright and Stocker (1992) is used to simulate the deep circulation of the world ocean. Under present‐day surface forcing, sinking occurs in the North Atlantic and the southern ocean, and realistic temperature and salinity structures are obtained in the Atlantic, Pacific, and Indian oceans. “Color” tracers and radiocarbon are used to identify the composition of the deepwater masses and the associated renewal time scales. While broad agreement with observations is found in all basins, the water masses in the southern ocean are too young. The global thermohaline circulation and the composition of the deepwater masses are sensitive to the buoyancy contrast between the southern ocean and the North Atlantic. This contrast can be modified by changing relaxation values of temperature and salinity at the northern and southern high latitudes. If the model is forced with the zonal averages of the observed surface salinity, North Atlantic Deep Water is the dominant deep ocean water mass, and hardly any Antarctic Bottom Water flows into the Atlantic. Choosing instead the observed salinities of the newly formed deep water as the restoring values, the model realistically simulates the penetration of Antarctic Bottom Water into the different ocean basins. This has a global effect through reducing both strength and depth of North Atlantic Deep Water formation. If higher surface salinity values are applied in the southern ocean, a steady state is obtained whose tracer distributions and overturning are consistent with reconstructions of the deep circulation during the last glacial maximum. The two states are stable also under mixed boundary conditions and transitions are possible by smoothly varying the surface freshwater flux of one state to that of the other. These experiments suggest the importance of modified high‐latitude forcing in glacial‐to‐interglacial transitions.

Low‐frequency internal oceanic variability under seasonal forcing

A series of numerical experiments are conducted using the Bryan‐Cox ocean general circulation model to investigate the potential existence of low‐frequency variability of the thermohaline circulation under seasonal forcing. Experiments are performed with different combinations of a seasonal cycle being present or not on the restoring temperature, the surface freshwater flux fields (mixed boundary conditions), and the surface wind forcing. Despite the presence of forcing on the dominant seasonal time scale, it is found that the system may oscillate at the decadal period or longer. The decadal variability is excited by changes in the net surface density flux which are due to the advection of temperature and salinity anomalies in the model domain. The magnitude of the seasonal cycle also plays an important role in determining the time scale of variability. Violent overturning events may occur on the century time scale under seasonal forcing. The magnitudes of the flushes are reduced compared with those found in similar experiments without the presence of a seasonal cycle.

On water mass exchange between the Southern Ocean and the World Ocean; emphasis on the Atlantic sector

We characterise the representation of the Southern Ocean water mass structure and sea ice within a suite of 15 global ocean-ice models run with the Coordinated Ocean-ice Reference Experiment Phase II (CORE-II) protocol. The main focus is the representation of the present (1988–2007) mode and intermediate waters, thus framing an analysis of winter and summer mixed layer depths; temperature, salinity, and potential vorticity structure; and temporal variability of sea ice distributions. We also consider the interannual variability over the same 20 year period. Comparisons are made between models as well as to observation-based analyses where available.The CORE-II models exhibit several biases relative to Southern Ocean observations, including an underestimation of the model mean mixed layer depths of mode and intermediate water masses in March (associated with greater ocean surface heat gain), and an overestimation in September (associated with greater high latitude ocean heat loss and a more northward winter sea-ice extent). In addition, the models have cold and fresh/warm and salty water column biases centred near 50°S. Over the 1988–2007 period, the CORE-II models consistently simulate spatially variable trends in sea-ice concentration, surface freshwater fluxes, mixed layer depths, and 200–700 m ocean heat content. In particular, sea-ice coverage around most of the Antarctic continental shelf is reduced, leading to a cooling and freshening of the near surface waters. The shoaling of the mixed layer is associated with increased surface buoyancy gain, except in the Pacific where sea ice is also influential. The models are in disagreement, despite the common CORE-II atmospheric state, in their spatial pattern of the 20-year trends in the mixed layer depth and sea-ice.

Effect of seasonal surface freshwater flux on sea surface temperature in the tropical Atlantic Ocean

In addition to its relationship to evaporative cooling and the atmosphere's radiative balance, anomalies of freshwater flux at the ocean surface have a potential impact on the stratification of the tropical mixed layer. Under light wind conditions when the thermocline is deep, excess rainfall can cause a low‐salinity layer to form, inhibiting vertical convection and mixing. It has previously been hypothesized that under conditions where the heat entering the mixed layer is balanced by cooling at the base of the mixed layer, such as occurs in upwelling regions, this effect could raise sea surface temperatures (Miller, 1976). Conversely, excess evaporation in such a region could destabilize the mixed layer, enhancing convection and lowering sea surface temperature. Here we report on a numerical experiment to test these ideas. We employ a model of the seasonal tropical Atlantic circulation which includes full representation of salinity and heat transport. Two simulations are compared. In one the surface boundary conditions include complete hydrological forcing by river discharge, rainfall, and evaporation. In the other simulation the vertical flux of water at the surface is set to zero. We find that seasonal changes in net freshwater flux at the surface do alter sea surface temperature by ≈±0.1°C. This change is too small to significantly alter the exchange of heat across the surface of the ocean. However, the effect is larger in areas strongly affected by river discharge and at the base of the mixed layer.

A Spin-Up Model as a Diagnostic Tool for Interpretation of Current and Density Measurements on the Continental Shelf of the Pacific Northwest

Abstract The primary goal of this paper is to prescribe the practical limitations of the two-dimensional, baroclinic, time-dependent model of Hamilton and Rattray (1978) for the Oregon-Washington continental shelf. A unique time series of density sections at approximately 3-day intervals over a 5-week period makes the test possible. The Columbia River effluent was modeled by a surface freshwater flux that allowed the plume to become detached from the coast during periods of northerly winds. Energy levels and phases of fluctuations in longshore currents and sea level could be effectively predicted with a vertically averaged version of the two-dimensional model whose essential longshore dynamics are a balance between local acceleration (multiplied by shelf depth), surface friction and bottom friction. With respect to simulation of the density field, the two-dimensional model was effective in predicting offshore displacement of surface isopycnals and vertical displacement of isopycnals below the surface laye...