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Young-Gyu Park

Abstract

In Stommel’s simple two-box model, which has provided an insight on the thermohaline circulation and climate instability mechanisms, a linear mass transport was used. However, a scaling law based on geostrophy and advective–diffusive heat balance suggests a nonlinear mass transport relation for the oceans. By including this nonlinear mass transport relation to Stommel’s box model, it is possible to study the effects of the thermocline, which was not considered before, on the stability of the thermohaline circulation while keeping the simplicity of Stommel’s box model. The results were compared with those obtained with the traditional model using a linear mass transport relation. The thermal mode circulation of the nonlinear model is significantly more stable than that of the linear model, suggesting the thermohaline catastrophe is less likely to occur in the present North Atlantic if the thermocline is considered. In the nonlinear model, the circulation removes density anomalies rapidly so that significantly higher haline forcing is needed to initiate the thermohaline catastrophe. A linear stability analysis shows that negative feedback from the mass transport law has the strongest effect on the stability within a parameter range relevant for the present North Atlantic. The analysis also shows that freshwater flux parameterization does not have significant effect on the stability excluding artificial stability due to the details of the salinity restoring boundary condition.

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Young-Gyu Park and Kirk Bryan

Abstract

Thermally driven ocean circulations in idealized basins are calculated with two well-known model codes, one based on depth-level coordinates and the other based on isopycnal coordinates. In addition, the two models have very different representations of convection. In the level-coordinate model, convective adjustment is used, while in the isopycnal-coordinate model, convection is simulated by a transformation of the surface layer to the layer below. Both models indicate a three-layer structure in the circulation. The lower and middle layers have a flow structure that corresponds with the classical abyssal circulation models. The upper flow is strongly constrained by the buoyancy flux field at the upper surface and the convective parameterization. The model with convective adjustment and level coordinates is dominated by an eastward flow, which sinks to subsurface level at the eastern boundary. It lacks any indication of a surface cyclonic flow, even in the vicinity of sinking at the northern wall. On the other hand, in the model based on density coordinates the eastward surface flow turns to the north at the eastern boundary and forms a pronounced cyclonic circulation at high latitudes. Due to the cyclonic circulation, the coldest surface water is found near the northwestern corner, while in the level model the coldest water is near the northeastern corner. The isopycnal model appears to be a more realistic representation of the real ocean since both wind and the thermohaline circulation are thought to contribute to the North Atlantic subarctic cyclonic gyre.

Although the zonally averaged buoyancy flux produced by the two model codes is the same, the actual patterns of buoyancy flux at the surface are not similar at high latitudes. This suggests that the two types of numerical models would indicate very different air–sea interaction if coupled to atmospheric models and used to simulate climate. The application of the Gent–McWilliams parameterization of mesoscale eddies to the model with z coordinates and convective adjustment reduces the differences between the surface circulation of the two models by a small amount.

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Young-Gyu Park and Kirk Bryan

Abstract

Two different types of numerical ocean circulation models are used in a classical idealized problem, the thermally induced circulation in an ocean basin bounded by two meridians to the east and west and by the equator and a line of constant latitude. A simple scaling theory exists for predicting poleward heat transport and the strength of meridional overturning as a function of vertical diffusivity and other external factors. However, previous studies have indicated conflicting results, and other scaling laws have been proposed. Experiments with two widely used types of numerical models, one based on depth coordinates and the other based on isopycnal layers, provide insight into the discrepancies of previous studies. In the numerical experiments vertical diffusivity is varied over a range of 200. The source of the difficulty in previous studies is in part traced to applying a fixed restoring coefficient at the upper boundary and considering the buoyancy forcing at the surface fixed irrespective of vertical diffusivity κ. Globally or zonally averaged results show a robust agreement between the two models and support the simple scaling law in a flat-bottom basin and a bowl-shaped basin, as long as the meridional circulation is estimated along isopycnal surfaces and in situ rather than externally imposed restoring density differences are used to estimate the geostrophic-scale velocity. Over the thermocline the vertical mean of the zonally averaged zonal baroclinic pressure gradient has constant ratio to the vertical mean of the zonally averaged meridional baroclinic pressure gradient, consistent with the scaling assumptions for a diffusive thermocline.

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Young-Gyu Park and J. A. Whitehead

Abstract

Convection experiments were carried out in a rectangular tank as a model of oceanic meridional overturning circulation. The objective was finding a relation between the meridional heat flux and thermal forcing. To make the meridional heat flux estimate possible, the heat flux was fixed at one bottom end of the tank using an electrical heater. Temperature was fixed at the other end using a cooling plate. All other boundaries were insulated. In equilibrium, the heat input to the fluid H was the same as the meridional heat flux (heat flux from the source to the sink), so it was possible to find a scaling law relating H to the temperature difference across the tank ΔT and rotation rate f. The experimental result suggests that the meridional heat transport in the experiment was mostly due to geostrophic flows with a minor correction caused by bottom friction. When the typical values of the North Atlantic are introduced, the geostrophic scaling law predicts meridional heat flux comparable to that estimated in the North Atlantic when the vertical eddy diffusivity of heat is about 1 cm2 s−1.

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Woo Geun Cheon, Chang-Bong Cho, Arnold L. Gordon, Young Ho Kim, and Young-Gyu Park

Abstract

An oscillation in intensity of the Southern Hemisphere westerly winds is a major characteristic of the southern annular mode. Its impact upon the sea ice–ocean interactions in the Weddell and Ross Seas is investigated by a sea ice–ocean general circulation model coupled to an energy balance model for three temporal scales and two amplitudes of intensity. It is found that the oscillating wind forcing over the Southern Ocean plays a significant role both in regulating coastal polynyas along the Antarctic margins and in triggering open-ocean polynyas. The formation of coastal polynya in the western Weddell and Ross Seas is enhanced with the intensifying winds, resulting in an increase in the salt flux into the ocean via sea ice formation. Under intensifying winds, an instantaneous spinup within the Weddell and Ross Sea cyclonic gyres causes the warm deep water to upwell, triggering open-ocean polynyas with accompanying deep ocean convection. In contrast to coastal polynyas, open-ocean polynyas in the Weddell and Ross Seas respond differently to the wind forcing and are dependent on its period. That is, the Weddell Sea open-ocean polynya occurs earlier and more frequently than the Ross Sea open-ocean polynya and, more importantly, does not occur when the period of oscillation is sufficiently short. The strong stratification of the Ross Sea and the contraction of the Ross gyre due to the southward shift of Antarctic Circumpolar Current fronts provide unfavorable conditions for the Ross Sea open-ocean polynya. The recovery time of deep ocean heat controls the occurrence frequency of the Weddell Sea open-ocean polynya.

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Woo Geun Cheon, Young-Gyu Park, J. R. Toggweiler, and Sang-Ki Lee

Abstract

The Weddell Polynya of the mid-1970s is simulated in an energy balance model (EBM) sea ice–ocean coupled general circulation model (GCM) with an abrupt 20% increase in the intensity of Southern Hemisphere (SH) westerlies. This small upshift of applied wind stress is viewed as a stand in for the stronger zonal winds that developed in the mid-1970s following a long interval of relatively weak zonal winds between 1954 and 1972. Following the strengthening of the westerlies in this model, the cyclonic Weddell gyre intensifies, raising relatively warm Weddell Sea Deep Water to the surface. The raised warm water then melts sea ice or prevents it from forming to produce the Weddell Polynya. Within the polynya, large heat loss to the air causes surface water to become cold and sink to the bottom via open-ocean deep convection. Thus, the underlying layers cool down, the warm water supply to the surface eventually stops, and the polynya cannot be maintained anymore. During the 100-yr-long model simulation, two Weddell Polynya events are observed. The second one occurs a few years after the first one disappears; it is much weaker and persists for less time than the first one because the underlying layer is cooler. Based on these model simulations, the authors hypothesize that the Weddell Polynya and open-ocean deep convection were responses to the stronger SH westerlies that followed a prolonged weak phase of the southern annular mode.

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