Young-Gyu Park

Comparison of Thermally Driven Circulations from a Depth-Coordinate Model and an Isopycnal-Layer Model. Part I: Scaling-Law Sensitivity to Vertical Diffusivity

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 o...

The Stability of Thermohaline Circulation in a Two-Box Model*

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 s...

Rotating Convection Driven by Differential Bottom Heating

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 DT 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 cm 2 s21.

The Relationship of Weddell Polynya and Open-Ocean Deep Convection to the Southern Hemisphere Westerlies

AbstractThe 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, ...

Comparison of Thermally Driven Circulations from a Depth-Coordinate Model and an Isopycnal-Layer Model. Part II: The Difference and Structure of the Circulations

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.

Origin of the Tsushima Warm Current in a high resolution ocean circulation model

ABSTRACT Park, Y.-G., Yeh, S.-W., Hwang, J. and Kim, T., 2013, East China Sea circulation from a high resolution ocean circulation model. Using the results from a high-resolution global ocean circulation model, the present study describes the circulation over the East China Sea and the origin of the Tsushima Warm Current. We focus on the interaction between the Taiwan Warm Current, the Tsushima Warm Current, and the Kuroshio intrusion north of Taiwan and east of Kyushu by comparing mass fluxes. Although the modeled transport of the Tsushima Warm Current is weaker than observed values by about 30 %, that of the Taiwan Warm Current is within the known range. The transport of the latter is large enough to supply the former. Thus, in this model, the net intrusion of the Kuroshio towards the shelf between Taiwan and Kyushu would not make a significant contribution to the Tsushima Warm Current. Conversely, the flows northeast of Taiwan and west of Kyushu show a strong presence of branch currents toward the inner shelf from the Kuroshio. To resolve this apparent contradiction, we estimated mass budgets for the areas around Taiwan and west of Kyushu. The Taiwan–Tsushima Current System and the Kuroshio are located very closely to each other along the shelf break, and if they are not differentiated properly using high-resolution data, one could overestimate the strength of the branch currents or the onshore intrusion of the Kuroshio. Considering the similarity between the apparent branch currents from this model and the previously reported results, the branch currents, especially west of Kyushu, may not be as strong as previously suggested.

The Role of Oscillating Southern Hemisphere Westerly Winds: Southern Ocean Coastal and Open-Ocean Polynyas

AbstractAn 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. We find 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 spin-up 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 We...