Abstract

The oceanic Ekman transport and pumping are among the most important parameters in studying the ocean general circulation and its variability. Upwelling due to the Ekman transport divergence has been identified as a leading mechanism for the seasonal to interannual variability of the upper-ocean heat content in many parts of the World Ocean, especially along coasts and the equator. Meanwhile, the Ekman pumping is the primary mechanism that drives basin-scale circulations in subtropical and subpolar oceans. In those ice-free oceans, the Ekman transport and pumping rate are calculated using the surface wind stress. In the ice-covered Arctic Ocean, the surface momentum flux comes from both air–water and ice–water stresses. The data required to compute these stresses are now available from satellite and buoy observations. But no basin-scale calculation of the Ekman transport in the Arctic Ocean has been done to date. In this study, a suite of satellite and buoy observations of ice motion, ice concentration, surface wind, etc., will be used to calculate the daily Ekman transport over the whole Arctic Ocean from 1978 to 2003 on a 25-km resolution. The seasonal variability and its relationship to the surface forcing fields will be examined. Meanwhile, the contribution of the Ekman transport to the seasonal fluxes of heat and salt to the Arctic Ocean mixed layer will be discussed. It was found that the greatest seasonal variations of Ekman transports of heat and salt occur in the southern Beaufort Sea in the fall and early winter when a strong anticyclonic wind and ice motion are present. The Ekman pumping velocity in the interior Beaufort Sea reaches as high as 10 cm day⁻¹ in November while coastal upwelling is even stronger. The contributions of the Ekman transport to the heat and salt flux in the mixed layer are also considerable in the region.

Abstract

Along the Taiwan Strait (<100 m in depth) a northeastward flow persists in all seasons despite the annually averaged wind stress that is strongly southwestward. The forcing mechanism of this countercurrent is examined by using a simple ocean model. The results from a suite of experiments demonstrate that it is the Kuroshio that plays the deciding role for setting the flow direction along the Taiwan Strait. The momentum balance along the strait is mainly between the wind stress, friction, and pressure gradient. Since both wind stress and friction act against the northward flow, it is most likely the pressure gradient that forces the northward flow, as noted in some previous studies. What remains unknown is why there is a considerable pressure difference between the southern and northern strait. The Kuroshio flows along the east coast of Taiwan, and thus the western boundary current layer dynamics applies there. Integrating the momentum equation along Taiwan’s east coast shows that there must be a pressure difference between the southern and the northern tip of Taiwan to counter a considerable friction exerted by the mighty Kuroshio. This same pressure difference is also felt on the other side of the island where it forces the northward flow through Taiwan Strait. The model shows that the local wind stress acts to dampen this northward flow. This mechanism can be illustrated by an integral constraint for flow around an island.

Abstract

According to observations, the Arctic Ocean circulation beneath a shallow thermocline can be schematized by cyclonic rim currents along shelves and over ridges. In each deep basin, the circulation is also believed to be cyclonic. This circulation pattern has been used as an important benchmark for validating Arctic Ocean models. However, modeling this grand circulation pattern with some of the most sophisticated ocean–ice models has been often difficult. The most puzzling and thus perhaps the most interesting finding from the Arctic Ocean Model Intercomparison Project (AOMIP), an international consortium that runs 14 Arctic Ocean models by using the identical forcing fields, is that its model results can be grouped into two nearly exact opposite patterns. While some models produce cyclonic circulation patterns similar to observations, others do the opposite. This study examines what could be possibly responsible for such strange inconsistency. It is found here that the flux of potential vorticity (PV) from the subarctic oceans strongly controls the circulation directions. For a semienclosed basin like the Arctic, the PV integral over the whole basin yields a balance between the net lateral PV inflow and the PV dissipation along the boundary. When an isopycnal layer receives a net positive PV through inflow/outflow, the circulation becomes cyclonic so that friction can generate a flux of negative PV to satisfy the integral balance. For simplicity, a barotropic ocean model is used in this paper but its application to the 3D models will be discussed. In the first set of experiments, the model with a realistic Arctic bathymetry is forced by observed inflows and outflows. In this case, there is a net positive PV inflow to the basin, due to the fact that inflow layer is thinner than that of outflow. The model produces a circulation field that is remarkably similar to the one from observations. In the second experiment, the model bathymetry at Fram Strait is modified so that the same inflows and outflows of water masses lead to a net negative PV flux into the Arctic. The circulation is reversed and becomes nearly the opposite of the first experiment. In the third experiment, the net PV flux is made to be zero by modifying again the sill depth at Fram Strait. The circulation becomes two gyres, a cyclonic one in the Eurasian Basin and an anticyclonic one in the Canada Basin. To elucidate the control of the PV integral, a second set of model experiments is conducted by using an idealized Arctic bathymetry so that the PV dynamics can be better explained without the complication of rough topography. The results from five additional experiments that used the idealized topography will be discussed. While the model used in this study is one layer, the same PV-integral constraint can be applied to any isopycnal layer in a three-dimensional model. Variables that affect the PV fluxes to this density layer at any inflow/outflow channel, such as layer thickness and water volume flux, can affect the circulation pattern. The relevance to 3D models is discussed in this paper.

Abstract

The WKBJ method and a multiple-scale expansion technique are used to study equatorially trapped waves propagating on a zonally sloping themocline. Assuming that variations of the main thermocline depth (MTD) are slow (the change of the MTD over one wavelength is smaller than the wave amplitude), wave reflections can be neglected and the amplitudes of equatorially trapped waves can be derived by using the energy conservation law. It is found that the wavelengths and amplitudes of free waves are significantly modified by the MTD variations. While propagating eastward in an ocean basin (where the MTD is shallower), Kelvin waves shrink meridionally and zonally but their amplitudes increase to preserve wave energy; short Rossby waves behave in the opposite way. The wavelength of westward-propagating long Rossby waves becomes longer when they propagate into the deeper western ocean. The response of a Yanai wave to the changing thermocline depends on the sign of phase speed.

A simple numerical method is designed to verify the WKBJ results and also to study the cast of a relatively steep thermocline profile where the WKBJ method breaks down. Reflection of a Kelvin wave impinging on a thermocline front is also investigated in this work.

Abstract

An interdecadal oscillation in a coupled ocean–ice system was identified in a previous study. This paper extends that study to further examine the stability of the oscillation and the sensitivity of its frequency to various parameters and forcing fields. Three models are used: (i) an analytical box model; (ii) a two-dimensional model for the ocean thermohaline circulation (THC) coupled to a thermodynamic ice model, as in the authors’ previous study; (iii) a three-dimensional ocean general circulation model (OGCM) coupled to a similar ice model. The box model is used to elucidate the essential feedbacks that give rise to this oscillation and to identify the most important parameters and processes that determine the period. Numerical experiments in the 2D THC–ice model show that the model stability is sensitive to the ocean–ice coupling coefficient, the eddy diffusivity, and the strength of the thermohaline-circulation feedback per unit surface-polar density perturbation. The coupled model becomes more stable toward low coupling, greater diffusion, and weaker THC feedback. The period of the oscillation is less sensitive to these parameters. Nonlinear effects in the sea-ice model become important in the higher ocean–ice coupling regime where the effective sea-ice damping associated with this nonlinearity stabilizes the model. Surface Newtonian damping is also tested. The 3D OGCM, which includes both wind stress and buoyancy forcings, is used to test this coupled ocean–ice mechanism in a more realistic model setting. This model generates an interdecadal oscillation whose characteristics and phase relations among the model variables are similar to the oscillation obtained in the 2D models. The major difference is that the oscillation frequency is considerably lower. This difference can be explained in terms of the analytical box model solution in which the period of the oscillation depends on the rate of anomalous density production by melting/cooling of sea ice per SST anomaly, times the rate of warming/cooling by anomalous THC heat advection per change in density anomaly. The 3D model has a smaller THC response to high-latitude density perturbations than in the 2D model, and anomalous velocities in the 3D case tend to follow the mean isotherms so the anomalous heat advection is reduced. This slows the ocean–ice feedback process, leading to the longer oscillation period.

Abstract

A coupled atmosphere-ocean model is used to investigate some important effects of a steep sloping thermocline in the, central Pacific. It is found that the coupled modes are very sensitive to the steepness of the thermocline in the central Pacific Ocean. The wave reflection and modal decomposition processes play an important role in the initial development of the unstable mode and dramatically affect the fates of the oscillation. A sensitivity test is conducted to test the role of western boundary reflection in this particular model. The insensitivity of the western boundary reflection seems to agree with the results of previous research that showed that coupled unstable modes do not necessarily depend on Rossby wave reflections.

Abstract

The structure of a Stommel type of frictional western boundary layer in a thermally driven two-layer model is examined. Instead of specifying the interfacial upwelling a priori, it is calculated as part of the solution subject to the dynamic and thermodynamic constraints. It is shown that upwelling prevails in the western boundary. Scaling analysis indicates that upwelling within the western boundary layer is two orders of magnitude stronger than that in the ocean interior. Furthermore, the total amount of upwelling within the western boundary layer constitutes a substantial part of the basin-integrated upwelling. Thus, our results suggest that it is important to study the dynamic role of western boundary upwelling in ocean circulation models.

Abstract

The East Greenland Current (EGC) had long been considered the main pathway for the Denmark Strait overflow (DSO). Recent observations, however, indicate that the north Icelandic jet (NIJ), which flows westward along the north coast of Iceland, is a major separate pathway for the DSO. In this study a two-layer numerical model and complementary integral constraints are used to examine various pathways that lead to the DSO and to explore plausible mechanisms for the NIJ’s existence. In these simulations, a westward and NIJ-like current emerges as a robust feature and a main pathway for the Denmark Strait overflow. Its existence can be explained through circulation integrals around advantageous contours. One such constraint spells out the consequences of overflow water as a source of low potential vorticity. A stronger constraint can be added when the outflow occurs through two outlets: it takes the form of a circulation integral around the Iceland–Faroe Ridge. In either case, the direction of overall circulation about the contour can be deduced from the required frictional torques. Some effects of wind stress forcing are also examined. The overall positive curl of the wind forces cyclonic gyres in both layers, enhancing the East Greenland Current. The wind stress forcing weakens but does not eliminate the NIJ. It also modifies the sign of the deep circulation in various subbasins and alters the path by which overflow water is brought to the Faroe Bank Channel, all in ways that bring the idealized model more in line with observations. The sequence of numerical experiments separates the effects of wind and buoyancy forcing and shows how each is important.

Abstract

The overflow of the dense water mass across the Greenland–Scotland Ridge (GSR) from the Nordic Seas drives the Atlantic meridional overturning circulation (AMOC). The Nordic Seas is a large basin with an enormous reservoir capacity. The volume of the dense water above the GSR sill depth in the Nordic Seas, according to previous estimates, is sufficient to supply decades of overflow transport. This large capacity buffers overflow’s responses to atmospheric variations and prevents an abrupt shutdown of the AMOC. In this study, the authors use a numerical and an analytical model to show that the effective reservoir capacity of the Nordic Seas is actually much smaller than what was estimated previously. Basin-scale oceanic circulation is nearly geostrophic and its streamlines are basically the same as the isobaths. The vast majority of the dense water is stored inside closed geostrophic contours in the deep basin and thus is not freely available to the overflow. The positive wind stress curl in the Nordic Seas forces a convergence of the dense water toward the deep basin and makes the interior water even more removed from the overflow-feeding boundary current. Eddies generated by the baroclinic instability help transport the interior water mass to the boundary current. But in absence of a robust renewal of deep water, the boundary current weakens rapidly and the eddy-generating mechanism becomes less effective. This study indicates that the Nordic Seas has a relatively small capacity as a dense water reservoir and thus the overflow transport is sensitive to climate changes.

Abstract

This paper examines the role of potential vorticity (PV) balance in source- and sink-driven flows between two basins. As shown in previous studies, PV advection into a basin, say a positive PV advection, requires a negative frictional torque to maintain a steady PV balance. This sense of torque may be provided by a cyclonic boundary current within the basin. The PV advection through a channel is due almost entirely to advection of planetary PV, f/H, where f is the Coriolis parameter and H is the column thickness. Therefore a localized change of depth, and thus H in the channel, directly affects the PV transport and will result in a basinwide change of the circulation pattern. For example, if the channel depth is made shallower while holding the transport fixed, the PV advection is then increased and the result may be a strong recirculation within the basin, as much as two orders of magnitude greater than the transport through the channel. When the basins are connected by two channels at different latitudes or with different sill depths, the throughflow is found to be divided between the two channels in a way that satisfies the integral constraint for flow around an island. The partition of the flow between two channels appears to be such as to minimize the net frictional torque. In still another set of experiments, the large-scale pressure difference (layer thickness) between the basins is specified and held fixed, while the throughflow is allowed to vary in response to changes in the frictional torque. The interbasin transport is strongly influenced by the length of the boundary or the magnitude of the viscosity in the sense that a greater PV frictional torque allows a greater PV transport and vice versa. This result is counterintuitive, if it is assumed that the throughflow is determined by viscous drag within the channel but is a straightforward consequence of the basin-scale PV balance. Thus, the important frictional effect in these experiments is on the basin-scale flow and not on the channel scale.