Abstract

The Indonesian Throughflow, weaving through complex topography, drawing water from near the division of the North Pacific and South Pacific water mass fields, represents a severe challenge to modeling efforts. Thermohaline observations within the Indonesian seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) offer an opportunity to compare observations to model output for these periods. The simulation used in these comparisons is the Los Alamos Parallel Ocean Program (POP) 1/6 deg (on average) global model, forced by ECMWF wind stresses for the period 1985 through 1995. The model temperature structure shows discrepancies from the observed profiles, such as between 200 and 1200 dbar where the model temperature is as much as 3°C warmer than the observed temperature. Within the 5°–28°C temperature interval, the model salinity is excessive, often by more than 0.2. The model density, dominated by the temperature profile, is lower than the observed density between 200 and 1200 dbar, and is denser at other depths. In the model Makassar Strait, North Pacific waters are found down to about 250 dbar, in agreement with observations. The model sill depth in the Makassar Strait of 200 m, rather than the observed 550-m sill depth, shields the model Flores Sea from Makassar Strait lower thermocline water, causing the Flores lower thermocline to be dominated by salty water from the Banda Sea. In the Maluku, Seram, and Banda Seas the model thermocline is far too salty, due to excessive amounts of South Pacific water. Observations show that the bulk of the Makassar throughflow turns eastward into the Flores and Banda Seas, before exiting the Indonesian seas near Timor. In the model, South Pacific thermocline water spreads uninhibited into the Banda, Flores, and Timor Seas and ultimately into the Indian Ocean. The model throughflow transport is about 7.0 Sv (Sv ≡ 10⁶ m³ s⁻¹) in August 1993 and 0.6 Sv in February 1994, which is low compared to observationally based estimates. However, during the prolonged El Niño of the early 1990s the throughflow is suspected to be lower than average and, indeed, the model transports for the non–El Niño months of August 1988 and February 1989 are larger. It is likely that aspects of the model bathymetry, particularly that of the Torres Strait, which is too open to the South Pacific, and the Makassar Strait, which is too restrictive, may be the cause of the discrepancies between observations and model.

Abstract

The WOCE Community Modeling Effort (CME) general circulation model of the North Atlantic was used to investigate the behavior, nature, and dynamics of 50-day oscillations seen in the meridional component of velocity between 5° and 11°N, 35° and 55°W. Oscillations of the meridional component of velocity, with a period of about 50 days, appear as the seasonal meander pattern of the North Equatorial Countercurrent starts to break down in December. They appear first near 35° and are advected westward. They have a westward phase velocity of about O.1 m s⁻¹, wavelength of about 6000 km, and a very slow eastward group velocity. Their period, phase speed and wavelength agree with recent observations. Calculation of the leading terms from the full vorticity equation following a model decomposition in the propagation region showed that the oscillations were first and second mode baroclinic Rossby waves. Repetition of the vorticity analysis on an undecomposed snapshot during the period of retroflection revealed the NECC meanders also to be baroclinic Rossby waves, the same as the 50-day oscillations. These findings, together with the time evolution of the individual flow components over an annual cycle, suggested that the 50-day oscillations were the westward advected residue of the NECC meander pattern that is released as the NECC slows in December. The retroflecting North Brazil Current produces Rossby waves with very slow eastward group velocity that are advected eastward by the NECC until they reach 35°W, where they dissipate. A standing wave pattern is established for several months, while the NECC is active. Once it slows, the waves am advected westward and disappear totally by May. Neither wind forcing nor barotropic instability were considered to be responsible for the oscillations in the mode1.

Abstract

The relationship between the southern annular mode (SAM) and Southern Ocean mixed layer depth (MLD) is investigated using a global 0.1° resolution ocean model. The SAM index is defined as the principal component time series of the leading empirical orthogonal function of extratropical sea level pressure from September to December, when the zonally symmetric SAM feature is most prominent. Following positive phases of the SAM, anomalous deep mixed layers occur in the subsequent fall season, starting in May, particularly in the southeast Pacific. Composite analyses reveal that for positive SAM phases enhanced surface cooling caused by anomalously strong westerlies weakens the stratification of the water column, leading to deeper mixed layers during spring when the SAM signal is at its strongest. During the subsequent summer, the surface warms and the mixed layer shoals. However, beneath the warm surface layer, anomalously weak stratification persists throughout the summer and into fall. When the surface cools again during fall, the mixed layer readily deepens due to this weak interior stratification, a legacy from the previous springtime conditions. Therefore, the spring SAM–fall MLD relationship is interpreted here as a manifestation of reemergence of interior water mass anomalies. The opposite occurs after negative phases of the SAM, with anomalously shallow mixed layers resulting. Additional analyses reveal that for the MLD region in the southeast Pacific, the effects of salinity variations and Ekman heat advection are negligible, although Ekman heat transport may play an important role in other regions where mode water is formed, such as south of Australia and in the Indian Ocean.

Abstract

High spatial resolution isopycnal diffusivities are estimated in the Kuroshio Extension (KE) region (28°–40°N, 120°–190°E) from a global ° Parallel Ocean Program (POP) simulation. The numerical float tracks are binned using a clustering approach. The number of tracks in each bin is thus roughly the same leading to diffusivity estimates that converge better than those in bins defined by a regular geographic grid. Cross-stream diffusivities are elevated in the southern recirculation gyre region, near topographic obstacles and downstream in the KE jet, where the flow has weakened. Along-stream diffusivities, which are much larger than cross-stream diffusivities, correlate well with the magnitudes of eddy velocity. The KE jet suppresses cross-stream mixing only in some longitude ranges. This study estimates the critical layer depth both from linear local baroclinic instability analysis and from eddy phase speeds in the POP model using the Radon transform. The latter is a better predictor of large mixing length in the cross-stream direction. Critical layer theory is most applicable in the intense jet regions away from topography.

ABSTRACT

The depth-integrated vorticity budget of a global, eddy-permitting ocean/sea ice simulation over the Antarctic continental margin (ACM) is diagnosed to understand the physical mechanisms implicated in meridional transport. The leading-order balance is between the torques due to lateral friction, nonlinear effects, and bottom vortex stretching, although details vary regionally. Maps of the time-averaged depth-integrated vorticity budget terms and time series of the spatially averaged, depth-integrated vorticity budget terms reveal that the flow in the Amundsen, Bellingshausen, and Weddell Seas and, to a lesser extent, in the western portion of East Antarctica, is closer to an approximate topographic Sverdrup balance (TSB) compared to other segments of the ACM. Correlation and coherence analyses further support these findings, and also show that inclusion of the vorticity tendency term in the response (the planetary vorticity advection and the bottom vortex stretching term) increases the correlation with the forcing (the vertical net stress curl), and also increases the coherence between forcing and response at high frequencies across the ACM, except for the West Antarctic Peninsula. These findings suggest that the surface stress curl, imparted by the wind and the sea ice, has the potential to contribute to the meridional, approximately cross-slope, transport to a greater extent in the Amundsen, Bellingshausen, Weddell, and part of the East Antarctic continental margin than elsewhere in the ACM.

Abstract

The Arabian Sea, influenced by the Indian monsoon, has many unique features, including its basin-scale seasonally reversing surface circulation and the Great Whirl, a seasonal anticyclonic system appearing during the southwest monsoon close to the western boundary. To establish a comprehensive dynamical picture of the Arabian Sea, we utilize numerical model output and design a full vorticity budget that includes a fully decomposed nonlinear term. The ocean general circulation model has 0.1° resolution and is mesoscale eddy-resolving in the region. In the western boundary current system, we highlight the role of nonlinear eddies in the life cycle of the Great Whirl. The nonlinear eddy term is of leading-order importance in this feature’s vorticity balance. Specifically, it contributes to the Great Whirl’s persistence in boreal fall after the weakening of the southwesterly winds. In the open ocean, Sverdrup dynamics and annual Rossby waves are found to dominate the vorticity balance; the latter is considered as a key factor in the formation of the Great Whirl and the seasonal reversal of the western boundary current. In addition, we discuss different forms of vertically integrated vorticity equations in the model and argue that the bottom pressure torque term can be interpreted analogously as friction in the western boundary and vortex stretching in the open ocean.

Abstract

Eulerian and Lagrangian statistics were calculated from the North Atlantic surface drifter dataset for the years 1993–97 and a high-resolution eddy-resolving configuration of the Los Alamos National Laboratory (LANL) Parallel Ocean Program (POP) model. The main purpose of the study was to statistically quantify the state of the surface circulation in the North Atlantic Ocean for this period and compare it with the equivalent modeled state. Diffusivities and time and length scales are anisotropic over most of the ocean basin, except in most of the subpolar regions. Typical time and length scales are 2–4 days and 20–50 km. Longest timescales are found in the energetically quiescent regions in the south and southeast sectors of the basin. The longest length scales are found in the energetic western boundary current system, the most dispersive region of the domain. In many respects the eddy-resolving model reproduced a surface circulation in good statistical agreement with that depicted by the drifters. Model time and length scales were also anisotropic, with typical timescales of 2–4 days and length scales of 20–50 km in the zonal direction, and 30–50 km in the meridional direction. An eddy-permitting POP simulation produced unrealistic time and length scales that were too long and too short relative to the drifter scales; these were attributed to the model being too stable hydrodynamically.

Abstract

A new method is proposed for extrapolating subsurface velocity and density fields from sea surface density and sea surface height (SSH). In this, the surface density is linked to the subsurface fields via the surface quasigeostrophic (SQG) formalism, as proposed in several recent papers. The subsurface field is augmented by the addition of the barotropic and first baroclinic modes, whose amplitudes are determined by matching to the sea surface height (pressure), after subtracting the SQG contribution. An additional constraint is that the bottom pressure anomaly vanishes. The method is tested for three regions in the North Atlantic using data from a high-resolution numerical simulation. The decomposition yields strikingly realistic subsurface fields. It is particularly successful in energetic regions like the Gulf Stream extension and at high latitudes where the mixed layer is deep, but it also works in less energetic eastern subtropics. The demonstration highlights the possibility of reconstructing three-dimensional oceanic flows using a combination of satellite fields, for example, sea surface temperature (SST) and SSH, and sparse (or climatological) estimates of the regional depth-resolved density. The method could be further elaborated to integrate additional subsurface information, such as mooring measurements.

Abstract

The authors present new estimates of the eddy momentum and heat fluxes from repeated high-resolution upper-ocean velocity and temperature observations in Drake Passage and interpret their role in the regional Antarctic Circumpolar Current (ACC) momentum balance. The observations span 7 yr and are compared to eddy fluxes estimated from a 3-yr set of output archived from an eddy-resolving global Parallel Ocean Program (POP) numerical simulation. In both POP and the observations, the stream-averaged cross-stream eddy momentum fluxes correspond to forcing consistent with both a potential vorticity flux into the axis of the Subantarctic Front (SAF) and a sharpening of all three main ACC fronts through Drake Passage. Further, the POP analysis indicates that the mean momentum advection terms reflect the steering of the mean ACC fronts and are not fully balanced by the eddy momentum forcing, which instead impacts the strength and number of ACC fronts.

The comparison between POP and observed eddy heat fluxes was less favorable partly because of model bias in the water mass stratification. Observed cross-stream eddy heat fluxes are generally surface intensified and poleward in the ACC fronts, with values up to approximately −290 ± 80 kW m⁻² in the Polar and Southern ACC Fronts. Interfacial form stresses F_T , derived from observed eddy heat fluxes in the SAF, show little depth dependence below the Ekman layer. Although F_T appears to balance the surface wind stress directly, the estimated interfacial form stress divergence is only an order of magnitude greater than the eddy momentum forcing in the SAF. Thus, although the eddy momentum forcing is of secondary importance in the momentum balance, its effect is not entirely negligible.

Abstract

A multiwavenumber theory is formulated to represent eddy diffusivities. It expands on earlier single-wavenumber theories and includes the wide range of wavenumbers encompassed in eddy motions. In the limiting case in which ocean eddies are only composed of a single wavenumber, the multiwavenumber theory is equivalent to the single-wavenumber theory and both show mixing suppression by the eddy propagation relative to the mean flow. The multiwavenumber theory was tested in a region of the Southern Ocean (70°–45°S, 110°–20°W) that covers the Drake Passage and includes the tracer/float release locations during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Cross-stream eddy diffusivities and mixing lengths were estimated in this region from the single-wavenumber theory, from the multiwavenumber theory, and from floats deployed in a global ° Parallel Ocean Program (POP) simulation. Compared to the single-wavenumber theory, the horizontal structures of cross-stream mixing lengths from the multiwavenumber theory agree better with the simulated float-based estimates at almost all depth levels. The multiwavenumber theory better represents the vertical structure of cross-stream mixing lengths both inside and outside the Antarctica Circumpolar Current (ACC). Both the single-wavenumber and multiwavenumber theories represent the horizontal structures of cross-stream diffusivities, which resemble the eddy kinetic energy patterns.