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Rosalind Echols and Stephen C. Riser

Abstract

Barrier layers (BLs) are a frequent occurrence in low-latitude oceans, but variations in identification methods and quantitative descriptors used, as well as analyses of their lifetimes, lead to differing views on their overall impact. Herein a new method is proposed for identifying BLs that relies on the vertical spice profile rather than arbitrary temperature thresholds. Using 13 years of Argo profiling float data from the Arabian Sea, this method is shown to produce reasonable assessments of BL characteristics in the region. Similarly, use of the spice variable to identify formation mechanisms shows promise for using limited observational data and climatology to reproduce mechanisms proposed from modeling studies. Upper-ocean stability calculations combining profiling float and atmospheric forcing data are used to suggest the most likely one-dimensional mechanisms for erosion of BLs and to calculate the expected lifetimes of BLs throughout the year. Consistent with the evidence about the seasonal spatial extent and frequency of BLs, their duration is expected to be 2–3 times longer during the northeast monsoon than the southwest monsoon. However, the most likely erosion mechanisms vary widely throughout the year, as do the associated changes to the upper-ocean structure and thus likely impacts on SST.

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Susan L. Hautala and Stephen C. Riser

Abstract

The horizontal mean circulation and diapycnal flux divergence field in the South Pacific between Tahiti and the East Pacific Rise are obtained from a nonconservative β-spiral inverse method applied to high quality hydrographic data. The diapycnal flux divergence term is found to be an essential part of the deep vorticity balance and proper resolution of this term is critical to the success of the β-spiral calculation. The subthermocline circulation in this region of the South Pacific consists of three vertical regimes. Between the thermocline and approximately 1800–2000 m a cyclonic circulation pattern exists. A zone of minimum motion is found between 1800 and 2000 m. Below this zone, northward flow along the rise and westward flow in the north indicate an anticyclonic gyre, centered on 15°S, that extends approximately 1000–2000 km to the west of the East Pacific Rise. The deep vorticity balance associated with this flow system is primarily between meridional advection of planetary vorticity and the nonconservative diapycnal flux divergence term. Below the zone of minimum motion, the water mass characteristics, flow, diapycnal flux divergence, and vorticity balance are consistent with those of a large-scale circulation actively driven by hydrothermal buoyancy flux from the East Pacific Rise.

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Fiammetta Straneo, Mitsuhiro Kawase, and Stephen C. Riser

Abstract

Intermediate, or deep, convection in a baroclinic flow occurs along slanted paths parallel to the alongflow absolute momentum surfaces. These surfaces are principally tilted due to the vertical shear in velocity but can be further modified by a nonvertical axis of rotation. An inviscid Lagrangian parcel model, using realistic parameters, is utilized to illustrate, qualitatively, the different scenarios resulting from the combined action of inertial and gravitational forces acting on sinking parcels of dense fluid. More quantitative results are derived from a series of numerical experiments using a zonally invariant, high-resolution, nonhydrostatic model. Convection occuring in a flow with tilted absolute momentum surfaces will mix properties along these slanted surfaces. This implies that the fluid can retain a weak vertical stratification while overturning and also, more importantly, that the evolution of the convective layer cannot be described in terms of one-dimensional, vertical mixing. The authors show, for conditions typical of the Labrador Sea, that the convective layer depth difference between that estimated by mixing vertically and one obtained allowing for slantwise mixing can be greater than 100 m; slantwise convection reaches deeper because of the reduced stratification along the slanted paths. An alternative slantwise mixing scheme, based on the assumption of zero potential vorticity of the convected fluid, is proposed.

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M. Susan Lozier and Stephen C. Riser

Abstract

Boundary layer potential vorticity dynamics for a quasi-geostrophic, eddy-resolving general circulation ocean model are studied using both Lagrangian and Eulerian analyses. Active western boundary layers are found not only in the upper wind-driven layer but also in the lower layers, despite the lack of a direct vorticity input to the deep ocean. At the western wall dissipative and inertial boundary regimes are exclusively controlled by the time-mean dynamics except for the deepest layer where eddy fluxes drive the mean flow across mean potential vorticity contours. Boundary layers formed at the southern wall in this model are dynamically distinct from the western boundary layers; they are controlled solely by the eddy flux of potential vorticity found in this region of active baroclinic instability. Basin-integrated vorticity balances reveal a strong contribution to the vorticity cycle by the lateral boundaries with such input overshadowed by the vorticity exchange across the midbasin gyre boundary in the surface layer.

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Susan L. Hautala and Stephen C. Riser

Abstract

We examine problems of steady abyssal circulation using an inviscid planetary geostrophic layered model. The model includes an active wind-driven upper layer and arbitrary topography; forcing is in the form of specified interlayer mass fluxes from which we model global upwelling and geothermal heating. The results considered in this study are appropriate to the shadow zone of a Pacific-sized, moderately wind-forced oceanic basin.

It is found that topographic effects, global upwelling and geothermal heating may all control the abyssal circulation, at least locally, within realistic values of their parameter ranges, suggesting that a better understanding of the geothermal and interior buoyancy flux fields is necessary for an understanding of abyssal circulation in the ocean. The model reduces to a nonlinear extension of the one-layer abyssal circulation model of Stommel and Arons, generalized to include mass flux structure and topography, and this limit is shown to be valid in an oceanic shadow zone for moderate topography and weak to moderate buoyancy forcing.

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Earle A. Wilson and Stephen C. Riser

Abstract

During each summer monsoon, the Bay of Bengal is inundated by a large amount of rain and river discharge. The effects of this freshening are gradually reversed over the course of the year, with near-surface salinities typically returning to their presummer monsoon levels before the start of the next rainy season. While the forcing responsible for the summertime freshening is clear, the processes that act to restore the bay’s salinity are not well understood. To examine these processes, the authors construct a basin-integrated, near-surface, seasonal salinity budget using data-assimilated output from the Hybrid Coordinate Ocean Model (HYCOM). From this salinity budget, it is deduced that vertical salt fluxes are primarily responsible for counterbalancing the near-surface freshening caused by the summertime freshwater fluxes. These vertical salt fluxes are largest during the months that immediately follow the summer monsoon, when the near-surface halocline is strongest. These results must be tempered with the knowledge that HYCOM misrepresents some key features of the bay’s salinity field. In particular, the model tends to overestimate salinity along the East Indian Coastal Current during its equatorward phase. Notwithstanding these biases, these results still suggest that vertical processes have a prominent role in the bay’s near-surface salinity budget.

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Alison R. Gray and Stephen C. Riser

Abstract

Using observations from the Argo array of profiling floats, the large-scale circulation of the upper 2000 decibars (db) of the global ocean is computed for the period from December 2004 to November 2010. The geostrophic velocity relative to a reference level of 900 db is estimated from temperature and salinity profiles, and the absolute geostrophic velocity at the reference level is estimated from the trajectory data provided by the floats. Combining the two gives the absolute geostrophic velocity on 29 pressure surfaces spanning the upper 2000 db of the global ocean. These velocities, together with satellite observations of wind stress, are then used to evaluate Sverdrup balance, the simple canonical theory relating meridional geostrophic transport to wind forcing. Observed transports agree well with predictions based on the wind field over large areas, primarily in the tropics and subtropics. Elsewhere, especially at higher latitudes and in boundary regions, Sverdrup balance does not accurately describe meridional geostrophic transports, possibly due to the increased importance of the barotropic flow, nonlinear dynamics, and topographic influence. Thus, while it provides an effective framework for understanding the zero-order wind-driven circulation in much of the global ocean, Sverdrup balance should not be regarded as axiomatic.

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Annie P. S. Wong and Stephen C. Riser

Abstract

Multiyear under-ice temperature and salinity data collected by profiling floats are used to study the upper ocean near the Wilkes Land coast of Antarctica. The study region is in the seasonal sea ice zone near the southern terminus of the Antarctic Circumpolar Current. The profiling floats were equipped with an ice-avoidance algorithm and had a survival rate of 74% after 2.5 yr in the ocean. The data show that, in this part of Antarctica, the rate of sea ice decay exceeds the rate of sea ice growth. During the sea ice growth period, the water column is weakly stratified because of brine rejection and is only marginally stable. The average winter mixed layer temperature is about 0.12°C above the surface freezing point, providing evidence of entrainment of warmer water from the permanent pycnocline. The average mixed layer salinity increases by 0.127 from June to October. A one-dimensional model is used to quantify evolution of the winter mixed layer under a sea ice cover. The local winter entrainment rate is estimated to be 49 ± 11 m over 5 months, supplying a heat flux of 34 ± 8 W m−2 to the base of the mixed layer in winter. Model output gives a thermodynamic sea ice growth of 28 ± 15 cm over the same period. The winter ocean–atmosphere heat loss through leads and sea ice is estimated to be 14–25 W m−2 in this area, which is broadly in line with other winter observations from the East Antarctic region.

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Dana D. Swift and Stephen C. Riser

Abstract

For timescales much greater than the local buoyancy period, the buoyant response of a RAFOS float is virtually dictated by its compressibility. As the compressibility of a thermally inert RAFOS float increases from zero, its oceanic equilibrium surface undergoes a smooth continuous deformation starting from an in situ density surface, eventually merging with an isopycnal surface and finally with a neutral surface. Thus there is a continuum of operational modes available to RAFOS floats; each mode is associated with a critical compressibility. Hypothetically, the compressee that transforms an isobaric RAFOS float into an isopycnal float can be modified to make a neutral-surface drifter by altering the critical value of its compressibility. The ballast procedure used to target a float to a prescribed equilibrium surface can be viewed as an accurate (±3%) laboratory measurement of the float's compressibility. For current “isobaric” RAFOS floats, the mean measured compressibility was approximately 2.71×10−6 db−1 (i.e., about 60% that of seawater), which can induce pressure deviations from an isobar as large as 125–175 db in the main thermoclines of the North Pacific and North Atlantic. Errors in ballasting a float yield targeting errors that depend on the float's compressibility and the local density stratification of the ocean. For isobaric floats deployed to 1000 db in the North Pacific Ocean, the targeting errors are approximately 23 db per gram of ballast error. By optimizing the ballast procedure, the ballast errors (and hence the targeting errors) can be minimized. For 59 shallow (1000 db) floats to which the optimized procedure was applied, preliminary estimates of the mean and maximum targeting errors are 25 and 50 db.

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Alison R. Gray and Stephen C. Riser

Abstract

This response addresses the three comments by A. Polonsky on “A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo.”

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