The Large-Scale Vorticity Balance of the Antarctic Continental Margin in a Fine-Resolution Global Simulation

ABSTRACTThe 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.

The Impact of Divergence Tilt and Meridional Flow for Cross-Equatorial Eddy Momentum Transport in Gill-Like Settings

This work investigates the sensitivity of the cross-equatorial eddy momentum flux and its rotational and divergent components to Hadley cell strength in simple variants of the Gill problem. An expression is derived linking the divergent momentum flux to the mean meridional wavenumber weighted by the spectrum of divergent eddy kinetic energy, supporting the relation between divergence phase tilt and momentum flux suggested by a previous study. Newtonian cooling makes the divergence tilt eastward moving away from the equator as observed, but this tilt is also sensitive to the Hadley cell. As the divergence tilt is enhanced in the downstream direction of the flow, wave propagation increases along that direction when the Hadley cell strengthens. The meridional flow also plays a second, important role for cross-equatorial propagation. With no Hadley cell, inviscid Sverdrup balance requires perfect compensation between the divergent and rotational momentum fluxes at the equator. The model can only produce cross-equatorial propagation when Sverdrup balance is violated, which in the linear, nearly inviscid limit requires vorticity advection by the mean flow. As the Hadley cell attenuates the geopotential tilt imparted by the divergent forcing, the compensation by the rotational momentum flux is reduced. The linear model can reproduce reasonably well previous nonlinear results by Kraucunas and Hartmann when linearized about their zonal-mean climatologies. The sensitivity of the cross-equatorial momentum fluxes to Hadley cell strength in these solutions is dominated by changes in the divergent flux and consistent with diagnosed changes in the divergence tilt.

Barotropic vorticity balance of the North Atlantic subpolar gyre in an eddy-resolving model

The circulation in the North Atlantic subpolar gyre is complex and strongly influenced by the topography. The gyre dynamics are traditionally understood as the result of a topographic Sverdrup balance, which corresponds to a first-order balance between the planetary vorticity advection, the bottom pressure torque, and the wind stress curl. However, these dynamics have been studied mostly with non-eddy-resolving models and a crude representation of the bottom topography. Here we revisit the barotropic vorticity balance of the North Atlantic subpolar gyre using a new eddy-resolving simulation (with a grid space of ≈2 km) with topography-following vertical coordinates to better represent the mesoscale turbulence and flow–topography interactions. Our findings highlight that, locally, there is a first-order balance between the bottom pressure torque and the nonlinear terms, albeit with a high degree of cancellation between them. However, balances integrated over different regions of the gyre – shelf, slope, and interior – still highlight the important role played by nonlinearities and bottom drag curls. In particular, the Sverdrup balance cannot describe the dynamics in the interior of the gyre. The main sources of cyclonic vorticity are nonlinear terms due to eddies generated along eastern boundary currents and time-mean nonlinear terms in the northwest corner. Our results suggest that a good representation of the mesoscale activity and a good positioning of mean currents are two important conditions for a better representation of the circulation in the North Atlantic subpolar gyre.

Barotropic vorticity balance of the North Atlantic subpolar gyre in an eddy-resolving model

The circulation in the North Atlantic Subpolar gyre is complex and strongly influenced by the topography. The gyre dynamics is traditionally understood as the result of a topographic Sverdrup balance, which corresponds to a first order balance between the planetary vorticity advection, the bottom pressure torque and the wind stress curl. However, this dynamics has been studied mostly with non-eddy-resolving models and a crude representation of the bottom topography. Here we revisit the barotropic vorticity balance of the North Atlantic Subpolar gyre using a high resolution simulation (≈ 2-km) with topography-following vertical coordinates to better represent the mesoscale turbulence and flow-topography interactions. Our findings highlight that, locally, there is a first order balance between the bottom pressure torque and the nonlinear terms, albeit with a high degree of cancellation between each other. However, balances integrated over different regions of the gyre – shelf, slope and interior – still highlight the important role played by nonlinearities and the bottom drag curls. In particular the topographic Sverdrup balance cannot describe the dynamics in the interior of the gyre. The main sources of cyclonic vorticity are the nonlinear terms due to eddies generated along eastern boundary currents and the time-mean nonlinear terms from the Northwest Corner. Our results suggest that a good representation of the mesoscale activity along with a good positioning of the Northwest corner are two important conditions for a better representation of the circulation in the North Atlantic Subpolar Gyre.

The Impact of the Asian Summer Monsoon Circulation on the Tropopause

Previous studies have identified two important features of summertime thermodynamics: 1) a significant correlation between the low-level distribution of equivalent potential temperature and the potential temperature θ of the extratropical tropopause and 2) a northwestward shift of the maximum tropopause θ relative to the maximum low-level . Here, the authors hypothesize these two features occur because of the Asian monsoon circulation. The hypothesis is examined using a set of idealized prescribed sea surface temperature (SST) aquaplanet simulations. Simulations with a zonally symmetric background climate exhibit a weak moisture–tropopause correlation. A significant correlation and northwestward shift occurs when a zonal wave-1 SST perturbation is introduced in the Northern Hemisphere subtropics. The equivalent zonal-mean subtropical warming does not produce a significant correlation. A mechanism is proposed to explain the moisture–tropopause connection that involves the circulation response to zonally asymmetric surface heating and its impact on the tropopause defined by the 2-potential-vorticity-unit (PVU; 1 PVU = 10−6 K kg−1 m2 s−1) surface. While the circulation response to diabatic heating is well known, here the focus is on the implications for the tropopause. Consistent with previous research, surface heating increases the low-level and produces low-level convergence and a cyclonic circulation. The low-level convergence is coupled with upper-level divergence via convection and produces an upper-level anticyclonic circulation consistent with Sverdrup balance. The anticyclonic vorticity lowers the PV northwest of the surface heating via Rossby wave dynamics. The decreased PV leads to a northwestward shift of the 2-PVU surface on fixed pressure levels. The θ value to the northwest of the surface heating is higher, and consequently the maximum tropopause θ increases.

Reply to “Comments on ‘A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo’”

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

Spatial and Temporal Scales of Sverdrup Balance*

Sverdrup balance underlies much of the theory of ocean circulation and provides a potential tool for describing the interior ocean transport from only the wind stress. Using both a model state estimate and an eddy-permitting coupled climate model, this study assesses to what extent and over what spatial and temporal scales Sverdrup balance describes the meridional transport. The authors find that Sverdrup balance holds to first order in the interior subtropical ocean when considered at spatial scales greater than approximately 5°. Outside the subtropics, in western boundary currents and at short spatial scales, significant departures occur due to failures in both the assumptions that there is a level of no motion at some depth and that the vorticity equation is linear. Despite the ocean transport adjustment occurring on time scales consistent with the basin-crossing times for Rossby waves, as predicted by theory, Sverdrup balance gives a useful measure of the subtropical circulation after only a few years. This is because the interannual transport variability is small compared to the mean transports. The vorticity input to the deep ocean by the interaction between deep currents and topography is found to be very large in both models. These deep transports, however, are separated from upper-layer transports that are in Sverdrup balance when considered over large scales.

On the Observability of Oceanic Gyres

This study examines the observability of a stratified ocean in a square flat basin on a midlatitude beta plane. Here, “observability” means the ability to establish, in a finite interval of time, the time-dependent ocean state given density observations over the same interval and with no regard for errors. The dynamics is linearized and hydrostatic, so that the motion can be decomposed into normal modes and the observability analysis is simplified. An observability Gramian (a symmetric matrix) is determined for the flows in an inviscid interior, in frictional boundary layers, and in a closed basin. Its properties are used to establish the condition for complete observability and to identify optimal data locations for each of these flows. It is found that complete observability of an oceanic interior in time-dependent Sverdrup balance requires that the observations originate from the westernmost location at each considered latitude. The degree of observability increases westward due to westward propagation of long baroclinic Rossby waves: data collected in the west are more informative than data collected in the east. Likewise, the best locations for observing variability in the western (eastern) boundary layer are near (far from) the boundary. The observability of a closed basin is influenced by the westward propagation and the boundaries. Optimal data locations that are identified for different resolutions (0.01 to 1 yr) and lengths of data records (0.2 to 20 yr) show a variable influence of the planetary vorticity gradient. Data collected near the meridional boundaries appear always less informative, from the viewpoint of basin observability, than data collected away from these boundaries.

A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo

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.