The paper “A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo” is devoted to extended analysis of the correctness of classic Sverdrup balance for steady meridional circulation in the World Ocean using absolute geostrophic velocities assessed from Argo and wind stress obtained from the satellite scatterometer (QuikSCAT). The authors tried to give a comprehensive discussion of the problem. I was especially satisfied that they confirmed the robustness of the classic theory for extended regions of the World Ocean and I would like to add the following comments to the authors’ results.

First of all the application of classic Sverdrup theory for the description of large-scale circulation in the real World Ocean was a matter of long-term debates. As far as I remember, one of the first extended critical analyses of Sverdrup theory was published by A. Sarkisyan in the mid-1960s. English-speaking readers can find the suitable references in the numerous papers and books of this author published from the 1970s to the 2010s with different coauthors in English (see, e.g., Marchuk and Sarkisyan 1988; Marchuk et al. 1973). It seems to me it was necessary to mention this in sections 1 and 2 (Introduction and Background). Sarkisyan believed that one of the principal restriction of Sverdrup theory is the assumption that h (the ocean depth or, in the other interpretation, the thermocline thickness) is a constant (not a function of x and y). The author insisted that the joint effect of baroclinicity and relief (which is absent in Sverdrup theory as a result of this assumption) is a crucial reason for observed peculiarities of steady, large-scale currents in the World Ocean. I am not absolutely sure that all his arguments are axiomatic. However, I believe that they should be taken into account when Sverdrup balance and large-scale oceanic dynamics were analyzed in section 5 (Results and discussion) of Gray and Riser (2014).

Gray and Riser (2014) considered h as a function of x and y and mentioned the possibility of an absence of such a no motion surface. At the same time they neglected a priori the additional term in the integral vortex equation. This term arises if one defines W_h, U_h, and V_h. It seems to me it was worth discussing this problem in detail. For example, the authors could assess the magnitude of W_h = U_h∂h/∂x + V_h∂h/∂y when they considered h as the depth of certain isopycnals. I think a map of the currents at depth h would be useful to assess the accuracy of the assumption that vertical motions vanish just at this depth. My impression that the technique applied to determine h based on the best fit leads to a formal minimization of the impact of total discrepancies and errors on the calculation results rather than to an evaluation of real, no motion depth. This assumption could also be assessed using h topography and absolute currents at this depth.

My second comment concerns the general accuracy of calculations that were not comprehensively discussed. The authors described the procedure of processing and the accuracy of geostrophic current assessment in detail. At the same time, there are at least two additional sources of errors. The first source is due to potential inaccuracy of the wind stress, which was not discussed at all. The second error stems from different space–time averaging of the wind and oceanic fields. In principle, this can lead to quite significant errors, taking into account the intense space–time variability of the analyzed fields. The authors wrote nothing about the interpolation procedure for the wind stress. Concerning temporal averaging they just briefly noted that “the longer averaging periods [not entirely overlapping] were adopted because they offered a slight reduction in the uncertainties and using the shorter contemporaneous periods produced only negligible differences” (Gray and Riser 2014, p. 1222). The last statement is not fully understood. I took part in several attempts to assess the Sverdrup transport variability in the North Atlantic since the early 1980s. Quite intense temporal variability of the Sverdrup transport was repeatedly found (see, e.g., Fig. 1a). So, the change of period of averaging by a few years can lead to quite strong variations of Sverdrup transport.

View Full Size

(a) Interannual variability of integral meridional Sverdrup transport [ is zonally integrated along the 35°N Sverdrup transport multiplied by −1; 1 Sverdrup (Sv) = 10⁶ m³ s⁻¹], (b) Gulf Stream transport , and (c) their cross-correlation function; negative lag means that leads. Thick lines are linear trends. Smoothed curves are polynomial approximation. Dashed lines show the 95% confidence interval (after Dzhiganshin and Polonsky 2009).

Citation: Journal of Physical Oceanography 45, 5; 10.1175/JPO-D-14-0127.1

• Download Figure
• Download figure as PowerPoint slide

It is well known from classic theory that the Sverdrup balance is broken in the western boundary layer. This is because of the inertia–viscous nature of oceanic dynamics and the importance of transient processes there (Stommel 1948; Munk 1950; Pedlosky 1987). In particular, the Gulf Stream recirculation reinforces the upstream jet and leads (together with thermohaline forcing) to jet intensification. Stationary wind-driven gyre forcing accounts for only about 30% of Gulf Stream transport (Figs. 1a,b). Lead–lag correlation of the integral Sverdrup and Gulf Stream transports is significant but not very high (Fig. 1c). At least in part this is because of more complicated oceanic dynamics within the western boundary layer than in the subtropical gyre interior. Mesoscale eddies are the principal element of horizontal mixing and are the cause of the Gulf Stream recirculation. Their role in oceanic dynamics was intensively discussed in the 1970–80s after prominent Soviet, U.K., and U.S. field experiments [POLYGON-70, Mid-Ocean Dynamics Experiment (MODE), and POLYMODE]. In fact, results published in that time emphasized the crucial importance of transient mesoscale processes for the large-scale oceanic dynamics (e.g., Nelepo et al. 1980). This discussion is still in progress (see, e.g., Bryan 1996; Lozier 2010; Chelton et al. 2011; Zhang et al. 2014). The existence of a Sverdrup balance over the extended ocean interior is a very important fact for the theory of large-scale oceanic circulation. It proves that mesoscale eddies are not crucially important for the large-scale dynamics everywhere. It seems to me this was worth a mention.

In conclusion, I would like to emphasize that the paper “A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo” is a valued application of Argo and satellite products for the study of the large-scale oceanic circulation. It is not restricted by formal utilization of recent oceanographic and satellite data for calculation of absolute currents in the World Ocean but tries to improve our knowledge about large-scale oceanic dynamics.