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CORRESPONDENCE Comments on ''A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo''

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CORRESPONDENCE Comments on ''A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo'' 1446 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 45 CORRESPONDENCE Comments on ‘‘A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo’’ ALEXANDER POLONSKY Marine Hydrophysical Institute, Sevastopol, Ukraine (Manuscript received 26 June 2014, in final form 2 December 2014) The paper ‘‘A Global Analysis of Sverdrup Balance large-scale currents in the World Ocean. I am not ab- Using Absolute Geostrophic Velocities from Argo’’ is solutely sure that all his arguments are axiomatic. devoted to extended analysis of the correctness of classic However, I believe that they should be taken into ac- Sverdrup balance for steady meridional circulation in count when Sverdrup balance and large-scale oceanic dy- the World Ocean using absolute geostrophic velocities namics were analyzed in section 5 (Results and assessed from Argo and wind stress obtained from the discussion) of Gray and Riser (2014). satellite scatterometer (QuikSCAT). The authors tried Gray and Riser (2014) considered h as a function of x to give a comprehensive discussion of the problem. I was and y and mentioned the possibility of an absence of especially satisfied that they confirmed the robustness of such a no motion surface. At the same time they the classic theory for extended regions of the World neglected a priori the additional term in the integral Ocean and I would like to add the following comments vortex equation. This term arises if one defines Wh, Uh, to the authors’ results. and Vh. It seems to me it was worth discussing this First of all the application of classic Sverdrup theory problem in detail. For example, the authors could assess for the description of large-scale circulation in the real the magnitude of Wh 5 Uh›h/›x 1 Vh›h/›y when they World Ocean was a matter of long-term debates. As far considered h as the depth of certain isopycnals. I think a as I remember, one of the first extended critical analyses map of the currents at depth h would be useful to assess of Sverdrup theory was published by A. Sarkisyan in the the accuracy of the assumption that vertical motions mid-1960s. English-speaking readers can find the suit- vanish just at this depth. My impression that the tech- able references in the numerous papers and books of nique applied to determine h based on the best fit leads this author published from the 1970s to the 2010s with to a formal minimization of the impact of total dis- different coauthors in English (see, e.g., Marchuk and crepancies and errors on the calculation results rather Sarkisyan 1988; Marchuk et al. 1973). It seems to me it than to an evaluation of real, no motion depth. This was necessary to mention this in sections 1 and 2 (In- assumption could also be assessed using h topography troduction and Background). Sarkisyan believed that and absolute currents at this depth. one of the principal restriction of Sverdrup theory is the My second comment concerns the general accuracy of assumption that h (the ocean depth or, in the other in- calculations that were not comprehensively discussed. terpretation, the thermocline thickness) is a constant The authors described the procedure of processing and (not a function of x and y). The author insisted that the the accuracy of geostrophic current assessment in detail. joint effect of baroclinicity and relief (which is absent in At the same time, there are at least two additional Sverdrup theory as a result of this assumption) is a sources of errors. The first source is due to potential crucial reason for observed peculiarities of steady, 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, Corresponding author address: Alexander Polonsky, Marine Hydrophysical Institute, 2 Kapitansksaya St., Sevastopol, Ukraine this can lead to quite significant errors, taking into ac- 299011. count the intense space–time variability of the analyzed E-mail: [email protected] fields. The authors wrote nothing about the interpolation DOI: 10.1175/JPO-D-14-0127.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 09/24/21 08:36 AM UTC MAY 2015 C O R R E S P O N D E N C E 1447 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 dy- namics and the importance of transient processes there (Stommel 1948; Munk 1950; Pedlosky 1987). In partic- ular, the Gulf Stream recirculation reinforces the up- stream 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 in- tegral 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 in- terior. 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 meso- scale 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 every- where. It seems to me this was worth a mention. FIG. 1. (a) Interannual variability of integral meridional Sverdrup In conclusion, I would like to emphasize that the pa- Sv 8 transport [Qy is zonally integrated along the 35 NSverdrup per ‘‘A Global Analysis of Sverdrup Balance Using 2 transport multiplied by 21; 1 Sverdrup (Sv) 5 106 m3 s 1], Absolute Geostrophic Velocities from Argo’’ is a valued (b) Gulf Stream transport (Syr), and (c) their cross-correlation application of Argo and satellite products for the study function; negative lag means that QSv leads. Thick lines are linear y of the large-scale oceanic circulation. It is not restricted trends. Smoothed curves are polynomial approximation. Dashed lines show the 95% confidence interval (after Dzhiganshin and by formal utilization of recent oceanographic and sat- Polonsky 2009). ellite data for calculation of absolute currents in the World Ocean but tries to improve our knowledge about large-scale oceanic dynamics. procedure for the wind stress. Concerning temporal aver- aging they just briefly noted that ‘‘the longer averaging REFERENCES periods [not entirely overlapping] were adopted because Bryan, K., 1996: The role of meso-scale eddies in the poleward they offered a slight reduction in the uncertainties and transport of heat by the oceans: A review. Physica D, 98, 249– using the shorter contemporaneous periods produced only 257, doi:10.1016/0167-2789(96)00119-4. Chelton, D. B., M. G. Schlax., and R. M. Samelson, 2011: Global negligible differences’’ (Gray and Riser 2014, p. 1222). The observations of nonlinear mesoscale eddies. Prog. Oceanogr., last statement is not fully understood. I took part in several 91, 167–216, doi:10.1016/j.pocean.2011.01.002. attempts to assess the Sverdrup transport variability in the Dzhiganshin, G. F., and A. B. Polonsky, 2009: Low-frequency North Atlantic since the early 1980s. Quite intense tem- variations of the Gulf Stream transport: Description and poral variability of the Sverdrup transport was repeatedly mechanisms. Phys. Oceanogr., 19, 151–169, doi:10.1007/ s11110-009-9047-5. found (see, e.g., Fig. 1a). So, the change of period of av- Gray, A. R., and S. C. Riser, 2014: A global analysis of Sverdrup eraging by a few years can lead to quite strong variations of balance using absolute geostrophic velocities from Argo. J. Phys. Sverdrup transport. Oceanogr., 44, 1213–1229, doi:10.1175/JPO-D-12-0206.1. Unauthenticated | Downloaded 09/24/21 08:36 AM UTC 1448 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 45 Lozier, M. S., 2010: Deconstructing the conveyor belt. Science, 328, Nelepo, B. A., N. P. Bulgakov, and I. Timchemko, 1980: Synoptic 1507–1511, doi:10.1126/science.1189250. Eddies in the Ocean (in Russian). Naukova Dumka, 288 pp. Marchuk, G. I., and A. S. Sarkisyan, 1988: Mathematical Modeling Pedlosky, J., 1987: Geophysical Fluid Dynamics. 2nd ed. Springer- of Ocean Circulation. Springer-Verlag, 292 pp. Verlag, 710 pp. ——, A. S. Sarkisian, V. P. Kochergin, 1973: Calculations of flows in a Stommel, H., 1948: The westward intensification of wind- baroclinic ocean: Numerical methods and results. Geophys. As- driven ocean currents. Eos Trans. Amer. Geophys. Union, trophys. Fluid Dyn., 5, 89–99, doi:10.1080/03091927308236109. 29, 202–206. Munk, W., 1950: On the wind-driven ocean circulation. Zhang, Z., W. Wang, and B. Qiu, 2014: Oceanic mass transport J. Meteor., 7, 80–93, doi:10.1175/1520-0469(1950)007,0080: by mesoscale eddies. Science, 345, 322–324, doi:10.1126/ OTWDOC.2.0.CO;2. science.1252418. Unauthenticated | Downloaded 09/24/21 08:36 AM UTC.
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