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The Zonal Momentum Balance of the Equatorial Undercurrent in the Central Pacific

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The Zonal Momentum Balance of the Equatorial Undercurrent in the Central Pacific University of South Florida Scholar Commons Marine Science Faculty Publications College of Marine Science 6-1997 The Zonal Momentum Balance of the Equatorial Undercurrent in the Central Pacific L. Qiao University of South Florida Robert H. Weisberg University of South Florida, [email protected] Follow this and additional works at: https://scholarcommons.usf.edu/msc_facpub Part of the Marine Biology Commons Scholar Commons Citation Qiao, L. and Weisberg, Robert H., "The Zonal Momentum Balance of the Equatorial Undercurrent in the Central Pacific" (1997). Marine Science Faculty Publications. 147. https://scholarcommons.usf.edu/msc_facpub/147 This Article is brought to you for free and open access by the College of Marine Science at Scholar Commons. It has been accepted for inclusion in Marine Science Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. 1094 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 27 The Zonal Momentum Balance of the Equatorial Undercurrent in the Central Paci®c L. QIAO AND R. H. WEISBERG Department of Marine Science, University of South Florida, St. Petersburg, Florida (Manuscript received 29 February 1996, in ®nal form 25 November 1996) ABSTRACT Current velocity data from an array of subsurface moorings deployed during the Tropical Instability Wave Experiment from May 1990 to June 1991 are used to diagnose the upper-ocean zonal momentum balance at 08, 1408W. The ¯ow ®eld and associated zonal momentum ¯ux divergence are fully three-dimensional over the upper 250 m, consistent with the earliest descriptions and theoretical ideas of the Equatorial Undercurrent (EUC). Estimates of the vertical stress divergence show dynamical ¯ow regimes that change between the surface and the base of the EUC, being essentially linear (modi®ed by nonlinearity) near the surface, weakly nonlinear at the EUC core, and fully nonlinear below the core. The vertical stress divergence is much larger over the lower portion of the EUC than previously reported, but this is consistent with the observed downstream deceleration of the EUC and the idea that vertical mixing is important in maintaining the thermostad. Nonlinearity becomes increasingly important with decreasing frequency, but tends to cancel upon vertical integration. 1. Introduction Global Atmosphere (TOGA) Tropical Instability Wave Experiment (TIWE). The array provides estimates of the The discovery by Cromwell et al. (1954) of a very vertical circulation (Weisberg and Qiao 1996, unpub- swift, subsurface current, ¯owing eastward on the equa- lished manuscript) and thus a three-dimensional view of tor in opposition to the winds, initiated an ongoing di- the ¯ow ®eld. The present paper uses these data to es- alog on the dynamics of this remarkable Equatorial Un- timate the zonal momentum ¯ux divergence and, com- dercurrent (EUC). Early descriptive studies by Knauss bined with other data from the TOGA-Tropical Atmo- (1960, 1966) showed the EUC to be continuous along, sphere Ocean (TOGA-TAO) array, to diagnose the upper- symmetric about, and tightly con®ned to the equator ocean zonal momentum balance. Section 2 reviews pre- with transports comparable to other major ocean cur- vious work on this topic. Section 3 describes the data rents. Early theoretical studies, beginning with the non- and methods. Section 4 attempts a quantitative diagnosis linear, inertial jet arguments of Fofonoff and Montgom- of the record-length averaged, depth-dependent zonal ery (1955) and the linear, frictional arguments of Arthur momentum balance and provides a description of how (1960) and Stommel (1960), were followed by numer- the dynamics change between the surface and the base ous articles combining these arguments into more com- of the EUC. Section 5 offers a more qualitative (owing plete theories. Central to all of these is the three-di- to data limitations) view of the time-dependent variations. mensionality of the ¯ow ®eld, driven by a depth-de- These ®ndings are then summarized and discussed in pendent zonal pressure gradient (ZPG) whose vertical section 6. integral tends to balance a westward surface wind stress. This three-dimensionality is what makes the circulation so important to contemporary climate-related studies, 2. Background because it largely determines the equatorial sea surface temperature distribution. But, it is also what makes a Knauss (1960, 1966) gives a comprehensive descrip- quantitative understanding of the EUC so dif®cult since tion of the EUC along with dynamical inferences. The it requires resolving the circulation's divergence. ¯ow is three-dimensional with a meridional conver- In May 1990, an array of ®ve subsurface acoustic gence upon the EUC core compensated by a vertical Doppler current pro®ling moorings was deployed about divergence away from the core. Intense vertical mixing 08, 1408W for 13 months as part of the Tropical Oceans is surmised on the equator to account for both the ob- served material property distributions in the meridional plane and the approximate geostrophic balance (meri- dionally) found for the near-equator zonal currents. Me- Corresponding author address: Dr. Robert H. Weisberg, Depart- ment of Marine Science, University of South Florida, 140 Seventh ridional convergence upon the EUC core is the essential Avenue South, St. Petersburg, FL 33701-5016. element in Fofonoff and Montgomery (1955) where the E-mail: [email protected] speed of the EUC is accounted for by conservation of q1997 American Meteorological Society JUNE 1997 QIAO AND WEISBERG 1095 absolute vorticity. This convergence is attributed to an of the ZPG and the estimated nonlinear accelerations eastward directed ZPG force owing to a westward wind were very similar to those of Bryden and Brady (1985), stress over a bounded basin. Recognizing that the effects as was the imbalance in the integrated ZPG and the of the wind-induced surface stress may extend vertically surface stress. The mean accelerations tended to oppose over the same region for which the ZPG is dynamically each other, suggesting that nonlinearity redistributes signi®cant, Arthur (1960) calculated a velocity pro®le momentum vertically in the zonal plane within the upper on the equator from the balance between the vertical 250 m. Assuming a small, constant vertical eddy vis- 24 2 21 stress divergence and the ZPG. Thus, the EUC core cosity coef®cient Av 5 1 3 10 m s (motivated by occurs where the stress is zero and the stress divergence the microstructure measurements of Peters et al. 1988), crosses zero together with the ZPG. Charney (1960) and the vertical stress at 250 m was calculated to be 100 Charney and Spiegel (1971) combined these inertial and times smaller than the surface stress. Thus, the vertically viscous effects in a constant density EUC model and integrated imbalance was not resolved. A time-depen- found that the relative importance of these terms greatly dent analysis showed that on intraseasonal timescales affects the resulting three-dimensional ¯ow ®eld. the vertically integrated ZPG varied with the surface Theory and observations con®rm that the zonal mo- stress to within about the same imbalance as the mean. mentum balance on the equator must entail conver- In an attempt to resolve the role of turbulent stress gences of momentum ¯ux and stress along with a ZPG, divergence, Hebert et al. (1991) used shipboard micro- but further advances have been hampered by data lim- structure and moored measurements collected between itations. Simultaneous data have been unavailable for 1408W and 1108W in spring 1987. With zonal advection estimating these constituents, leaving uncertainty in the being the only calculable nonlinear acceleration term, zonal momentum balance for both analytical and nu- correspondences were not achieved between the esti- merical model results. mated ZPG, acceleration, and turbulent stress diver- In a diagnostic study, Bryden and Brady (1985) used gence. It was suggested that annual averages are nec- historical hydrographic data for a box bounded by 58S essary for comparing estimates of turbulent stress di- and 58N, 1508W and 1108W, and 500 db and the surface. vergence with the diagnostic calculation of Bryden and The mean horizontal pressure gradient was referenced Brady (1985). to 500 db; the horizontal velocity components were es- Turbulent stress divergence occurs over synoptic as timated from the horizontal pressure gradient compo- well as microstructure and intermediate scales. At syn- nents and the climatological wind stress using geo- optic scales, the tropical instability waves are particu- strophic and Ekman assumptions, and the vertical ve- larly important. For example, the horizontal Reynolds locity component (w) was then calculated by mass con- stress divergence on the equator in the central Paci®c servation. The ZPG decreased monotonically to zero is a signi®cant fraction of the wind-stress-induced body between the surface and the lower portion of the EUC force (Hansen and Paul 1984), and similarly for the and was slightly westward below. Its vertical integral Atlantic (Weisberg and Weingartner 1988). Additional balanced the surface wind stress to within about 80%. supporting evidence is found in Lukas (1987), Wilson The vertical integral of the nonlinear accelerations over- and Leetmaa (1988), Bryden and Brady (1989), and compensated the surface wind stress/ZPG imbalance, Luther and Johnson (1990). However, even when av- implying a signi®cant stress (and stress divergence) at eraging over a record length suf®cient to include the least to the base of the EUC. On the equator upwelling synoptic scale (Johnson and Luther 1994), the role of was found above 180 db with smaller downwelling be- the turbulent stress divergence on the equator has re- low, and the ¯ow was described as being primarily along mained an unresolved issue. isopycnals. In summary, the available datasets show an approx- The w pro®le of Bryden and Brady (1985) has been imate balance on the equator between the vertically in- used in subsequent studies for estimating the vertical tegrated ZPG force and the surface stress.
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