• Ando, K., and M. J. McPhaden, 1997: Variability of surface layer hydrography in the tropical Pacific Ocean. J. Geophys. Res., 102 , 2306323078.

    • Search Google Scholar
    • Export Citation
  • Bloomfield, P., 1976: Fourier Decomposition of Time Series: An Introduction. Wiley, 258 pp.

  • Bonjean, F., and G. S. E. Lagerloef, 2002: Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean. J. Phys. Oceanogr., 32 , 29382954.

    • Search Google Scholar
    • Export Citation
  • Bryden, H. L., and E. C. Brady, 1985: Diagnostic model of the three-dimensional circulation in the upper equatorial Pacific Ocean. J. Phys. Oceanogr., 15 , 12551273.

    • Search Google Scholar
    • Export Citation
  • Chelton, D. B., and Coauthors, 2001: Observations of coupling between surface wind stress and sea surface temperature in the eastern tropical Pacific. J. Climate, 14 , 14791498.

    • Search Google Scholar
    • Export Citation
  • Chereskin, T. K., 1995: Direct evidence for an Ekman balance in the California Current. J. Geophys. Res., 100 , 1826118269.

  • Chereskin, T. K., and D. Roemmich, 1991: A comparison of measured and wind-derived Ekman transport at 11°N in the Atlantic Ocean. J. Phys. Oceanogr., 21 , 869878.

    • Search Google Scholar
    • Export Citation
  • Chereskin, T. K., W. D. Wilson, H. L. Bryden, A. Ffield, and J. Morrison, 1997: Observations of the Ekman balance at 8°30′N in the Arabian Sea during the 1995 southwest monsoon. Geophys. Res. Lett., 24 , 25412544.

    • Search Google Scholar
    • Export Citation
  • Cronin, M. F., and W. S. Kessler, 2002: Seasonal and interannual modulation of mixed layer variability at 0°, 110°W. Deep-Sea Res. I, 49 , 117.

    • Search Google Scholar
    • Export Citation
  • Cronin, M. F., N. Bond, C. Fairall, J. Hare, M. J. McPhaden, and R. A. Weller, 2002: Enhanced oceanic and atmospheric monitoring underway in eastern Pacific. Eos, Trans. Amer. Geophys. Union, 83 , 205. doi:10.1029/2002EO000137. 210–211.

    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., W. G. Large, J. J. Tribbia, P. R. Gent, B. P. Briegleb, and J. C. McWilliams, 2006: Diurnal coupling in the tropical oceans of CCSM3. J. Climate, 19 , 23472365.

    • Search Google Scholar
    • Export Citation
  • Ekman, V. W., 1905: On the influence of the earth’s rotation on ocean-currents. Ark. Mat. Astron. Fys., 2 , 152.

  • Fairall, C. F., E. F. Bradley, J. E. Hare, A. A. Grachev, and J. B. Edson, 2003: Bulk parameterization of air–sea fluxes: Updates and verification for the COARE algorithm. J. Climate, 16 , 571591.

    • Search Google Scholar
    • Export Citation
  • Flament, P., and L. Armi, 2000: The shear, convergence, and thermohaline structure of a front. J. Phys. Oceanogr., 30 , 5166.

  • Flament, P., S. Kennan, R. Knox, P. Niiler, and R. Bernstein, 1996: The three-dimensional structure of a tropical instability wave. Nature, 383 , 610613.

    • Search Google Scholar
    • Export Citation
  • Freitag, H. P., Y. Feng, L. J. Mangum, M. J. McPhaden, J. Neander, and L. D. Stratton, 1994: Calibration procedures and instrumental accuracy estimates of TAO temperature, relative humidity and radiation measurements. Tech. Memo. ERL PMEL-104 (PB95–174827), NOAA, Pacific Marine Environmental Laboratory, Seattle, WA, 32 pp.

    • Search Google Scholar
    • Export Citation
  • Freitag, P., M. McPhaden, C. Meinig, and P. Plimpton, 2003: Mooring motion bias of point Doppler current meter measurements. Proc. IEEE Seventh Working Conf. on Current Measurement Technology, San Diego, CA, IEEE, 155–160.

    • Search Google Scholar
    • Export Citation
  • Garrett, C. J. R., and J. W. Loder, 1981: Dynamical aspects of shallow sea fronts. Philos. Trans. Roy. Soc. London, 302A (1472) 563581.

    • Search Google Scholar
    • Export Citation
  • Gordon, R. L., 1996: Acoustic Doppler Current Profilers, Principles of Operation: A Practical Primer. R. D. Instruments, 51 pp. [Available from R. D. Instruments, 9855 Businesspark Ave., San Diego, CA 92131.].

    • Search Google Scholar
    • Export Citation
  • Halpern, D., and H. P. Freitag, 1987: Vertical motion in the upper ocean of the equatorial Pacific. Oceanol. Acta, (Special Vol.), 19–26.

    • Search Google Scholar
    • Export Citation
  • Johnson, G. C., M. J. McPhaden, and E. Firing, 2001: Equatorial Pacific Ocean horizontal velocity, divergence, and upwelling. J. Phys. Oceanogr., 31 , 839849.

    • Search Google Scholar
    • Export Citation
  • Kug, J-S., I-S. Kang, and S. I. An, 2003: Symmetric and antisymmetric mass exchanges between the equatorial and off-equatorial Pacific associated with ENSO. J. Geophys. Res., 108 , 3284. doi:10.1029/2002JC001671.

    • Search Google Scholar
    • Export Citation
  • Lien, R-C., D. R. Caldwell, M. C. Gregg, and J. N. Moum, 1995: Turbulence variability at the equator in the central Pacific at the beginning of the 1991–1993 El Niño. J. Geophys. Res., 100 , 68816898.

    • Search Google Scholar
    • Export Citation
  • McPhaden, M. J., M. F. Cronin, and D. C. McClurg, 2008: Surface mixed layer temperature balance on seasonal time scales in the eastern tropical Pacific. J. Climate, 21 , 32403260.

    • Search Google Scholar
    • Export Citation
  • Meinen, C. S., M. J. McPhaden, and G. C. Johnson, 2001: Vertical velocities and transports in the equatorial Pacific Ocean during 1993–99. J. Phys. Oceanogr., 31 , 32303248.

    • Search Google Scholar
    • Export Citation
  • Nagai, T., A. Tandon, and D. L. Rudnick, 2006: Two-dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large-scale deformation. J. Geophys. Res., 111 , C090938. doi:10.1029/2005JC002964.

    • Search Google Scholar
    • Export Citation
  • Pacanowski, R. G., and S. G. H. Philander, 1981: Parameterization of vertical mixing in numerical models of the tropical ocean. J. Phys. Oceanogr., 11 , 14431451.

    • Search Google Scholar
    • Export Citation
  • Peters, H., M. C. Gregg, and J. M. Toole, 1988: On the parameterization of equatorial turbulence. J. Geophys. Res., 93 , 11991218.

  • Price, J. F., and M. A. Sundermeyer, 1999: Stratified Ekman layers. J. Geophys. Res., 104 , 2046720494.

  • Price, J. F., R. A. Weller, and R. Pinkel, 1986: Diurnal cycling: Observations and models of the upper ocean response to diurnal heating, cooling and wind mixing. J. Geophys. Res., 91 , 84118427.

    • Search Google Scholar
    • Export Citation
  • Price, J. F., R. A. Weller, and R. R. Schudlich, 1987: Wind-driven ocean currents and Ekman transport. Science, 238 , 15341538.

  • Santiago-Mandujano, F., and E. Firing, 1990: Mixed-layer shear generated by wind stress in the central equatorial Pacific. J. Phys. Oceanogr., 20 , 15761582.

    • Search Google Scholar
    • Export Citation
  • Schneider, N., and P. Müller, 1994: Sensitivity of the surface equatorial ocean to the parameterization of vertical mixing. J. Phys. Oceanogr., 24 , 16231640.

    • Search Google Scholar
    • Export Citation
  • Smyth, W. D., D. Hebert, and J. N. Moum, 1996: Local ocean response to a multiphase westerly wind burst. 1. Dynamic response. J. Geophys. Res., 101 , 2249522512.

    • Search Google Scholar
    • Export Citation
  • Stommel, H., 1960: Winddrift near the equator. Deep-Sea Res., 6 , 298302.

  • Thompson, L., 2000: Ekman layers and two-dimensional frontogenesis in the upper ocean. J. Geophys. Res., 105 , (C3). 64376451.

  • Weisberg, R. H., and L. Qiao, 2000: Equatorial upwelling in the central Pacific estimated from moored velocity profiler. J. Phys. Oceanogr., 30 , 105124.

    • Search Google Scholar
    • Export Citation
  • Wijffels, S., E. Firing, and H. Bryden, 1994: Direct observations of the Ekman balance at 10°N in the Pacific. J. Phys. Oceanogr., 24 , 16661679.

    • Search Google Scholar
    • Export Citation
  • Willett, C. S., R. R. Leben, and M. F. Lavín, 2006: Eddies and tropical instability waves in the eastern tropical Pacific: A review. Prog. Oceanogr., 69 , 218238.

    • Search Google Scholar
    • Export Citation
  • Wyrtki, K., 1981: An estimate of equatorial upwelling in the Pacific. J. Phys. Oceanogr., 11 , 12051214.

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Near-Surface Shear Flow in the Tropical Pacific Cold Tongue Front

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  • 1 NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington
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Abstract

Near-surface shear in the Pacific cold tongue front at 2°N, 140°W was measured using a set of five moored current meters between 5 and 25 m for nine months during 2004–05. Mean near-surface currents were strongly westward and only weakly northward (∼3 cm s−1). Mean near-surface shear was primarily westward and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was subtracted from the observed shear, the residual ageostrophic currents relative to 25 m were northward and had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a vertically uniform front. According to this “frontal Ekman” model, the ageostrophic Ekman spiral is forced by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A diurnal jet was observed that was on average 12 cm s−1 stronger at 5 m than at 25 m, even though daytime stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night than daytime and decreases with depth. A “generalized Ekman” model is also developed that assumes that viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the features of the observed mean shear and is valid both in frontal regions and at the equator.

Corresponding author address: Meghan F. Cronin, NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115-6349. Email: meghan.f.cronin@noaa.gov

Abstract

Near-surface shear in the Pacific cold tongue front at 2°N, 140°W was measured using a set of five moored current meters between 5 and 25 m for nine months during 2004–05. Mean near-surface currents were strongly westward and only weakly northward (∼3 cm s−1). Mean near-surface shear was primarily westward and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was subtracted from the observed shear, the residual ageostrophic currents relative to 25 m were northward and had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a vertically uniform front. According to this “frontal Ekman” model, the ageostrophic Ekman spiral is forced by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A diurnal jet was observed that was on average 12 cm s−1 stronger at 5 m than at 25 m, even though daytime stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night than daytime and decreases with depth. A “generalized Ekman” model is also developed that assumes that viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the features of the observed mean shear and is valid both in frontal regions and at the equator.

Corresponding author address: Meghan F. Cronin, NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115-6349. Email: meghan.f.cronin@noaa.gov

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