• Cushman-Roisin, B., 1981: Effects of horizontal advection on upper ocean mixing: A case of frontgenesis. J. Phys. Oceanogr.,11, 1345–1356.

  • ——, 1984: On the maintenance of the Subtropical Front and its associated countercurrent. J. Phys. Oceanogr.,14, 1179–1190.

  • Dewar, W. K., 1991: Arrested fronts. J. Mar. Res.,49, 21–52.

  • ——, 1992: Spontaneous shocks. J. Phys. Oceanogr.,22, 505–522.

  • Hasunuma, K., and K. Yoshida, 1978: Splitting the subtropical gyre in the western North Pacific. J. Oceanogr. Soc. Japan,34, 160–172.

  • Huang, R. X., 1988: On boundary value problems of the ideal fluid thermocline. J. Phys. Oceanogr.,18, 619–641.

  • ——, 1990: On the three-dimensional structure of wind-driven circulation in the North Atlantic. Dyn. Atmos. Oceans,15, 117–159.

  • ——, and S. Russell, 1994: Ventilation of the Subtropical North Pacific. J. Phys. Oceanogr.,24, 2589–2605.

  • Inui, T., K. Takeuchi, and K. Hanawa, 1999: A numerical investigation of the subduction process in response to an abrupt intensification of the westerlies. J. Phys. Oceanogr., in press.

  • Kubokawa, A., 1995: Stationary Rossby waves and shocks on the Sverdrup coordinate. J. Oceanogr.,51, 207–224.

  • ——, 1997: A two-level model of subtropical gyre and subtropical countercurrent. J. Oceanogr.,53, 231–244.

  • ——, and T. Inui, 1999: Subtropical countercurrent in an idealized ocean GCM. J. Phys. Oceanogr.,29, 1303–1333.

  • Levitus, S., 1982: Climatological Atlas of the World Ocean. NOAA Prof. Paper No. 13, U.S. Govt. Printing Office, 173 pp.

  • Luyten, J. R., J. Pedlosky, and H. Stommel, 1983: The ventilated thermocline. J. Phys. Oceanogr.,13, 292–309.

  • Pedlosky, J., 1983: Eastern boundary ventilation and the structure of the thermocline. J. Phys. Oceanogr.,13, 2038–2044.

  • ——, and W. R. Young, 1983: Ventilation, potential-vorticity homogenization and the structure of the ocean circulation. J. Phys. Oceanogr.,13, 2020–2037.

  • ——, and P. Robbins, 1991: The role of finite mixed-layer thickness in the structure of the ventilated thermocline. J. Phys. Oceanogr.,21, 1018–1031.

  • ——, W. Smith, and J. R. Luyten, 1984: On the dynamics of the coupled mixed layer–thermocline system and determination of the oceanic surface density. J. Phys. Oceanogr.,14, 1159–1171.

  • Rhines, P. B., and W. R. Young, 1982: A theory of the wind-driven circulation. I. Mid-ocean gyres. J. Mar. Res.,40 (Suppl.), 559–596.

  • Roden, G. I., 1980: On the variability of surface temperature front in the western Pacific, as detected by satellite. J. Geophys. Res.,85 (C), 2704–2710.

  • Stommel, H., and F. Shott, 1977: The beta spiral and the determination of the absolute velocity field from hydrographic data. Deep-Sea Res.,24, 325–329.

  • Suga, T., K. Hanawa, and Y. Toba, 1989: Subtropical mode water in the 137°E section. J. Phys. Oceanogr.,19, 1605–1618.

  • Takeuchi, K., 1984a: Numerical study of the Subtropical Front and the Subtropical Countercurrent. J. Oceanogr. Soc. Japan,40, 371–381.

  • ——, 1984b: An attempt to explain the formation mechanism of the Subtropical Front using a two layer model (in Japanese with English abstract). Geophys. Bull. Hokkaido Univ.,44, 77–84.

  • Talley, L. D., 1988: Potential vorticity distribution in the North Pacific. J. Phys. Oceanogr.,18, 89–106.

  • Uda, M., and K. Hasunuma, 1969: The eastward Subtropical Countercurrent in the Western North Pacific Ocean. J. Oceanogr. Soc. Japan,25, 201–210.

  • Welander, P., 1981: Mixed layer and fronts in simple ocean circulation model. J. Phys. Oceanogr.,11, 148–152.

  • Williams, R. G., 1989: The influence of air–sea interaction on the ventilated thermocline. J. Phys. Oceanogr.,19, 1255–1267.

  • ——, 1991: The role of the mixed layer in setting the potential vorticity of the main thermocline. J. Phys. Oceanogr.,21, 1803–1814.

  • Williams, R. G., M. A. Spall, and J. C. Marshall, 1995: Does Stommel’s mixed layer “demon” work? J. Phys. Oceanogr.,25, 3089–3102.

  • Yoshida, K., and T. Kidokoro, 1967a: A subtropical countercurrent in the North Pacific—An eastward flow near the Subtropical Convergence. J. Oceanogr. Soc. Japan,23, 88–91.

  • ——, and ——, 1967b: A subtropical countercurrent (II)—A prediction of eastward flows at lower subtropical latitudes. J. Oceanogr. Soc. Japan,23, 231–236.

  • Young, W. R., and P. B. Rhines, 1982: A theory of the wind-driven circulation. II. Gyre with western boundary layers. J. Mar. Res.,40 (Suppl.), 849–872.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 172 172 7
PDF Downloads 117 117 9

Ventilated Thermocline Strongly Affected by a Deep Mixed Layer: A Theory for Subtropical Countercurrent

View More View Less
  • 1 Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan
© Get Permissions Rent on DeepDyve
Restricted access

Abstract

The mixed layer thickness generally increases in the northward direction, but its change seems to occur rather sharply in a narrow transition zone, referred to as a mixed layer front here. The author investigates the effects of the mixed layer front on the ventilated thermocline structure, especially focusing on the generation mechanism of the subtropical countercurrent. If the mixed layer front is not parallel to the surface density contour, the fluid with minimum isopycnal potential vorticity is formed around the intersection of the mixed layer front and outcrop line. If the mixed layer front slants northeastward and surface density is zonally uniform, as seen in a numerical experiment carried out by Kubokawa and Inui in which the subtropical countercurrent was reproduced, the minimum potential vorticity on a high-density isopycnal occurs in the east, while that in a low-density isopycnal occurs in the west. In the present study, analytic solutions for a simple three-layer model are presented first to demonstrate that such an inhomogeneous distribution of isopycnal potential vorticity can generate a subtropical countercurrent. Then, a multilayer ventilated thermocline model coupled with the mixed layer is solved numerically. For the northeastward slanting mixed layer front, as advected southward, the low potential vorticity fluids in different layers converge in the horizontal plane, forming a thick ventilated layer in the central western region of the subtropical gyre. This thick ventilated layer lifts the base of the surface layer and generates a subtropical countercurrent along the southeastern edge of this region. On the other hand, a southeastward slanting mixed layer front can also generate a countercurrent.

Corresponding author address: Dr. Atsushi Kubokawa, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan.

Email: kubok@ees.hokudai.ac.jp

Abstract

The mixed layer thickness generally increases in the northward direction, but its change seems to occur rather sharply in a narrow transition zone, referred to as a mixed layer front here. The author investigates the effects of the mixed layer front on the ventilated thermocline structure, especially focusing on the generation mechanism of the subtropical countercurrent. If the mixed layer front is not parallel to the surface density contour, the fluid with minimum isopycnal potential vorticity is formed around the intersection of the mixed layer front and outcrop line. If the mixed layer front slants northeastward and surface density is zonally uniform, as seen in a numerical experiment carried out by Kubokawa and Inui in which the subtropical countercurrent was reproduced, the minimum potential vorticity on a high-density isopycnal occurs in the east, while that in a low-density isopycnal occurs in the west. In the present study, analytic solutions for a simple three-layer model are presented first to demonstrate that such an inhomogeneous distribution of isopycnal potential vorticity can generate a subtropical countercurrent. Then, a multilayer ventilated thermocline model coupled with the mixed layer is solved numerically. For the northeastward slanting mixed layer front, as advected southward, the low potential vorticity fluids in different layers converge in the horizontal plane, forming a thick ventilated layer in the central western region of the subtropical gyre. This thick ventilated layer lifts the base of the surface layer and generates a subtropical countercurrent along the southeastern edge of this region. On the other hand, a southeastward slanting mixed layer front can also generate a countercurrent.

Corresponding author address: Dr. Atsushi Kubokawa, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan.

Email: kubok@ees.hokudai.ac.jp

Save