Climate Variability Inferred from a Continuously Stratified Model of the Ideal-Fluid Thermocline

Rui Xin Huang Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

Search for other papers by Rui Xin Huang in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

Climate variability in the subtropical gyre interior induced by anomalous surface thermal forcing, Ekman pumping, mixed layer depth variability, and anomalous subpolar water formation is examined, using a continuously stratified model of the ideal-fluid thermocline. Cooling (heating) induces a negative (positive) potential vorticity perturbation in the ventilated thermocline, and the associated density perturbations propagate downstream in the form of second and higher baroclinic modes. The second baroclinic mode resembles the traditional second baroclinic mode because it has a thermal structure with cooling (warming) in the upper thermocline and warming (cooling) in the lower thermocline.

Anomalous Ekman pumping can also induce density perturbations that propagate westward in the form of the first baroclinic mode. In addition, if the outcrop lines are nonzonal, there are density perturbations that propagate downstream in the form of the second or third baroclinic modes. Perturbations in the sea surface elevation are mostly confined to the region of anomalous forcing. On the other hand, when the low potential vorticity anomaly in the subpolar mode water spreads into the subtropical basin, both the unventilated and ventilated thermocline move downward. Consequently, temperature at a given depth seems to increase.

Corresponding author address: Rui Xin Huang, Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: rhuang@whoi.edu

Abstract

Climate variability in the subtropical gyre interior induced by anomalous surface thermal forcing, Ekman pumping, mixed layer depth variability, and anomalous subpolar water formation is examined, using a continuously stratified model of the ideal-fluid thermocline. Cooling (heating) induces a negative (positive) potential vorticity perturbation in the ventilated thermocline, and the associated density perturbations propagate downstream in the form of second and higher baroclinic modes. The second baroclinic mode resembles the traditional second baroclinic mode because it has a thermal structure with cooling (warming) in the upper thermocline and warming (cooling) in the lower thermocline.

Anomalous Ekman pumping can also induce density perturbations that propagate westward in the form of the first baroclinic mode. In addition, if the outcrop lines are nonzonal, there are density perturbations that propagate downstream in the form of the second or third baroclinic modes. Perturbations in the sea surface elevation are mostly confined to the region of anomalous forcing. On the other hand, when the low potential vorticity anomaly in the subpolar mode water spreads into the subtropical basin, both the unventilated and ventilated thermocline move downward. Consequently, temperature at a given depth seems to increase.

Corresponding author address: Rui Xin Huang, Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: rhuang@whoi.edu

Save
  • Cane, M. A., and E. S. Sarachik, 1976: Forced baroclinic ocean motions. I. The linear equatorial unbounded case. J. Mar. Res.,34, 629–665.

  • Deser, C., M. A. Alexander, and M. S. Timlin, 1996: Upper-ocean thermal variability in the North Pacific during 1970–1991, J. Climate,9, 1840–1855.

  • Huang, R. X., 1984: The thermocline and current structure in subtropical/subpolar basins. Ph.D. thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 218 pp. [WHOI Tech. Rep. WHOI-84-42.].

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

  • ——, 2000: Parameter study of a continuously stratified model of the ideal-fluid thermocline. J. Phys. Oceanogr.,30, 1372–1388.

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

  • ——, and J. Pedlosky, 1999: Climate variability inferred from a layered model of the ventilated thermocline. J. Phys. Oceanogr.,29, 779–790.

  • Iselin, C. O’D., 1939: The influence of vertical and lateral turbulence on the characteristics of the waters at mid-depths. Trans. Amer. Geophys. Union,20, 414–417.

  • Joyce, T. M., R. S. Pickart, and R. C. Millard, 1999: Long-term hydrographic changes at 52 and 63°W in the North Atlantic subtropical gyre and Caribbean. Deep-Sea Res. II,46, 245–278.

  • Liu, Z., 1999: Forced planetary wave response in a thermocline gyre. J. Phys. Oceanogr.,29, 1036–1055.

  • McCreary, J. P., 1985: Modeling equatorial ocean circulation. Annu. Rev. Fluid Mech.,17, 359–409.

  • Schneider, N., A. J. Miller, M. A. Alexander, and C. Deser, 1999: Subduction of decadal North Pacific temperature anomalies: Observations and dynamics. J. Phys. Oceanogr.,29, 1056–1070.

  • Tourre, Y. M., Y. Kushnir, and W. B. White, 1999: Evolution of interdecadal variability in sea level pressure, sea surface temperature, and upper ocean temperature over the Pacific Ocean. J. Phys. Oceanogr.,29, 1528–1541.

  • Zhang, R. H., 1998: Decadal variability of temperature at a depth of 400 meters in the North Pacific Ocean. Geophys. Res. Lett.,25, 1197–1120.

  • ——, L. M. Rothstein, and A. J. Busalacchi, 1998: Origin of upper-ocean warming and El Nino change on decadal scales in the tropical Pacific Ocean. Nature,391, 879–883.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 142 29 2
PDF Downloads 33 13 2