Modulation of cross-isothermal velocities with ENSO in the tropical Pacific cold tongue

View More View Less
  • 1 National Center for Atmospheric Research, Boulder, Colorado
© Get Permissions
Open access

Abstract

The tropical Pacific cold tongue (CT) plays a major role in the global climate system. The strength of the CT sets the zonal temperature gradient in the Pacific that couples with the atmospheric Walker circulation. This coupling is an essential component of the El Niño Southern Oscillation (ENSO). The CT is supplied with cold water by the equatorial undercurrent that follows the thermocline as it shoals toward the east, adiabatically transporting cold water towards the surface. As the thermocline shoals, its water is transformed through diabatic processes producing water mass transformation (WMT) that allows water to cross mean isotherms. Here, we examine WMT in the cold tongue region from a global high resolution ocean simulation with saved budget terms that close its heat budget exactly. Using the terms of the heat budget, we quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, solar penetration), and find that vertical mixing is the single most important contribution in the thermocline, while solar heating dominates close to the surface. Horizontal diffusion is much smaller. During El Niño events, vertical mixing, and hence cross-isothermal flow as a whole, is much reduced, while during La Niña periods strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances cooling of the CT from below the surface.

Denotes content that is immediately available upon publication as open access.

National Center for Atmospheric Research, Boulder, Colorado

NOAA-PMEL, Seattle, Washington

University of Washington,Seattle, Washington

Corresponding author address: Anna-Lena Deppenmeier. E-mail: deppenme@ucar.edu

Abstract

The tropical Pacific cold tongue (CT) plays a major role in the global climate system. The strength of the CT sets the zonal temperature gradient in the Pacific that couples with the atmospheric Walker circulation. This coupling is an essential component of the El Niño Southern Oscillation (ENSO). The CT is supplied with cold water by the equatorial undercurrent that follows the thermocline as it shoals toward the east, adiabatically transporting cold water towards the surface. As the thermocline shoals, its water is transformed through diabatic processes producing water mass transformation (WMT) that allows water to cross mean isotherms. Here, we examine WMT in the cold tongue region from a global high resolution ocean simulation with saved budget terms that close its heat budget exactly. Using the terms of the heat budget, we quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, solar penetration), and find that vertical mixing is the single most important contribution in the thermocline, while solar heating dominates close to the surface. Horizontal diffusion is much smaller. During El Niño events, vertical mixing, and hence cross-isothermal flow as a whole, is much reduced, while during La Niña periods strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances cooling of the CT from below the surface.

Denotes content that is immediately available upon publication as open access.

National Center for Atmospheric Research, Boulder, Colorado

NOAA-PMEL, Seattle, Washington

University of Washington,Seattle, Washington

Corresponding author address: Anna-Lena Deppenmeier. E-mail: deppenme@ucar.edu
Save