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| Equatorial Pacific Cold Tongue | |
|---|---|
| Name | Equatorial Pacific Cold Tongue |
| Caption | Sea surface temperature anomalies in the tropical Pacific |
| Location | Pacific Ocean, along the Equator from the coast of Ecuador and Peru westward toward the central Pacific Ocean basin |
| Type | Cold sea surface temperature anomaly |
Equatorial Pacific Cold Tongue is a zonal band of anomalously cool sea surface temperatures in the eastern and central Pacific Ocean along the Equator that plays a central role in tropical climate and ocean dynamics. It links coastal upwelling near Peru and Ecuador with open-ocean thermocline processes, interacts strongly with the El Niño–Southern Oscillation phenomenon, and modulates regional weather patterns affecting societies in Australia, Indonesia, Chile, and the United States. The feature is a focal point for research at institutions such as Scripps Institution of Oceanography, NOAA, CSIRO, and Woods Hole Oceanographic Institution.
The cold tongue is defined as a persistent zonal region of reduced sea surface temperature that extends westward from the eastern Pacific Ocean coast along the Equator and is evident in climatologies produced by projects like Hadley Centre and NOAA's reanalysis datasets. It is characterized in observational programs led by TAO/TRITON and ARGO and is distinct from eastern-boundary upwelling systems found off California Current and Humboldt Current coasts. The tongue’s climatological expression is described in works from IPCC assessments and long-term studies at California Institute of Technology and Princeton University.
The cold tongue typically occupies the tropical eastern and central Pacific Ocean between roughly 160°W and the coasts of Ecuador and Peru, straddling the Equator and influencing SST gradients toward the Date Line. Its mean sea surface temperature minima and zonal SST gradient are measured in datasets from AVHRR, MODIS, SeaWiFS, and GHRSST, and correlate with thermocline depth observed by ARGO floats and XBT transects. The tongue’s cross-equatorial asymmetry and seasonal migration are documented in studies from University of Miami, MIT, and Lamont-Doherty Earth Observatory.
Formation arises from a suite of processes: persistent east-west trade winds associated with the Walker circulation, equatorial upwelling driven by near-surface divergence and Ekman dynamics described in classic work by Henry Stommel and Walter Munk, and subsurface thermocline shoaling influenced by equatorial wave dynamics investigated by Philander and Jin. Ocean-atmosphere coupling involves feedbacks with the Intertropical Convergence Zone, convective tendencies over Indonesia and Papua New Guinea, and remote forcing from the Indian Ocean and Atlantic Ocean via atmospheric teleconnections studied at NOAA/PMEL and Yale University.
The tongue’s strength modulates and is modulated by the El Niño–Southern Oscillation cycle described in canonical literature from NOAA and Scripps Institution of Oceanography, with El Niño events weakening or collapsing the tongue and La Niña events strengthening it. Its seasonal peak in boreal spring contributes to the spring predictability barrier discussed in predictive research at European Centre for Medium-Range Weather Forecasts and National Center for Atmospheric Research, and it interacts with decadal variability modes examined by Paleoceanography groups at Geological Survey of Japan and University of California, Los Angeles.
By controlling surface temperature, nutrient upwelling, and primary productivity, the tongue strongly influences marine ecosystems off Peru and Ecuador, affecting fisheries exploited by fleets from Chile and international companies documented by FAO assessments and research programs at Instituto del Mar del Perú. Variability in the tongue alters habitats for species studied by Smithsonian Tropical Research Institute and shifts distributions tracked by conservation groups like WWF and IUCN.
Monitoring leverages moored arrays such as TAO/TRITON and PIRATA, satellite missions including NOAA's GOES, ERS, and Jason series, and in situ networks like ARGO and shipboard hydrography coordinated by CLIVAR. SST, sea level, and thermocline data from AVHRR, MODIS, TOPEX/Poseidon, and GRACE inform operational products at NOAA and research at Scripps Institution of Oceanography.
General circulation models developed at GFDL, ECMWF, NCAR, and various universities struggle to simulate the tongue’s mean state and seasonal cycle due to biases in equatorial upwelling, thermocline representation, and air-sea coupling highlighted in IPCC model intercomparison projects like CMIP6. Predictability is limited by the spring predictability barrier and model errors in representing processes identified by researchers at Princeton University and Massachusetts Institute of Technology, motivating high-resolution coupled modeling, data assimilation efforts at ECMWF and NOAA, and targeted field campaigns coordinated with WHOI and Scripps Institution of Oceanography.