• Abarbanel, H. D. I., D. D. Holm, J. E. Marsden, and T. Ratiu, 1984: Richardson number criterion for the nonlinear stability of three-dimensional stratified flow. Phys. Rev. Lett., 52, 23522355, https://doi.org/10.1103/PhysRevLett.52.2352.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Bosse, A., P. Testor, P. Damien, C. Estournel, P. Marsaleix, L. Mortier, L. Prieur, and V. Taillandier, 2021: Wind-forced submesoscale symmetric instability around deep convection in the northwestern Mediterranean Sea. Fluids, 6, 123, https://doi.org/10.3390/fluids6030123.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Boyer, T. P., and Coauthors, 2013: World Ocean Database 2013. NOAA Atlas NESDIS 72, 208 pp., http://doi.org/10.7289/V5NZ85MT.

  • Canuto, V. M., A. Howard, Y. Cheng, and M. S. Dubovikov, 2001: Ocean turbulence. Part I: One-point closure model—Momentum and heat vertical diffusivities. J. Phys. Oceanogr., 31, 14131426, https://doi.org/10.1175/1520-0485(2001)031<1413:OTPIOP>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Charney, J. G., and S. L. Spiegel, 1971: Structure of wind-driven equatorial currents in homogeneous oceans. J. Phys. Oceanogr., 1, 149160, https://doi.org/10.1175/1520-0485(1971)001<0149:SOWDEC>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Christensen, N., 1971: Observations of the Cromwell Current near the Galapagos Islands. Deep-Sea Res. Oceanogr. Abstr., 18, 2733, https://doi.org/10.1016/0011-7471(71)90013-1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Clayson, C. A., and L. H. Kantha, 1999: Turbulent kinetic energy and its dissipation rate in the equatorial mixed layer. J. Phys. Oceanogr., 29, 21462166, https://doi.org/10.1175/1520-0485(1999)029<2146:TKEAID>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Crawford, W. R., 1982: Pacific equatorial turbulence. J. Phys. Oceanogr., 12, 11371149, https://doi.org/10.1175/1520-0485(1982)012<1137:PET>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Cromwell, T., R. B. Montgomery, and E. D. Stroup, 1954: Equatorial undercurrent in Pacific Ocean revealed by new methods. Science, 119, 648649, https://doi.org/10.1126/science.119.3097.648.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Cushman-Roisin, B., and J.-M. Beckers, 2011: Introduction to Geophysical Fluid Dynamics: Physical and Numerical Aspects. 2nd ed. Academic Press, 828 pp.

    • Search Google Scholar
    • Export Citation
  • Dunkerton, T. J., 1981: On the inertial stability of the equatorial middle atmosphere. J. Atmos. Sci., 38, 23542364, https://doi.org/10.1175/1520-0469(1981)038<2354:OTISFT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ertel, H., 1942: Ein neuer hydrodynamischer Erhaltungssatz. Naturwissenschaften, 30, 543544, https://doi.org/10.1007/BF01475602.

  • Farrar, J. T., 2011: Barotropic Rossby waves radiating from tropical instability waves in the Pacific Ocean. J. Phys. Oceanogr., 41, 11601181, https://doi.org/10.1175/2011JPO4547.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Fofonoff, N. P., and R. B. Montgomery, 1955: The equatorial undercurrent in the light of the vorticity equation. Tellus, 7, 518521, https://doi.org/10.3402/tellusa.v7i4.8910.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Forryan, A., A. C. Naveira Garabato, C. Vic, A. J. G. Nurser, and A. R. Hearn, 2021: Galápagos upwelling driven by localized wind–front interactions. Sci. Rep., 11, 1277, https://doi.org/10.1038/s41598-020-80609-2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gill, A. E., 1982: Atmosphere-Ocean Dynamics. Academic Press, 664 pp.

  • Gouriou, Y., and J. Toole, 1993: Mean circulation of the upper layers of the western equatorial Pacific Ocean. J. Geophys. Res., 98, 22 49522 520, https://doi.org/10.1029/93JC02513.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gula, J., M. J. Molemaker, and J. C. McWilliams, 2015: Gulf Stream dynamics along the southeastern U.S. seaboard. J. Phys. Oceanogr., 45, 690715, https://doi.org/10.1175/JPO-D-14-0154.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Haine, T. W. N., and J. Marshall, 1998: Gravitational, symmetric, and baroclinic instability of the ocean mixed layer. J. Phys. Oceanogr., 28, 634658, https://doi.org/10.1175/1520-0485(1998)028<0634:GSABIO>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Holmes, R. M., L. N. Thomas, L. Thompson, and D. Darr, 2014: Potential vorticity dynamics of tropical instability vortices. J. Phys. Oceanogr., 44, 9951011, https://doi.org/10.1175/JPO-D-13-0157.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., 1974: The role of potential vorticity in symmetric stability and instability. Quart. J. Roy. Meteor. Soc., 100, 480482, https://doi.org/10.1002/qj.49710042520.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Jakoboski, J., R. E. Todd, W. B. Owens, K. B. Karnauskas, and D. L. Rudnick, 2020: Bifurcation and upwelling of the equatorial undercurrent west of the Galápagos Archipelago. J. Phys. Oceanogr., 50, 887905, https://doi.org/10.1175/JPO-D-19-0110.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Johnson, E. S., and D. S. Luther, 1994: Mean zonal momentum balance in the upper and central equatorial Pacific Ocean. J. Geophys. Res., 99, 76897705, https://doi.org/10.1029/94JC00033.

    • Crossref
    • 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, https://doi.org/10.1175/1520-0485(2001)031<0839:EPOHVD>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Johnson, G. C., B. M. Sloyan, W. S. Kessler, and K. E. McTaggart, 2002: Direct measurements of upper ocean currents and water properties across the tropical Pacific during the 1990s. Prog. Oceanogr., 52, 3161, https://doi.org/10.1016/S0079-6611(02)00021-6.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Karnauskas, K. B., R. Murtugudde, and A. J. Busalacchi, 2007: The effect of the Galápagos Islands on the equatorial Pacific cold tongue. J. Phys. Oceanogr., 37, 12661281, https://doi.org/10.1175/JPO3048.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Karnauskas, K. B., R. Murtugudde, and A. J. Busalacchi, 2010: Observing the Galápagos–EUC interaction: Insights and challenges. J. Phys. Oceanogr., 40, 27682777, https://doi.org/10.1175/2010JPO4461.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Karnauskas, K. B., J. Jakoboski, T. M. S. Johnston, W. B. Owens, D. L. Rudnick, and R. E. Todd, 2020: The Pacific equatorial undercurrent in three generations of global climate models and glider observations. J. Geophys. Res. Oceans, 125, e2020JC016609, https://doi.org/10.1029/2020JC016609.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kessler, W. S., 2006: The circulation of the eastern tropical Pacific: A review. Prog. Oceanogr., 69, 181217, https://doi.org/10.1016/j.pocean.2006.03.009.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kessler, W. S., L. M. Rothstein, and D. Chen, 1998: The annual cycle of SST in the eastern tropical Pacific, diagnosed in an ocean GCM. J. Climate, 11, 777799, https://doi.org/10.1175/1520-0442(1998)011<0777:TACOSI>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Knauss, J. A., 1959: Measurements of the Cromwell current. Deep-Sea Res., 6, 265286, https://doi.org/10.1016/0146-6313(59)90086-3.

  • Knauss, J. A., and J. E. King, 1958: Observations of the Pacific equatorial undercurrent. Nature, 182, 601602, https://doi.org/10.1038/182601a0.

  • Kundu, P. K., I. M. Cohen, D. R. Dowling, and G. Tryggvason, 2015: Fluid Mechanics. 6th ed. Academic Press, 921 pp.

  • Leslie, W. R., and K. B. Karnauskas, 2014: The equatorial undercurrent and TAO sampling bias from a decade at SEA. J. Atmos. Oceanic Technol., 31, 20152025, https://doi.org/10.1175/JTECH-D-13-00262.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lukas, R., 1986: The termination of the equatorial undercurrent in the eastern Pacific. Prog. Oceanogr., 16, 6390, https://doi.org/10.1016/0079-6611(86)90007-8.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lukas, R., and E. Firing, 1984: The geostrophic balance of the Pacific equatorial undercurrent. Deep-Sea Res., 31A, 6166, https://doi.org/10.1016/0198-0149(84)90072-4.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lyman, J. M., D. B. Chelton, R. A. DeSzoeke, and R. M. Samelson, 2005: Tropical instability waves as a resonance between equatorial Rossby waves. J. Phys. Oceanogr., 35, 232254, https://doi.org/10.1175/JPO-2668.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lyman, J. M., G. C. Johnson, and W. S. Kessler, 2007: Distinct 17- and 33-day tropical instability waves in subsurface observations. J. Phys. Oceanogr., 37, 855872, https://doi.org/10.1175/JPO3023.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Molemaker, M. J., J. C. McWilliams, and W. K. Dewar, 2015: Submesoscale instability and generation of mesoscale anticyclones near a separation of the California Undercurrent. J. Phys. Oceanogr., 45, 613629, https://doi.org/10.1175/JPO-D-13-0225.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Moum, J. N., D. Hebert, C. A. Paulson, and D. R. Caldwell, 1992: Turbulence and internal waves at the equator. Part I: Statistics from towed thermistors and a microstructure profiler. J. Phys. Oceanogr., 22, 13301345, https://doi.org/10.1175/1520-0485(1992)022<1330:TAIWAT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Palacios, D. M., 2004: Seasonal patterns of sea-surface temperature and ocean color around the Galápagos: Regional and local influences. Deep-Sea Res. II, 51, 4357, https://doi.org/10.1016/j.dsr2.2003.08.001.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pedlosky, J., 1987a: An inertial theory of the equatorial undercurrent. J. Phys. Oceanogr., 17, 19781985, https://doi.org/10.1175/1520-0485(1987)017<1978:AITOTE>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pedlosky, J., 1987b: Instability theory. Geophysical Fluid Dynamics, Springer, 490623.

  • Pedlosky, J., 1998: Equatorial dynamics of the thermocline: The equatorial undercurrent. Ocean Circulation Theory, Springer, 321337.

  • Philander, S. G. H., 1976: Instabilities of zonal equatorial currents. J. Geophys. Res., 81, 37253735, https://doi.org/10.1029/JC081i021p03725.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Philander, S. G. H., 1978: Instabilities of zonal equatorial currents, 2. J. Geophys. Res., 83, 36793682, https://doi.org/10.1029/JC083iC07p03679.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pietri, A., P. Testor, V. Echevin, A. Chaigneau, L. Mortier, G. Eldin, and C. Grados, 2013: Finescale vertical structure of the upwelling system off southern Peru as observed from glider data. J. Phys. Oceanogr., 43, 631646, https://doi.org/10.1175/JPO-D-12-035.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Proehl, J. A., 1998: The role of meridional flow asymmetry in the dynamics of tropical instability. J. Geophys. Res., 103, 24 59724 618, https://doi.org/10.1029/98JC02372.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Qiao, L., and R. H. Weisberg, 1996: The zonal momentum balance of the equatorial undercurrent in the central Pacific. J. Phys. Oceanogr., 27, 10941119, https://doi.org/10.1175/1520-0485(1997)027<1094:TZMBOT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Rudnick, D. L., 2016: Ocean research enabled by underwater gliders. Annu. Rev. Mar. Sci., 8, 519541, https://doi.org/10.1146/annurev-marine-122414-033913.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Rudnick, D. L., and S. T. Cole, 2011: On sampling the ocean using underwater gliders. J. Geophys. Res., 116, C08010, https://doi.org/10.1029/2010JC006849.

    • Search Google Scholar
    • Export Citation
  • Rudnick, D. L., R. E. Davis, and J. T. Sherman, 2016: Spray underwater glider operations. J. Atmos. Oceanic Technol., 33, 11131122, https://doi.org/10.1175/JTECH-D-15-0252.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Rudnick, D. L., W. B. Owens, K. B. Karnauskas, and T. M. S. Johnston, 2020: Repeat observations by gliders in the equatorial region [data set]. Scripps Institution of Oceanography, Instrument Development Group, accessed 30 June 2020, https://doi.org/10.21238/S8SPRAY0090.

    • Search Google Scholar
    • Export Citation
  • Rudnick, D. L., W. B. Owens, T. M. S. Johnston, K. B. Karnauskas, J. Jakoboski, and R. E. Todd, 2021: The equatorial current system west of the Galápagos Islands during the 2014–16 El Niño as observed by underwater gliders. J. Phys. Oceanogr., 51, 317, https://doi.org/10.1175/JPO-D-20-0064.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schubert, W., E. Ruprecht, R. Hertenstein, R. N. Ferreira, R. Taft, C. Rozoff, P. Ciesielski, and H.-C. Kuo, 2004: English translations of twenty-one of Ertel’s papers on geophysical fluid dynamics. Meteor. Z., 13, 527576, https://doi.org/10.1127/0941-2948/2004/0013-0527.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sherman, J., R. E. Davis, W. B. Owens, and J. Valdes, 2001: The autonomous underwater glider “Spray.” IEEE J. Oceanic Eng., 26, 437446, https://doi.org/10.1109/48.972076.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Smyth, W. D., J. N. Moum, L. Li, and S. A. Thorpe, 2013: Diurnal shear instability, the descent of the surface shear layer, and the deep cycle of equatorial turbulence. J. Phys. Oceanogr., 43, 24322455, https://doi.org/10.1175/JPO-D-13-089.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Stevens, D. E., 1983: On symmetric stability and instability of zonal mean flows near the equator. J. Atmos. Sci., 40, 882893, https://doi.org/10.1175/1520-0469(1983)040<0882:OSSAIO>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Thomas, L. N., J. R. Taylor, R. Ferrari, and T. M. Joyce, 2013: Symmetric instability in the Gulf Stream. Deep-Sea Res. II, 91, 96110, https://doi.org/10.1016/j.dsr2.2013.02.025.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Thompson, A. F., A. Lazar, C. Buckingham, A. C. N. Garabato, G. M. Damerell, and K. J. Heywood, 2016: Open-ocean submesoscale motions: A full seasonal cycle of mixed layer instabilities from gliders. J. Phys. Oceanogr., 46, 12851307, https://doi.org/10.1175/JPO-D-15-0170.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Thomson, W., 1871: XLVI. Hydrokinetic solutions and observations. London Edinburgh Dublin Philos. Mag. J. Sci., 42, 362377, https://doi.org/10.1080/14786447108640585.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Thyng, K., C. Greene, R. Hetland, H. Zimmerle, and S. DiMarco, 2016: True colors of oceanography: Guidelines for effective and accurate colormap selection. Oceanography, 29, 913, https://doi.org/10.5670/oceanog.2016.66.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Todd, R. E., D. L. Rudnick, M. R. Mazloff, R. E. Davis, and B. D. Cornuelle, 2011: Poleward flows in the southern California Current System: Glider observations and numerical simulation. J. Geophys. Res., 116, C02026, https://doi.org/10.1029/2010JC006536.

    • Search Google Scholar
    • Export Citation
  • Todd, R. E., W. B. Owens, and D. L. Rudnick, 2016: Potential vorticity structure in the North Atlantic western boundary current from underwater glider observations. J. Phys. Oceanogr., 46, 327348, https://doi.org/10.1175/JPO-D-15-0112.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Todd, R. E., D. L. Rudnick, J. Sherman, W. B. Owens, and L. George, 2017: Absolute velocity estimates from autonomous underwater gliders equipped with Doppler current profilers. J. Atmos. Oceanic Technol., 34, 309333, https://doi.org/10.1175/JTECH-D-16-0156.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • von Helmholtz, H., 1868: XLIII. On discontinuous movements of fluids. London Edinburgh Dublin Philos. Mag. J. Sci., 36, 337346, https://doi.org/10.1080/14786446808640073.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Webber, J. B. W., 2012: A bi-symmetric log transformation for wide-range data. Meas. Sci. Technol., 24, 027001, https://doi.org/10.1088/0957-0233/24/2/027001.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wyrtki, K., 1974: Equatorial currents in the Pacific 1950 to 1970 and their relations to the trade winds. J. Phys. Oceanogr., 4, 372380, https://doi.org/10.1175/1520-0485(1974)004<0372:ECITPT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wyrtki, K., and B. Kilonsky, 1984: Mean water and current structure during the Hawaii-to-Tahiti shuttle experiment. J. Phys. Oceanogr., 14, 242254, https://doi.org/10.1175/1520-0485(1984)014<0242:MWACSD>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yu, Z., J. P. McCreary, and J. A. Proehl, 1995: Meridional asymmetry and energetics of tropical instability waves. J. Phys. Oceanogr., 25, 29973007, https://doi.org/10.1175/1520-0485(1995)025<2997:MAAEOT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yu, Z., P. S. Schopf, and J. P. McCreary, 1997: On the annual cycle of upper-ocean circulation in the eastern equatorial Pacific. J. Phys. Oceanogr., 27, 309324, https://doi.org/10.1175/1520-0485(1997)027<0309:OTACOU>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zaron, E. D., and J. N. Moum, 2009: A new look at Richardson number mixing schemes for equatorial ocean modeling. J. Phys. Oceanogr., 39, 26522664, https://doi.org/10.1175/2009JPO4133.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
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Potential Vorticity and Instability in the Pacific Equatorial Undercurrent West of the Galápagos Archipelago

Julie JakoboskiaMIT–WHOI Joint Program in Oceanography, Cambridge, Massachusetts

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Robert E. ToddbWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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W. Brechner OwensbWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Kristopher B. KarnauskascUniversity of Colorado Boulder, Boulder, Colorado

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Daniel L. RudnickdScripps Institution of Oceanography, University of California, San Diego, La Jolla, California

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Abstract

The Galápagos Archipelago lies on the equator in the path of the eastward flowing Pacific Equatorial Undercurrent (EUC). When the EUC reaches the archipelago, it upwells and bifurcates into a north and south branch around the archipelago at a latitude determined by topography. Since the Coriolis parameter (f) equals zero at the equator, strong velocity gradients associated with the EUC can result in Ertel potential vorticity (Q) having sign opposite that of planetary vorticity near the equator. Observations collected by underwater gliders deployed just west of the Galápagos Archipelago during 2013–16 are used to estimate Q and to diagnose associated instabilities that may impact the Galápagos Cold Pool. Estimates of Q are qualitatively conserved along streamlines, consistent with the 2.5-layer, inertial model of the EUC by Pedlosky. The Q with sign opposite of f is advected south of the Galápagos Archipelago when the EUC core is located south of the bifurcation latitude. The horizontal gradient of Q suggests that the region between 2°S and 2°N above 100 m is barotropically unstable, while limited regions are baroclinically unstable. Conditions conducive to symmetric instability are observed between the EUC core and the equator and within the southern branch of the undercurrent. Using 2-month and 3-yr averages, e-folding time scales are 2–11 days, suggesting that symmetric instability can persist on those time scales.

Significance Statement

The Pacific Ocean contains fast-moving currents near the equator and below the surface that result in instabilities and mixing. The Galápagos Archipelago lies directly in the path of the eastward-flowing Pacific Equatorial Undercurrent. There are few observations of what happens to the current when it reaches the Galápagos Archipelago, so theories and models of the instabilities and mixing resulting from these strong currents have not been well verified. The Repeat Observations by Gliders in the Equatorial Region (ROGER) project deployed autonomous underwater gliders to observe the current system in this region. The results show that a range of instabilities may be responsible for the cold sea surface temperature of the Galápagos Cold Pool and the generation of tropical instability waves.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Jakoboski’s current affiliation: MetOcean Solutions, Raglan, New Zealand.

Corresponding author: Julie Jakoboski, jjakobos@alum.mit.edu

Abstract

The Galápagos Archipelago lies on the equator in the path of the eastward flowing Pacific Equatorial Undercurrent (EUC). When the EUC reaches the archipelago, it upwells and bifurcates into a north and south branch around the archipelago at a latitude determined by topography. Since the Coriolis parameter (f) equals zero at the equator, strong velocity gradients associated with the EUC can result in Ertel potential vorticity (Q) having sign opposite that of planetary vorticity near the equator. Observations collected by underwater gliders deployed just west of the Galápagos Archipelago during 2013–16 are used to estimate Q and to diagnose associated instabilities that may impact the Galápagos Cold Pool. Estimates of Q are qualitatively conserved along streamlines, consistent with the 2.5-layer, inertial model of the EUC by Pedlosky. The Q with sign opposite of f is advected south of the Galápagos Archipelago when the EUC core is located south of the bifurcation latitude. The horizontal gradient of Q suggests that the region between 2°S and 2°N above 100 m is barotropically unstable, while limited regions are baroclinically unstable. Conditions conducive to symmetric instability are observed between the EUC core and the equator and within the southern branch of the undercurrent. Using 2-month and 3-yr averages, e-folding time scales are 2–11 days, suggesting that symmetric instability can persist on those time scales.

Significance Statement

The Pacific Ocean contains fast-moving currents near the equator and below the surface that result in instabilities and mixing. The Galápagos Archipelago lies directly in the path of the eastward-flowing Pacific Equatorial Undercurrent. There are few observations of what happens to the current when it reaches the Galápagos Archipelago, so theories and models of the instabilities and mixing resulting from these strong currents have not been well verified. The Repeat Observations by Gliders in the Equatorial Region (ROGER) project deployed autonomous underwater gliders to observe the current system in this region. The results show that a range of instabilities may be responsible for the cold sea surface temperature of the Galápagos Cold Pool and the generation of tropical instability waves.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Jakoboski’s current affiliation: MetOcean Solutions, Raglan, New Zealand.

Corresponding author: Julie Jakoboski, jjakobos@alum.mit.edu
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