Ocean Science www.ocean-sci.net 1812-0784 1812-0792 4 1 2008 10.5194/os-4-73-2008 http://www.ocean-sci.net/4/73/2008/ http://www.ocean-sci.net/4/73/2008/os-4-73-2008.html http://www.ocean-sci.net/4/73/2008/os-4-73-2008.pdf 73 88 2008-02-19 Variability in the subtropical-tropical cells and its effect on near-surface temperature of the equatorial Pacific: a model study J. F. Lübbecke jluebbecke@ifm-geomar.de C. W. Böning A. Biastoch Leibniz Institute of Marine Sciences at the University of Kiel, (IfM-GEOMAR), Kiel, Germany A set of experiments utilizing different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-) tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Niño3-region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-) decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Niño episodes. The behaviour is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems. Adcroft, A., Hill, C., and Marshall, J.: Representation of topography by shaved cells in a height coordinate ocean model, Mon. Weather Rev., 125, 2293–2315, 1997. Alory, G., Cravatte, S., Izumo, T., and Rodgers, K B.: Validation of a decadal OGCM simulation for the tropical pacific, Ocean Model., 10, 272–282, 2005. Barnier, B., Siefridt, L., and Marchesiello, P.: Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analyses, J Marine~Syst., 6(4), 363–380, 1995. Barnier, B., Madec, G., Penduff, T., Molines, J.-M., Treguier, A.-M., Beckmann, A., Biastoch, A., Böning, C., Dengg, J., Gulev, S Le Sommer, J., Remy, E., Talandier, C., Theetten, S., and Maltrud, M.: Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy permitting resolution, Ocean Dynam., 56(5–6), 543–567, 2006. Blanke, B. and Delecluse, P.: Variability of the tropical Atlantic ocean simulated by a general circulation model with two different mixed layer physics, J Phys Oceanogr., 23, 1363–1388, 1993. Butt, J. and Lindstrom, E.: Currents off east coast of New Ireland, Papua-New Guinea and their relevance to regional undercurrents in the western equatorial Pacific Ocean, J Geophys Res., 99, 12 503–12 514, 1994. Capotondi, A. and Deser, C.: Why are there Rossby wave maxima in the Pacific at 10° S and 13° N?, J Phys Oceanogr., 33, 1549–1563, 2003. Capotondi, A., Alexander, M A., Deser, C., and McPhaden, M J.: Anatomy and Decadal Evolution of the Pacific Subtropical-Tropical Cells (STCs), J Climate, 18(18), 3739–3758, 2005. Chang, P., Yamagata, T., Schopf, P., Behera, S K., Carton, J., Kessler, W S., Meyers, G., Qu, T., Schott, F., Shetye, S., and Xie, S P.: Climate Fluctuations of Tropical Coupled Systems – The Role of Ocean Dynamics, J Climate, 19, 5122–5174, 2006. Cheng, W., McPhaden, M J., Zhang, D., and Metzger, E J.: Recent Changes in the Pacific Subtropical Cells Inferred from an Eddy-Resolving Ocean Circulation Model, J Phys Oceanogr., 37, 1340–1356, 2007. Fichefet, T. and Morales-Marqueda, M A.: Sensitivity of a global sea ice model to the treatment of ice thermodynamics and dynamics, J Geophys Res., 102, 12 609–12 646, 1997. Gent, P R. and McWilliams, J.: Isopycnal mixing in ocean circulation models, J Phys Oceanogr., 20, 150–156, 1990. Griffies, S., Böning, C W., and Treguier, A M.: Design Considerations for Coordinated Ocean-Ice Reference Experiments, WGSF/WCRP Flux News, 3, 3–5, 2007. Haney, R L.: Surface Thermal Boundary Condition for Ocean Circulation Models, J Phys Oceanogr., 1(4), 241–248, 1971. Hazeleger, W., de~Vries, P., and van Oldenbourgh, G J.: Do tropical cells ventilate the Indo-Pacific equatorial thermocline?, Geophys Res Lett., 28(9), 1763–1766, 2001. Hazeleger, W., Seager, R., Cane, M., and Naik, N H.: How can tropical Pacific ocean heat transport vary?, J Phys Oceanogr., 34, 320–333, 2004. Huang, B. and Liu, Z.: Pacific subtropical–tropical thermocline water exchange in the National Centers for Environmental Prediction ocean model, J Geophys Res., 104 (C5), 11 065–11 076, 1999. Johnson, G C. and McPhaden, M.: Interior Pycnocline Flow from the Subtropical to the Equatorial Pacific Ocean, J Phys Oceanogr., 29(12), 3073–3089, 1999. Johnson, G C., Sloyan, B M., Kessler, W S., and McTaggert, K E.: Direct measurements of upper ocean currents and water properties across the tropical pacific during the 1990s, Progr Oceanogr., 52, 31–61, 2002. Kim, Y Y., Qu, T., Jensen, T., Miyahima, T., Mitsudera, H., Wang, H.-W., and Ishida, A.: Seasonal and interannual variations of the North Equatorial Current bifurcation in a high-resolution OGCM, J Geophys Res., 109, C03040, doi:10.1029/2003JC002013, 2004. Kleeman, R., McCreary, J P., and Klinger, B A.: A mechanism for generating ENSO decadal variability, Geophys Res Lett., 26(12), 1743–1746, 1999. Large, W. and Yeager, S.: Diurnal to decadal global forcing for ocean and sea-ice models: the data sets and flux climatologies, NCAR Technical Note: NCAR/TN–460+STR, CGD Division of the National Center for Atmospheric Research, 2004. Large, W G.: Core Forcing for Coupled Ocean and Sea – Ice Models, WGSF/WCRP Flux News, 3, 2–3, 2007. Lee, T. and Fukumori, I.: Interannual-to-decadal variations of the tropical-subtropical exchange in the Pacific Ocean: Boundary versus interior pycnocline transports, J Climate, 16, 4022–4042, 2003. Levitus, S., Boyer, T P., Conkright, M E., O'Brian, T., Antonov, J., Stephens, C., Stathopolos, L., Johnson, D., and Gelfeld, R.: World Ocean Database 1998, NOAA Atlas NESDID, 18, 1998. Liu, Z. and Philander, G.: Tropical-extratropical oceanic exchange pathways, in: Ocean Circulation and Climate: Observing and Modeling the Global Ocean, edited by: Siedler, G., Church, J., and Gould, J., 247–257, Academic Press, 2001. Liu, Z., Philander, S. G H., and Pacanowski, R C.: A GCM Study of Tropical-Subtropical Upper-Ocean Water Exchange, J Phys Oceanogr., 24, 2606–2623, 1994. Lohmann, K. and Latif, M.: Tropical Pacific Decadal Variability and the Subtropical-Tropical Cells, J Climate, 18, 5136–5178, 2005. Madec, G. and Imbard, M.: A global ocean mesh to overcome the North Pole singularity, Clim Dyn., 12, 381–388, 1996. Madec, G., Delecluse, P., Imbard, M., and Levy, C.: OPA 8.1 general circulation model reference manual, Notes de l'IPSL, Universit$\acutee$ P. et M. Curie, B102 T15-E5, 4 place Jussieu, Paris cedex 5, N°S 11, 91 p., 1998. McCreary, J P. and Lu, P.: Interaction between the Subtropical and Equatorial Ocean Circulations: The Subtropical Cell, J Phys Oceanogr., 24, 466–496, 1994. McPhaden, M J. and Zhang, D.: Slowdown of the meridional overturning circulation in the upper Pacific Ocean, Nature, 415, 603–608, 2002. McPhaden, M J. and Zhang, D.: Pacific Ocean circulation rebounds, Geophys Res Lett., 31, L18301, doi:10.1029/2004GL020727, 2004. Nonaka, M., Xie, S.-P., and McCreary, J.: Decadal variations in the Subtropical Cells and equatorial Pacific SST, Geophys Res Lett., 29(7), 20–1–20–3, 2002. Rothstein, L M., Zhang, R.-H., Busalacchi, A., and Chen, D.: A Numerical Simulation of the Mean Water Pathways in the Subtropical and Tropical Pacific Ocean, J Phys Oceanogr., 28(2), 322–343, 1997. Rowe, G D., Firing, E., and Johnson, G C.: Pacific Equatorial Subsurface Countercurrent Velocity, Transport, and Potential Vorticity, J Phys Oceanogr., 30, 1172–1187, 2000. Schott, F., Wang, W., and Stammer, D.: Variability of Pacific subtrobical cells in the 50-year ECCO assimilation, Geophys Res Lett., 34, L05604, 2007. Schott, F A., McCreary, J. P., and Johnson, G.: Shallow overturning circulation of the tropical-subtropical oceans, in: Earth Climate: The Ocean-Atmoshere Interaction, edited by: Wang, C., Xie, S.-P., and Carton, J A., 261–304, American Geophysical Union, 2004. Sloyan, B M., Johnson, G C., and Kessler, W S.: The Pacific Cold Tongue: A Pathway for Interhemisheric Exchange, J Phys Oceanogr., 33(5), 1027–1043, 2003. Smith, S R., Legler, D M., and Verzone, K V.: Quantifying Uncertainties in NCEP Reanalysis Using High-Quality Research Vessel Observations, J Climate, 14, 4062–4072, 2001. Springer, S R., McPhaden, M J., and Busalacchi, A J.: Oceanic Heat Content Variability in the Tropical Pacific During the 1982 –1983 El Niño , J Geophys Res., 95, 22 089–22 101, 1990. Steele, M., Morley, R., and Ermold, W.: PHC: A Global Ocean Hydrography with a High-Quality Arctic Ocean, J Climate, 14(9), 2079–2087, 2001. Toole, J M., Millard, R C., Wang, Z., and Pu, S.: Observations of the Pacific North Equatorial Current bifurcation at the Philippine coast, J Phys Oceanogr., 20, 307–318, 1990. Tsuchiya, M.: Subsurface countercurrents in the eastern equatorial Pacific Ocean, J Marine~Res., 33, 145–175, 1975. Tsuchiya, M., Lukas, R., Fine, R., Firing, E., and Lindstrom, E.: Source waters of the Pacific Equatorial Undercurrent, Progr Oceanogr., 23, 101–147, 1989. Wyrtki, K. and Kilonsky, B.: Mean water and current structure during the Hawaii-to-Tahiti Shuttle Experiment, J Phys Oceanogr., 14, 242–254, 1984. Zebiak,S. ~E.: Oceanic heat content variability and El Niño cycles, J Phys Oceanogr., 19, 475–486, 1989.