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Sources of the Cold Tongue in the Eastern Tropical Pacific Ocean

Principal Investigator: Bernadette M. Sloyan

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July 1 2002 - June 30, 2003

PIs: Bernadette M. Sloyan
Woods Hole Oceanographic Institution
Woods Hole, MA 0243

Gregory C. Johnson, and William S. Kessler
NOANPacific Marine Environmental Laboratory
Seattle, WA 98115-6349

Program Manager, Michael Patterson, OGP

Research Activities and Results
The overall goal of this research is to make data--based models of the mean and seasonal or interannual cycles of the eastern tropical Pacific ocean circulation. The primary aim of the project is to locate and quantify the circulation components that contribute to the cold tongue. This project is a collaboration between Dr. Sloyan (WHOI) nnd Drs. Johnson and Kessler (NOANPMEL). The original grant began in February 2000, when Dr. Sloyan arrived at PMEL. In August 2001 Dr. Sloyan officially began employment at WHOI, and in November 2001 physically moved to from PMEL to WHOI. The collaboration on this project among Drs. Sloyan, Johnson and Kessler continued after Dr. Sloyan moved to WHOI and a final manuscript is now nearing completion.

Dr. Sloyan is responsible for the development, implementation and analysis of the core inverse model of this work. The first component of the study, formulating the mean inverse model, using CTD/ADCP sections, CTD and XBT data, as well as wind and air-sea flux climatologies showed that the tropical Pacific can be divided into three distinct regions: western; central; and eastern. In the western region the Equatorial Undercurrent and subsurface countercurrents develop. In the central and eastern regions the shallow  tropical cells are seen. The Pacific cold tongue in the eastern region results from diapycnal upwelling through all layers of the Equatorial Undercurrent, which preferentially exhausts the lightest (warmest) layers of the Equatorial Undercurrent between 125W and 95W, allowing the denser (colder) layers to upwell east of 95W and adjacent to the American coast (Sloyan et aI., 2003). This work also highlighted the role of the cold tongue in the interhemispheric transport.

In the last 12 months Dr. Sloyan has developed an interannual ENSO inverse model. This model has resolved the evolution of the currents and circulation across the tropical Pacific Ocean, and the effect of this circulation on the cold tongue during a composite 1990's ENSO cycle. ENSO modulates the currents of the tropical Pacific, in particular the SEC(N,S) are weaker than normal in the western and central Pacific, and surface westward current develop in the western and central Pacific. Only a small reduction to the EVC transport and a slight increase in the transport of the NSCC in the western tropical Pacific are seen. Surface transport on the western warm pool moves the warm water pool into the central Pacific, which is advected into the eastern region at the end of the ENSO year+O (Sloyan et al. 2003). The manuscript describing the results of this model will be submitted shortly.

Dr. Johnson's primary responsibility was analysis of the CTD/ADCP data from TAO cruises that are a vital component of this project. During the early part of this project, Dr. Johnson finished up work using these data to study the mean velocity, divergence, and upwelling in the cold tongue (Johnson et aI., 2001). He also contributed to a study using ADCP data, TAO winds, and scatterometer stress estimates to show how tropical ocean currents can play an important role in modifying air-sea momentum fluxes (Kelly et aI., 2001), to a study that used the TAO mooring data for a nove1look at seasonal and interannual variability of equatorial upwelling (Meinen et aI., 2001), to a review of global tropical ocean circulation (Godfrey et aI., 2001), especially the Pacific and Indian basins, and to a study comparing and contrasting tropical circulation in the Atlantic to the Pacific (Molinari et aI., 2003). He used the surface drifter data to look at th.~ surface pathways and time scales of flow in the Pacific subtropical cell, as well as patterns of equatorial divergence related to the cold tongue and the tropical cells (Johnson, 2001). He used the CTD/ADCP sections to analyze the mean, seasonal, and interannual variability of the zonal currents and water property structure across the entire tropical Pacific (Johnson et aI.,2002b). He also used the two-decade long historical deep CTD data record to revisit the Pacific Equatorial Deep Jets, finding very long temporal and zonal coherence for these features, as well as significant but very slow downward vertical propagation (Johnson et aI., 2002a). Most recently he has contributed to a study using the velocity information from the CTD/ADCP data to explain how the zonal currents in the equatorial Pacific modify Rossby wave dynamics, allowing an asymmetric response in the annual Rossby wave, as observed with altimeter data (Chelton et aI., 2003).

Dr. Kessler has worked with the historical XBT and CTD data that are vital to the east Pacific inverse, and showed that the NE tropical Pacific (essentially the EPIC region of interest) is a region of strong off-equatorial upwelling that contributes significantly both to mean SST patterns and to the general circulation of the Pacific (Kessler, 2002). This work also showed that OGCM models of the Pacific forced by climatological winds typically develop an unrealistic discontinuity in the NECC in the central Pacific. Examination of the reasons for this show that the error is due to poor resolution of the ITCZ curl, which is too weak and too broad in in situ products, and that scatterometer wind forcing produces a more realistic NECC simulation. He also helped to study annual and ENSO modulation of near-surface stratification and the diurnal cycle of SST based on high resolution time series from the 0, 110W TAO mooring (Cronin and Kessler 2002). During final stages of the EI Nino of 1997-98, the establishment of shallow barrier layers allowed warm conditions to persist at 110W, even as the thermocline shoaled. Recently, using the ADCP velocity data as well as satellite winds and a numerical moded, he showed that the Pacific South Equatorial Current is amplified by curl due to east Pacific air-sea interaction, and by nonlinear vorticity advection (Kessler et aI, 2003). He is al,so working on a review of the eastern tropical Pacific circulation (Kessler 2003).

Publications and Presentations of Results Fully or Partially Supported by this Grant
Chelton, D. B., M. G. Schlax, J. M. Lyman, and G. C. Johnson. 2003. Equatorially trapped Rossby waves in the presence of meridionally sheared baroclinic flow in the
Pacific Ocean. Progress in Oceanography, 56, 323-380.

Cronin, M. F. and W. S. Kessler. 2002. Near surface conditions at 0° ,1100W during the onset of the 1997-98 EI Nino. Deep-Sea Research I, 49, 1-17.

Godfrey, J. S., G. C. Johnson, M. J. McPhaden, G. Reverdin, and S. Wijffels. 2001. The Tropical Ocean Circulation. In: Ocean Circulation and Climate - Observing and
Modelling the Global Ocean, G. Siedler, J. Church and J. Gould, eds., Academic Press, London, 715 pp.

Johnson, G. C. 2001. The Pacific Ocean subtropical cell surface limb. Geophysical Research Letters, 28,1771-1774.

Johnson, G. C., E. Kunze, K. E. McTaggart, and D. W. Moore. 2002a. Temporal and spatial structure of the Equatorial Deep Jets in the Pacific Ocean. Journal of Physical
Oceanography, 32, 3396-3407.

Johnson, G. C., M. J. McPhaden, and E. Firing. 2001. Equatorial Pacific Ocean horizontal velocity, divergence, and upwelling. Journal of Physical Oceanography, 31,

Johnson, G. C., B. M. Sloyan, W. S. Kessler, and K. E. McTaggart. 2002b. Direct measurements of upper ocean currents and water properties across the tropical Pacific
during the 1990s. Progress in Oceanography, 52, 31-61.

Kelly, K. A., S. Dickinson, M. J. McPhaden, and G. C. Johnson. 2001. Ocean currents evident in satellite wind data. Geophysical Research Letters, 28, 2469-2472.

Kessler. W. S. 2002. Mean three-dimensional circulation in the northeast tropical Pacific. Journal of Physical Oceanography, 32,2457-2471.

Kessler, W.S. 2003. Circulation of the eastern tropical Pacific: A review. Progress in Oceanograpy, in preparation.

Last updated: August 19, 2008

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