a synthetic drifter analysis of upper-limb meridional overturning circulation interior ocean...

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  • A Synthetic Drifter Analysis of Upper-Limb Meridional Overturning Circulation Interior Ocean Pathways in the Tropical/Subtropical Atlantic

    George Halliwell, MPO/RSMAS, University of Miami, FLRobert Weisberg, University of South Florida, St. PetersburgDennis Mayer, NOAA/AOML, Miami, FL

    Atlantic Ocean simulations are performed using the new Hybrid-Coordinate Ocean Model (HYCOM) to study the upper limb of the Meridional Overturning Circulation (MOC).

    One goal of the project is to study dynamical processes that govern pathways taken by the upper limb water, in particular processes associated with: crossing the Equatorinteractions between the MOC and the seasonally-varying wind-driven gyre circulation.Another goal is to quantify water mass transformations along the pathways.

  • Initial AnalysisThe initial focus is on upper-limb water particles that follow the interior pathway in the tropical North Atlantic.

    May account for a substantial fraction of upper limb transport

    The model was seeded with synthetic drifters to trace upper limb pathways.

  • Schematic of MOC Upper-Limb Pathways.

  • Specific Goals of the Interior Pathway AnalysisIt will be demonstrated that key processes that cause an upper limb fluid particle to take the interior pathway are:

    Equatorial Upwelling

    Seasonal variability of the Equatorial and Tropical Gyres, including the NECC

    Northward Ekman transport north of 5N

    Ekman pumping in the subtropical North Atlantic

    The use of a low-resolution model is adequate for a initial study of these processes

  • Model Simulations

    DomainAtlantic Ocean, 30S to 70NResolution: 1.4 degrees horizontal, 25 layers verticalForcingDerived from the 1948-2000 NCEP/NCAR reanalysis climatologyvector wind stress10m wind speedfriction velocity2m air temperature2m atmospheric specific humidityprecipitationnet longwave radiationshortwave radiationModel PropertiesKPP MixingSimple energy loan ice modelSurface salinity relaxation to Levitus climatology20-year spinup from Levitus climatology was performed first

  • Model interfaces and density contours

    Colored bands outline density contoursThick line is mixed layer base

  • Meridional Overturning Streamfunction

  • Importance of Seasonal Gyre VariabilityThe Tropical and Equatorial gyres are strong during summer and fall.

    Strong eastward transport by the NECC stores heat along the gyre boundary.

    During the subsequent winter, the gyres weaken to permit the northward release of this stored heat by the wind-driven ageostrophic flow

    This storage and release mechanism has been described by Philander.

    We hypothesize that much of the water carrying the stored heat is upper-limb water following the interior pathway

  • Mean MeridionalHeat Flux (PW)

  • Cumulative Heat Flux MapsThe following two figures show maps of the cumulative heat flux integrated from the western boundary.

    1. The mean cumulative heat flux; the cumulative heat along the eastern boundary equals the basin-wide integrated meridional heat flux shown in the previous figure.

    2. Four seasonal mean maps of the cumulative heat flux. The storage and release of heat at the latitude of the NECC is evident.

  • Southern Hemisphere Drifter ReleaseDrifters were released in the western South Atlantic within a box through which most of the upper-limb water flows.

    Release longitudes: 33W to 29W, one degree interval

    Release latitudes: 5S to 14S, one degree interval

    Release depths: 25m to 300m, 25m interval, plus 400m

    Release times: 12 monthly releases beginning 1 January

    Simulation run for 8 years

  • Drifterreleasebox

  • Some Characteristic Drifter PathwaysThe following three figures show small subsets of the released drifters that followed three characteristic pathways:

    1. Interior pathway after traveling eastward along the Equator

    2. Interior pathway after not traveling eastward along the Equator

    3. Western boundary pathway

    These figures collectively illustrate the importance of equatorial upwelling and subtropical Ekman pumping to drifters from the Southern Hemisphere that take the interior pathway.

  • Initial Census7920 Drifters Released

    After Eight Years:

    51% of drifters never cross 5N

    20% eventually enter the Caribbean and proceed northward in the subtropical gyre western boundary flow.

    12% directly follow the western boundary8% follow an interior pathway

  • This figure shows the seasonality of nearsurface drifters in the western boundary north of the Equator intaking either the western boundary pathway or the interior pathway.

  • Consequences of Vertical Drifter MotionThe following two figures show the consequences of subtropical Ekman pumping in the interior North Atlantic

    1. Lagrangian drifters moving northward in the nearsurface Ekman drift subduct north of 15N, then eventually loop to the south and enter the North Equatorial Current

    2. Isobaric drifters released near the surface continue moving northward until they become trapped in the subtropical convergence.

  • Individual Drifter PathsThe following four figures show the history of individual drifters. The dots shown along the paths represent 1 January positions.

  • Conclusions (1)The importance of the following processes to upper limb water following the interior pathway was verified:

    Equatorial Upwelling

    Seasonal variability of the Equatorial and Tropical Gyres, including the NECC

    Northward Ekman transport north of 5N

    Ekman pumping in the subtropical North Atlantic

    We intend to continue these studies using HYCOM at high resolution. We expect details of these results to change, but hypothesize that the processes listed above will remain very important.

  • Conclusions (2)Drifters following the interior pathway spend most of their lives in the upper-ocean mixed layer.

    Water particle density and PV are not approximately conserved

    Only Lagrangian drifters were capable of entering the subtropical North Atlantic western boundary circulation.

    Preceding conclusions have implications for studying upper-limb pathways in the tropical/subtropical Atlantic with in-situ drifters.

    Must be Lagrangian drifters