initial analysis
DESCRIPTION
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, FL Robert Weisberg, University of South Florida, St. Petersburg Dennis Mayer, NOAA/AOML, Miami, FL. - PowerPoint PPT PresentationTRANSCRIPT
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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. Petersburg
Dennis 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 Equator– interactions between the MOC and the seasonally-varying wind-driven
gyre circulation.• Another goal is to quantify water mass transformations along the
pathways.
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Initial Analysis
• The 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.
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Schematic of MOC Upper-Limb Pathways.
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Specific Goals of the Interior Pathway Analysis
• It 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
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Model Simulations
• Domain– Atlantic Ocean, 30S to 70N– Resolution: 1.4 degrees horizontal, 25 layers vertical
• Forcing– Derived from the 1948-2000 NCEP/NCAR reanalysis climatology
• vector wind stress• 10m wind speed• friction velocity• 2m air temperature• 2m atmospheric specific humidity• precipitation• net longwave radiation• shortwave radiation
• Model Properties– KPP Mixing– Simple energy loan ice model– Surface salinity relaxation to Levitus climatology
• 20-year spinup from Levitus climatology was performed first
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Model interfaces and density contours
Colored bands outline density contoursThick line is mixed layer base
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Meridional Overturning Streamfunction
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Importance of Seasonal Gyre Variability
• The 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
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Mean MeridionalHeat Flux (PW)
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Cumulative Heat Flux Maps
• The 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.
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Southern Hemisphere Drifter Release
• Drifters 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
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Drifterreleasebox
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Some Characteristic Drifter Pathways
• The 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.
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Initial Census
• 7920 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 boundary• 8% follow an interior pathway
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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.
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Consequences of Vertical Drifter Motion
• The 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.
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Individual Drifter Paths
• The following four figures show the history of individual drifters. The dots shown along the paths represent 1 January positions.
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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.
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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