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 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.

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.

Schematic of MOC Upper- Limb Pathways.

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

Model Simulations Domain –Atlantic Ocean, 30S to 70N –Resolution: 1.4 degrees horizontal, 25 layers vertical Forcing –Derived from the 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

Model interfaces and density contours Colored bands outline density contours Thick line is mixed layer base

Meridional Overturning Streamfunction

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

Mean Meridional Heat Flux (PW)

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.

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

Drifter release box

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.

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

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

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.

Individual Drifter Paths The 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