Cross-Gyre Thermohaline Transport in the Tropical Atlantic: The role of NBC Rings Bill Johns Zulema Garraffo Division of Meteorology and Physical Oceanography RSMAS, University of Miami

Cross-gyre Thermohaline Flow Pathways in the Tropical Atlantic Questions: What fraction of the total cross-gyre transport is carried by NBC Rings? What is the vertical structure of the ring-induced transport? How is the remaining transport split between interior and coastal current pathways?

Fratantoni et al. (2000): Schematic of MOC cross- gyre pathways, from 1/4°, 6- layer, NRL Atlantic Basin model with 14 Sv MOC: In the surface layer, the transport is divided evenly between the interior and western boundary layer. Rings carry most of the transport in the western boundary layer (~3 Sv). The remaining transport occurs via an intermediate western boundary current.

The North Brazil Current Rings Experiment:  Several articles published in:”Interhemispheric Water Exchange in the Atlantic”, Elsevier Oceanography Series (2003)

Shipboard CTD/ADCP Surveys: Four NBC Rings surveyed with different vertical structures

Ship-surveyed Rings 1 and 2: Stations in the ring cores (blue/green curves) show relatively undiluted South Atlantic water (SAW). Stations on the perimeter show mixed SAW/NAW. The fraction of SAW on each density surface is given by: Frac SAW = (S-S N )/(S S -S N )

SAW Volume Calculations: High concentrations of SAW are usually trapped within the ring core, inside the radius of maximum velocity (r max ). The fraction of SAW is integrated over the area of the ring to estimate the total amount of SAW trapped and transported in each ring. Ring 1 Ring 2 Ring 1 Ring 2

Rings observed by CM/CTD moorings: A pair of current meter/CTD moorings placed in the ring translation corridor continuously sampled passing NBC rings. Ring tracks are derived from merged TOPEX/ERS altimetry (Goni).

A total of 11 surface- intensified rings, and 3 subsurface rings, were identified during the 20- month time series record. The surface rings show varying degrees of vertical penetration, with the deepest - reaching rings occurring in boreal fall and winter.

Examples of “virtual” cross- sections of swirl velocity and SAW percentage in NBC rings, derived from the moored CM and CTD observations. Ring 2 is a deep reaching surface intensified ring; Ring 3a is a subsurface ring with core at 800m. Moored Ring 2 Moored Ring 3a Moored Ring 2 Moored Ring 3a

Ring Transport: The total cross- gyre watermass transport by rings is estmated to be ~9 Sv on an annualized basis. Subsurface rings, though formed less frequently, account for ~40% of the total ring transport.

MICOM: COADS Forcing, 1/12° resolution, 16 layers NBC Rings are formed year-round, with varying vertical structures, similar to observations. “Shallow” “Intermediate” “Deep”“Subsurface”

Model Rings: Watermass analysis is used to estimate the trapped SAW core volumes carried in NBC Rings, as for observations.

Ring Type Number of Rings in 6 model years Average # rings/year Average Transport per ring (Sv) Annualized Transport (Sv) Shallow Intermediate Deep Subsurface Total Summary of MICOM Rings: Average number of rings/year = 8 Total Ring transport = 7.5 Sv (approx. 20% in subsurface rings) Interannual variability: 5.6 – 9.2 Sv  Conclude: Both model and observations indicate a large (7-9 Sv) contribution by NBC Rings.

Other Pathways (interior transport and coastal “leakage”)? MICOM: Net meridional transport across 10° N (by layer), and its partitioning between the western boundary layer and interior Surface Upper Thermocline Lower Thermocline Upper Intermediate Lower Intermediate

Western Boundary Transport MICOM: Western boundary transport across 10° N (by layer), divided into “ring” and “non-ring” components

MICOM Cross-gyre Pathways Differences from Fratantoni et al: Surface layer: WB transport dominated by coastal current “leakage” Thermocline: Ring transport is significant, offset by reverse STC transport Intermediate: Also significant ring transport, but WBC is dominant. Question: Is the MOC too big in MICOM (22 Sv)? Or does the consistency with observed ring transport estimates suggest the MOC must be of O(20 Sv)?

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Ring Volumes and Seasonality: No clear seasonality is evident in ring formation frequency, but the ring-induced transport is largest in fall and winter. Larger and more deeply-penetrating rings form in this season.

Zonal Distribution of Transport in layers: Upper thermocline (layers 3-5): flow is confined to the eastern boundary Lower thermocline (layers 6-7): flow is distributed throughout the basin

Interior Transport: Why is the interior transport in the surface layer so weak (<< Ekman)? Surface geostrophic transport opposes Ekman transport Subsurface geostrophic transport is northward in the interior

Model Rings: Similar watermass analysis is used to estimate the trapped SAW core volumes carried in NBC Rings.