Advective time scales of Agulhas leakage to the North Atlantic in surface drifter observations and the 3D OFES model

The advective transit time of temperature-salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, as it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult because most observational and high-resolution model data is too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “super-trajectories” of any required length using a Monte Carlo simulation. These super-trajectories allow analysis of the circulation patterns and time scales based on Lagrangian data, in this case observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OFES model. The observational super-trajectories can only be used to study the two-dimensional surface flow, while the numerical super-trajectories can be used to study the full three-dimensional circulation. Results for the surface circulation indicate that the super-trajectories starting in the Agulhas Current and ending in the North Atlantic take at least 4 years and most complete the journey in 30 to 40 years. This time scale is, largely due to convergence and subduction in the subtropical gyres, longer than the 10 to 25 years it takes the 3D numerical super-trajectories to complete the journey.

Locations of 10,000 super-trajectories as a function of time. The super-trajectories are constructed from data from observational drifting buoys in the real ocean. All super-trajectories shown here start in the Agulhas Current and end either at the Greenland-Scotland Ridge or in the Labrador Sea within 100 years.

Figure 4: Surface advection time scales from the observational super-trajectories. (a) Map of the chance that a super-trajectory that starts in the Agulhas Current (circles) and ends in one of the areas of Deep Water formation (black lines in the North Atlantic) crosses a particular grid cell. The ocean grid cells without trajectories are shaded white. (b) Distribution of the transit time of the 2D observational Monte Carlo super-trajectories of the surface circulation. The bulk of the observational super-trajectories reach the North Atlantic in 25 to 40 years, while the fastest super-trajectory reaches the North Atlantic in 4 years.