# Presentation: 164097

## The role of vorticity fluxes in the dynamics of the Zapiola Anticyclone

**The role of vorticity fluxes in the dynamics of the Zapiola Anticyclone**
**Denis Volkov and Lee-Lueng Fu**
*NASA Jet Propulsion Laboratory, California Institute of Technology*

**Brazil Current**

**Malvinas**
**Current**

**ACC**

**BMC**

**Figure 1: **The Argentine Basin, its depth-averaged circulation estimated from an ECCO-2 output (red arrows) , and altimetry-derived eddy kinetic energy. Bottom topography is shown every 1000 m. Abbreviations: ACC – Antarctic Circumpolar Current, BMC – Brazil-Malvinas Confluence zone, D – domain of study.

- Eddy Kinetic Energy (cm2/s2)

- In the frictionless barotropic geostrophic flow the local time change of relative vorticity is balanced by the relative and planetary vorticity fluxes:

- where, 	         – horizontal velocity of the flow, and *β**=**∂f/∂y ***–*** *meridional gradient of planetary vorticity, and *f **– *planetary vorticity. The first two terms on the right side of the equation represent the relative vorticity flux while the last term is the planetary vorticity flux.
- The time change of the relative vorticity of the Zapiola Anticyclone appears to be almost fully balanced by vorticity fluxes. The relative vorticity fluxes are found to be occasionally coherent with the strength of the anticyclone on sub-annual timescales. The planetary vorticity fluxes are highly correlated with the circulation in the anticyclone on both sub-annual and inter-annual timescales. The long term linear intensification of the Zapiola Anticyclone from 1992 to 2006 is found to be determined solely by the planetary vorticity fluxes.

**3. Vorticity Balance**

**D**

**4. Conclusions**

**1. Outline**

- The Argentine Basin in the South Atlantic Ocean (Figure 1) is one of the most energetic regions in the ocean with complicated dynamics, which plays an important role in the global climate. A number of observations have discovered an intense anticyclonic gyre of barotropic circulation (with uniform current velocity from surface to bottom, Figure 2) around the Zapiola Rise in the center of the basin. Theoretical studies have shown that the Zapiola Anticyclone represents an eddy-driven flow controlled by bottom friction. Recent advances in high-resolution global-ocean data syntheses, performed using NASA supercomputing facilities under the project Estimating the Circulation and Climate of the Ocean, Phase II (ECCO-2), provide realistic simulations of the circulation and variability in the Argentine Basin (Figures 3-4). ECCO-2 aims to produce accurate syntheses of all available global-scale ocean and sea-ice data at an eddy-permitting resolution with a mean horizontal grid spacing of 18 km. Using these simulations we analyzed the vorticity balance (Figures 5) and investigated what physical mechanisms drive the variability of the Zapiola Anticyclone.

**Figure 2: **Average vertical zonal velocity (m/s) profile across 45°W between 40°S and 50°S.
- Within the Zapiola Anticyclone, between about 42°S – 48°S, zonal velocities are almost uniform from surface to bottom confirming the barotropic structure of the flow. The flow is highly variable with amplitude of about 100 Sv (1Sv=106 m3/s). The long term average eastward transport between 45°S and 48°S is around 95 Sv while the average westward transport between 42°S and 45°S is approximately 60 Sv. These values agree well with observational evidence. This is a very promising result because there are a very few global models capable to realistically reproduce the Zapiola Anticyclone.

**Figure 5: **
- Components of the time integral of the vorticity equation averaged over the domain D in the Zapiola Anticyclone: **(top)** – the time series of *ζ**D *(blue) and of the absolute vorticity flux (red), **(middle)** – the time series of *ζ**D *(blue) and of the relative vorticity flux (red), **(bottom)** – the time series of *ζ**D *(blue) and of the planetary vorticity flux (red). The dashed lines are their linear trends. Correlation coefficients between the compared curves are shown.

**Figure 3:**** **In the area of the Zapiola Anticyclone the model performs well: the variations of the simulated (red curve) and altimeter-measured (blue curve) sea levels are highly coherent (correlation = 0.54, which is significant at 99% confidence). Due to geostrophic balance the gyre spins up when the sea level rises and spins down when the sea level decreases.

***Zapiola Anticyclone***

- As a quantity describing the circulation in the Zapiola Anticyclone we used the vertical curl *ζ**=**∂v/∂x – ∂u/∂y* of the horizontal velocity (relative vorticity) averaged over the domain D in Figure 1 within 40°W-50°W and 42°S-48°S (Figure 4, blue curve). Positive values denote an anticyclonic character of the circulation, which is dominant in the Zapiola Anticyclone, while negative values indicate a cyclonic flow. The linear trend (dashed lines) shows that from 1992 to 2006 the gyre was spinning up.

- Assuming geostrophy, when 					    *ζ**  *can also be
- computed from sea level data (Figure 4, red curve):

- and

- where *u* and *v* are the horizontal velocity components, *h* is the sea surface height, *g* – gravity, and *f = 2**ω**sin**φ* – Coriolis parameter.

- The eddy-permitting ECCO-2 simulations realistically reproduce the circulation of the Zapiola Anticyclone and its variability.
- Vorticity fluxes are found to determine changes in the strength of the Zapiola Anticyclone. The relative vorticity fluxes do effect the variability of the anticyclone predominantly on sub-annual time scales while the major part of the variabiity including the linear acceleration of the Zapiola Anticyclone from 1992 to 2006 is accounted for by the planetary vorticity fluxes.

**2. Circulation in the Zapiola Anticyclone**

**Figure 4: **The relative vorticity averaged over the Zapiola Anticyclone (domain D in Figure 1) estimated from the depth-averaged velocities (blue curve) and from sea surface height data (red curve).

**Corresponding author:** Denis Volkov, _[dvolkov@caltech.edu](mailto:dvolkov@caltech.edu)_, Jet Propulsion Laboratory, California Institute of Technology, MS 300-314, 4800 Oak Grove Dr., Pasadena CA 91109-8099