Oceanic Turbulence and Stochastic Models from Subsurface Lagrangian Data for the Northwest Atlantic Ocean

Abstract

The historical dataset provided by 700-m acoustically tracked floats is analyzed in different regions of the northwestern Atlantic Ocean. The goal is to characterize the main properties of the mesoscale turbulence and to explore Lagrangian stochastic models capable of describing them. The data analysis is carried out mostly in terms of Lagrangian velocity autocovariance and cross-covariance functions. In the Gulf Stream recirculation and extension regions, the autocovariances and cross covariances exhibit significant oscillatory patterns on time scales comparable to the Lagrangian decorrelation time scale. They are indicative of sub-and superdiffusive behaviors in the mean spreading of water particles. The main result of the paper is that the properties of Lagrangian data can be considered as a superposition of two different regimes associated with looping and nonlooping trajectories and that both regimes can be parameterized using a simple first-order Lagrangian stochastic model with spin parameter ⍀. The spin couples the zonal and meridional velocity components, reproducing the effects of rotating coherent structures such as vortices and mesoscale eddies. It is considered as a random parameter whose probability distribution is approximately bimodal, reflecting the distribution of loopers (finite ⍀) and nonloopers (zero ⍀). This simple model is found to be very effective in reproducing the statistical properties of the data.

Cyclonic and anticyclonic motion in the upper ocean

“…For loopers in the core of a coherent eddy, W is a good approximation of (half) the relative vorticity, z, z $ 2W [Veneziani et al, 2005a]. For all loopers, W provides direct information on particle rotation and polarity [Veneziani et al, 2004[Veneziani et al, , 2005b, with positive (negative) W associated with cyclonic (anticyclonic) motion in the Northern hemisphere (opposite sign in the Southern hemisphere).…”

“…Here we use an automated method [Veneziani et al, 2004[Veneziani et al, , 2005bDoglioli et al, 2006] that identifies loopers as trajectories with an average value of spin greater than a cut-off value W c (section 2.2).…”

Low‐frequency processes and turbulence structure in a perturbed boundary layer

“…When these conditions occur, low-frequency (typically 30-60 min) horizontal wind oscillations cause the meandering of the mean wind direction (Maryon, 1998;Mahrt, 2007). In this case, the wind velocity autocorrelation functions do not fit an exponential decay but show an oscillating behaviour , probably determined by horizontal coherent structures (Veneziani et al, 2004).…”

A path integral method for data assimilation

“…Thousands of floats are in use at the moment and plans are presently being drawn up for the deployment of many more of these measuring devices, capable of sampling the ocean dynamics on paths, rather than on grids (see Veneziani et al (2004) and references contained therein).…”