Diurnal Cycling of Submesoscale Dynamics: Lagrangian Implications in Drifter Observations and Model Simulations of the Northern Gulf of Mexico

Abstract

The diurnal cycling of submesoscale circulations in vorticity, divergence, and strain is investigated using drifter data collected as part of the Lagrangian Submesoscale Experiment (LASER) experiment, which took place in the northern Gulf of Mexico during winter 2016, and ROMS simulations at different resolutions and degree of realism. The first observational evidence of a submesoscale diurnal cycle is presented. The cycling is detected in the LASER data during periods of weak winds, whereas the signal is obscured during strong wind events. Results from ROMS in the most realistic setup and in sensitivity runs with idealized wind patterns demonstrate that wind bursts disrupt the submesoscale diurnal cycle, independently of the time of day at which they happen. The observed and simulated submesoscale diurnal cycle supports the existence of a shift of approximately 1–3 h between the occurrence of divergence and vorticity maxima, broadly in agreement with theoretical predictions. The amplitude of the modeled signal, on the other hand, always underestimates the observed one, suggesting that even a horizontal resolution of 500 m is insufficient to capture the strength of the observed variability in submesoscale circulations. The paper also presents an evaluation of how well the diurnal cycle can be detected as function of the number of Lagrangian particles. If more than 2000 particle triplets are considered, the diurnal cycle is well captured, but for a number of triplets comparable to that of the LASER analysis, the reconstructed diurnal cycling displays high levels of noise both in the model and in the observations.

Submesoscale Mixing Across the Mixed Layer in the Gulf of Mexico

“…A brief validation of the model representation of temperature and salinity in the upper 300 m of the water column is provided in the Supplementary Material ( Supplementary Figure 1) against CTD profiles collected in 2015. The configuration further improves that in previous works and the validation of other fields or periods can be found in Luo et al (2016); Cardona and Bracco (2014), Cardona et al (2016b), and Sun et al (2020).…”

“…As note of caution, a resolution of 1 km is insufficient to capture the rate of convergence at the ocean surface in the Gulf of Mexico (Barkan et al, 2019;Sun et al, 2020) and higher than 1 km horizontal resolution may be needed. Additionally, the upper few centimeters of the water column are characterized by very strong vertical velocity magnitudes (Laxague et al, 2018), and therefore large convergence/divergence fluxes, that are not captured by current hydrostatic ocean models.…”

“…Submesoscale circulations are commonly found in the form of small (smaller than 10 km in diameter) eddies and rapidly changing vorticity filaments and fronts (Capet et al, 2008;Thomas and Ferrari, 2008;McWilliams, 2016;Sun et al, 2020). They form preferentially in the upper and bottom turbulent boundary layers (Lévy et al, 2012) and are characterized by O(1) Richardson and Rossby numbers, with horizontal scales between 100 m and a few kilometers, and characteristic time scales from hours to days .…”

Submesoscale Vorticity and Divergence in the Alboran Sea: Scale and Depth Dependence

“…Even though estimate precision increases with increasing number of drifters (Essink, 2019), the recent work by Berta et al (2020b) has shown that statistical estimates from triplets are robust, based on the comparison with estimates from larger drifter clusters and from concurrent independent observations based on Eulerian X-band radar. These findings are also confirmed by modeling results by Sun et al (2020).…”

“…Drifter triplets are re-sampled every 15 min (only triplets sharing at most one drifter are considered) and followed for the time interval Dt over which the KPs are estimated. Using independent chance triplets for a short time window, instead of following original triplets for longer time, allows to maximize the statistics and to minimize the effect of inhomogeneous sampling driven by convergent regions (Pearson et al, 2020;Sun et al, 2020). The KPs results presented in the following are expressed in terms of the local Coriolis parameter, f (8.572 × 10 −5 s −1 at the reference latitude 36 • N).…”