Current effects on wind generated waves near an Ocean Eddy Dipole

TL;DR

The paper investigates how surface currents, particularly in ocean eddy dipoles with central jets, modify wind-generated waves. Using WW3 in both idealized setups and a two-month hindcast, it demonstrates that dipoles act as focusing lenses, increasing $H_s$ via refraction and advection, with shorter-period waves showing stronger amplification. It compares current inputs from HYCOM, SSalto/Duacs, and GlobCurrent, finding HYCOM captures a broader energy range due to ageostrophic dynamics but can misplace the central jet, while ADT-derived fields (SSalto/GlobCurrent) provide more reliable local wave fields in geostrophic-dominated regions. Denoised SeaState-CCI altimeter $H_s$ data enable high-resolution evaluation, revealing that current-field choice substantially affects predicted wave amplification and energy distribution near dipoles. The results have practical implications for ship routing and hazard assessment in dipole-rich regions and underscore the importance of selecting current products that balance resolution, geostrophy, and ageostrophic processes in wave forecasting.

Abstract

Ocean eddy dipoles are among the most common mesoscale features and may be ubiquitous across the global oceans. However, wave-current interactions in their proximity have not been extensively studied. Here we examine the impact of surface currents on the wave field near an ocean eddy dipole. Using the WW3 wave model, we conducted idealized numerical simulations to assess the influence of different configurations on the spatial variability of Significant Wave Height ($H_s$). Additionally, a two-month hindcast of a strong dipole event in the Southwestern Atlantic Ocean was performed using three distinct surface current products: SSalto/Duacs, HYCOM NCODA and GlobCurrent. Among these, HYCOM, which incorporates ageostrophic effects, provided a more detailed representation of oceanic energy compared to GlobCurrent and SSalto/Duacs, which primarily reflect geostrophic components. The hindcast assessment employed denoised altimeter-derived $H_s$ data, with a spatial resolution of approximately 6~km. The greatest increase in wave energy occurs in the region between the peak values of positive and negative vorticity, where the opposing surface currents reach their maximum intensity. Therefore, dipoles act as converging lenses for surface waves, channeling their refraction towards the central jet. Despite its poorer spatial and temporal resolutions, SSalto-Duacs surface current data provides more reliable $H_s$ fields, in the study region where geostrophic dynamics are expected to be significant or even dominant. HYCOM captures a broader range of dynamical processes, essential for accurately representing the total energy, though discrepancies with SSalto/Duacs data may arise from assimilation inaccuracies and model limitations.

Figures (8)

• Figure 1: Key topographic features and main surface circulation with an example of a typical dipole composed of a cyclonic eddy (black) and an anticiclonic eddy (red).
• Figure 2: [a] is the map of significant wave height ($H_s$) for the 7 s waves ($\epsilon=0.0822$). The arrows indicate the surface current direction and magnitude with its scale displayed at the left top corner. [b] is the Rossby number --- the surface relative vorticity $\zeta$ normalized by the Coriolis parameter $f$ --- in the region shown as a black rectangle in [a]. [c d] are the relative difference of $H_s$, in the black rectangle, between the runs using the current input and the run with no current (respectively for the 7 s and 15 s waves). The arrows indicate wave propagation direction scaled by $H_s$.
• Figure 3: Averaged wavenumber spectra (in cycles per km) computed along the main axis (E-W) in the black rectangle in Figure \ref{['fig:fig9n']} for $H_s$ (solid line). The wavenumber $H_s$ spectrum without current input is displayed in red. Averaged wavenumber spectrum of surface current speed is the dashed line.
• Figure 4: Fields of Absolute Dynamic Topography (ADT) from SSalto/Duacs and the derived geostrophic currents superimposed with the tracks shown as straight colored lines. [abcd] are four out of the 14 cases during the period of August and September 2010 that the satellite tracks crossed the dipole's central jet, respectively tracks: #1; #4 and #5; #8; #13 (Table \ref{['tab:sat']}). The scale of the vectors is displayed on the top right corner.
• Figure 5: Example of one event that HYCOM misplaced the position of the strongest currents associated with the dipole --- see also Figure \ref{['fig:fig4']}. Top row: surface current field from GlobCurrent (left) and HYCOM (right) for 08-Sep-2010 at 07:00:00. The arrows indicate the current direction and magnitude sub-sampled every four bins. Red circles are the ground track of Jason-2 cycle 80. Bottom row: $H_s$ relative difference between runs with and without current for GlobCurrent (left) and HYCOM (right). The results employing the SSalto current field are similar to GlobCurrent.