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Characterizing the transition from balanced to unbalanced motions in the Southern California current

TitleCharacterizing the transition from balanced to unbalanced motions in the Southern California current
Publication TypeJournal Article
Year of Publication2019
AuthorsChereskin TK, Rocha C.B, Gille ST, Menemenlis D., Passaro M.
JournalJournal of Geophysical Research-Oceans
Date Published2019/03
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000464656900040
Keywordscurrent system; decomposition; delay-doppler; internal gravity waves; mesoscale; north pacific; ocean; oceanography; satellite altimetry; sea-surface height; submesoscale; transition; variability; wave-number spectra

As observations and models improve their resolution of oceanic motions at ever finer horizontal scales, interest has grown in characterizing the transition from the geostrophically balanced flows that dominate at large-scale to submesoscale turbulence and waves that dominate at small scales. In this study we examine the mesoscale-to-submesoscale (100 to 10km) transition in an eastern boundary current, the southern California Current System (CCS), using repeated acoustic Doppler current profiler transects, sea surface height from high-resolution nadir altimetry and output from a (1/48)degrees global model simulation. In the CCS, the submesoscale is as energetic as in western boundary current regions, but the mesoscale is much weaker, and as a result the transition lacks the change in kinetic energy (KE) spectral slope observed for western boundary currents. Helmholtz and vortex-wave decompositions of the KE spectra are used to identify balanced and unbalanced contributions. At horizontal scales greater than 70km, we find that observed KE is dominated by balanced geostrophic motions. At scales from 40 to 10km, unbalanced contributions such as inertia-gravity waves contribute as much as balanced motions. The model KE transition occurs at longer scales, around 125km. The altimeter spectra are consistent with acoustic Doppler current profiler/model spectra at scales longer than 70/125km, respectively. Observed seasonality is weak. Taken together, our results suggest that geostrophic velocities can be diagnosed from sea surface height on scales larger than about 70km in the southern CCS. Plain Language Summary Observations and numerical models are now able to resolve changes in velocity or sea surface height over distances of just a few kilometers. Geostrophically balanced flows dominate at large scales, while turbulence and waves dominate at small scales. One challenge is to characterize the transition between large and small scales. This study employs spectral analysis of high horizontal resolution observations and a numerical model simulation in order to determine the length scale of the transition from balanced to unbalanced motions in the southern California Current System. We find that in the California Current System, geostrophic motions dominate on scales larger than about 70km. Seasonality in the transition scale is also examined and is found to be weak.

Short TitleJ Geophys Res-Oceans
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