Diagnostic Characteristics of Submesoscale Coastal Surface Currents

Abstract

Submesoscale kinetic energy (KE) spectra and fluxes at the length scales ranging from 2 to 25 km are estimated from hourly and O(1) km‐scale coastal surface current maps observed from shore‐based high‐frequency radars off southern San Diego. The one‐dimensional wave number‐domain KE spectra of the surface currents have decay slopes between k−2 and k−3 at a wave number (k) of 0.5 km−1. The KE spectra exhibit anisotropy associated with anisotropic circulation, which is constrained by the shoreline and bottom bathymetry. Moreover, the KE spectra exhibit weak seasonality related to the regional submesoscale eddies and low‐frequency circulation with weak seasonal variability. The estimated KE fluxes are categorized into four cases of purely forward cascades and inverse cascades at all wave numbers, inverse‐then‐forward cascades with a single zero‐crossing within the range of wave numbers (0.04 to 0.5 km−1), and residuals, which account for approximately 33%, 39%, 19%, and 9% of the total number of realizations, respectively. An injection scale where forward enstrophy cascade and inverse energy cascade occur is estimated to be 5 to 10 km from the cases of the inverse‐then‐forward cascade, which is consistent with the length scales of the regional submesoscale eddies. Thus, the regional submesoscale processes are initiated by surface frontogenesis due to the weak seasonal low‐frequency surface circulation and topography‐related currents, then maintained by baroclinic instabilities associated with the seasonal mixed layer and O(10) km‐scale submesoscale eddies with weak seasonal variability.

The Kinematic Similarity of Two Western Boundary Currents Revealed by Sustained High‐Resolution Observations

“…Indeed, the EAC system is unique among WBCs for its large EKE-to-KE ratio (Boland & Hamon, 1970;Godfrey et al, 1980). In the coastal ocean, Lekien and Coulliette (2007) found k À3 using HF radar in Monterey Bay, and Soh and Kim (2018) found k À2 to k À3 from HF radar offshore San Diego. The observed slopes are similar to those reported by Callies and Ferrari (2013) in the Gulf Stream extension but steeper than in the Kuroshio (k À2.3 ; Qiu et al, 2017) and in the subtropical North Pacific (k À2 ; Callies & Ferrari, 2013).…”

Submesoscale Surface Tidal, Vortical, and Residual Circulations in a Semienclosed Bay

“…Temperature and salinity profiles off Yeosu Bay during a 20-year period (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) were sampled approximately bimonthly from regional hydrographic surveys of conductivity, temperature, and depth (CTD), and they are regressed with basis functions of a temporal mean, trigonometric functions at the annual and semiannual frequencies, and a linear trend to constitute an improved climatology of the vertical stratification (e.g., Lee & Kim, 2018;Lee et al, 2019;Yoo et al, 2018). Since the sampled CTD data have uneven tempo- In the shallow water environment (less than 50-m depth), the vertical stratification appears to be weak with nearly well-mixed conditions during the winter (December to March), as shown by vertical isopycnals in time ( Figure 3); on the other hand, the stratification appears to be strong during the summer (April to November), as reflected by sloped isopycnals in time, and delayed temperate peaks occur in the upper and lower layers but are dissimilar.…”

“…The estimated stream functions in this paper are used as a proxy of the circular flows to identify submesoscale eddies using flow geometry. The nearly closed contours of the stream functions are clustered with the same sign of vorticity and connected as a time series when the center of the cluster is in a range of less than the typical translating distance (see Kim, 2010;Yoo et al, 2018, for more details). The HFR-derived radial velocity maps and MOHID-simulated surface currents are decomposed into stream functions, velocity potentials, and their corresponding current components using the Helmholtz decomposition.…”

“…The submesoscale characteristics are summarized by a diameter of 5 to 10 km and nearly symmetric normalized vorticity with a maximum between 0.3 and 1.2 (Figures 10a, 10b, and 10e). The persistency of the identified eddies is distributed as an exponential decay shape with a decorrelation time scale of less than 10 hr (Figure 10g); some of these eddies are dynamically unstable ( < −1; Figure 10e; e.g., Buckingham et al, 2016;Kim, 2010;Shcherbina et al, 2013;Yoo et al, 2018). However, the identified eddies in this region do not show much asymmetry of the normalized vorticity because the short-lived and soon-to-be dissipated submesoscale eddies, including unstable eddies, are mainly detected in our observations and numerical model.…”

“…The regional circulation in the study domain is described in terms of the tidal currents in and out of Yeosu Bay; the nearly constant, northeastward, low-frequency geostrophic currents are part of the Tsushima Current and are intermittent wind-driven events (e.g., Kim & Min, 2008;Yoo et al, 2018). As the tides and tide-driven currents at shallow depths (less than 50 m) are mostly deterministic signals that can be filtered by a harmonic analysis at the primary tidal constituents, including annual and semiannual frequencies, we examine the spatial structures of the tidal amplitudes and phases of the HFR-derived radial velocities and vector currents.…”