Mesoscale Eddy Variability Research Articles

The Kuroshio Extension System: Its Large-Scale Variability and Role in the Midlatitude Ocean-Atmosphere Interaction

The Kuroshio Extension and its recirculation gyre form an interconnected dynamic system. The system is located at a crossroads where the meso-scale and large-scale oceanic variability are highest, and where the ocean-atmosphere interaction is most active in the Pacific Ocean outside of the tropics. Following a brief review of the mean flow and meso-scale eddy variability, this study describes in detail the large-scale structural change (an oscillation between an elongated and a contracted state) observed in the Kuroshio Extension system. Causes for this structural change are explored next, and it is argued that the basin-wide external wind forcing and the nonlinear dynamics associated with the inertial recirculation gyre are both important factors. Data analysis results are reviewed and presented, emphasizing that the surface Kuroshio Extension is not simply a well-mixed layer passively responding to heat flux anomalies imposed by the atmosphere. It is argued that large-scale changes in the Kuroshio Extension system influence the surface ocean heat balance and generate wintertime sea surface temperature (SST) anomalies through both horizontal geostrophic heat advection and re-emergence to the surface mixed layer of sequestered mode water temperature anomalies.

Effects of Eddy Variability on the Circulation of the Japan/East Sea

The effect of mesoscale eddy variability on the Japan/East Sea mean circulation is examined from satellite altimeter data and results from the Naval Research Laboratory Layered Ocean Model (NLOM). Sea surface height variations from the Geosat-Exact Repeat Mission and TOPEX/POSEIDON altimeter satellites imply geostrophic velocities. At the satellite crossover points, the total velocity and the Reynolds stress due to geostrophic mesoscale turbulence are calculated. After spatial interpolation the momentum flux and effect on geostrophic balance indicates that the eddy variability aids in the transport of the Polar Front and the separation of the East Korean Warm Current (EKWC). The NLOM results elucidate the impact of eddy variability on the EKWC separation from the Korean coast. Eddy variability is suppressed by either increasing the model viscosity or decreasing the model resolution. The simulations with decreased eddy variability indicate a northward overshoot of the EKWC. Only the model simulation with sufficient eddy variability depicts the EKWC separating from the Korean coast at the observed latitude. The NLOM simulations indicate mesoscale influence through upper ocean-topographic coupling.

Continuous assimilation of Geosat altimetric sea level observations into a numerical synoptic ocean model of the California Current

In this study, real Geosat altimetric sea level observations for the 1‐year period extending from January to December 1987 were assimilated into a realistic wind‐driven numerical synoptic ocean model of the California Current. The objective was to evaluate the effectiveness of using a realistic synoptic ocean model to interpolate (dynamically) real altimetric sea level observations from the Geosat ERM onto a regular grid. First, the model mean sea level was shown to simulate qualitatively the observed mean sea level found in in situ CaICOFI hydrographic data. Second, the model was shown to simulate approximately the space‐time statistics of mesoscale eddy variability contained within the altimetric sea level observations. Third, prior to assimilation the altimetric sea level observations were referenced using this model mean. Then the referenced altimetric sea level observations were assimilated into the model, with the resulting sea level residuals compared with estimates from in situ (expendable bathythermograph) observations. This comparison yielded nearly exact agreement at low frequency (i.e., the semiannual cycle), but less agreement on month‐to‐month time scales of variability (yet still significant), probably owing to the unaltered nature of the in situ estimates. Subsequent comparisons of the dynamically interpolated altimetric sea level residuals with those obtained from the statistical interpolation yielded marked improvement in the latter over the former; i.e., dynamical interpolation allowed observed mesoscale variability to be conducted from the Geosat ERM repeat tracks into the regions between tracks with little change in magnitude and structure, not possible with the statistical interpolation. The latter tended to produce extrema on grid points that coincided with the track lines. As such, dynamical interpolation demonstrated marked improvement in resolution, time‐space continuity, intensity, and gradient structure of mesoscale eddy activity over that possible with statistical interpolation. Model‐data assimilation also allowed for the determination of the vertical structure of the mesoscale eddy activity. Moreover, it yielded a forecasting capability that had a statistically significant improvement over persistence in specifying mesoscale eddy activity 2–3 weeks into the future.

Spatial and temporal variability of major ocean currents and mesoscale eddies

A global picture of ocean current variability may be obtained by analyzing surface drift currents in terms of their mean and eddy kinetic energies. High values for both quantities are found in the western boundary currents and in the equatorial current system; low values are found in the interior of major gyres. However, nowhere are eddy energies less than 200 cm2 s−2, indicating that, even in the least energetic parts of the oceans, surface speeds of ~20 cm s−1 prevail. Recent experimental studies also support the widespread occurrence of mesoscale mid-oceanic eddies. Another type of eddy is abundant in the vicinity of boundary currents: examples include Gulf Stream Rings, the ‘Great Whirl’ of the Somali Current, and disturbances of the predominantly zonal equatorial flow manifested by large-scale meandering about the equator. Recent numerical models using low-viscosity and high-resolution computational grids also reveal the ubiquitous existence of mesoscale structures. The importance of eddies is that they seem to be energetic enough and sufficiently widespread so as to play some part — not yet understood — in the circulation of the world ocean. Speculative analogies to the atmosphere suggest that the mesoscale ocean eddies are ‘the storms and weather systems of the sea’. We need global statistics on their distribution, their occurrence in various oceanic regions, their dimensions, and their lifetimes. The prospect of even a single global oceanic ‘weather map’, comparable to those obtained daily for the atmosphere, is hopeless in terms ofin situ oceanographic observations. Remote sensing may provide a partial solution.