Charles C. Eriksen

UNDERWATER GLIDERS FOR OCEAN RESEARCH

Underwater gliders are autonomous vehicles that profile vertically by buoyancy control and move horizontally on wings. Gliders are reviewed, from their conception by Stommel as an extension of autonomous profiling floats, through their development in 3 models, and including their first deployments singly and in numbers. This paper discusses the basics of glider function as implemented by University of Washington, Seaglider, Scripps Institution of Oceanography, and Webb Research in Slocum. Preliminary results are presented from a recent demonstration project that used a network of gliders off Monterey. A wide range of sensors has already been deployed on gliders, with many under development, and a wider range of future possibilities. Glider networks appear to be among the best approaches to achieving subsurface spatial resolution necessary for ocean research.

Upper-Ocean Inertial Currents Forced by a Strong Storm. Part I: Data and Comparisons with Linear Theory

Abstract A strong, isolated October storm generated 0.35–0.7 m s−1 inertia] frequency currents in the 40-m deep mixed layer of a 300 km×300 km region of the northeast Pacific Ocean. The authors describe the evolution of these currents and the background flow in which they evolve for nearly a month following the storm. Instruments included CTD profilers, 36 surface drifters, an array of 7 moorings, and air-deployed velocity profilers. The authors then test whether the theory of linear internal waves propagating in a homogeneous ocean can explain the observed evolution of the inertial frequency currents. The subinertial frequency flow is weak, with typical currents of 5 cm s−1, and steady over the period of interest. The storm generates inertial frequency currents in and somewhat below the mixed layer with a horizontal scale much larger than the Rossby radius of deformation, reflecting the large-scale and rapid translation speed of the storm. This scale is too large for significant linear propagation of the...

Implications of Ocean Bottom Reflection for Internal Wave Spectra and Mixing

Abstract A linear internal wave model for reflection off a sloping bottom applied to a field of horizontally isotropic waves typical of the deep ocean leads to a strongly perturbed frequency-vertical wavenumber energy spectrum. The spectrum is dominated by a nonintegrable singularity at the internal wave critical frequency characteristic of the environment and bottom slope. An observational requirement that the internal wave spectrum near the bottom relax to the open deep-ocean level and shape within a few hundred meters vertically implies a flux imbalance normal to the boundary. The flux that must be redistributed over the internal wave spectrum, or lost from it, amounts to O(10−2 W m−2), larger than for most other energy transfer mechanisms estimated for internal waves. A small fraction of this flux imbalance applied to mixing can account for a basin-averaged effective vertical diffusivity of 10−4 m2 s−1. Bottom reflection represents not only a likely and powerful sink for internal wave energy, but a me...

Moored observations of upper-ocean response to the monsoons in the Arabian Sea during 1994-1995

The role of surface forcing in the semiannual evolution of the upper-ocean temperature, salinity, and velocity fields in the Arabian Sea is examined. To do so, variability in the upper ocean in the central Arabian Sea was observed from an array of moorings deployed from October 1994 to October 1995. The Northeast (NE) Monsoon was characterized by moderate winds, clear skies, and dry air; sea-surface temperature (SST) dropped by 31C; the ocean lost an average of 19.7 W m � 2 and the mixed layer deepened by 100 m in response. The Southwest (SW) Monsoon was accompanied by strong winds, cloudy skies, and moist air; the ocean gained an average of 89.5 W m � 2 but SST dropped by 5.51C and the mixed layer deepened to almost 80 m. The response to the NE Monsoon included daily cycling in the depth of the mixed layer in response to the diurnal cycle in the buoyancy forcing and a weak local, wind-driven response. Stronger windforcing during the SW Monsoon dramatically reduced diurnal restratification, and a clearer signal of local, wind-driven flow in the upper ocean was found. The strongest velocity signal in the upper ocean, however, was the flow associated with mesoscale geostrophic features that passed slowly through the moored array, dominating the current meter records in the first part of the NE Monsoon and again in the latter part of the SW Monsoon. One-dimensional heat and freshwater balances, which held at other times through the year, broke down during the passage of these features. r 2002 Published by Elsevier Science Ltd.

Buoyant Eddies Entering the Labrador Sea Observed with Gliders and Altimetry

Abstract Intense, buoyant anticyclonic eddies spawned from the west Greenland boundary current were observed with high-resolution autonomous Seaglider hydrography and satellite altimetry as they entered the Labrador Sea interior. Surveys of their internal structure establish the transport of both low-salinity water in the upper ocean and warm, saline Irminger water at depth. The observed eddies can contribute significantly to the rapid restratification of the Labrador Sea interior following wintertime deep convection. These eddies have saline cores between 200 and 1000 m, low-salinity cores above 200 m, and a velocity field that penetrates to at least 1000 m, with 0–1000-m average speeds exceeding 40 cm s−1. Their trajectory, together with earlier estimates of the gyre circulation, suggests why the observed region of deep convection is so small and does not occur where wintertime cooling by the atmosphere is most intense. The cyclostrophic surface velocity field of the anticylones from satellite altimetry...

Internal wave reflection and mixing at Fieberling Guyot

The structure of internal wave reflection off a sloping bottom on the steep flank of a tall North Pacific Ocean seamount is observed in year-long moored array records to differ substantially from the form predicted by linear theory, although linear theory accounts for several qualitative features of the process. This study documents new features of wave reflection as described below. Wave reflection is detectable as far as 750 m above the bottom. Motions are dominated by a single empirical mode whose phase structure obeys linear internal wave dispersion but whose amplitude decays with a scale comparable to the wavenumber. While the dominant mode has scales appropriate to the reflection of a first baroclinic mode wave incident from the open ocean, its decay from the bottom is such that current vectors in the vertical plane rotate clockwise in time when viewed with shallow water to the right. This flow resembles the lower half of the deepest cell pattern predicted by linear reflection from a uniform slope. At the local internal wave critical frequency, the dominant mode has nearly a vanishing wavenumber rather than the infinite wavenumber predicted by linear reflection. Reflected waves are aligned parallel to the bottom slope measured on wave spatial scales, rather than shorter ones. Wave reflection causes large, frequent density overturns, implying mixing. The rate and strength of these overturns imply a local vertical eddy viscosity of 2–6 × 10−4 m2/s over the bottom few hundred meters. The contribution of bottom-intensified mixing to the open deep ocean is roughly equivalent to that found in situ, although reflection from gentler slopes or at lower latitude may produce greater contribution from internal wave-reflection-induced mixing.

Observations of amplified flows atop a large seamount

Current meter records from a mooring atop a large seamount in the eastern North Pacific (Fieberling Guyot) are dominated by diurnal fluctuations at depths close to the seamount summit depth. These oscillations are driven by diurnal tides to have amplitudes of 0.2–0.4 m/s or more and exhibit characteristics of seamount trapped waves (i.e., coastal trapped waves for a submerged periodic coast). These waves rectify to produce anticyclonically directed low-frequency currents in the vicinity of the seamount summit of magnitude comparable to the rms wave amplitude. Mean currents over several months are directed across isobaths at depths more than 200 m shallower than the seamount summit. Currents closer to the bottom have a component directed toward deeper water and are ageostrophic as a result of wave rectification. Internal wave band currents are more energetic with increasing proximity to the local bottom, presumably as a result of wave reflection and focussing.

Subthermocline Eddies over the Washington Continental Slope as Observed by Seagliders, 2003–09

AbstractIn the California Current System, subthermocline, lenslike anticyclonic eddies generated within the California Undercurrent (CU) are one mechanism for lateral transport of the warm, saline waters of the CU. Garfield et al. established the name “Cuddies” for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of CU water. This study presents observations of subthermocline eddies collected from a time series of Seaglider surveys in the northern California Current System. Gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5.5 yr. Close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones. Water properties at the core of anticyclonic eddies were similar to those in the core of the CU over the continental slope; these anticyclones are examples of Cuddies. Anticyclonic (cyclonic) eddies had average radii of 20.4 (20.6) km, peak azimuthal current speeds of 0.25 (0.23) ...

Mesoscale eddies, coastal upwelling, and the upper-ocean heat budget in the Arabian Sea

Estimation of the terms in the upper-ocean heat budget from a moored array in the central Arabian Sea shows periods when a rough balance between the temperature trend and the horizontal advection of heat exists. Altimetry and sea-surface temperature imagery are used to demonstrate that these episodes of strong horizontal advection are associated with mesoscale features. During the wintertime Northeast (NE) Monsoon these are capped-off mesoscale eddy features generated during the previous summertime Southwest (SW) Monsoon and have little horizontal transport of heat within the mixed layer. During the SW Monsoon the major contribution is strong offshore export of coastally upwelled water in a filament with a strong surface presence. Temperature and salinity properties from the moored array and a SeaSoar survey during the formation of the coastal filament confirm the offshore transport of the upwelled water mass to the site of the moored array, more than 600 km offshore. Estimates of the filament section heat flux are several percent of the total estimated heat flux due to upwelling along the Arabian Peninsula, and remote sensing data show that similar mesoscale variability along the coast is enhanced during the SW Monsoon. This points to the importance of mesoscale-modulated transports in not only the observed heat budget at the moored array, but in the overall upper-ocean heat budget in the Arabian Sea.

Deep Currents and Their Interpretation as Equatorial Waves in the Western Pacific Ocean

Abstract Vertical profiles of current and density made within 5° latitude of the equator along longitudes 168 and 179°E (the vicinity of the Gilbert Islands) reveal multiple deep current reversals which are confined to the equator. These current jets have amplitudes of roughly 5–20 cm s−1, vertical scales of order hundreds of meters, and time scales longer than the one month of measurements during the cruise. Spectral analysis of profiles indicates that 1) meridional trapping varies roughly as the square root of the vertical scale, 2) both east and north currents are coherent over meridional separations of 0°45′ within roughly 1°30′ of the equator; and 3) zonal current lags vertical displacement by π/4 in depth within 0°45′ of the equator. Zonal coherence scales are smaller than the smallest separation of 3°30′ along the equator. Kinetic energy decreases near the island chain. Current-meter records taken over nearly two years near the islands suggest strong quasi-annual variability in deep currents near t...