Abstract

Velocity time series are used to study cross-shelf circulation on the northern California shelf and to examine classical ideas of locally wind-forced cross-shelf circulation. A simple linear two-dimensional model of cross-shelf transport is compared to estimates of cross-shelf transport in the near surface, interior, and near bottom. In winter, when wind forcing is brief and episodic, model transports are highly correlated to the total surface flow and show some skill in predicting subsurface cross-shelf flow. The same model does not work well below the surface in summer when persistent upwelling is observed. This suggests a two-dimensional wind-forced model of cross-shelf circulation may have more applicability to the brief wind events observed in winter than to the persistent wind events observed in summer. The reason for this is unclear. Numerous factors not included in the simple linear wind-forced model such as mesoscale features, upwelling fronts, the interaction of flow with topography, baroclinic pressure gradients, remote forcing, and small-scale wind stress all affect cross-shelf circulation. It is possible some of these are more pronounced on the northern California shelf in summer.

Abstract

Objective streamfunction and velocity potential maps of three commonly observed flow states near Point Conception, California, are derived from moored current-meter and drifter observations. The states are defined using current-meter data. Drifter data are sorted by flow state and averaged within spatial bins to resolve their spatial structures. Spatial correlations of drifter data show the along-shelf and cross-shelf decorrelation distances are approximately 45 and 25 km, respectively. This information is used to make the objective streamfunction and velocity potential maps. The total velocity is represented as the sum of the rotational, nondivergent streamfunction component and the irrotational, divergent velocity potential component. The velocity derived from the streamfunction is much stronger than that derived from the velocity potential. The streamfunction is sufficiently well resolved to make meaningful vorticity maps. The velocity potential indicates a source term in the western Santa Barbara Channel consistent with wind stress curl-driven upwelling, but divergence maps derived from the velocity potential are very noisy, making comparisons with wind stress curl mapped on the same scales problematic.

Abstract

Month-long simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) with a horizontal resolution of 9 km have been used to investigate perturbations of topographically forced wind stress and wind stress curl during upwelling-favorable winds along the California and Baja California coasts during June 1999. The dominant spatial inhomogeneity of the wind stress and wind stress curl is near the coast. Wind and wind stress maxima are found in the lees of major capes near the coastline. Positive wind stress curl occurs in a narrow band near the coast, while the region farther offshore is characterized by a broad band of weak negative curl. Curvature of the coastline, such as along the Southern California Bight, forces the northerly flow toward the east and generates positive wind stress curl even if the magnitude of the stress is constant. The largest wind stress curl is simulated in the lees of Point Conception and the Santa Barbara Channel. The Baja California wind stress is upwelling favorable. Although the winds and wind stress exhibit great spatial variability in response to synoptic forcing, the wind stress curl has relatively small variation. The narrow band of positive wind stress curl along the coast adds about 5% to the coastal upwelling generated by adjustment to the coastal boundary condition. The larger area of positive wind stress curl in the lee of Point Conception may be of first-order importance to circulation in the Santa Barbara Channel and the Southern California Bight.

Abstract

Several simple laboratory experiments have been conducted to study the dynamic behavior of the vector-averaging current meter (VACM) compass and vane follower. They demonstrate that the behavior of the compass and vane follower can be modeled as a damped linear harmonic oscillator for small-amplitude forcing. The combined eddy-current and bearing-friction torque nearly critically damps the free oscillation of the compass and vane follower. This frictional torque is proportional to the angular-velocity difference between the instrument magnet assembly and housing. Dynamic experiments on five compasses indicate a mean (undamped) resonant period of 3–5 s at 41°N. Similar experiments on two vane followers indicate a resonant period of 2–3 s.

For the VACM dynamic compass experiments, frictional torque allowed an angular oscillation of the compass housing to drive an oscillation of the compass magnet, and at resonant forcing, the compass magnet oscillates exactly in phase with its housing. For the VACM vane-follower experiments, angular motion of the vane magnet directly drove the vane follower. For resonant forcing of the vane magnet, the vane-follower oscillation overshoots the forcing slightly and is 90° out of phase with the forcing. The damped linear harmonic oscillator model suggests that a small-amplitude angular forcing of the compass or vane-follower housing (which may occur in the field due to mooring motion) should not cause any error in the vector-averaged headings. However, periodic angular oscillations near the resonant frequencies of the compass or vane follower could cause an error in the magnitude of the vector-averaged velocity. Forcing at frequencies lower than the well-defined resonant frequencies of the instruments should have little effect since directional errors do not exceed the angular resolution of the instruments at periods of ≥10 s.

Abstract

This study demonstrates the long-term stability of salinity measurements from Argo floats equipped with inductive conductivity cells, which have extended float lifetimes as compared to electrode-type cells. New Argo float sensor payloads must meet the demands of the Argo governance committees before they are implemented globally. Currently, the use of CTDs with inductive cells designed and manufactured by RBR, Ltd., has been approved as a Global Argo Pilot. One requirement for new sensors is to demonstrate stable measurements over the lifetime of a float. To demonstrate this, data from four Argo floats in the western Pacific Ocean equipped with the RBRargo CTD sensor package are analyzed using the same Owens–Wong–Cabanes (OWC) method and reference datasets as the Argo delayed-mode quality control (DMQC) operators. When run with default settings against the standard DMQC Argo and CTD databases, the OWC analysis reveals no drift in any of the four RBRargo datasets and, in one case, an offset exceeding the Argo target salinity limits. Being a statistical tool, the OWC method cannot strictly determine whether deviations in salinity measurements with respect to a reference hydrographic product (e.g., climatologies) are caused by oceanographic variability or sensor problems. So, this study furthermore investigates anomalous salinity measurements observed when compared with a reference product and demonstrates that anomalous values tend to occur in regions with a high degree of variability and can be better explained by imperfect reference data rather than sensor drift. This study concludes that the RBR inductive cell is a viable option for salinity measurements as part of the Argo program.