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Abstract: This paper investigates the transport structure of surface currents around the Monterey Bay, CA region. Currents measured by radar stations around Monterey Bay indicate the presence of strong, spatial and temporal, nonlinear patterns. To understand the geometry of the flow in the bay, Lagrangian coherent structures (LCS) are computed. These structures are mobile separatrices that divide the flow into regions of qualitatively different dynamics. They provide direct information about the flow structure but are geometrically simpler than particle trajectories themselves. The LCS patterns were used to reveal the mesoscale flow conditions observed during the 2003 Autonomous Ocean Sampling Network (AOSN-II) experiment. Drifter paths from the AOSN experiment were compared to the patterns induced by the LCS computed from high-frequency radar data. We verify that the fate of the drifters can be better characterized based on the LCS than direct interpretation of the current data. This property can be exploited to optimize drifter deployment. [Copyright &y& Elsevier]

Abstract: Impact of HF radar surface-current assimilation on ocean circulation model predictions in the Monterey Bay area is studied and evaluated during the time frame of the Autonomous Ocean Sampling Network (AOSN-II) experiment (August–September 2003). In the first instance, a previously described method for assimilation of surface current data is applied to 33-h low-pass-filtered data and a non-tidal version of the circulation model. It is demonstrated that assimilation of surface velocity data significantly improves the surface and subsurface correlation of model currents with moored current observations. These results from the AOSN-II period illustrate that surface-current assimilation is beneficial even in cases for which very high-resolution (3km) atmospheric forcing is utilized. The assimilation approach is also tested with hourly, unfiltered, CODAR-type HF radar-derived surface currents within a model configuration that includes tidal forcing. It is shown, that assimilation of unfiltered (with tides) surface-current observations into the model with tides improves the sub-tidal model predictions to the level comparable with the assimilation of filtered data into the non-tidal model, which is significant with respect to options for designing real-time nowcast and forecast systems. Finally, the approach is extended and evaluated for the direct assimilation of HF radar-derived radial velocity components. The model runs that included assimilation of radials from at least two HF radar sites show better correlations with observations than the non-assimilative run, especially those runs that included radials from the Santa Cruz site. Directions of radials for that site coincide with the directions of dominant southward flow during upwelling events and the northward flow during relaxation events. Direct assimilation of radial currents extends the range of influence of the data into regions covered by only one HF radar site. [Copyright &y& Elsevier]

As part of a program to determine the location and geometry of the San Andreas Fault (SAF) buried beneath shallow sediments near its northern landward limit, three >20-m-long parallel trenches were constructed at positions distributed over a distance of ∼55 m. The majority of excavated material comprised unconsolidated fluvial sediments deposited in a number of paleochannels. Single zones of active faulting identified in each of the trenches were initially interpreted in terms of a solitary strand of the SAF. To map the SAF between and beyond the trenches and to detect other active fault zones hidden by the young sedimentary cover, we collected a dense ground-penetrating radar (georadar) data set across a 23.2×72 m area. The data were recorded using a semi-automated acquisition system that included a conventional georadar unit coupled to a self-tracking laser theodolite with automatic target recognition capabilities. Since these data were plagued by system ringing as a result of the moderate-to-high electrical conductivities of the surficial sediments, an extensive data processing scheme was required to extract meaningful subsurface information. The final processed georadar volume (cuboid) contained numerous subhorizontal and trough-shaped reflections that originated from the fluvial paleochannels. Using the geological interpretation of the trench walls as a guide to pick semi-automatically the times of the most important reflecting horizons, we discovered that alignments of the nearly linear boundaries of these horizons defined two NW–SE trending strands of the SAF within the survey area. The georadar expression of the eastern SAF strand could only be traced over a distance of ∼38 m. It had been intersected in the northern trench. In contrast, the western SAF strand extended over the entire length of the georadar volume and had been intersected in the central and southern trenches. Prominent reflections on georadar cross sections were found to be vertically displaced by 0.2–0.3 m across both SAF strands. A conspicuous linear-trending feature observed on horizontal sections at 3.3–3.6 m depth was laterally offset by 4.5–5.5 m along the eastern SAF strand. The interpreted vertical and horizontal offsets could have been generated by the 1906 San Francisco earthquake and/or earlier events. Undetermined amounts of aseismic slip may also have occurred along the newly defined SAF strands. [Copyright &y& Elsevier]

The features of the 1997–1998 El Nin˜o event were analyzed by Empirical Orthogonal Functions (EOF) statistical methods applied to the remotely sensed sea surface temperature anomalies (SSTA) measured by AVHRR radiometers; anomalies of water circulation derived from sea surface height anomalies (SSHA) measured by TOPEX/Poseidon radar altimeter; and meteorological information (air temperature, upwelling index, and wind stress curl). EOF statistics demonstrated the features of an El Nin˜o event during the second half of 1997 and the first half of 1998, with sea level elevated along the coast and with SSHA gradients, indicating a retarding of both the equatorward California Current and the alongshore poleward Southern California Countercurrent. The positive SST anomaly developed first in the Southern California Bight and then in the zone of upwelling to the north of Point Conception. The anomalies of upwelling index and the wind stress curl pattern also changed during the El Nin˜o event, but these changes occurred later than hydrographic variations and were too weak to explain the observed changes in SSTA and SSHA. We conclude that off central and southern California oceanic teleconnection (i.e., the consequences of propagation northward of coastally trapped downwelling waves) was responsible for the 1997–1998 El Nin˜o event. [Copyright &y& Elsevier]

The timing and intensity of the effects of the 1997–98 El Nin˜o on sea-surface temperature (SST) and coastal sea level along the US west coast are examined using in situ time-series measurements, and the effects on upper ocean currents on the continental shelf and slope off Oregon and northern California are examined using repeated shipborne ADCP transects, a mid-shelf mooring off Newport Oregon and an HF surface current radar. An initial transient positive anomaly was observed in both adjusted sea level and SST during May–June 1997, followed by anomalously high coastal sea levels, generally strongest during September 1997 through February 1998 and abruptly returned to normal in late February 1998, and by positive temperatures anomalies over the mid-shelf that persisted longer, into April 1998. Low-frequency coastal sea-level anomalies propagated poleward at 2.1 m/s. Poleward flow over the shelf and slope was enhanced at most depths during the El Nin˜o, compared with following years. Northward currents in the upper 12 m over the continental shelf off Newport, Oregon averaged 13.7 cm/s stronger during August 1997 through February 1998 than during the same period the following year. Enhanced poleward flow was present at all latitudes sampled during November 1997 and February 1998, particularly over the continental slope. These transects also provided clear views of a fall/winter equatorward undercurrent, which was both strongest and had the most alongshore similarity of form, during the ENSO. Finally, subsurface-intensified anticyclonic eddies originating in the poleward undercurrent appear to be a recurrent feature of the circulation off Newport late in the upwelling season. [Copyright &y& Elsevier]