Analysis of AVHRR, CZCS and historical in situ data off the Oregon Coast

The original scientific objectives of this grant were to: (1) characterize the seasonal cycles and interannual variability for phytoplankton concentrations and sea surface temperature (SST) in the California Current using satellite data; and (2) to explore the spatial and temporal relationship between these variables and surface wind forcing. An additional methodological objective was to develop statistical methods for forming mean fields, which minimize the effects of random data gaps and errors in the irregularly sampled CZCS (Coastal Zone Color Scanner) and AVHRR (Advanced Very High Resolution Radiometer) satellite data. A final task was to evaluate the level of uncertainty in the wind fields used for the statistical analysis. Funding in the first year included part of the cost of an image processing system to enable this and other projects to process and analyze satellite data. This report consists of summaries of the major projects carried out with all or partial support from this grant. The appendices include a list of papers and professional presentations supported by the grant, as well as reprints of the major papers and reports

Atmospheric conditions during the spring and fall transitions in the coastal ocean off western United States

We examine large-scale atmospheric behavior around the time of the spring and fall transitions in the coastal ocean off the west coast of North America. Records of adjusted sea level (ASL), coastal wind stress, sea level atmospheric pressure (SLP), and 500-mbar heights for the years 1971-1975 and 1980-1983 are analyzed. The records cover periods of 91 days, centered on the dates of the spring and fall transitions as determined from coastal adjusted sea level data. Empirical orthogonal functions are obtained from the ASL, coastal wind stress, and SLP records. The two dominant modes of the ASL and coastal wind stress are similar around the times of both the spring and fall transitions, and the time series for these modes are highly correlated with one another. Transitionlike behavior is evident in the time series of the first modes in both spring and fall, but the spring transition is more pronounced. Principal estimator patterns, formed from the dominant empirical orthogonal functions show the spatial patterns of SLP which force the ASL and coastal wind stress during the transitions. The SLP pattern which coincides with the spring transition is the formation of a high-pressure system centered at 45°N and 140°W along with the development of a low-pressure cell over the southwest continental United States. Inspection of the 500-mbar height composites for 91 days surrounding the spring transition for the 9 years reveals the formation of a ridging pattern and diffluent flow over the western United States at the time of the transition; following the transition, the ridging relaxes but the diffluent flow over the continent remains for the duration of the 45 days examined here. The fall transition is characterized by a rise in ASL, particularly north of 40°N, and a change from southward to northward wind stress. The SLP pattern which coincides with the fall transition involves the appearance of a low-pressure system off western North America centered at 50°N and 140°W, representing the passage of synoptic storms through the region. Prior to the fall transition, the 500-mbar heights are somewhat diffluent and show a trough over the southwestern United States; after the transition the 500-mbar flow over the northeast Pacific and North America is nearly zonal.Keywords: spring transition, fall transition, atmospheric conditions, western United State

The large-scale summer circulation of the California Current

Satellite data from the Geosat altimeter and the Advanced Very High Resolution Radiometer (AVHRR) are used to show the large-scale structure of the surface circulation of the California Current System in summer. These data show the connection between an equatorward jet and temperature front off Oregon that lies within 100 km of the coast, similar to that first observed in the 1960’s and 1970’s and a jet that meanders along the convoluted offshore edge of a temperature front off California, as repeatedly observed in the 1980’s

Seasonal and interannual variablity of pigment concentrations across a California Current frontal zone

Previously published physical and biological data document a zonally oriented frontal region within the California Current system separating colder and more eutrophic water north of ≈33°N from warmer; more stratified, and oligotrophic water farther to the south. Satellite images of phytoplankton pigment from the coastal zone color scanner from 1979-1983 and 1986 are used to examine the seasonal and interannual variability of both the latitudinal position of this front and the pigment concentrations associated with it. Many temporal and spatial characteristics of the pigment structure are repeated in different years, and a general seasonal cycle is described. Variations in the frontal structure are controlled primarily by changes in pigment concentrations north of the front. Seasonality is minimum south of the front where concentrations remain low (2.0 mg m⁻³) are present north of the front. Lower pigment concentrations within the sampled region (0.5-1.0 mg m⁻³) north of the front in mid-late summer (June-August), resulting from a seasonal shift in the cross-shelf distribution of pigment, reduce and often eliminate the pigment gradient forming the front. Concentrations greater than 1.0 mg m⁻³ typically extend 150-250 km farther offshore in spring (April) than in summer (June-July). Superimposed on this general seasonality is strong interannual variability in the magnitude of the frontal gradient, its latitudinal position, and the seasonal development of higher biomass in regions north of the front. Pigment concentrations during the El Niño year of 1983 are distinctly lower than those of other years. The patterns evident in the satellite data are compared with available in situ measured hydrographic data and nutrient and phytoplankton concentrations. A comparison of the seasonal and interannual variability of these patterns to surface wind shows little direct relation between frontal strength or position and wind forcing.Keywords: phytoplankton pigment, California CurrentKeywords: phytoplankton pigment, California Curren

Comparison of velocity estimates from advanced very high resolution radiometer in the Coastal Transition Zone

Two methods of estimating surface velocity vectors from advanced very high resolution radiometer (AVHRR) data were applied to the same set of images and the results were compared with in situ and altimeter measurements. The first method used and automated feature –tracking algorithm and the second method used an inversion of the heat equation. The 11 images were from 3 days in July 1988 during the Coastal Transition Zone field program and the in situ data included acoustic Doppler current profiler (ADCP) vectors and velocities from near surface drifters. The two methods were comparable in their degree of agreement with the in situ data, yielding velocity magnitudes that were 30-50% less than drifter and ADCP velocities measured at 15-20 m depth, with rms directional differences of about 60°. These differences compared favorably with a baseline difference estimate between ADCP vectors interpolated to drifter locations within a well-sampled region. High correlations between the AVHRR estimates and the coincident Geosat geostrophic velocity profiles suggested that the AVHRR methods adequately resolved the important flow features. The flow field was determined to consist primarily of a meandering southward flowing current, interacting with several eddies, including a strong anticyclonic eddy to the north of the jet. Incorporation of sparse altimeter data into the AVHRR estimates gave a modest improvement in comparisons with in situ data

Blended sea level anomaly fields with enhanced coastal coverage along the U.S. West Coast.

We form a new data set of fields of sea level anomalies by combining gridded daily fields derived from altimeter data with coastal tide gauge data. Within approximately 50-80 km of the coast, the altimeter data are discarded and replaced by a linear interpolation between the tide gauge and remaining offshore altimeter data. A 20-year mean is subtracted from each time series (tide gauge or altimeter) before combining the data sets to form the merged sea level anomaly data set. Geostrophic velocity anomaly fields are formed from the surface heights. Daily mean fields are produced for the period 1 January 1993 - 31 December 2014. The primary validation compares geostrophic velocities calculated from the height fields and velocities measured at four moorings covering the north-south range of the new data set. The merged data set improves the alongshore (meridional) component of the currents, indicating an improvement in the data set

Wind-driven surface transport in stratified closed basins: Direct versus residual circulations

A numerical model has been used to investigate the wind-driven circulation in a stratified basin of moderate size, Lake Tahoe, California-Nevada. Two types of circulation are identified: "direct" circulations, in which the current directions remain relatively constant and the mean circulation formed over several days resembles the instantaneous circulation, and "residual" circulations, in which the currents fluctuate continuously and the mean circulation is characterized by small net displacements of parcels after large oscillations. Previous studies of stratified closed basins have emphasized residual circulations caused by cyclonically propagating internal basin modes, resulting in a single cyclonic mean gyre during light to moderate winds. Observations at Lake Tahoe have shown currents which are more constant in direction, with a double gyre pattern of surface circulation, dominated by an anticyclonic northern gyre. Model experiments of Lake Tahoe demonstrate that the curl of the wind stress must be included to obtain a direct, double gyre circulation similar to the observations. Horizontally uniform winds cause a residual circulation, similar to that reported at other lakes. Use of the model to calculate the vorticity budget clarifies the role of the wind stress curl in creating the direct double gyre.Copyrighted by American Geophysical Union

Spatial and interannual variability in mesoscale circulation in the northern California Current System

We used wavelet analyses of sea surface height (SSH) from >13 years of satellite altimeter data to characterize the variability in mesoscale circulation in the northern California Current (35°N–49°N) and explore the mechanisms of variability. We defined ‘‘mesoscale’’ circulation as features, such as eddies and filaments, which have 50- to 300-km length scales and 4- to 18-week temporal scales. Fluctuations in SSH caused by such features were reflected in wavelet analyses as power (energy). Spatial and interannual variation in mesoscale energy was high. Energy was highest at ∼38°N, decreasing to the north and south. Between ∼43°N and 48°N, energy was low. Zonally, mesoscale energy was highest between ∼125°W and 129°W at latitudes south of 44°N; very little power occurred in the deep ocean west of 130°W. Energy peaked during summer/fall in most years. The primary climate signals were suppressed energy during La Nin˜a and cold years and increased energy during El Niño events. Energy was not strongly linked to upwelling winds, but did correspond to climate indices, indicating that basin-scale processes play a role in controlling mesoscale circulation. We hypothesize that climate affects mesoscale energy through changes in both potential and kinetic energy in the form of density gradients and coastal upwelling winds. The relationship between mesoscale circulation and climate was complex: no single climate, transport, or upwelling index explained the variability. These results are relevant to ecosystem dynamics and the global carbon cycle because mesoscale circulation features deliver nutrient-rich water and coastal organisms from productive upwelling areas to the deep sea

Dynamical analysis of the upwelling circulation off central Chile

In this article we analyze the momentum and vorticity balances of a numerical simulation of the upwelling circulation off central Chile (34° –40°S) and its response to interannual local wind changes. Our analysis indicates that the path of the upwelling jet is strongly controlled by the bottom topography. This topographic steering causes the jet to separate from the coast at the Punta Lavapie cape (~37°S). Although the zeroth-order momentum balance is dominated by the geostrophic terms, the circulation is also affected by nonlinear processes, which lead to the formation of large meanders and the shedding of cyclonic eddies north of Punta Lavapie during periods of wind relaxation. The relative contributions of the zeroth-order vorticity balance and the advective terms are also strongly controlled by changes in the coastline geometry and the bottom topography. Vorticity is created along the current axes and transported toward the coast and the Peru-Chile Trench, where it dissipates. South of Punta Lavapie the across-shelf transports are weaker with equatorward flows that are more stable than in the north. Additional numerical simulations indicate that during periods with El Niño conditions, the area is affected with a general weakening of the currents and upwelling activity, although the northern region still shows the formation of eddies. During years with relatively stronger winds, in contrast, the upwelling activity and across-shelf transport processes are significantly increased. The results show that the Punta Lavapie cape has a large effect on the spatial and temporal variability of the coastal currents in the region off central Chile

Estimates of sea surface height and near-surface alongshore coastal currents from combinations of altimeters and tide gauges

Present methods used to retrieve altimeter data do not provide reliable estimates of sea surface height (SSH) in the nearshore region, resulting in a measurement gap of 25–50 km next to the coast. In the present work, gridded SSH fields produced by Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) in the offshore region are combined with coastal tide gauge time series of SSH to improve estimation in that gap along the west coast of the United States in the northern California Current System between 40° and 45°N and 123.8° and 126°W. To assess the increase in skill provided by this procedure, the geostrophic alongshore currents, calculated from the new SSH fields in the gap region, are compared to three in situ, nearshore current measurements, resulting in correlation coefficients of 0.73–0.83 and standard deviations of the differences of 11.6–12.6 cm/s, substantially improved from the AVISO-only results. When the Ekman current components are estimated and added to the geostrophic currents, comparisons to the 10 m deep acoustic Doppler current profiler velocities are only slightly improved. The Ekman components make a more significant contribution when compared to HF radar surface current measurements, providing correlations of 0.94 and standard deviations of the differences of 6.4–9.5 cm/s. These results represent a dramatic improvement in the quality of the SSH fields and estimated alongshore currents when additional, realistic SSH data from the coastal region are added. Here we use coastal tide gauges to provide the additional SSH data but also discuss more general approaches for altimeter SSH retrievals in coastal regions where tide gauge data are not available