Goals and Objectives Questions and Hypotheses MethodsOver the course of a 5-7 year period, much of the interannual variability within the CCS may be related to the ENSO cycle. Examining the biological response to ENSO-related physical variability provides an approach for identifying and quantifying biophysical interactions within the CCS. The intensive field studies will provide detailed "snapshots" of the biophysical interactions, but unless they are extremely fortuitously timed, the field studies will not resolve the full evolution of biological responses to an ENSO cycle. While some features of the biological responses and physical causes are relatively well known, the study of the biophysical linkages has been piecemeal in the past, relying on observation systems existing for other purposes. Present monitoring of the conditions of the CCS consist mostly of surface or coastal measurements and are primarily of physical parameters only. Existing monitoring may be sufficient to provide the rough physical context of the ENSO cycle, but it is clearly not adequate for monitoring the biophysical interactions that occur. The CalCOFI surveys in Region III off Southern California provide an important exception. To place the process studies within the context of the interannually varying biophysical interactions, occuring over the 5-7 year period, it will be necessary to augment monitoring of both biological and physical measurements, especially in regions other than the Southern California Bight.
As in most coastal ocean locations, a number of physical parameters are measured on a regular basis along the west coast of the U.S., providing a good basis for designing an augmented monitoring system. These include sea level at various tide gauge stations, surface meteorology (met) over coastal land stations and at buoys 10-20 km offshore (over the shelf), SST and wave statistics at other nearshore buoys, coastal SST and salinity at selected locations, and surface met and SST from merchant vessels. Subsurface temperature has been measured from a small number of merchant vessels along a few ship tracks in the past, but this activity has decreased in the last decade. Under cloud-free conditions, NOAA satellites measure SST four times each day and the SeaWiFS color sensor will provide an estimate of surface pigment concentrations every two days. At present sea surface height is measured by altimeters along tracks separated by 100-300 km, which can be used to estimate the large-scale surface circulation patterns starting 50 km offshore, with one realization every 10-35 days. Satellite scatterometers provide estimates of wind stress fields starting 50 km offshore, with resolution of 50-100 km every 1-4 weeks. Existing weather forecast models provide estimates of surface wind stress and heat fluxes over the ocean every 6-12 hours, with horizontal grid spacings of 100-500 km. The value of these measurements to U.S. GLOBEC EBC is limited, because they are: (1) almost entirely physical; (2) made predominantly at the surface; and (3) either shore-based or provide only very coarse resolution off-shore.
The suggested biological-physical interaction monitoring should augment existing monitoring programs. Most of the augmentation will deal with specific biological responses to physical variability at daily to interannual time scales and mesoscale phenomena at the coast and at the core of the California Current.
Sediment trap data. To fully exploit the potential of the high-resolution sediment records to study climate change and its impact on marine biota, it is important to monitor the processes that create the sediment record. This is done using sediment trap moorings near the sites of deposition. Sediment trap studies provide information on how the seasonally varying input of biogenic and terrigenous material relates to changing environmental conditions, and what proxies of biological and physical processes are preserved in the sediments. Time series sediment trapping, combined with hydrographic measurements and remotely sensed observations of surface ocean conditions provide an ideal means to investigate these questions and should be an integral part of the monitoring efforts of the U.S. GLOBEC EBC program so that we may link the mesoscale and regional process studies to the seasonal, interannual, and interdecadal changes preserved in the sediment records. A spatial array of traps moored near and distant from sediment records could give an indication of the spatial variability of sedimentation, thus the degree to which the sediment time series represents historical variability on larger space scales.
Physical forcing and processes of importance to marine populations. Inflow/Outflow at the boundaries-Altimeter alongtrack heights allow the calculation of cross-track geostrophic surface velocity along tracks that are more than 20-50 km from the coast. By defining a volume inshore of a system of tracks, one can calculate geostrophic inflow/outflow at the surface. An altimeter does not sense wind-driven Ekman drift, which may be calculated from wind stress-most likely from model winds, since the scatterometer may alias storms. Thus, with data already available from satellites and/or surface buoys, surface currents can be calculated (except within 20-50 km of the coast). To examine currents over the shelf (nearshore) and vertical current shear, either CTD data or ADCP current data (at buoys) are needed. Augmenting the offshore meteorological buoys with downward looking ADCPs would be extremely valuable, especially if located under altimeter tracks or crossover points, for verifying the winds and the baroclinic shear in the upper ocean that the altimeter is missing.
Wind stress, surface mixing, wind stress curl, timing of seasonal events-The present system of meteorological buoys located 10-20 km from the coast, along with model winds, probably provide everything needed except the wind stress curl. The model winds underestimate the curl and smooth it over large distances due to their coarse spatial resolution. The cross-shelf component of the curl in spring and summer could probably be measured by placing a met buoy farther offshore (approximately 100 km) and another located only 1 km from the shore, but a denser (in the along-shore direction) array of buoys would be needed to capture the curl in winter.
Water temperature and stratification-AVHRR images, met buoys and merchant vessels all provide SST. To document subsurface temperature changes, including stratification, we need to add subsurface temperature measurements at met buoys, strengthen the XBT program and make periodic vertical slices (transects), either cross-shelf at a few locations and alongshelf at physical boundaries.
Transport-The altimeter does not sample well within 50 km of the coast and is presently unusable within 100-200 km of the coast off Oregon and Washington because of incorrect tidal corrections. Offshore CTD transects will measure the mean geostrophic transport relative to some depth and ADCP measurements will improve the calculations. Cross-shelf transects will reveal jets or eddies crossed along the way, if sampling is sufficient. Towed, undulating vehicles with CTD sensors provide the best coverage. Complete small-scale 3-D surveys are needed to define the eddy field (scales of 10-200 km), which are prohibitive for a monitoring component. Drifters provide information on the eddy field statistics and typical velocities. Presently available winds are probably adequate to describe the surface Ekman transport. Rough estimates of upwelling and vertical transport made from geostrophic wind fields are probably adequate.
Movement of regional physical boundaries-When clouds are absent, the surface signature of these boundaries sometimes may be monitored from satellite SST and color images. However, some of the regional boundaries do not have surface expressions. To describe these boundaries, an alongshelf transect (CTD, ADCP) will be most effective.
Vertical structure-Buoys may provide continuous profiles of temperature structure and velocity at a point, providing information on the forcing of internal mixing. Merchant vessel XBTs provide stratification periodically along regular routes. Regular transects can provide information about the vertical structure of chlorophyll and nutrients. The depths of the thermocline and nutricline are of particular interest, since they may change in some climate change scenarios.