Seasonal upwelling from March until October is responsible for much of the high primary production along the central California coast. ENSO events will therefore usually impact this upwelling region in the spring when the temperatures along the equator are slowly returning to normal. Due to the unusual timing of the current ENSO event, some effects may have been observed during the late summer and early autumn of 1997. Results from local cruises and quarterly regional cruises during 1997-1998 are being examined and compared with historical data and a local time series that started in 1989. In addition to the shipboard data, moorings have been providing a continuous record of physical and optical parameters. The sea surface partial pressure of carbon dioxide was also measured from these moorings during most of this event. Upwelling started exceptionally early in 1997. Elevated partial pressure of carbon dioxide and decreasing temperature were evident at one of the moorings in late February and a cruise in early March confirmed these indicators. As the year progressed, a warm anomaly developed and the partial pressure of carbon dioxide fell below atmospheric values and has remained there with very minor short-lived exceptions. River runoff from strong winter rains in this ordinarily dry region may be responsible for some of the carbon dioxide decreases in the late winter of 1998.
The Central California Coast at the latitude of Monterey Bay (~37°N) usually experiences multiple upwelling and relaxation events between April to September. Monterey Bay is located at the eastern edge of the California Current, on the eastern boundary of the North Pacific gyre. While northwesterly (upwelling favorable) winds prevail over most of the year, their intensity is strongest during the late spring. A cold upwelling plume is frequently found to originate just to the north of Monterey Bay and extend across the mouth of the bay, while warmer water is found inside the bay. To the south of Monterey Bay another upwelling center that frequently terminates in an offshore cold filament can usually be found after several days of favorable winds.
Upwelling waters tend to come from depths of 100-150 meters and can be as cold as 9.5°C with a salinity of about 33.8 psu. The poleward flowing California Undercurrent tends to be the source of these waters. Offshore California Current waters have considerably lower salinity (~32.5 psu) and summertime surface temperatures of about 15°C. Nitrate and dissolved silicon concentrations in the upwelling waters can be in excess of 25”moles kg-1. Sea surface partial pressure of CO2 can be in excess of 750 ”atm in freshly upwelled water. During moderate upwelling events, high primary productivity tends to reduce the sea surface nutrient and CO2 levels on time scales of several days and length scales of tens of kilometers.
Phytoplankton production and communities tend to have a distinct seasonal cycle. Productivity is lower and uniform (~ 500 mgCm-2d-1) from October to February when picoplankton dominate the non-upwelling winter period. During the upwelling season from March to September, diatoms dominate the biomass and productivity is high and variable (~2000 mgCm-2d-1)
Three interdisciplinary moorings have been deployed off the central California coast by MBARI. These moorings report the temperature structure of the upper 300 meters and are equipped with optical and chemical sensors near the sea surface. Meteorological and current data is also available from these systems. Data are transmitted to shore on an hourly basis. The most complete data record from this ENSO event is from the innermost mooring (M1). The M1 mooring is usually directly downstream from a major upwelling center and is therefore a good indicator of upwelling intensity. The travel time of surface water (as measured by drifters) between the nearest cold water source to the north and M1 is about two days.
The temperature record from the 1997-1998 season clearly demonstrates
the anomaly associated with the ENSO event. The onset of the spring upwelling
season has been delayed in 1998 and the apparent intensity is relatively
low. The contrast between the 1997 and 1998 onset of upwelling is illustrated
We have been conducting a sampling program during monthly cruises between the coast and the M2 mooring. Quarterly cruises that extend to 400 km offshore have been sampled since March of 1997. Additional shiptime has been scheduled for 1998 to obtain greater detail during the present ENSO event. The main hydrographic sampling line is oriented approximately perpendicular to the coast with a near-shore station in the center of Monterey Bay. Some stations along this line have been sampled since 1938 and a more extensive data collection effort was undertaken as part of the CalCOFI program starting in 1949. The data presented here is from cruises during the spring upwelling seasons of 1997 and 1998. Sea surface pCO2 levels did not exceed atmospheric levels at any location in 1998 while the results of strong upwelling can be observed within about 230 km of the coast in 1997. A similar pattern was found for nitrate. The only detectable sea surface nitrate concentrations in 1998 were found very close to the shore and may have been related to river discharge.
Many of the water mass characteristics that were observed during these cruises are characteristic of the water masses usually found in this region. In the 1998 data, there were some aspects that may influence the distribution of nutrients and inorganic carbon when upwelling does take place. The high salinity water that is representative of the poleward undercurrent was found offshore, while the lower salinity water that may be of sub-arctic origin was found near the coast. This is the opposite of the 'normal' situation. Since the water found closest to the coast has the largest influence over the composition of the upwelling waters and sub-arctic waters have relatively lower nutrient content, primary productivity could be relatively lower during upwelling. Another significant difference seen in 1998 is the presence of very low salinity ( <33.0) water near the surface, a possible result of a very wet winter.
No significant difference in the relationship between nutrients and water masses was observed between the two years. The differences in integrated nitrate available for upwelling is due to the density and water mass structure near the coast. In 1998, integrated nitrate between the surface and the coldest water that has been observed to upwell in this region during intense events (~9șC) was about half of the 1997 value.
The central California coast is a semi-arid region. Most of the annual rainfall occurs during the winter months and some of that water is usually held back in reservoirs. River runoff is seldom noticeable in the ocean during the spring upwelling season. The rains associated with the 1997-1998 ENSO event were more than twice the long-term average and conspicuous freshwater plumes formed off the coast.
These two enhanced AVHRR images demonstrate the sediment load that was flushed into the coastal ocean after the 1998 rains started. The sediment load probably sinks out of the surface waters after a few days, but the freshwater signal could be observed for weeks to months.
During our 1998 cruises the freshwater plumes appeared to be a significant source of variability at the sea surface. Low surface salinity values were observed as far as 300 km from the coast. These plumes were associated with increased phytoplankton biomass and considerable pCO2 undersaturation when compared with surrounding waters. These events were also observed in the M1 and M3 mooring data.
The low pCO2 observed was probably a result of biological consumption. Measurements made in local rivers indicate CO2 concentrations that are higher than those in the ocean. The nutrients that could fuel such blooms may have been derived from terrestrial runoff. Nitrate concentrations in the Salinas River, which discharges into Monterey Bay, ranged from 100 to 300 ”M. When mixed with seawater, this nitrate input is sufficient to produce the observed pCO2 deficits in the low salinity water. Since the nitrate concentrations in the headwaters are very low, one possible source for these nutrients was leaching from the large agricultural operations in this drainage basin. Much of the freshwater that we observed offshore was probably discharged through San Francisco Bay and may be representative of the California Central Valley drainage, another region of intense agriculture. Elevated nitrate and phosphate concentrations are frequently found in the waters moving through San Francisco Bay. Preliminary calculations indicate that in the spring of 1998 the flow of the Sacramento River alone could generate a 400 km2 patch of low salinity water (~31.5) on a daily basis. During years of normal upwelling and average rains, these terrestrial inputs are probably not significant when compared to the nutrient flux from depth.
We wish to thank the David and Lucile Packard Foundation for their support of this work. Information about river flows was obtained from the US Geological Survey and some of the river nutrient data was provided by K. Johnson of Moss Landing Marine Labs. P. Durkee from the Naval Postgraduate School made the enhanced AVHRR images available.