Satellites provide a unique perspective for viewing and understanding the Monterey Bay National Marine Sanctuary. They
provide a regional view, showing patterns that are difficult to see from shipboard observations, and they provide a context in time, showing details of changes since the satellites were launched. Using satellite data allows analysis of patterns, trends and
variability at resolutions not possible from ship and over areas
not captured by moored instruments. This permits evaluation
of events such as El Niño and clearer identification of unusual
Satellites collect different types of data of use for the Monterey Bay area. The Advanced Very High Resolution Radiometer (AVHRR), which has operated on National Oceanic and Atmospheric Administration (NOAA) weather satellites
for more than 20 years, measures thermal infrared radiation,
which allows sea-surface temperature to be determined. Ocean color sensors, such as the Sea-Viewing Wide Field-of-View
Sensor (SeaWiFS), launched in 1997, measure the amount
of light at different wavelengths (colors). Color analysis allows estimation of the amount of chlorophyll (i.e., algae) and the
amount of turbidity produced by sediment in the water. The
satellites collect data nearly every day, with samples (pixels)
every 1 to 4 kilometers where there are no clouds. As a result,
the satellites collect hundreds to thousands of samples over
the sanctuary on every clear day -- and during cloud breaks
on cloudy days.
Upwelling is one of the most important factors influencing
the region. From March to October, steady winds from the
northwest cause surface water to move offshore and be replaced
by nutrient-rich sub-surface water from offshore. The sub-surface water is cooler, so upwelling causes the coldest water temperatures to be delayed until April
and May rather than mid-winter. (See Figure 1, p. 12.) An El Niño event significantly reduces the winds that cause upwelling, while its opposite, La Niña, strengthens them. The 1997-1998 El Niño was severe and significantly reduced upwelling, resulting in warmer sea-surface
temperatures. The lack of upwelled nutrients reduced chlorophyll concentrations through 1998 (see Figure 2, p. 12), potentially altering the entire food chain. In contrast, the strong 1999 La Niña, which might have been expected to intensify upwelling, had little influence on chlorophyll. In 2001, a much different event occurred: sub-surface, nutrient-rich water that is normally found near Alaska moved much further south. Through upwelling, this water produced an infusion of nutrients into the California coastal system, resulting in much higher chlorophyll concentrations than normal.
|Figure 1. Sea-surface temperature from satellite for the entire sanctuary, showing the
long-term monthly averages from 1985 to 2004 and the monthly means
|Figure 2. Mean chlorophyll concentration by month for sanctuary areas less than 200 meters deep. Chlorophyll concentration on average follows the upwelling season of March to October, with highest chlorophyll concentration in the spring. The El Niño led to depressed chlorophyll through 1998. The highest chlorophyll, in 2001, resulted from nutrients coming in from Alaskan water.
Winter is, of course, the wettest time of the year in California. While El Niño is known for reducing upwelling, it also causes wetter winters, and the rainfall has a dramatic effect on sediment flowing into the sanctuary. (See Figure 3.) The 1998 El Niño produced a 500 percent increase in turbidity due to sediment in the coastal area of the sanctuary. Turbidity indicates areas of levels of nutrients and pollutants, which ran off the land, increased sedimentation and decreased light for kelps, other algae and sea grasses. The additional nutrients did not appear to offset the lack of upwelled nutrients in the coastal part of the sanctuary. Between the El Niño impact and the Alaskan water impact of 2001, the satellite data indicate that offshore sources of nutrients are far more important regionally to the sanctuary than land-based nutrients, although land-based nutrients may influence some parts of Monterey Bay.
|Figure 3. Turbidity and precipitation for sanctuary areas less than 200 meters deep. Turbidity is caused by
sediment loading. Normally there is only a slight impact from winter rainfall, and this is localized in parts of Monterey Bay. The wet El Niño winter produced much higher than normal sediment loads through the area.
We are beginning to compare the satellite data with other data sets, including seabirds and marine mammals, in order to identify patterns that can be linked to behavior and mortalities. In addition, ocean color satellites are a potential tool in detecting and monitoring harmful algal blooms, which may permit more rapid response to these events.
Richard P. Stumpf
National Oceanic and Atmospheric Administration,
National Ocean Service