Climate

San Mateo and Santa Cruz Counties are within California's Mediterranean climate, which is generally characterized by hot, dry summers and cool, wet winters. Along the coast and as far as 10 miles inland, summers are cool with morning and evening fog. Precipitation is primarily due to rainfall from Pacific storms that move east across the area from November through March. Fog also contributes a significant amount of moisture along the coast, especially in the summer months. Maximum precipitation occurs between December and February. There is usually no precipitation in June, July, August and September.

A storm blows in at Gazos Creek Beach, San Mateo County. Photo courtesy of Erik Geotze, at VirtualParks

San Mateo County

The climate in San Mateo County is reflective of the diversity of topographic features found there. San Francisco Bay moderates the temperatures of the northeast section of the county. There, temperature varies between an average of 50°F in the coldest winter months to an average of 65°F in the summer. Farther south and somewhat inland the weather is not as influenced by the bay, and temperatures are more extreme. For example, in the town of Woodside, temperatures range from lows in the 40s to highs in the 70s (°F). In the Santa Cruz Mountains, which make up the majority of the central and western parts of the county, the climate is generally cooler and wetter. To the west, along the coast, temperatures are also relatively moderate; in the coastal town of Half Moon Bay, temperatures vary from 48 to 61°F. Precipitation across the county is also variable. Rainfall averages from 20 inches" per year near the San Francisco Bay to about 41 inches in the Santa Cruz Mountains to about 38 inches in the south region of the county to about 34 inches along the coast (Western Regional Climate Center 2003).

Santa Cruz County

Air temperatures range annually from highs around 100°F in the summer to lows in the 20s (°F) in the winter, but temperatures are typically moderate in Santa Cruz County. Average winter temperatures are in the 50s (°F) on the coast and high 40s (°F) inland and average summer temperatures range from the 70s (°F) at the coast to 80s (°F) inland. Temperatures are generally less extreme along the coast than inland; in the summer the coast is cooler and in the winter, it is warmer. Daily differences range between 5 and 15°F. Average annual rainfall ranges widely – from 30 inches in the city of Santa Cruz to 50 inches in Ben Lomond. On a few days in most years, light snowfall may occur above 2000 feet (Golden Gate Weather Service 2000).

Climate, including mean annual precipitation and air temperature (°F) as shown here, vary with topology in San Mateo and Santa Cruz Counties.

Global Climate Change

The climate of our world is changing. Over the past century, the earth has become warmer. Temperatures around the world are higher than they have been in the past thousand years. It is an accepted fact that human activities add to natural warming processes by contributing large amounts of carbon dioxide to the atmosphere. The carbon dioxide and other gasses cause the "greenhouse effect" by trapping heat near the earth's surface (NOAA 2000). Scientists disagree, however, about the rate of global warming and the significance of anthropogenic activities (The Exploratorium 2002).

In California, there is a great likelihood that winters will become warmer and wetter in the next century. This means that there is likely to be more rain than snow in the winter, diminishing the water stored as snow and potentially creating an even greater shortage of water during the summer months. The summer months are likely to remain hot and dry and possibly become hotter and drier. These changes may have profound effects on water availability and usage throughout the state (Union of Concerned Scientists 2002), with unknown but potentially significant effects on salmonids. For example, hotter summers and decreased water quantity could significantly degrade salmonid habitat (see the Factors Limiting Salmonid Production section for more information). Longer, more severe storm events could scour gravel beds and contribute to erosion and landslides, making suitable spawning locations more difficult to find (see the Factors Limiting Salmonid Production section). If existing limiting factors are not adequately addressed, the stresses brought about by climate change could further contribute to salmonid population declines.

Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is an El Nino-like shift in ocean temperature. It results in large-scale temperature oscillations in the Pacific every 15–20 years and an additional temperature shift that takes place on a longer time scale of 50–70 years. The longer PDO shift is associated with gentle temperature decreases and gradual increases. The PDO is distinct from the El Nino / Southern Oscillation (ENSO) in that 1) PDO events persist for much longer than the typical 6–18 month duration of ENSO and 2) PDO events are most visible in the North Pacific/North American region while secondary effects are observed in the tropics. ENSO events are most visible in the tropics with secondary effects observed in the North Pacific (Mantua 2003).

Typical wintertime Sea Surface Temperature (colors), Sea Level Pressure (contours) and surface windstress (arrows) anomaly patterns during warm and cool phases of PDO. Image created by Steven Hare, International Pacific Halibut Commission. Used with permission from the University of Washington's Joint Institute for the Study of the Atmosphere and Oceans.

Major changes in the Pacific coastal marine ecosystems are correlated with phase changes in the PDO. Warm eras increase coastal ocean productivity in Alaska and inhibit productivity on the west coast of North America. Cold eras increase productivity on the west coast of North America and decrease productivity in Alaska. Although current meteorological data only show one cycle of the 50–70 year temperature shift, tree rings and fishery data from historic records show a similar time scale shift (Jet Propulsion Laboratory 2000).

Causes for the PDO and the techniques to predict it are currently unknown. From a societal perspective, however, it is important to recognize the PDO. Its discovery shows that "normal" climate conditions can vary over time periods comparable to a human lifespan. When considering salmonids and their response to anthropogenic factors and global climate change, the PDO adds another layer of complexity. When the PDO is going through a cold phase, salmonid fisheries on the west coast of North America increase (Hare 1996; Hare et al. 1999). Coastal managers, scientists, and policy makers must use caution in using short-term population increases to make long-term predictions about future salmonid populations. These changes might be a result of a cold PDO rather than an increase in sustainability due to restoration or management activities. Alternatively, a decline in salmonid populations could be a response to a warm PDO. A warm PDO was in effect from 1977 through at least the mid-1990s and may be continuing presently, or we may be at the start of a cool cycle. An awareness of the PDO and ENSO events is necessary to make sound decisions regarding salmonid recovery plans and their implementation.

References

Golden Gate Weather Service. 2000. San Francisco Bay Area Climate and Weather [Web page]. Golden Gate Weather Service [cited December 15, 2003]. View on-line source.

Hare, S.R. 1996. Low Frequency Climate Variability and Salmon Production. PhD Dissertation, School of Fisheries, University of Washington, Seattle, WA.

Hare, S. R., N.J. Matua, and R.C. Francis. 1999. Inverse production regimes: Alaska and West Coast pacific salmon. Fisheries 24:6-14.

Jet Propulsion Laboratory. 2000. "Pacific Decadal Oscillation Packs a One-Two Punch." California Institute of Technology.

Mantua, N. 2003. The Pacific Decadal Oscillation [Web page]. The Joint Institute for the Study of the Atmosphere and Oceans [cited November 12 2003]. View on-line source.

NOAA. 2000. A Paleo Perspective on Global Warming: A Final Word [Web site]. NOAA [cited January 8, 2004]. View on-line source.

The Exploratorium. 2002. Overview of Climate Change Research [Web page] [cited November 14, 2003]. View on-line source.

Union of Concerned Scientists. 2002. Confronting Climate Change in California [Web page] [cited 2003]. View on-line source.

Western Regional Climate Center. 2003. Northern California Climate Summaries [Web page] [cited November 12, 2003]. View on-line source.

Additional Resources for Topography, Climate, and Weather Information

This part of the Golden Gate Weather Service's Web site provides a detailed description of the topography of the Bay Area and weather patterns: http://ggweather.com/sf/narrative.htm.

This Web site summarizes the essential climate variables (precipitation, temperature) into monthly averages of the five longest running weather stations in San Mateo County (Half Moon Bay, Redwood City, San Francisco Airport, San Gregorio, Woodside) and three weather stations in Santa Cruz (Ben Lomond, Santa Cruz, Watsonville). It also contains a general map with station locations: http://ggweather.com/climate/.

This Web site is primarily for current weather and road conditions but also has useful weather-related variables including flood status with a river level map, tide information, and wave heights: http://ceres.ca.gov/theme/county_weather.html.

This site, run by the Integrated Pest Management Program at University of California at Davis, has daily averages for temperature and precipitation but does not have monthly summaries: http://www.ipm.ucdavis.edu/WEATHER/wxretrieve.html.

This NOAA Web site has the most complete list of weather observation stations for the county including past stations that are no longer recording data. While this has the most complete information, it can be somewhat technical to use and there is a small fee for access to on-line data summaries. There is a large amount of data available here with variables not found anywhere else, such as a record of storm events like flash floods, tornados, and heavy rainfall events: http://lwf.ncdc.noaa.gov/oa/climate/stationlocator.html.

This is the Web site for the Western Regional Climate Center administered by NOAA. They have on-line climate summaries for temperature, sunshine, sky cover, station pressure, humidity, precipitation, and wind: http://www.wrcc.dri.edu/climsum.html.

This is the NOAA Web site for Climate Change Data and Detection. Current research and requests for proposals are posted on the site: http://www.ogp.noaa.gov/mpe/ccdd/index.htm .

This is the last page in a NOAA Web site about global warming. It summarizes NOAA's position on the causes of global warming: http://www.ngdc.noaa.gov/paleo/globalwarming/end.html.

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