PART VIIC: Temperature of Bay water


Like salinity, the temperature of Bay water varies spatially and temporally and is an indicator of mixing and the relative contributions of fresh and salt water. The story is somewhat more complicated, however, because sometimes during the year the coldest water comes from the ocean and sometimes it comes through the Delta. Colder water is more dense (heavier) than warm water and tends to sink to the bottom. This effect is very strong in the open ocean, where salinity variations are small. In San Francisco Bay, where salinity variations are large, salinity has a greater impact on water mass sinking and mixing than temperature.

Water temperature is measured by the USGS because it is an indicator of mixing and because many biological processes (including fish migrations) respond to temperature changes. The seasonal range of water temperature in the Bay is from about 8°C to about 23°C. The diagram below is an example of two vertical profiles collected by thermometers on the USGS submersible instrument package at two different locations in the Bay during the same sampling cruise in July 1996.

The temperature of Bay water fluctuates with the seasonal changes in air temperature and solar radiation.

Description of circled numbers:

1. This profile was measured in Central Bay on July 17, 1996. It illustrates the condition in which temperature is nearly uniform from top to bottom -- a temperature gradient is small or absent -- because mixing is faster than surface heating. There are, however, small, irregular temperature changes with depth.

2. This profile was measured in San Pablo Bay on July 17, 1996. It illustrates a condition in which the upper layer of the water is warmer than the lower layers. This example shows that water temperature is not necessarily uniform from top to bottom -- a temperature gradient can exist in the water. In this case, the surface water was about two degrees Celsius (or almost four degrees Fahrenheit) warmer than the bottom water, and mixing is not as rapid as surface heating.

1a. Using the following formula, calculate the temperature range in the Bay (8°C to 23°C) in °F.

°F = (9/5 °C) + 32

1. What processes cause Bay waters to become better mixed from top to bottom?

2. Why might the Central Bay be colder and better mixed than San Pablo Bay on the same summer day? (To see a map of the two locations, go to the map of USGS sampling locations.)


The figure below shows how temperature varies both spatially, along the length of the Bay, and temporally, from season to season and year to year. These data are from the USGS Water Quality program and combine measurements from three years that had high natural variability. For example, 1994 was a dry year, while 1995 was a wet year.

The upper panel displays the Delta Outflow Index for 1993-1995. The lower panel shows the changing distribution of water temperature at the Bay surface along the USGS sampling transect. Color is proportional to temperature, with darker (red) shadings indicating warm water and lighter (blue) shadings indicating cold water. The vertical axis represents variability in space from the Sacramento River (top of image), to the Central Bay, and then to the lower South Bay (bottom of image). The horizontal axis represents change over time from 1993 through 1995.

Description of Circled Numbers:

1. Water temperature changes with season, and are warmest in August when temperatures reach 22-23°C (72­73° F).

2. Bay waters are coldest in December and January, reaching minimum temperature of about 8°C (46° F).

3. In summer, the water is warmer in the Sacramento River than in the Central Bay (Bay Bridge). This shows the effect of mixing between warmer Bay waters and colder waters from the Pacific Ocean.

4. Sometimes there are sharp temperature gradients, like this one near Hunter's Point. These gradients suggest regions of slow horizontal mixing. In this case a bump in the sea floor, the San Bruno Shoal, acts to slow mixing between the South Bay and Central Bay, allowing the South Bay waters to warm up faster than the Central Bay waters which are close to the colder Pacific Ocean.

3. Summarize the summer and winter temperature differences.

4. During which season is there a stronger temperature gradient along the length of the Bay, summer or winter? Why?

5. During the summer, Bay water is coldest near the Golden Gate, where ocean water enters. You may notice that in the winter, the water near the Bay entrance is slightly warmer than in the rest of the Bay, particularly the north Bay, just the opposite relationship to summertime conditions. Explain why this difference would occur.


The plots below show temperature variation along the length of the Bay and from the surface to the Bay bottom. Temporal variability is shown by plots from different months and different years. Temperatures shown are for the same months as the salinity data in Part VIIB.

Temperature in degrees Celsius

April 19, 1994

April 4, 1995

September 27, 1994

September 21, 1995

6. Describe the difference in temperature distribution in different months of the same year.

7. Compare data from the same months of different years. Why were water temperatures warmer in April 1994 than in April 1995? (Hint: think of the difference in climate during the two years.)

8. In the September months, why is the coldest water in the Central Bay?




This diagram shows the variation in surface water temperature along the length of the Bay in November 1996 (blue symbols; a normal year) and in November 1997 (red symbols; an El Niño year). The horizontal axis corresponds to the USGS sampling transect. Negative numbers are distances south of the Central Bay and positive numbers are distances northeast of the Central Bay. The vertical distribution of temperatures at a particular location represents the range of temperatures between the surface and bottom waters at that site.

4a. Describe the temperature differences between the two years. How can these differences be explained in light of global climate conditions?




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