The winds in San Francisco Bay, at least in the summer, are 
characterized by very strong diurnal variations; that is, a 
land/sea breeze that is driven by the temperature difference 
between the land and the sea. During the winter, when storms 
come through, this whole pattern is upset, and there are very 
strong winds from the southwest and sometimes from the north¬ 
west. This has a tendency to create perturbations in the cir¬ 
culation, after which time it returns to a more steady rate. 
Let's talk in more detail about tides. The variations in 
sea level at the Golden Gate create a tidal wave which propa¬ 
gates into the Bay. The tidal characteristics between the north 
and south ends of the Bay are quite different. The south end of 
the Bay has what one would call a standing wave; that is, as you 
look at the windward side of a fixed object in the water, you 
would see a reflected wave and an incident wave. These two 
waves propagate against each other, creating a standing wave. 
Now, one characteristic of a standing wave is that the maximum 
velocities occur when the tide is in the midpoint between its 
extreme. 
The characteristics of the tide in the northern region is a 
combination of a standing and a progressive wave; that is, the 
current speed is maximum at the crests and troughs, like wind 
waves on a lake. There is quite a phase lag as the wave propa¬ 
gates up the reach, perhaps three or four hours before it comes 
to Suisun Bay. 
Sea level is, of course, continuous across central San 
Francisco Bay. However, the water currents have a different 
phasing as you traverse the central bay area. What you find is 
that the tide turns first in South Bay and then later in the 
northern reach. For instance, when the tide turns and starts to 
flood, it will change first and start flooding in South Bay be¬ 
cause it changes at the mid-tide level, whereas it's not going 
to change in the northern part until several hours later. Water 
flows out to the northern reach, into the South Bay, while the 
water is starting to flood into Golden Gate. Eventually, the 
water will turn and start propagating up the northern reach. 
What you have is an unusual but very effective way to pump water 
between the north and south reach of the Bay. 
Looking at the tides as they propagate through the Bay from 
the Golden Gate, you can see, if you look at tidal amplitudes, 
that the amplitude of the wave is increased as it goes south, 
which is a characteristic of a standing wave. Phase differences 
here are small, that is, everything happens simultaneously. In 
the northern reach, there is a tendency for local reflections to 
occur, for example, at the eastern end of San Pablo Bay. In 
fact, the phase increases in a monotonic manner up the northern 
reach. The tidal currents in San Francisco Bay are quite large. 
At Golden Gate, there is a constriction, leading to currents of 
up to 5 knots. 
25 
