On the patterns of global fluid flows

?On the patterns of global fluid flows 

The earth is rotating continuously, from west to east. The solid part of the earth rotates together. The liquid part (water and air) is a little bit drag. You might expect liquid on the earth primarily flow from east to west. This is indeed the case.

The drag is the strongest near equator, where the velocity is the highest. Worldwide, the ocean currents around the equator are mostly east toward west. For details, see the following figure, which is Figure 1 in 

 https://www.coastalwiki.org/wiki/Ocean_circulation

Air circulation exhibits similar patterns. Areas with latitudes lower than 30 degree, as well as higher than 60 degree, mostly have east winds.

When the west marching warm equatorial water bumps into a land, it forces to higher latitude areas and turn back to fill the volume vacated by the west moving equatorial water. This forms a circuit of circulation. Many circuits can be seen in the figure.

Ocean currents moving from low latitude to high latitude are warm water. Ocean currents moving from high latitude to low latitude are cold water. In the figure, warm currents are in red and cold currents are in blue. 

Armed with this figure, and the knowledge of global wind patterns, we can understand some of the climate patterns. 

The latitude of San Francisco is about 38 degree, in the zone of westerlies . West of San Francisco is the vast Pacific Ocean. Yet San Francisco is very dry. Why?

Although San Francisco is in the zone of westerlies, the ocean water from the North Pacific current is cold. Cold water generates little vapor. That is why San Francisco is dry.

Warm and cold are relative. Cold water in San Francisco is warm water in Vancouver. Vancouver’s temperature is much lower than that of San Francisco. The saturation point of air in Vancouver is much lower than that of San Francisco. That is why Vancouver is rainy in winter while San Francisco is a lot drier.