As the fires in Southern California continue to rage, consuming thousands of homes, tens of thousands of acres of land and displacing nearly a million residents, it may help to better understand how the winds in this region have historically made these natural disasters worse than they would otherwise be. And also to see how this circulatory phenomenon affects the coastal marine ecosystem.
Santa Anas are seasonal, occurring from mid Autumn to early Spring when the California coast is warmer than the inland deserts. To understand how these powerful winds form, we first need to get a handle on the lay of the land. Southern California abuts the Pacific ocean, and moving inland the land gently slopes upward from sea level. There is then a sharp increase in altitude as we hit the Sierra Nevada Mountains. East of the Sierras lies the enormous Rocky Mountains. Between the Sierras and Rockies lies a large low lying, dry area known as the Great Basin.
You’ve all seen those meteorological maps on the news showing high pressure and low pressure weather systems. Winds form because air moves from an area of high pressure to an area of low pressure. The pressure difference between these adjacent air masses can be caused by temperature. Basically what happens is the air rises as it warms and cooler air rushes in to take its place. The larger the temperature difference, the faster the air moves, and thus the stronger the winds are.
Normally, because of the hot air over the deserts and the cool air over the ocean, the winds blow from West to East (onshore winds). However, when the desert becomes cooler than the air over the ocean the winds can reverse direction. This happens around this time of year, as the Great Basin between the mountain ranges cools compared to the coast. Because we are dealing with an air mass located between two mountain ranges, there is another phenomenon that occurs because the cold air atop the mountains sinks rather rapidly downslope. These are known as katabatic winds.
Ok, so what do we have here? Warm air over the ocean and cooler air in the Great Basin, trapped between two mountain ranges. And to throw in one more factor, by this time of year the region has usually experienced several months of little rainfall. The air in the Great Basin is pulled towards the Pacific Ocean and is funneled through the narrow passes in the Sierra Nevadas. This intensifies these offshore winds, causing them to move at near hurricane speeds. To clarify the terminology, winds are named for the direction they came from, not the direction they are going. For example, Northeasterlies in New England (often called "nor’easters") are moving from northeast to southwest. To make matters more confusing though, offshore winds are moving from land towards the ocean, while onshore winds move from the ocean to the land (toward the directions the wind blows).
Santa Ana winds don’t start wildfires, but because they are dry, originating over the deserts, and often occur after droughts, any fires that have started are fueled and quickly spread by the powerful winds. In cases like the fires this fall, if multiple fires occur the winds can cause them to collide and essentially join forces. These wind-fueled fires are strong enough to jump fire barriers, highways and destroy electrical lines.
So how do Santa Anas affect marine life? I’ve mentioned in several previous diaries that one of the conditions needed to produce abundant plankton growth is upwelling. Upwelling occurs when vertical layers of ocean water inverts, with bottom water rising to the surface and the top layer sinking to the bottom. This inversion pulls up nutrients, which would otherwise be trapped in the sediment and be unavailable for surface-dwelling plankton to utilize.
Because of the strength and continuity of the winds blowing towards the ocean, the Santa Anas actually move the warm surface waters out to sea. This creates a vacuum that is filled by deep cold water rising up to take its place, pulling nutrients to the surface along with it. These nutrients, once trapped at the ocean bottom, can now be used by phytoplankton and result in a population explosion known as a plankton bloom. The phytoplankton bloom is followed by an increase in zooplankton which feed on it. This in turn provides a rich source of food for animals higher up in the food chain, especially pelagic schooling fish such as sardines and anchovies. I wrote more about plankton blooms and upwelling here.
This is a satellite image of the California wildfires as seen from space.
No poll today as Friday’s diary, "J. Livingston Cracks A Clam", has already been decided on.
Other diaries in this series can be found here.