12.3: Local and Regional Winds

In the sections that follow, we’ll use the concepts of pressure, the PGF, and the Coriolis force to understand local, regional, and global winds. Local winds, such as sea and land breezes, operate over the mesoscale—a few to a hundred kilometers. These winds affect coastal air temperatures and local wave conditions. Regional winds, such as Santa Ana winds and monsoonal circulation, operate over synoptic scales, on the order of hundreds to a thousand kilometers. These winds directly impact weather and coastal ocean processes. Global winds, including trade winds, westerlies, and jet streams, operate over global scales greater than a thousand kilometers. These winds drive the ocean currents.

Sea and Land Breezes

A wind coming onto the shore of a coastline, known as an onshore wind, provides a perfect opportunity for a robotross to take flight. Unlike most birds, albatrosses (and our robotross) cannot just flap their wings and take flight. They require some wind or, at the very least, a good runway so they can trot into the wind. The sea breeze, a thermally driven onshore wind, provides an ideal launching pad.

The sea breeze forms as a result of uneven heating of land and water along a coastline. Land, compared to water, heats quickly, so a temperature gradient forms across the land–water boundary. Heating land surfaces causes the air over the land to rise, creating lower pressure at the surface. A pressure gradient forms, causing higher-pressure air over the ocean to move onto the shore. Aloft, the rising air over land creates a higher pressure than that of the corresponding altitude over water. So air at altitude begins to move offshore, from the land to the ocean. A circulation cell thus develops, with ocean-cooled air coming onto the shore at the surface and land-warmed air moving offshore aloft. Because heating of land generally becomes most intense in the afternoon, the sea breeze generally peaks at that time. At night the opposite situation occurs. Land cools more quickly than water, so an area of lower pressure develops over the water. A pressure gradient forms, directed from land to ocean, and an offshore wind develops—a wind that blows from land to ocean. We call this wind a land breeze, a thermally driven wind that blows from land toward the ocean.

Sea breezes and land breezes can moderate coastal temperatures. Sea breezes cool coastlines during the day, and land breezes warm them at night. Sea and land breezes can also generate local waves. An afternoon wind swell is a familiar feature to Southern California surfers, especially during the summer, when the difference between land and ocean temperatures is greatest. Thermal circulations, such as sea and land breezes, require relatively stable atmospheric conditions to form. They also require a strong temperature gradient between the land and the ocean. A stable atmosphere and strong temperature gradient typically occur during the summer months, so sea and land breezes are most common in the summer.

On- and Offshore Winds

Winds that blow from the ocean to land—onshore winds—and winds that blow from the land to the ocean—offshore winds—can also be caused by pressure gradients over synoptic scales. Pressure gradients develop when air mass movements and other factors cause pressure differences between inland and coastal or offshore locations. These pressure gradients may enhance or overpower thermally driven, local wind patterns. On- and offshore flows exert a strong influence over weather along coastlines and can modify conditions within coastal waters.

Television weather forecasts in coastal California cities such as San Diego, Los Angeles, and San Francisco include a reference to on- or offshore winds. Onshore winds typically bring with them the marine layer, a low-altitude cover of stratus clouds that forms over a cool ocean. In places like San Francisco, onshore winds in summer can bring fog, simply defined as a cloud formed near the ground. Because nighttime ocean temperatures are generally warmer than nighttime land temperatures, an onshore flow brings warm, moist air in contact with the cooler land surface, and fog forms. Like a cat, the fog sweeps quietly inland until warmer daytime temperatures cause it to dissipate. Meteorologists often refer to onshore winds as “nature’s air-conditioner,” especially in summer, when the deck of clouds or fog they bring keeps land temperatures cooler than they otherwise would be.

An offshore flow brings higher temperatures to coastal cities. Without a reflective layer of near-surface clouds and cool air from the ocean, offshore winds allow the land to heat faster. Offshore winds also carry airborne particles and air pollution across the ocean. The particles these winds carry can have impacts on marine life. Nitrogen compounds in the air may stimulate phytoplankton growth. On the other hand, toxic metals may inhibit the growth of some organisms (Mahowald et al. 2018).

Santa Ana Winds

Locally infamous offshore winds, the Santa Ana winds, deserve special attention. Named in 1880 for their occurrence through Santa Ana Canyon (along I-91 between Chino Hills and the Santa Ana Mountains), the Santa Ana winds have become the stuff of Southern California legend. They’ve appeared in books and movies, grabbed the attention of songwriters and artists, and struck fear into anyone living at the edge of the chaparral. Santa Ana winds can bring hot, dry, dust- and smoke-filled, hurricane-strength wind gusts across the Southland. They’ve been responsible for some of the worst wildfires in Southern California history. Perhaps that’s why some locals call them “devil winds” (e.g., Masters 2012).

Scientists define Santa Ana winds as “episodic pulses of easterly, downslope, offshore flows over the coastal topography of Southern California and Northern Baja California” (e.g., Guzman-Morales et al. 2016). Let’s dissect this definition.

First, Santa Ana winds are episodic, meaning they occur at irregular intervals. On average Southern California experiences 32 Santa Ana wind events per year, typically from October to April. However, September and May events are not uncommon.

Second, Santa Ana winds blow from the east or northeast at speeds (generally 10 to 30 mph; 4.5 to 13.5 m s^-1) that exceed the local wind field for more than 12 hours. Most Santa Ana wind events last from a few to 6 days; about 10 percent last up to 12 days (e.g., Guzman-Morales et al. 2016). Extreme events can generate gusts up to 80 miles per hour (35 m s^-1; Fovell and Gallagher 2018).

Third, and perhaps most important, Santa Ana winds are downslope winds, meaning they blow from higher to lower elevations. The movement of air from a higher to a lower elevation causes it to undergo an increase in pressure (because, as we learned above, air pressure is greatest at sea level). This increase in air pressure results in a process called compressional heating (also known as adiabatic heating). It’s the same as what happens when you add air to a bicycle or car tire. Ever feel the valve stem when adding air? It gets hot because you are compressing a given volume of air and the energy it contains into a smaller volume. Compressional heating (and its opposite, expansional cooling) also happens in the ocean, but because air is more compressible than water, the effect is not as dramatic. Compressional heating causes Santa Ana winds to heat up as they travel from east-northeast toward the coast. In fact, during Santa Ana wind events, coastal cities are hotter than areas a few or even several miles inland by a few degrees or more. A few hundred miles or more inland, air temperatures may be quite chilly. The heat of Santa Ana winds comes from compression, not from blowing across a desert. Compressional heating also results in very low humidity values—a measure of the water vapor in a parcel of air. High temperatures and low humidity values increase fire danger.

Finally, like land breezes, Santa Ana winds blow offshore due to the PGF that develops when inland areas have higher air pressure than along the coast. Specifically, Santa Ana winds form when a ridge of high pressure occupies the Great Basin region, an expanse of basins and mountain ranges that cover parts of California, Idaho, Nevada, Oregon, Utah, and Wyoming. The resultant pressure gradient generates winds that blow from the Great Basin toward Southern California and Baja California Norte (i.e., Northern Baja). Because they encounter a barrier of mountains surrounding Southern California (the San Gabriel and San Bernardino ranges), they accelerate as they funnel through the narrow mountain passes and canyons that open onto the coastal plain.

The elevated heat, single-digit humidity, and near-hurricane-strength winds bring a host of discomforts. The heat and low humidity dry out your skin, irritate your eyes, and, according to one author, “curl your hair and make your nerves jump” (Chandler 1946). The high winds send plumes of dust and debris into the air, making it difficult to drive and even breathe. And, of course, their peak season occurs at the end of summer, when vegetation is driest, and the danger of wildfire is greatest. Some of Southern California’s worst wildfires have been stoked or even caused by Santa Anas and similar downslope winds. High winds can knock down power lines or cause them to spark. Once the vegetation ignites, a wildfire in the presence of Santa Ana winds can be very difficult to stop. As one firefighter puts it, “About the only firebreak that works is the Pacific Ocean” (Wolansky 2016).

Their numerous negative qualities aside, Santa Ana winds do bring some benefits, especially to Southern California’s coastal waters, where trace metals such as iron (Fe) may limit phytoplankton productivity. Offshore winds, and especially Santa Ana winds, deliver dust and biologically important micronutrients (namely metals, such as manganese and iron) in quantities sufficient to enhance the growth of phytoplankton. A study in 2017 suggested that as much as 15 percent of phytoplankton growth during winter and spring could be attributed to additional micronutrients supplied by Santa Ana winds (Felix-Bermudez et al. 2017). A number of studies also suggest that atmospheric dust particles delivered to the ocean can act as “ballast” for particulate organic carbon and enhance the sinking rates of these carbon-rich particles (Bressac et al. 2014; Pabortsava et al. 2017; van der Jagt et al. 2018). Such effects—phytoplankton fertilization and ballasting of carbon to the deep sea—have implications for the ocean carbon cycle and climate change. Next time you’re complaining about the Santa Ana winds, just remember that they are fertilizing phytoplankton that feed fishes and removing carbon dioxide from the atmosphere, a benefit to us, the ocean, and the planet.