Today we are anchored in a little bay on the west coast of the Baja peninsula. We are hiding from 30-40 knot north winds and a nasty swell that have been building for the past two days. The little fishing village at the head of the bay, San Juanico, is enveloped in a sand storm. It is a mile away and a dinghy ride in these winds would be far too wet for a visit. So, we will hide here at least for the afternoon. These north winds are called the “Baja Bash,” because any boater foolish enough to try to get to San Diego by this coastal route is bashed unmercifully. Adding to the wind bashing is the surprising cold; sea and air temperatures have dropped by 10°C (18°F) in the past two days and we have said goodbye to tropical nights.
In this report I will describe how the this sudden change in our conditions — polar fleece, sleeping bags, wool caps — is the end result of a chain of air and ocean currents that begins on a global scale with solar heating of the tropics and ends on a local scale with coastal winds and the upwelling of deep, cold ocean water.
The famous Coriolis force lies at the heart of it all. We will talk about this mysterious force then we will use a simple weather chart to demonstrate the interrelationship between the tropics, the Mid-Pacific Gyre, the Garbage Patch, coastal north winds, and, finally, coastal upwelling.
Figure 1. Current weather. A weather chart for the North Pacific ocean shows the surface pressure lines, wind barbs, and colored areas for precipitation. Our hiding spot is marked by the black circle at 26°N on the Baja coast. Labels have been added to the chart for reference. The marked features are the Mid-Pacific High (H), the North Pacific Gyre, the trade wind regimes (T), the Intertropical Convergence Zone (ITCZ), and upwelling areas along the coast (U) which are driven by the northerly winds. Click on the graph for a detailed view. Image credit: buoyweather.com.
Reading the weather chart.
The weather chart, Figure 1, was downloaded as we left Cabo San Lucas and prepared for a week-long transit to San Diego. This chart has, in one graphic, just about everything I have mentioned in many previous reports on air and ocean currents.
Symbols on the chart define the barometric pressure, winds, and areas of rain. Constant pressure lines are called isobars and, in the chart, the isobar circles define the mid-Pacific high pressure which is centered at 34°N, 150°W and has a central pressure of about 1028 millibars. The green wind barbs show wind speed and direction. Barbs are like arrows with only half their feathers. They point in the direction of the surface wind, and the number of barbs give the wind speed. One half a barb is 10 knots, a full barb is 20 knots, and one and a half barbs show a 30 knot wind. Precipitation areas are colored according to the scale on the right hand side. Rates from zero to forty-eight millimeters (2 inches) per hour are shown.
What follows here is a brief and highly simplistic description of the labeled features on the weather chart. However, before anything else you must understand the very important Coriolis force.
Coriolis force is central to understanding how winds and currents interact and none of the explanations here make sense without the most rudimentary understanding of this mysterious force.
Coriolis force is not actually a force, like gravity. In actuality it is a complicated gyroscopic reaction to the rotating Earth that pushes air or water in motion to the right of its path. It seems like a force because of its effect on motion. As an example, if water tries to move in a westerly direction, it veers off course, toward the north, as though a force was pushing it. (In the Southern Hemisphere the veer is to the left.)
The next time you are in the playground — in raising four sons I have spent quite a bit of time in the playground — stand on the merry-go-round and have someone turn it gently in a clockwise direction. Now close your eyes and take a small step in any direction. Surprise; you will not step forward, rather your foot will land to the right of where you planned. That is Coriolis force.
Figure 2. Global wind fields. A schematic of the major global wind fields. The red dashed boxes are the area of interest. The equatorial Hadley Cell (HC) is created by solar heating at the equator. Starting at the equator, warm, moist air rises to a high altitude. Winds at the surface converge to replace the rising air (the trade winds). The region of convergence is called the doldrums or the ITCZ. The rising air loses all its water as precipitation, then moves in a northwesterly direction to mid latitudes where it descends back to the surface as dry, cold (relatively) air. The area of descending air concentrates into the mid-ocean high regions. Image credit: David Jessey, Cal Poly, Pomona.
Trade winds (T). Figure 2 shows the equatorial Hadley Cell (marked by HC) which is made up of the Intertropical Convergence Zone (ITCZ), the northwest moving “westerlies” at high altitude, the descending dry air at mid latitudes, and the trade winds. The trade winds are steady winds, about 15-20 knots, that are directed to the equator. In the northern hemisphere they are directed to the south and in the southern hemisphere they are directed to the north. Due to Coriolis force (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere), the trades bend to the west in both hemispheres. The resulting wind fields (by convention winds are described by the direction they come from) are the NE trades above the equator and the SE trades below the equator. Or simply the Easterlies.
Intertropical Convergence Zone (ITCZ). The popular name for the ITCZ is the doldrums. The doldrums is a region near the equator where the north and south trade winds converge. As the Northeast and the Southeast trade winds flow toward each other they take up water by evaporation and become saturated with water. They warm in the intense tropical sun. The name doldrums means low spirits, a feeling of boredom or depression which is well suited to these conditions.. Clouds in this region reflect the high humidity and the sudden release of energy that accompanies the rainfall.
When the winds meet (converge) they rise up into the atmosphere and lose their water by precipitation. The doldrums is the narrow band of towering clouds, squalls, and high humidity that is formed from the convergence. Because of differences in ocean coverage between the two hemispheres, the trade winds do not converge exactly at the equator, but usually they between two and seven degrees north approximately. In Figure 1, the doldrum band is clearly identified by the high precipitation region around +5°N. The trade winds and the doldrums were discussed in a previous report.
Mid-Pacific High (H). Coriolis forces concentrate the dry, descending air of the Hadley Cell into mid-ocean high pressure areas. In the Pacific Ocean this is the Mid-Pacific High. Figure 1 shows the Mid-Pacific High centered around 30-40°N latitude. The center of the high and its central pressure vary in time according to season and in response to other processes such as the Jet Stream to the north.
As a rule, air flows from high pressure toward low pressure and we expect to see winds flowing away from the high. However, Coriolis force pushes the flow to the right, so as the winds blow away from the high they are turned to the right. As a result, the winds around the high blow in a clockwise direction (see the green wind barbs) around the high cell.
Note; the same thing happens in reverse for a hurricane. Air that is heated by a warm ocean forms a low pressure center. As air flows toward the low it is turned to the right and begins a counter-clockwise vortex. As the vortex becomes stronger, winds increase and the storm intensifies to become a hurricane.
North Pacific Gyre, the Garbage patch. Around the Mid-Pacific High, the clockwise winds blow across thousands of miles of ocean. There is a small amount of friction between the air and the water so, by friction, the winds try to push the ocean surface in the same circle. However, water flow has the same Coriolis force to the right that air does, so the resulting ocean currents, on the surface, are toward the high center (there is no shame in doodling these directions on a piece of scrap paper).
Floating objects, pollution, garbage, and plastics are carried with the resulting ocean currents, and over time they spiral into the high. The garbage is captured in the gyre and remains there for years. The famous “garbage patch” has attracted considerable attention lately. At last accounting the garbage patch covered an area roughly the size of the United States. In Figure 1 the North Pacific Gyre roughly occupies the area shown by the dashed line.
Coastal winds and the Baja Bash. Finally, we come to the important coastal upwelling region on the west coast of the U.S.A. and Mexico. As I write this report Ocean Watch is hiding in a little bay called Bahia San Juanico on the west coast of the Baja peninsula.
You can see from the isobars and the wind barbs in Figure 1 that for the entire trip up the coast Ocean Watch has sailed against winds of 20 to 30 kts. These winds have been blowing across at least 500 miles of open ocean and over such a long distance a large ocean swell can develop. They have and we can vouch for it.
This route up the Baja coast is well known to sailors, and is avoided as a matter of course. The ride is notoriously known as “The Baja Bash.” It’s fun to sail down to Mexico; not so much fun coming home.
Last night the isobars tightened more and the head winds increased to above 30 knots. Ocean Watch was riding over the waves with ease, but for the crew the ride was not comfortable. Our mainsail was pulled down into its third and final reef, and as conditions worsened, we decided to hide in this little bay, at least until tomorrow.
Along with the winds, waves and the bashing, it has become cold. For the first time in months we are wearing our polar fleece jackets on deck, at least at night. The air temperature has dropped from 30°C (86°F) to 14°C (57°F), and in the wind over the ocean, that is bone chilling.
Coastal Upwelling (U) and Sea-Surface Temperature
The reason the air is cold is that the ocean is cold. We are in the regime of coastal upwelling. The upwelling area, see ‘U’ on the chart, extends up the coast of California and is one of several upwelling regions around the world. My previous report on upwelling and El Niño talked about upwelling around the world. Figure 3 tells the story for the California coast. Since leaving Puerto Vallarta the sea temperature has dropped from about 25°C (77°F) to 15°C (59°F) and most of that drop occurred in just the last few hundred miles, from Cabo San Lucas to our current anchorage in Bahia San Juanico.
Figure 3. SST on the coast. A map of the sea surface temperature (SST) along the coast of Baja. The anchorage at San Juanico is shown marked with “OW” and the Cabo San Lucas at the end of the Baja peninsula. The temperature decrease along the Baja western shore is a result of coastal upwelling, a response to the strong northerly coastal winds. Click on the image for a larger view. Image credit: buoyweather.com.
Coastal upwelling is brought about by north winds and Coriolis force. As discussed above, wherever the wind blows, there is a tendency for the resulting current to flow to the right of the wind. (Even icebergs flow about 20° to the right of the wind.) The wind blows along the coast and the surface water moves offshore. Something has to replace the water leaving the coast and that is water from below; cold fertile water from below the thermocline. The thermocline was introduced in science report 24 April 2010).
The world and its workings cannot be taken in pieces. Everything in Nature, the winds, currents, animals, chemicals, and even the motion of the stars, play a role in how this grand machine plays its song. In this simple description I wanted to show how the cold water on our coast, the coastal winds, the oceanic pressure fields, the trade winds, and the Sun’s heat in the tropics (heat from the Sun drives the whole engine) all relate to each other. That is the lesson we must remember. Tinker with any tiny piece of the whole at a risk to us all.
Remember: all views, ideas, and comments here are ad hoc, off the cuff, minimally researched, and subject to revision at any moment.
Michael Reynolds, Ph.D., RMR Company