Background
With apologies, this document is about three weeks late. Ocean Watch is now in the much colder waters of Mar de Plata where the temperature is in the mid 20’s,–that’s in degrees Celsius. In Fahrenheit it is in the upper 70’s ºF. See my note on units below. It is partly cloudy, and the Nuevo Año celebrations are cranking up. Life is good-today.
However, we were not such happy sailors on December 4. We were into probably the most depressing part of the voyage. Those of you who followed our daily situation, especially the courageous daily blogs that Herb and David delivered with a stamina worthy of the best war correspondents, are aware of the situation. Hindsight is the best viewpoint and misery is so quickly forgotten. But this report was written in the heart of the drive to Rio. To fully appreciate it, read this in a sauna.
December 4, 2009. We are currently at 2º-35′S,40º-50′W plowing along the north coast of Brazil. We are trying to progress in a southeast direction along the coast, then round the corner and head south down to Rio. In San Juan we began to hear about the difficulties of this portion of our voyage. The current and winds would likely be against us and the going would be very slow and very uncomfortable. Well, there was nothing to do but press on and so we departed Puerto Rico on November 8. After a short visit to the island nation of St. Lucia, we began the push south and, just as expected, the going was tough. The slow going required a fuel stop and we selected Cayenne, French Guyana as the best option. A visit to this French territory with its good food and pleasant people was fun but put us further behind schedule. On November 22, at 6:30 in the morning we weighed anchor and sailed out for seven very tough days.
In this report I want to give you a picture of our world that week. Herb and Mark have made numerous references to this difficult sailing situation. They talked of the heat, rough seas, and head winds, but my report will talk about the oceanography and might give some insight into the sort of oceanography going on in this little corner of the Atlantic ocean. So, take a breath and dig in. Stay with me for a two-minute course in tropical oceanography and to paraphrase Betty Davis, wear your seat belts; it’s going to be a bumpy ride.
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| Fig 1. The wind regions if there were no continents. If the Earth were not rotating the heating from the sun would force a single convection cell from the equator to both poles. The rotation forces the flow into smaller cells; the trade wind easterlies, the westerlies, and the polar easterlies. The continents add to the complexity. |
Trade Winds Drive the Equatorial Currents (See Fig. 1.)
We begin with the trade winds. In a world without continents, the global wind systems, shown in Fig. 1, is one of the most efficient means of transporting heat from < ;script type=”text/javascript”> // –> the equator where the sun-light is direct to the much colder north and south poles. The rotation of the Earth guides the air flow into cells; the trade winds, the westerlies, and the polar easterlies. The continents complicate things considerably but the basic patterns still exist.
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 the rotation of the Earth, they tilt 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. In Oceanographic terms “forcing” is the push the winds give to the surface currents. The direction of the forcing is in the direction toward which the wind blows. The trade winds produce a constant westward forcing across all the tropical ocean. Bear in mind, this is all a very general “average” picture of a turbulent fluid field.
The doldrums are a region near the equator where the north and south trade winds converge. North and south of the equator the trade winds flow toward each other and along the way they take up water by evaporation, becoming thick with humidity. When they meet they rise up into the atmosphere and lose their water by precipitation. Therefore the doldrums are the narrow band of towering clouds, squalls, and high humidity. (Actually the trade winds do not meet exactly at the equator, but usually within a few degrees north.) The name doldrums, meaning low spirits, a feeling of boredom or depression, is well suited. The doldrums lie between two and seven degrees north, approximately. Clouds in this region reflect the high humidity and the sudden release of energy that accompanies the rainfall. An example of towering cumulus clouds and an anvil cloud where the tower reaches the stable top of the atmospheric troposphere is given in Fig. 2 (Please refer to main photo above. This photograph was taken at sunset as Ocean Watch makes its way south to the doldrums. The convergence creates towering cumulus clouds that precipitate and grow to the top of the atmosphere.)
The westward wind forcing creates ocean currents called the equatorial currents that flow along the equator to the
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| Fig 3. A sketch of the major ocean currents in the central Atlantic Ocean. A red dot shows the location of interest. The system of equatorial currents is well defined in the east and the North Brazil Current, fed from the different south equatorial currents, dominates the west. This current is the primary way surface water is exchanged between the north and south Atlantic Oceans. |
west. The two wind-driven equatorial currents push quite a bit of water toward Brazil where it piles up and looks for somewhere to go. One result of the western currents is a thin counter current, the equatorial counter current, that snakes between the north and south equatorial currents.
The equatorial currents meet Brazil (See Fig. 3.)
Most importantly, the North Brazil Current is formed and pushes huge amounts of water northward into the north Atlantic where it joins the Gulf Stream. The North Brazil Current is much smaller than the Gulf Stream, about 1/4 the volume, but it is important because almost all of the exchange of surface water from the southern to the northern hemisphere takes place here. Without the North Brazil current the North Atlantic Ocean would be an almost closed body of water.
Gyres in the current (See Fig. 4.)
Like eddies in a stream, the North Brazil Current, Gulf Stream, and all the other boundary currents often form swirls called rings. Fig. 4 shows the currents we faced leaving Cayenne on November 22. It was not a good picture. A huge ring (also called a gyre) was sitting right in front of us. Currents in the gyre were as high as 4 knots to the north. We considered two choices. Either we could leave Cayenne and turn south to hug the coast and push against the constant 1-2 knot currents or we could try sailing well offshore to the other side of the ring and then catch a ride south. This was a good plan but it required conditions to cooperate.
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| Fig 4. Currents during our Cayenne-Sao Luis passage. A large gyre dominates the area. The current direction in the gyre is clockwise. The plan for as Ocean Watch was to go well offshore and into the low velocity core then turn south and catch the south currents. East winds and slower than expected going force a compromise to the plan then a dive into Sao Luis. The current map is a product of the University of Miami called HYCOM. It is available for any location through http://buoyweather.com. |
No help from the winds
Such was not the case. Fig. 4 shows our location each day from November 22 to November 29. Winds seldom allowed good sailing. Instead of the nice NE trade winds we expected and hoped for, we faced winds that were almost directly from east or even southeast. Do you remember I said above that the doldrums, the place where the north and south trades converge is just above the equator? It so happens this line was almost exactly at 3 degrees N. As a result our tack out was depressing (the doldrums), gray, thick with squalls and dotted with severe wind shifts. Head winds prevented any pure sailing except for short durations. Fuel was low. We were in the middle of nowhere. It was time to head to a port and so on November 29 we entered the harbor of Itaqui.
Around the horn with a new plan
We left Itaqui, the port for Sao Luis, Brazil on December 1. This time we had a different plan. We would sail as close to the coast as possible. There was no secret for escaping this situation altogether — we had to choose the better of bad options. We ended up heading in a SE direction into SE winds and NE currents. Southbound boaters to Rio, beware the North Brazil Current.
Michael Reynolds, Ph.D.
michael@rmrco.com
Remember: all views, ideas, and comments here are ad hoc, off the cuff, minimally researched, and subject to revision at any moment.
Reference for Figures 1 & 3.
Regional Oceanography: An Introduction. M.Tomczak and S.Godfrey
(2001) http://www.es.flinders.edu.au/~mattom/regoc/pdfversion.html
Notes on Units:
Knot: A knot is the universal maritime measure of distance. It is based on the spherical Earth where a nautical mile is one minute of latitude, that is 1/60 of a degree. A knot is one nautical mile per hour. Therefore, is we are headed due south averaging 5 kts for a full day we will cross two degrees of latitude. Roughly a knot is half a meter per second and a little bit more than one mile per hour.
1 kt = 0.5144 m/s = 1.15 mph.
Centigrade: In America we cling to the old Fahrenheit degree while the rest of the world and the world of science has been using the Celsius (or Centigrade) degree for the past decades. Americans will adapt, they must, so why not now? A quick rule for converting from ºC to ºF: double the temperature, subtract 10%, and add 32.
For me I remember that 0C = 32F, 10C = 50F, and 28C ~= 82F. Then you get an idea of the rest.
This crew log submitted by Iridium OpenPort and Stratos
