27 March 2006

Jumping off from Ixtapa.. why?

Ron Larson (Tom’s brother) here. I want to add a post to the blog here to explain to visitors why Tom and Amy are leaving for the South Pacific from this part of Mexico at this time of year.

If you notice, they are now situated firmly between the Tropic of Cancer and the equator, right around the time of the year of the vernal equinox. This is by design. They have put themselves there and then in order to catch a ride on the trade winds and currents that flow from these latitudes, east to west, across the Pacific Ocean.

I will attempt to explain how, and why, this works. It is very interesting.

Important latitudes
The earth is divided into lines of latitude (or sometimes called parallels) which measure the distance between the poles and the equator on the surface of the earth. There are 90 degrees of latitude between the north pole and the equator (the northern hemisphere). And there are of course 90 degrees on latitude between the south pole and the equator (the southern hemisphere). The equator is called zero degrees. The north pole is 90 degrees north. And the south pole is 90 degrees south.

The Equator
The equator is situated half way between the north and south poles. The equator crosses South America through the Amazon basin on the east, and the country of Equador on the west. As everyone knows, countries near the earth’s equator are best known for their moist, tropical weather and dense jungles.

The 45th Parallel
If you are half way between the north pole and the equator, you would be at one half of 90, which is 45. The 45th parallel runs just south of Portland, Oregon and across the northern US states.

The Tropic of Cancer
The Tropic of Cancer is the parallel of latitude that lies 23.439444 degrees north of the Equator. It passes close to Cabo san Lucus in Baja. The Tropic of Cancer is the farthest northern latitude at which the sun can appear directly overhead, which occurs on the summer solstice. North of this line is the subtropics and Northern Temperate Zone.

The Tropic of Capricorn
The Tropic of Capricorn is at the opposite latitude south of the Equator. South of the Tropic of Cancer and north of the Tropic of Capricorn are the Tropics.

Only at latitudes between the Tropics is it possible for the sun to be at the zenith. This means that the water and land at these latitudes receive the most direct sunlight on earth.

Tom & Amy are currently at 17.6666 degrees north. The equator is of course at zero degrees. What this means is that they are now firmly in the tropical latitudes of the northern hemisphere.

The Vernal Equinox
The vernal equinox occurs on 20-March this year (2006). On this day, at the equator, the sun will be directly overhead at noon. The day and night of earth are equal on this, 12 hours each. For the next 3 months, the latitude where the sun will be directly overhead at noon will drift north a few degrees. On 20-June, the summer solstice, this latitude will reach the Tropic of Cancer, when it will turn south and drift towards the Tropic of Capricorn over the next 6 months.

The Coriolis Effect
The Coriolis effect is an apparent deflection of a moving object in a rotating frame of reference. It puts a spin on moving fluids and gases on earth. There are examples of this effect in everyday life, such as the direction of rotation of cyclones. Due to the effect, cyclones rotate counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere.

The Time and Place!
What this means is that for the next 3 months, the northern tropical latitudes get the most direct sunlight on the planet. The waters of the Pacific ocean are soaking up all this energy from the sun, getting warmer and warmer. That is exactly where Tom & Amy are.

The Hadley Cell
The major driving force of atmospheric circulation in the tropical regions is solar heating. Because of the Earth's 23.5 ° axial tilt, the sun is never more than a few tens of degrees from directly overhead at noon in the tropics; as a consequence, incident solar radiation provides maximum energy at the equator. This heat is largely transported into the atmosphere as latent heat via convection in daily thunderstorms that form in this weather belt.

The Hadley cell is a closed circulation cell. It carries heat and moisture from the tropics to the northern and southern mid-latitudes.

Warm, moisture-bearing air at the equator is pumped upwards in thunderstorms. The moisture and heat from the air on the ocean surface goes to the upper troposphere. These air masses eventually reach the tropopause, the boundary between the troposphere and the stratosphere, at a height of between 12 and 15 km, and cannot proceed upward. Nor, because of the upward-welling air below them, can they sink. As a consequence, they are forced either north or south of the equator.

The Horse Latitudes
The airflow loses heat as it travels, and at about 30° north/south of the equator, it begins to descend. As it descends it is compressed, increases in temperature from adiabatic heating and thus the relative humidity decreases, so skies in this high-pressure weather regime tend to be cloud-free, and windless days are common.

This region marks the zone of separation between the Hadley cell and the temperate zone Ferrel cell, and is known as the "horse latitudes". According to the story, in times when ship's captains relied upon the wind to reach their destinations, finding themselves becalmed was usually bad news for any horses aboard, which were thrown overboard in order to conserve precious water.

The Intertropical Convergence Zone
The Intertropical Convergence Zone (ITCZ), also known as the Intertropical Front or the Equatorial Convergence Zone, is a belt of low pressure girdling Earth at the equator. It is formed, as its name indicates, by the convergence of warm, moist air from the latitudes above and below the equator.

The air is drawn in to the intertropical convergence zone by the action of the Hadley cell. Regions in the intertropical convergence zone receive precipitation more than 200 days in a year.

The location of the intertropical convergence zone varies over time, as it moves back and forth across the equator in a semiannual pattern, following the sun's zenith point. The ITCZ usually sits right over the equator in March and September. Between March and June it drifts north to around 10 degrees latitude.

The Doldrums:
Because of the strength of the Hadley cells on either side of it, weather systems familiar to mid-latitude dwellers do not have the chance to form, and as a result, there are no prevailing winds. Horizontal motion is due entirely to air from the trade winds replacing that carried aloft by convection, a slow, languorous process at best.

Early sailors named this belt of calm the doldrums because of the low spirits they found themselves in after days of no wind. To find oneself becalmed in this region in a hot and muggy climate could mean death in the era when wind was the only motive force.

The Sweet Spot:
The area between the ICTZ and the horse latitudes is where the air moves along the surface of the earth. The ICTZ marks the south end of the northern hemisphere’s Hadley cell. The Horse latitudes mark the north end of the northern hemisphere’s Hadley cell. The ICTZ has low pressure, and the horse latitude has high pressure. So the air on the surface moves south towards the equator, picking up heat and moisture until it hits the ICTZ and shoots back up into the high altitudes. This area of the Hadley cell is the sweet spot for sailors. It is where the consistent winds come from, called The Trade Winds.

The Trade Winds
But, the trade winds do not blow north to south as the Hadley cell would have you believe. They blow east to west. The reason for this is because of the spinning of the earth.

There are two forces causes the winds to blow west. The primary force is the Coriolis force twists the downfalling air at the horse latitudes. It is at this point the Coriolis force evidences itself, leading to a clockwise rotation in the northern hemisphere and a counterclockwise rotation on the southern hemisphere.

The other force is friction. At about 2 km above the surface of the Earth, the subsiding air enters the boundary layer, a region of the atmosphere where winds are subject to the influence of surface topography. Winds blow more strongly over grasslands and ocean because there is nothing to block their movement. So as the earth spins from west to east, it appears to the observer standing on earth that it is the air, and not him, which is moving.

The airmass spreads out over the surface of the Earth, moving in a westerly direction. It begins its journey back toward the equator, closing the Hadley cell, and appears as the Trade Winds.

Another factor in their decision on where to go and when to do it are tropical cyclones, aka hurricanes in North America. There are normally no cyclones within 10 degrees of the equator because the Coriolis effect is too weak there to start the spinning needed.

The North Atlantic hurricane season runs between June and November. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. So this time of year is the best time avoid getting hit by a cyclone before getting into the safer cyclone-free latitudes closer to the equator.

The Great Ocean Currents
Besides the wind, sailors must also factor in the ocean currents too. For the Sandpiper, they also get the benefit of favourable surface currents along with the trade winds which will push them west across the Pacific Ocean at these latitudes.

North Pacific Gyre
The North Pacific Gyre (also known as the North Pacific Subtropical Gyre) is a swirling vortex of ocean currents comprising most of the northern Pacific Ocean.

The North Pacific Gyre is located between the equator and 50º N latitude. It is formed by the clockwise circular pattern of the prevailing ocean currents: the North Pacific Current to the north, the California Current to the east, the North Equatorial Current to the south, and the Kuroshio Current to the west. It occupies an area of approximately ten million square miles.

The California Current
The California Current is a Pacific Ocean current that moves south along the western coast of North America, beginning off southern British Columbia, and ending off southern Baja California. It is part of the North Pacific Gyre.

North Equatorial Current
The North Equatorial Current is a significant Pacific and Atlantic Ocean current that flows east-to-west between the equator and 10° north. It's caused by the rotation of the Earth towards the east, the resulting westerly current brought about by the relative motion of the water with respect to the earth.

Counter Currents
There are a couple of counter currents that are of concern to Tom & Amy. The first is a narrow California counter current that moves sub-tropical water northwards during the winter months. They will have to buck this current for a short while. But because of this time of year they are leaving, the impact of this current is minimized.

As they head south and approach the equator, they hit the Equatorial Counter Current. It is a significant current in the Pacific, Atlantic and Indian Oceans that flows west-to-east at approximately five degrees north. The Counter Currents result from the need to balance the movement of water to the west of each ocean, caused by the westerly flowing North and South Equatorial currents.

So there you have it. The sun is pumping energy into the Pacific Ocean, which in drives an enormous “hamster wheel” of air in the atmosphere of the tropical latitudes. And the North Pacific Ocean is slowly spinning clockwise, like a giant toilet bowl, dragging all that floats in it along for the ride. Tom and Amy plan to catch a ride on this machine, making their voyage across the Pacific as easy as possible.

Here is a nice little animated introduction to all of this. I hope you found this useful!

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