Wednesday, July 22, 2009

Day 42- Wind and ocean currents

Noon position: 40°50'29.98"N 173°19'55.80"W

We are 5,200 miles into our journey and back in the western hemisphere, starting to make our way south with a heading of 115. Our speed over ground (SOG) is 7.6 knots and we’re getting winds out of the west at 20-25 knots. These are the winds we want to see-the Captain and crew are anxious to make it to Honolulu. We all have prior commitments that have been put in jeopardy by the weather conditions.

The winds started really picking up yesterday evening, jumping from 6-8 knots in the AM to 20-25 in the PM as we headed north. For a while the winds were hovering around 30 knots, throwing large rolling swells at us which caused a sudden, violent collapse of the sail. The crew donned life vests and rushed to the foredeck to fish the sail out of the water. The damage to the sail was pretty severe. We lost the most efficient sail for the current weather conditions-which lowered the morale a bit.

Part of our issue is that the nature of the area in which we have been sampling is a high pressure zone. High pressure zones are characterized by light winds. We needed to spend the fuel to motor through these calms to in order to even run the Manta trawls. The past several days have been working towards an escape from these conditions, using sail power and not trying not to use the ever dwindling fuel supply. Our plan of escape: sailing north. This may seem a bit illogical since our destination is way south of us, but there just so happens to be a rhyme to our reason. Here’s Joel on the subject:

“Since we are a sailing vessel and we must sail to get home, we need to find where the favorable winds are. The wind and currents in the North Pacific Ocean are dominated by the large gyre at its center. The wind in the California Current flows from north to south and is the eastern edge of the gyre, the Trade Winds blowing toward the west and southwest make the southern boundary, the Kurshio current flows south to north off the east coast of Asia makes the western edge of the gyre while the westerlies blow back toward the West Coast of North America and make the northern boundary of the gyre. (note: keep in mind that winds are named for the direction they originate from. Thus westerlies come from the west, easterlies from the east etc.). All together they make a giant clockwise circular current and wind system with high atmospheric pressure and calmer variable winds at the center known as the North Pacific Subtropical Gyre. It is one of five major gyres in the world’s oceans. There are three in the northern hemisphere (The North Pacific, North Atlantic, and the Indian Ocean) which spin clockwise. In the southern hemisphere the South Atlantic, South Pacific and Indian oceans spin counter clockwise.

The most direct course home would have been to motor into the trade winds in the southern part of the gyre, but since we need to sail to get home we must go north and find the westerlies which blow to the east. Once we go far enough east we will make a right hand turn and will have to pass again through the center of the high pressure system which is blocking our path to Hawai’i. The light and variable winds from the gyre will eventually hook us up with the trade winds and we can use them to hastily return us to Oahu.”

The Corilolis Effect is the driving force behind the current and wind systems Joel described above. In the most basic sense, the Coriolis Effect is a result of the west to east rotation of the earth. This means that air flow is affected by this motion. For example, an object on a trajectory directly from the North Pole directly south would be deflected to east. In the southern hemisphere, an object traveling from the South Pole northward would be deflected to the west. Air pushes water to create surface currents, thus the deflection I just mentioned from high pressure systems, combined with boundaries created by earth’s landmasses encourage the dominant currents, like those described by Joel to maintain their clockwise (northern hemisphere) and counterclockwise (southern hemisphere) rotations.

Furthermore there are several convection cells of air resulting from the combination of the rotation of the earth and the differential heating of the earth. So the sun hits different parts of the earth at different angles. The more direct the angle of sun exposure, the more that surface will be heated. This is why it is so hot along the equator, where the sun hits directly for much of the year, and so cold towards the poles where there is little direct sun exposure. This differential heating results in the rising and sinking of air. Cool air sinks, warm air rises. So as air is sinking and rising (on the macroscale) as well as being subject to the Corilolis force dominate wind patterns, such as the Tradewinds (named for the reliability of the winds for trade) are established.

On another note, reports from the wildlife front: Christiana dissected two fish today-a large flying fish (we think it’s Hirundichthys albimaculatus based on the key we have on board) and the first tuna of the trip (it was an Albacore). No plastic to report in either of them.

Two nights ago we were visited by a half dozen friendly Leach’s Petrels. Well not so much friendly as they were attracted to our running lights. It’s a dangerous situation for the birds to be hovering so close to the moving vessel, there are far too many lines and hard surfaces for them to maneuver around. But it’s also not an option for us to shut off our running lights-that would put us in a lot of danger. The story ending happily though-none of the birds were hurt and we were entertained by their spastic flapping and their almost monkey-like chirps.

We are still finding debris, although it is significantly sparser. The areas we are transiting through at the moment are thought to be contributing waters and not accumulation zones of debris. Visual observations from the crew confirm this theory-although the waters have been quite choppy which makes anything but the large and prominent debris very difficult to spot. Bottom line, we still encounter debris after a few minute stint on deck and we have yet to pull in a trawl (we’ve done 42 so far) without plastic fragments.
From the Pacific,

1 comment:

Anonymous said...

Are you guys returning soon? Your trip has been awesome to follow, and Drew's writing and photographs are phenomenal. Looks like the density of debris is consistent across the gyre. I'm really curious to see what the densities are. Best of luck through the last few days of your voyage.

Come home safely,