Monsoon Blues
Posted by: Dr. Will Sager
Monday, July 16, 2007
2:00 PM Indian Ocean
Monsoon Season in the Indian Ocean
Its' that time here at sea. While that usually means lots of rain in South Asia, out here our skies are relatively clear. But the rain isn’t the big problem. It’s the wind. Monsoon season also means strong pressure gradients between weather systems and out where we are that translates into strong southeast tradewinds. Throughout our cruise, we have had strong tradewinds, even near the equator where calmer winds are more typical. Most of the time we have had winds of 15-20 knots, but in the last five days, the winds kicked up to 25-30 knots consistently. Although the Revelle is a big blue and white kite in winds like that, the crew can compensate by steering off the wind. Sometimes we might steam along with the bow pointed 20° off the direction we are heading because the ship is fighting the wind. But it’s not the wind that makes the most problems, it’s the waves that the winds push along. When the winds kicked up to 25-30, the wave height also kicked up. We had been seeing 4-6 foot swells typically, but with the wind came swells of 8-12 feet. Yesterday evening, I stood on the main deck, which is itself 8-9 feet above sea level, and I had to look up to see some swells go by. They must have been 15-16 feet tall.
It is certainly more of a challenge to live on a ship in rough weather, but more importantly, it is also harder to do science. The multibeam sonar does not work well when the ship moves around a lot. That’s for two reasons. First, the sonar has gyroscopes to tell it which way it is pointing, but when the ship gyrates, the outgoing and returning sound waves don’t match up well. The worst problem is that a ship slamming into waves gets air bubbles under the hull and those little air bubbles absorb the sonar sound pulses. As a result, when we are going into the waves in rough weather, the sonar soundings are often erroneous and sometimes the sonar has trouble finding the bottom. One of the people on geophysics watch has to baby-sit the sonar and help it by changing sonar software parameters. So far the rough seas have not had a really bad effect on the seismic reflection profiling, but we did have to leave off the magnetometer on the Site 214 area survey because Lee was concerned that the big swells would tangle the magnetometer cable with the air guns. Moreover, we could see lots of wave noise in the streamer output. We can do some noise reduction processing later, but noise is bad. And you probably already read about the big wave we took over the stern while recovering the dredge yesterday. The captain says no more dredging with the stern towards the waves until the seas come down! We agree; safety is still always the most important thing at sea. (even when it limits our dredging options)
Just living with the ship motion also becomes more challenging.
The ship is moving around all of the time, but in calmer weather you compensate more easily. Imagine that you live in a house with a gremlin below that constantly pointed gravity in a different direction, so that you didn’t know from one moment to the next which way is down. Much of the time you can get a feeling for which way the walls and deck are going next and you compensate. But sometimes the roll is bigger or in a slightly different direction and you go flying into the wall. In rough weather you just have to do everything with the ship’s motion in mind. At 4 am when that big roll sends your stuff flying from the desk to the deck, you keep the ship’s motion in mind from the second your foot hits the deck. Trying to get your pants on? Sitting is also a good idea for that. Going through a door. Wait until the ship tilts that way or you are pushing a spring-loaded door uphill. But be careful when the ship tilts the other way so that you don’t go flying through. It’s actually amazing to watch all of the people who work out here and to marvel that there aren’t more crashes. Just in the crowded mess hall at mealtime people are zigging and zagging with the ship’s motion and it’s rare that there is a mess. It’s like some sort of aquatic ballet – without the music.
You get used to the movement after a while and don’t give it much thought. Sometimes you get irritated that it won’t stop and you wish the swells would go down. In fact, I’m wishing that right now. But when the cruise is over and your reach the shore, you have a new treat in store. It’s called “dock rock.” When you get on solid terra firma, your brain is so used to the ship’s motion that it registers the land as moving too. It goes away after a few hours or so of stability, but it can be a little odd to someone who has never experienced it before.
JULY
7/30 - Final day at sea: on the way into Singapore
7/26 - A Day in the Life of a Teacher at Sea
7/18 - Checking our Position (Latitude) with a Sextant
7/16 - Monsoon Blues in the Indian Ocean
7/15 - WAVES, Rough Weather, but an exciting day... WHALES
7/11 - Not the Best Day for Science at Sea
7/9 - Discoveries on the 90 East Ridge
7/5 - Closer to Home, Life Here Onboard
7/2 - Ocean Careers: Texas to Russia
JUNE
6/30 - Easy Cruising Day: Now in the Southeast Trade Winds
6/27 - We Crossed the Equator Today, Heading South.
6/21 - Night Time Fun in the Indian Ocean
6/20 - Transit Gale in the Indian Ocean
6/17 - Arrived in Phuket, Thailand (finally)
