Active vs. Break Monsoon

A week ago, during a dark night, a rainstorm didn’t just rain, but swept sheets of water across the deck.  Recording a peak rain rate of 140 mm/hour, or almost half a foot of water, we think the ship was thoroughly washed.

monsoon_rainsS

Yesterday we had a rare half-day of clear skies, instead of the usual hazy cover of humidity. Clouds quickly grew in the morning, sheared and swept away by the surface southwesterly monsoon winds. The wind and waves have been weakening over the past couple days, though it is still not calm.

cloud

Pitch, Roll, Rain

Do you picture the tropical seas as placid glassy surfaces? Not quite what we are seeing!

The current conditions in the Bay of Bengal are on the rougher side, for the tropics. (Although certainly not close to conditions during a cyclone or Southern Ocean storm or wintertime North Seas… and it’s still muggy hot!)

The Monsoon “waves” or oscillations appear in 10-14 day active periods where rain can be heavy, sky very cloudy, and winds intense. In between are somewhat longer periods called “break” periods where rain is much less, its sunny and hot, and winds can often be low.

Well, we certainly have been in an active period for last 3-4 days, and will remain active till about June 13-14th here in the Central Bay of Bengal! We are near 14°N, 88°E .

The winds have been consistently hovering around 30 knots (30 nautical miles per hour is about 54 km/h) with occasional wind gusts of 50 knots (90 km/h). The wave heights have been above 6 m almost all day today, touching 7 m. Those are some significant numbers – a 7m wave height is about a 21feet high wall of water.

The ship rides very well, and the scientists and the crew are in good spirits. Two days ago we saw a rain squall which lasted for maybe 20-30 minutes with a rain rate exceeding 140 mm/hour at the peak heavy rain! In other units = drenched in 5 seconds. This is all good news in terms of the scientific data we are collecting.

rvthompson_pitching

Ballons [Balloons! pardon my french]

How fast are the winds blowing, high above us? How warm are they, and how much moisture do they carry? The best way to find that out is with a balloon! We attach a thermometer, a humidity sensor, a GPS tracker, and a radio antenna (all integrated in a small lightweight box) to a helium-filled balloon, and release it. The instrument (called a radiosonde) rises up ~300 meters every minute, into thinner and colder layers of the atmosphere, often reaching up to 20’000 meters (65’000 feet) above us. The high-quality latex balloon expands up to 4-5m across, before it eventually pops! (and invariably falls somewhere..). The sonde transmits data, valuable especially over the oceans, back in real time.

Mooring adrift

How much heat does the ocean supply to the atmosphere? And how much water evaporates in the Bay of Bengal? Both are critical for the dynamics of the monsoon, and the rainfall over land. One of the instrument platforms deployed to answer these questions is a drifting “mooring”: a floating buoy tethered to a long line with a weight at the end. The buoy is decked out with gauges measuring weather conditions at the surface. Below the water, attached to the long line, are instruments measuring ocean currents, temperature, salinity, and turbulence at many different depths, from right at the surface to 200 m below. Normally such a mooring is held in place by a heavy weight sitting at the bottom of the ocean (often old railroad wheels!) for a year. However this mooring needs not to stay for an entire year, and also not stay precisely at one point. So a much easier solution was found: instead of anchored to the seafloor, it is “anchored” to the slow currents at 200 m depth, by large x-shaped wings that get dragged by the water. So even if the winds are howling at 15 m/s, or 30 knots, the buoy won’t be sailing away, but is stuck in place with the much slower moving deep ocean. Check out its current location and live data, courtesy of the Upper Ocean Processes group at the Woods Hole Oceanographic Institution: http://uop.whoi.edu/currentprojects/MISO/misodata.html