We continue to ride out the bad weather which has once again prevented all scientific work. The storm, which reached its peak on Thursday morning, has passed. However, although the wind has dropped significantly, a considerable swell still remains.
Last night most had a poor nights sleep owing to the roll of the ship in the 7-8 metre swell and
occasional larger waves. A popular method of preventing yourself being thrown out of bed is to put a life jacket under one side of the mattress. I tried this last night and had a relatively sound sleep though I did wake this morning to find my floor littered with the contents of my shelves and much of what was on my desk!
As there is no scientific work to report on I thought I’d write about the storm and the strategy of riding it out. The first signs of the approaching storm came on Tuesday 31st July when we experienced a long low swell from the south west which wasn’t accompanied by any wind or rain. This swell was being generated by a depression (a region of low pressure) to our north. Officers on board began monitoring this depression via synoptic charts (weather maps), provided by the Ocean Prediction Centre (updated every 6 hours), and data from satellites. Using this information the path of the storm was tracked and its course predicted. The swell continued to increase in size and on the Wednesday 1st August the Barograph started to decrease rapidly. The Barograph (see Fig.1) plots barometric pressure over time. A rapid decrease in pressure indicated we were moving under the depression. At midnight on Wednesday wind speeds began to increase from about 10 knots to 15-20 knots as pressure decreased further.
The pressure “cusped” on Thursday morning at about 09:00hr (it curved and started to increase again). During storms the strongest winds come as the pressure “cusps” and is accompanied by a change in wind direction. The wind changed from SSW to WSW accompanied by winds of 35-40 knots and then 50 knots with occasional gusting of 55-57 knots (~65 mph). As the pressure continued to increase through Thursday and during the course of today, wind speeds died away and are now (Friday afternoon) about 25 knots.
High wind speeds generate large waves; in this case the swell was about 8 metres on Thursday with occasional 10-12 metre waves. The strategy of riding out a storm is to point the bow in to the dominant wave direction. The ship then moves as slow as possible whilst maintaining speed enough to turn (a rate of ~3-4 knots). This allows control of the ship whilst preventing the vessel from hitting waves too hard and causing damage. As the depression moves and the wind direction changes, officers change the direction in which the ship travels to maintain the wave direction on the bow. Fig.5 shows a plot of the direction in which the ship has travelled since 03:00hr this morning showing adjustments to compensate for the changing wind direction.
Due to changes in wind direction and the persistence of waves in deep water, swell often comes from different directions. This is known as “confused swell” and can cause the vessel to roll…something which it has been doing quite a lot of…soup for lunch was definitely a bad choice!
The duration of the storm and the strategy of riding it out means we have travelled 67 miles from the north eastern station. This evening at 18:30hr we shall turn and steam for this station, arriving sometime before midnight. We will immediately deploy the semi balloon otter trawl (OTSB) and start fishing…at last!!!
University of Newcastle