JC31: 'Ingredients' of the Southern Ocean


Cruise diary

Tuesday 17 February 2009
Location: Drake Passage (62.81°S/63.62ºW)


As we approach the end of the first section of our journey, events have taken a dramatic turn, with a great depression now dominating the environment around the ship. Fortunately this is a depression of the meteorological type, and as yet nobody has been spotted weeping in the lab. The pressure has dropped rather dramatically over the last 24 hours, as shown in the photo of the barograph, with the air pressure being 950hPa at midday local time.

Low pressure systems are typically associated with unsettled weather. The average atmospheric pressure is about 1013hPa, whilst during the great storm of 1987 the pressure in the centre of the depression is recorded to have dropped to 953hPa. Depressions are formed by rising air reducing the pressure at the earth's surface. As the air rises, it cools and condenses, encouraging cloud formation. Low pressure systems are hence also often associated with rain. Winds blow in a direction acting to equalise pressure, and were it not for the effect of the Coriolis force, would blow directly from high to low pressure. (The Coriolis force deflects the wind, which thus flows anticlockwise around a low pressure system in the northern hemisphere, and clockwise in the southern hemisphere.) The pressure contours around a depression are hence tightly spaced, and associated with strong winds. This was the most noteworthy aspect of the great storm of 1987, where gusts of up to 100 knots were measured in the UK.

At the moment, we are experiencing some swell and are waiting for conditions to settle a little before we continue with the final measurements of this leg of the journey. One of the more hardened seafarers on board reports having seen the pressure drop to 930hPa before and we are quite some way off the lowest ever recorded pressure of 870hPa, reported in 1979. We are hoping that conditions will settle calmly, allowing us to finish this section of the journey uneventfully!

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