The Drake Passage, the narrow passage that extends from the tip of South America to the tip of the Western Antarctic Peninsula is well known for treacherous sea conditions. Many sailing ship’s of past centuries ended their careers while sailing around “the Horn” (Patagonia) and into the Drake. The past 24 hours has seen the kind of wind and seas for a sustained period of time that must have caused sailors grave concern. It has certainly cost us valuable “ship time” (the number of cruise days alloted to our project by the NSF). We have been standing by near our Stn #19, which is at the western end of Bransfield Strait, waiting for weather and sea conditions to moderate to a level where we can carry out our planned work. Similar to most stations, we want to do a series of observations and sample collections: CTD cast to 1000 m, MOCNESS tow to a 1000 m, IKMT to shallower depths, and an acoustic “Towfish” as we steam toward the next station (see ours blog s for Nov. 10th, 17th, and 18th).
|Figure 1. Wind speeds between 30 – 50 kts, barometric pressure dropping, and other weather data shown on the ship’s DAS (Data Acquisition System) screen for the previous 24 hrs from 11:00 am (1300 GMT) Nov. 19th.|
For the past 24 hours, we have had sustained winds between 30 and 50 kts out of the East (Fig. 1). The barometric pressure had been up around 1004 mb 2 days ago, when we had flat seas, clear skies, and excellent working conditions. It had dropped to 969.2 mb by this morning and seems to be bottoming out. This may be the longest stretch of high winds and seas we have had this cruise.
In planning our cruise, selecting our station locations, and laying out the cruise track, we assumed Bransfield Strait would be protected from the prevailing winds, which are usually out of the N-NW. We expected the early Austral Spring would be stormy, but we did expect that stations in the lee of the islands would be workable despite high winds because of the short fetch (distance the wind blows across the water to whip up the waves). Instead, there is an intense low pressure to our North, and the winds are coming from the East and blowing directly down the Strait. The long fetch has built the large swells that we are experiencing now.
So where did this severe weather originate? The isobar images that we get daily on the ship provide an answer. The low that settled in over the Drake Passage formed as a relatively weak low in the South Pacific Ocean off the west coast of South America on Nov 15th. It began moving SE towards the Drake Passage on Nov. 16th (Fig. 2A). The system intensified while still West of the southern tip of South America on Nov. 17th (Fig. 2B); it moved into the Drake Passage and pushed our good weather off to the East during Nov. 18th (Fig. 2C). The latest image shows the low pressure intensified, with barometer reading of 976 mb at the center (Fig. 2D).
|Figure 3. Forecast wind speeds and directions for the period 19-20 November 2011. Our area (Stn 19 is marked by the yellow dot) is forecast to have light winds for the next 24 hours or so. Note the red LMG dot on the chart is an old LMG position.|
-- Peter Wiebe, Woods Hole Oceanographic Institution