Broken, loose, slack, rotten, soft, solid and impenetrable

They don’t mention what it sounded like, but the log of the Thetis does say a lot about sea-ice. This isn’t surprising: they were sent to rescue the Lady Franklin Bay Expedition in 1884 because two previous resupply expeditions (in 1882 and 1882) had been blocked by the sea ice, and one ship (the Proteus) had been crushed by the ice and sunk. So manoeuvring through the ice was their major challenge.

We’d like to compare the Thetis’ observations with modern observations of sea-ice in the same place – as we do with temperature, pressure wind etc. This is a challenge, because modern observations come mostly from satellites; they are precise and quantitative and tell us the fraction of the sea (in each region) that was covered with ice on each day. Thetis’ log, on the other hand, contains statements like ‘working through slack ice and broad leads’ or ‘forcing a lead through heavy pack ice’; is this more or less than ‘75% coverage’? The best place to start is generally just to plot the data, so the video below shows the movements and ice observations we’ve taken from the log, and a modern satellite ice climatology (HadISST).

We can tell quite a lot just from this: it looks as if they first met serious ice around Disko island, on May 22nd, while the modern estimates suggest that we’d expect this a little further north. But in general they are seeing ice in much the same times and places that the satellites do.

It’s not quite enough to compare the ship records with a single satellite field. Sea-ice severity varies from year to year, so we’d like to compare Thetis’ experiences with a range of modern years, not just an average year (a climatology).

Iceberg plot for USRC Thetis 1884

Thetis’ ice observations (black text) compared with max, mean and min coverages from HadISST 1979-2004 (light, mid, and dark blue curves)

I’m calling this an iceberg plot. You’ll need to look at the bigger version, but it shows that the period where the ship made observations of ice fits pretty well with modern expectations, but that those observations generally imply heavy ice cover, and the modern observations suggest heavy ice cover only in particularly severe years. So we can tentatively conclude that, in summer 1884 in Baffin bay, there was more sea-ice than usual (by modern standards) but that the coverage was within the expected current range of year-to year variability. This is consistent with their air temperature observations, which were colder than the modern average, but not dramatically so.

A remaining oddity, however, is the time of year of this voyage. Thetis set out in May, and reached her farthest north in June. Sea-ice cover in Baffin bay is still heavy in June, and doesn’t reach its minimum until September. If they had waited two more months before setting out they could have reached their destination with much less effort (look at the ice-cover at the end of the video, when they are back in New York). Thetis could not wait without leaving the men they were rescuing to starve, but the resupply missions in 1882 and 1883 could have travelled at more favourable times of year. If Thetis could get there in June 1884, but Neptune, Proteus and Yantic could not get there in August of 1882 or 1883 – does that mean that 1882 and 1883 were remarkably heavy sea-ice years?

I wonder if the National Archives have the logs from the voyages in 1882 and 1883.

Trackbacks / Pingbacks

  1. Ice station Jeannette | Old Weather Blog - August 8, 2013
  2. Melt season | Old Weather Blog - September 10, 2013

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