Seals as Probes - Revisited

As per my last post, I was very happy that Chris Benjamin wrote up a considerably better researched version of my very first blog post for Science Friday last year regarding the use of seals in climate and weather monitoring/research. As I recently was able to attend a seminar on the updated results from the seal project I thought this would be a good opportunity to explore the subject a little further.

  Something that was perhaps missing from my original post is the importance of monitoring ocean conditions. Ocean observations are very important in terms of measuring their heat uptake. According to the IPCC Assessment Report 5 the top 700m of the oceans take up 93% of global warming heat content (see below).
 
Given this, you can perhaps see why it might be interesting (and important) to study the Antarctic circumpolar current (ACC). This is the most important current in the Southern Ocean and the only current that flows completely around the globe. Incidentally, it was discovered by Edmund Halley, the British astronomer (who says astronomers have no real-world use?). The ACC is equivalent in flow to all of the rivers in world combined (Chidichimo et al, 2014).

Measuring the temperature and salinity of the water in the ACC, in addition to other, less well-travelled water has become easier and more widespread with the ARGO float system. However, in order to take the same measurements beneath the variable sea ice which extends beyond the land it is necessary to recruit the services of seals, as covered by myself and Chris Benjamin.

The seal measurements allow forecasts to be made for the Antarctic area, this is particularly useful for military pilots who, under certain circumstances, must where survival 'dry suits' when flying. These suits are very uncomfortable, particularly in cramped cockpits and so knowing whether or not they are necessary is useful information for the pilots!

Mapping ocean currents, particularly in a predictive sense, can be immensely useful in a 'man overboard' search and rescue mission. The disappearance of flight MH370 last year is another example of the importance of this information.

The measurements made by the seals, or rather by the instruments attached to the seals, are made on the upward portion of the seals foraging dives. The data loggers consist of a pressure sensor (in addition to other sensors) which is on constantly, this notes when seals begin to ascend from their dive and alerts the other sensors to turn on and begin measurements. The record dive of an Elephant seal is over 2000 metres, over four times the average of 500 metres. It's not really understood how such deep dives are possible, although the seals have ribs made of cartilage which means that they can collapse their chests, the impact on the seals' blood chemistry is still very significant.

Dive profiles consist of seventeen points which are split into four messages, the transmission of which is attempted when the seal surfaces. A minimum of 160 seconds is needed to send these four messages via the Argos satellite tracking system, unfortunately the mean elephant seal surface period only lasts for 130 seconds, meaning that transmissions are often incomplete, resulting in poor positional information and/or incomplete profiles of the water temperature and salinity. An alternative to transmit the messages via the Iridium satellite constellation exists but depletes the data logger batteries at an increased rate.

  By running reanalysis experiments of past weather conditions the usefulness of the seal observations can be evaluated. It turns out that incorporating the temperature and salinity measurements tends to lead to an overestimation of the water salinity. This is difficult to verify without independent observations (of which there are none) but in the Kerguelen region where there is some overlap between ARGO floats and seal data it was found that the ARGO floats had the opposite bias - where the sea measured fresher than predicted by the ocean model.

  It is notable that the primary difference in instrumentation between the floats and the seal data loggers is that the former use capacitance to measure salinity while the latter use inductive conductivity sensors. The seal sensors are found to have a high bias (as seen) when the sensor is close to any surface (like a seal for example...). This effect can be accounted for by calibrating the sensor for each individual seal, unfortunately this requires a long data baseline of around a year. At which time the sensors are falling off the seals in any case!

  All this means that the salinity profiles obtained from the seals are partially useful for research purposes but can't really be used for meteorological forecasting.

  What is lucky is that even incorporating only the temperature profiles from the seals data loggers results in a significant improvement for temperature and salinity forecasts, as well as improvements in estimates of global temperatures, extending far beyond the regions sampled by the seals.

It has been noticed that the location and strength of ocean fronts are impacted in models by the seal-measured temperature. However, without good quality independent observations it is difficult to assess whether this is a beneficial change or not.