January 2011 Shark Bytes
Posted on Monday, 31 January 2011
I can hardly believe that we are already at the end of January 2011! We have had a very tough time with the wind this month but even so we have managed 5 pelagic shark trips. The weather seems to be seriously affected by La Nina. This is quite interesting so you’ll find some info on this below, for the scientifically minded. Chris & I have also signed up to join an expedition to look for a snow leopard in India this February so you’ll find some info on this trip below as well.
Pelagic Shark Trips
After returning from a trip to Botswana in December we were revitalized and ready to get back out to the creatures of the ocean.
Almost as if a warning nature said slow down, catch your breath and relax. A year going flat out all the time drains you and if nothing else the incessant gales and wind we have had since our return has at least allowed us time to spend with family, friends and most importantly our own animals.
We did manage a number of pelagic trips which have yielded high numbers of blue sharks, but low numbers of makos. The reason for this is probably two fold, the first being low numbers of tuna and game fish, so not much prey. The second being the very cold 10-13c up welled water that has pushed into the area we normally works which is typically 19-23c. On one trip we had to travel a massive 32 miles from Cape Point, our longest pelagic trip ever! And even so we only ended up finding 15 C water. I can tell you we were all very surprised when we ended up with 15 blue sharks throughout the day as we thought this had all the ingredients for a “no show” trip.
The Effects of the Gale Force South East Winds
We have never had so much wind in December/January as far as I can recall and it is this wind that causes major upwelling offshore of land masses and in turn chills the surrounding water as cool subsurface water moves in to take the warm waters place that has been blown offshore. Currents and winds then distribute this water into the area where we work.
The following is a possible explanation for the increased wind and also just how severe it has been. If you are not that interested in the science behind it all then skip the next paragraph or two.
Whilst the average temperatures of ocean currents are the general measure to see whether we are going into an El Nino or La Nina cycle this can be confusing and apparently is not the accurate way of doing things. The following excerpt is from Neville Nicholls a professor in climate research in Australia which is having the worst floods in over 100 years. http://www.abc.net.au/unleashed/neville-nicholls-42862.html
Secondly, the general ocean warming we have seen over the past 50 or so years, due to anthropogenic enhancement of atmospheric greenhouse gases, confounds the use of these temperatures to compare a recent episode of cool temperatures with cool temperatures earlier in the record – the global warming may have offset some cooling associated with the strong, recent La Niña. This would bias any comparison between the 2010 event and earlier events, prior to the strong global warming of the second half of the 20th century.
But we can use the Southern Oscillation Index, or SOI to compare the strength of La Niña episodes across time. The SOI is the standardised difference in surface atmospheric pressure between Tahiti and Darwin. Monthly SOI values are available at www.bom.gov.au/climate/current/soihtm1.shtml. Positive values of the SOI (low pressures at Darwin and high pressures at Tahiti) indicate a La Niña event.
There is no a priori reason to expect that global warming has necessarily led to long-term SOI changes that would confound our results if we use the SOI to compare historical and recent La Niña events. And values of the SOI are available from the end of the 19th century.
The SOI values confirm that we are in the middle of either the strongest La Niña event on record, or the second strongest. The SOI values for October 2010 and December 2010 were each the largest positive values on record for those months, as was the three-month average October-December 2010. If we take a longer perspective (July-December) then 1917 was stronger than 2010, but 2010 was still the second strongest in the historical record. Using either the October-December or the longer July-December periods, the strong La Niña events on 1973 and 1975 were both ranked as weaker than the 2010 event.
So basically we have exceptional conditions and right now and as I write this the wind at Cape Point is a breezy 56knots. It was 60 knots or more (up to at least 84knots) at least on a dozen days in the past month! Whilst warm water currents continue to flow along our East coast the strength a duration of wind has led to unprecedented amounts of up welled cold water being blown into our work area.