Saturday, 15 October 2016

Global Drifter Program(me)

Over the last two days, I have been in a technical workshop with a range of contributors to the Global Drifter Program. Drifters are deployed around the ocean, and most measure sea surface temperature (SST), surface atmospheric pressure and (via their changing position) the speed of ocean currents. The data are transmitted back, mostly hourly, and shared around the meteorological and oceanographic centres as inputs to forecasts of the weather and ocean conditions. There are around 1400 drifters working in this way around the world as I write (see below).
Drifter status on 10/102016. For update to most recent image from the GDP, click here.

This in situ system is highly complementary to the satellite system, and both are needed to meet the needs of meteorology and understanding of climate. One of the presentations at the workshop was by chief of a group in NOAA that generates near-real time SST products, by empirically correlating radiances from meteorological satellites at drifter SST locations with the drifter SST measurements. This is a very effective means of using satellite data effectively and rapidly to interpolate the drifter SSTs spatially, capturing the fronts, eddies and structures across the global oceans that can't be deduced from ~1400 point measurements.

We had the chance to see the Scripps Institute of Oceanography's lab for assembling their drifters, and learned a lot about the detailed thinking and experimentation that goes into creating drifters that can operate in the harsh environment of the ocean. Tiny design decisions are crucial to the success of their deployment.

Picture of drifter from our visit, showing where the SST sensor sits. In (rare!) calm conditions at sea, the water line is by design around the transition between blue anti-fouling paint and the white paint (to minimise solar heating) on the hull. So the drifter measures SST at a depth of 17 cm in a dead calm, or a little deeper once biomass starts to accumulate on the perennially submerged components, weighing the drifter down.

In SST CCI, we don't try to blend satellite and drifter data. Instead, we treat the satellite and drifter array as two independent systems that can tell us about marine climate change. If the measurements made by two completely different technologies (remote sensing and in-water thermistors) tell us the same story of change then this greatly increases our confidence that we have accurately quantified the changing marine environment.

That is the importance of our figure in the last assessment report of IPCC, below.

Change in global mean SST temperature in satellite data only (red lines, derived from Along Track Scanning Radiometers) and in in situ measurements only (ensemble of black lines, mostly from drifters). From Chapter 2 of the fifth IPCC assessment report of the physical basis of climate change.

Perfect agreement is not expected, since all measurement systems have some level of uncertainty. Nonetheless, it is clear that these independent datasets agree closely about a lot of the year-to-year changes in the overall temperature of the oceans, and about the general rate of change.

The satellite-based curve has been updated within the current SST CCI project, and we are still working on further extension back in time. A previous blog discussed how, last year, some of the US datasets blending various data sources were revised in a manner that brought them into closer agreement with these results, obtained a few years earlier.

The discussions in the workshop over the past two days have been very informative. We 'satellite folks' learned a lot about nature of the drifter array from the manufacturers and deployers of the drifters.  This will greatly help us when we use drifters to validate the uncertainties we attach to satellite SST data, for example. Turning the raw data from either system into a curve describing global change is tricky task. We all now appreciate better the challenges faced when extracting useful information, needed by society, from both satellite and in situ systems.

Thanks to Luca Centurioni and Lance Braash for hosting us all and showing us around -- and to all the manufacturers of drifters who gave their time to explain their work and listen to how the satellite community use their data.

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