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.

Thursday, 25 February 2016

Sea surface temperatures in 2015

For SST folks, 2015 was an interesting year. There was the flurry around a certain paper in Science and of course the emergence of a major El Nino event. In SST CCI, the focus is to do a careful job on historical records. But, with some support from the National Centre for Earth Observation, Owen Embury has done a quick and rough (for us!) look at the SST record up to 2015. This builds on our recent experimental reprocessing, but brings the record more up to date by using some short cuts. (These will be superseded by the culmination of the SST CCI project, so these results are provisional, and just posted for interest.)

First, the global average sea surface temperature change over time since 1991, as shown below:

We see 2015 as the warmest year for SST, with a marked increase since 2012. A similar sharp rise was seen in 1997/8, which was also the result of a major El Nino event. As then, it is likely that the global SST will drop down again during 2016 to temperatures more typical of recent years. 

The warm water associated with the El Nino event is obvious right across the equatorial Pacific in the map for December 2015, corresponding to the last point on the time series above:

We have the recent El Nino as being the record event in the classic El Nino "3.4 region" -- although the margin compared to the 1997/8 event is close enough that we won't be confident of its record-holding status until we have done the proper job on SST CCI reprocessing:

In contrast, the anomaly in the "4 region" is clearly stronger in our data than for the earlier event, indicating that the hot action was located further west than in 1997/8. 

On a final note, it is a pity that the next dual-view instrument, Sea and Land Surface Temperature Radiometer (SLSTR), missed the action of 2015, but in the project we are all very pleased that it has successfully been launched. Looking forward to working on the data.

Friday, 29 January 2016

Paid internship looking at industrial thermal plumes in coastal waters

Elevated temperatures from use of seawater as coolant by large coastal installations can be seen in high resolution thermal imagery.

Note that the SSTs in this image are not quantitatively correct. The image is based on LandSat8 and the thermal channels unfortunately have calibration issues that prevent quantitative water surface temperature determination. However, it is interesting to look qualitatively at the data, since it is at ~100 m resolution the in infrared bands, and resolves features like this that are not seen in 1 km data typically used for SST.

EDF UK are offering a student summer paid internship for a project working with me to scope the potential in principal for thermal remote sensing at high resolution to help in monitoring and understanding the physical and ecological impacts of coolant plumes from power stations around the UK. 

It would be excellent to have well calibrated multi-channel thermal resolution observations at ~100 m scale resolution for applications like this and also remote sensing of lakes. It is good that there is interest around Europe in extending the capabilities of future Sentinel 2 missions in this direction. 

Closing date for internship applications: 8th February. Don't delay!

Image credit: Shaun Lee, University of Reading. (Thanks, Shaun.)