Prediction from the University of Sheffield

Will the RAPID team find more large amplitude overturning excursions?

By Grant Bigg, Jose Roberto Ayala Solares and Hua-Liang Wei, University of Sheffield

Reconstructed  (blue) and predicted (red) AMOC time series.

At the University of Sheffield we have been working to reconstruct the time-series of Atlantic Meridional Overturning Circulation (AMOC), based on a control systems modelling approach using our knowledge of changing oceanic and meteorological conditions over the North Atlantic and neighbouring seas. By developing such a model using validation with the observed AMOC series over April 2004 to March 2014 we have produced hindcasts of the monthly variability of the AMOC from April 2014 until September 2015.

Our reconstructed AMOC time-series (BLUE line) reproduces both the disturbed annual cycle and the long-term trend of a decreasing overturning, suggesting that the combination of the ocean density and atmospheric circulation fields contain the underlying forcing factors producing the AMOC.

Our reconstructed AMOC time-series includes a prediction (RED line) out to September 2015. Given the ability of the model to reproduce the previous 10 years of observations of the AMOC, we therefore make the following three predictions:

1. The RAPID observations collected this autumn will show a mean overturning which is similar to that of the previous 5 years, with a value of ~ 16 Sv.
2. The observations will not show a significant anomaly such as was seen in early 2010, and indeed the maximum AMOC of the collected data will occur during November-December 2014, although we also predict a late maximum in September 2015 as well.
3. Despite the mean AMOC being of a similar level to recent years there will be short-lived excursions, both positive and negative, leading to maxima and minima approaching those of the previous decade’s observations. We have already given our predicted timing of the greatest positive excursion around the end of 2014 and the smallest value is expected be in mid-summer 2015. Our model suggests the latter to be ~ 80% of the mean AMOC.

How the predictions were made

We use Nonlinear Auto-Regressive Moving Average with eXogenous inputs (NARMAX) system identification modelling to produce a model for the observed variation of the AMOC using two large-scale environmental variables as inputs. The first use of this technique for environmental sciences can be found in Bigg et al. (2014). To represent atmospheric variability we use the North Atlantic Oscillation index, while for oceanic variability we combine a linear measure of the surface density of the northward-flowing Gulf Stream with a linear measure of the likelihood of deep-water formation through the surface density of the Labrador and Norwegian Seas.  The surface densities were calculated from the GODAS ocean reanalysis sea surface temperature and salinity. 

The NARMAX system identification model uses a forward regression orthogonal least squares algorithm to build models term by term from recorded datasets. This is achieved by using the Error Reduction Ratio (ERR), which shows the contribution that each selected model term makes to the variance of the dependent variable  (the observed AMOC here) expressed as a percentage, taking account of the noise in the data. The NARMAX method searches through an initial library of model terms, which typically includes linear and non-linear lagged variables, and selects the most significant terms to include in the final model. The model in this instance leads to 47 terms, with those having the greatest ERR representing quadratic terms of the oceanic input with time lags between 0 and 8 months.

Bigg, G. R., H. Wei, D. J. Wilton, Y. Zhao, S. A. Billings, E. Hanna, V. Kadirkamanathan, 2014, A century of variation in the dependence of Greenland iceberg calving on ice sheet surface mass balance and regional climate change, Proc. Roy. Soc Ser. A, 470, 20130662, doi:10.1098/rspa.2013.0662.

We’ve finished!

45 days at sea and we’re now in Nassau. The final moorings were recovered yesterday after a short hold up due to the weather. Now the work on the data starts in earnest and we calculate how the AMOC has behaved over the last 18 months.

It’s been a long trip but successful and now we can relax briefly before flying home!

 We'll leave you with a poster from our in-house artist (Gerard).

What Jason has been working on - ADCPs

Shenjie (or Jason) gives us a summary of what he has been spending a lot of time working on during this cruise. In addition to helping with completing logsheets during mooring recoveries and deployments and detaching and downloading instruments from the CTD frame in the small hours he has been in charge of processing data on the currents measured beneath the ship as we steam along.

Shenjie Zhou – what I have been working on

RAPID cruise dy039 is the very first official research cruise that I have ever joined in my whole life. The RRS Discovery sailed from Southampton on the 17 October 2015, heading to Tenerife for our first clearance (and a little maintenance).  After Tenerife, we started our journey deep into the subtropical Atlantic Ocean to recover the RAPID moorings deployed 18 months ago and deploy new ones. Every mooring consists of a beacon and light for emergency recovery, glass floats providing enough buoyancy for the moorings to keep all sensors at their designed depth working properly, MicroCAT (CTD) sensors, different models of current meters and acoustic releases at the bottom of each mooring attached with anchors.

Meanwhile, two vessel-mounted Acoustic Doppler Current Profilers (vmadcp) are working constantly to record the horizontal velocities of the ocean current flowing below the ship. These two vmadcps are mounted on the starboard side on the fore-bottom of the ship.

 

The vessel mounted ADCP transducers (photo taken in dry dock)

The coordinates below each sensor are referenced to the centre of the ship. Two instruments work on different frequencies; 75 kHz and 150 kHz. The former has a better penetration into the water to around 700m depth while the latter one reaches a shallower depth (400m) with an improved vertical resolution. Four beams are launched from the transducers with a beam angle of 30º.

Data measured by vmadcps are recorded via the vessel-mounted Data Acquisition System (VmDas) software installed on two PCs in the main laboratory.  The recording is stopped everyday for the software to make a copy of the recorded data onto the ship server for daily processing. Every time the software is restarted, a new sequence will be created as a tracker on this routine event. All the operations and commands made on the PCs in main lab will be sent the deck unit of the vmadcp system. These boxes then send signals to the transducer to execute the commands from the PC terminals. The top one is for the 75 kHz instrument and middle one is for 150 kHz, the bottom one is a backup for 75 kHz.

A typical view of the screen of the PCs controlling the ADCP measurements

The measurements from vmadcps when in deep water are calibrated during processing from the bottom track data collected during shallow water transits. Bottom track is basically to measure the heading (position) and speed of the centre of the ship as measured by the ADCP when referenced to the sea floor. It provides the correction estimates on the amplitude and angle for the vmadcp measurements when the bottom cannot be “seen” within the range of the instrument.  Constrained by the maximum bottom track range, bottom track mode can only be working functionally in shallow waters (within the maximum range and also depending on the setup for each instrument).  

The ADCP deck units

 

Apart from the vessel-mounted ADCPs, we also deployed one moored-version ADCP with one of our moorings at the normal offshore extent of the Deep Western Boundary Current (WB4) to collect currents data above the mooring which otherwise would be missing when the mooring is knocked down in strong currents. Two more are deployed at the continental shelf-edge (WBADCP) to measure the currents inshore of WB1.

A self logging version of an ADCP being prepared for deployment before being mounted in a float

Discovery Friday (Captain Scott's Discovery)

As I write this final instalment of Discovery Friday, we are hove to, waiting for weather to pass to finish our work on the RAPID moorings. The waiting is frustrating but should only be an extra few days—nothing in comparison to the original RRS Discovery, which spent an extra year trapped in ice in Antarctica waiting to be free. 

Captain Scott built his ship from donations to the Antarctic expedition. It was the last wooden ship built in Britain, built by the Dundee Shipbuilder’s Company and launched in 1901. Discovery was built for life in the ice of Antarctica. She had a flat bottomed hull and bow designed to ride up over the ice to break it with pressure. She had a steam engine but relied heavily on sail power to provide sufficient propulsion. The propeller and rudder could be hauled out of the water to avoid damage. Built for the ice, she handled badly in open water and was prone to large rolls.

Scott and his crew set sail in August 1901 and sighted Antarctica in January 1902. They spent a month mapping the coastline before anchoring in McMurdo Sound. They would stay there, trapped in the ice for two years. The idea of being confined in a 50 metre long vessel for two years, trapped in ice, with 47 men would not be something I can imagine enjoying.

The expedition was a success remapping the location of the magnetic South Pole and achieving a record furthest south point of 82º 18’. The ship broke free of the ice in February 1904 and arrived back to the UK in September. The expedition was a success.

The ship took part in various roles for the next 20 years before returning to research and exploration in 1923. It was at this point she was christened a Royal Research Ship, really beginning the lineage that our current ship continues today.

Much like Discovery II would do, this original RRS Discovery spent much of her time in the Antarctic mapping whale populations. This formed part of the Discovery Investigations from which started the Discovery Collections, samples from which are still being used for scientific research to this day.

There are a number of photos and paintings of this old ship on the new RRS Discovery, including two nice watercolours in the conference room. My favourite, however, is in the mess, taken from Discovery’s time as a research vessel rather than her more famous time as Scott’s Antarctic exploration vessel. It shows two men taking sampling with nets over the side. They are clad in gear that you might expect more to be wore by someone on safari in the early 20^th Century. Perhaps these were the precursors of the hard hats and safety footwear we wear today. It is a nice picture to finish on as it completes the lineage of scientific research from the original Discovery through to our current home.

Written by Gerard (posted by Darren - slightly late due to a slow connection!)

 

Meet the science party (part 8)

Meet the science party (part 7)

Discovery Friday (Discovery II)

For me, the most enigmatic of all the Discoveries is Discovery II. I have sailed on the two latest editions of RRS Discovery and read about the first RRS Discovery with Captain Scott in the Antarctic (more about this next week). But Discovery II, built in Glasgow in 1928, is the ship I know least about. It was interesting then that Discovery II features so heavily in the photographs on the bulkheads in the new Discovery.

Discovery II - The first purpose-built research ship

Discovery II is the only ship in the series that seems to have kept its number in the sequence—we don’t refer to our current home as Discovery IV, just Discovery. When it was built, it was the first purpose built research vessel in the world. She was 80m long with a single screw propeller that could notch up a dizzying 13.5 knots! By contrast, the current Discovery has a spec of 12 knots.

Discovery II spent much of her life working in the Southern Ocean. At the time, there was much focus on the whale trade. This lead to a lot of interest in the Southern Ocean. Discovery II did much work on mapping whale populations and krill abundance. She also gathered valuable hydrographic and chemical data from these infrequently visited waters.

The drama of sailing around Antarctica is captured in a number of the photographs on board here. There is Discovery II trapped in the ice while a crewman lies on the ice, ably viewed by 17 other seamen! Or Discovery II sitting in a bay surrounded by snowy mountains. Penguins and the skeletal remains of a victim of the whale trade lie in the foreground.

Discovery II in the ice

Providing scientific support for whaling seems very odd for me as a modern scientist. But in many ways this was the origins of British oceanography. In her later years, Discovery II moved northwards. One of her final contributions was a transatlantic hydrographic section that formed part of the International Geophysical Year in 1957. The important data gathered on that expedition was used near the start of the RAPID project. And formed one of the five hydrographic sections analysed by Harry Bryden and coauthors in a paper in the journal Nature that indicated the Atlantic overturning circulation had slowed from 1957 to 2004. This study provided great motivation for the RAPID project, thus linking Discovery II with what we are doing here and now.

It reminds me of a great quote from Prof. Carl Wunsch in MIT about making observations of the ocean or any part of the climate system. He says that “adequately sampled, carefully calibrated, quality controlled, and archived (observational) data for key elements of the climate system will be useful indefinitely.” And surely this has proved true for the scientists who sailed on Discovery II. 

Discovery II in Antarctica with penguins in the foreground

Written by Gerard (posted by Darren)