First week at sea and a short stop in Tenerife

Mooring equipment on the aft deck of RRS Discovery as she leaves Southampton.

The CTD frame being manoeuvred on deck
 before being deployed for the first profile.

Since leaving Southampton the RRS Discovery has made its way south to the Canaries.  We have not yet arrived at the first of the RAPID moorings but all on board have nonetheless been busy getting ready for the work ahead.  Each time the RRS Discovery starts a new expedition the participants need to install and setup the equipment needed for their work.  The moorings technicians have prepared dozens of instruments and many miles of wires for the moorings.   Scientists have set up the systems for gathering data underway, installed equipment for the analysis of samples, and prepared their computers for processing the mooring data that will soon be acquired.     We have also completed our first three CTD stations required to calibrate instruments prior to deployment.

A short port call in Santa Cruz provides an opportunity for additional scientists, technicians and crew to join the ship.  Some personnel will be leaving us too.  Steve has completed his work setting up the Myrtle Lander and Mike has finished work in the meteorology lab initiating the underway measurement of methane and carbon dioxide.

A number of other unofficial participants have left the ship since we arrived in port.    During our passage Discovery had become home to a surprising variety of birds.    A robin made himself at home in the main scientific lab and was often seen perching on a keyboard or monitor.    Also spotted on the ship were a falcon, two owls, and a number of finches.

One of several birds who hitched a ride on RRS Discovery 

Where is RRS Discovery now?

At the time of this post, the RRS Discovery is rapidly approaching the Canary Islands. The current location of the ship can be seen below.

RRS Discovery leaves Southampton

Last weekend the RRS Discovery left Southampton for her 6-week expedition to service the RAPID array of moorings across the Atlantic at 26°N.   

The Royal Research Ship Discovery before leaving for the RAPID expedition

The RAPID array monitors the Atlantic Meridional Overturning Circulation (AMOC) -  a system of surface and deep currents which together act like a conveyor belt transporting salt,  fresh-water, carbon and much else around the ocean.  Measurements of the AMOC are important for better long-term weather predictions, and for understanding the role of the global ocean in Earth’s climate system.

In addition to collecting data from existing sensors on the RAPID moorings, and redeploying these, the expedition will be adding biogeochemical sensors to some of the moorings as part of the ABC Fluxes project.  This aims to calculate time series of inorganic carbon and nutrient fluxes in the Atlantic at 26°N.

The team onboard will also be deploying a recently developed telemetry system for transmitting data to shore at regular intervals throughout the 18-month deployment period. 

Updates from the expedition will be posted on this blog, and the team will be tweeting using #rapidAMOC.

The Met Office ocean analysis

Predicting what the RAPID team will find using an ocean analysis

by Laura Jackson, Chris Roberts and Drew Peterson, Met Office

At the Met Office we've been examining how well our different products (ocean analysis, decadal prediction and various density and temperature metrics) compare with the observed AMOC variability. In particular we've found that an ocean analysis developed from our Forecast Ocean Assimilation Model (FOAM) and  used in our seasonal forecasting model (GloSea) is exceptionally good at reproducing the recent AMOC changes.

The figure  shows the period from April 2004, when the RAPID data starts. Each panel shows the existing observed data from RAPID (red) alongside values from the GloSea analysis (blue). As well as the actual AMOC strength we have also calculated the AMOC in the same way as RAPID by calculating the components from the Florida Straits flow (the northwards flow between mainland USA and the Bahamas), Ekman flow (driven by the wind) and Upper Mid Ocean (flow throughout the remainder of the Atlantic  from geostrophic interior flow and velocities in the western boundary region).

The Ekman flow (not shown) is identical since observational winds are used to drive the analysis. The Florida Straits (FS) and Upper mid ocean (UMO) components don't match individually as well as their sum, suggesting that although the FS and UMO components are not perfect, the analysis as a whole is doing a good job of constraining the combined sum of transports through the FS and Antilles currents. It is possible that a higher resolution might improve the representation of flows in the shelf region.

Since our model compares so well with the observations, we wanted to use it to predict what the observations being collected at the moment might show, and to enter the survey for the MOC strength from October 2014-September 2015. Unfortunately our analysis finishes in May 2015, so we have tried to estimate the last few months of data (see below for method).

The result of this analysis suggests an increase of the AMOC from Oct 2014-Sept 2015 from the 2009-2014 average by 0.7 Sv, which is made up of an Ekman increase of 0.6 Sv and an increase of 0.1Sv from the UMO + FS. It will be interesting to see how the new RAPID data compares!

Method for last few months 

For the Ekman component, we do not have wind stress for the full period, however we found that the Ekman component had a good correlation with the NCEP zonal wind strength at the same latitude. Hence we used a simple linear regression model with the NCEP wind strength which we have for the missing months. For other components we used the seasonal climatology plus the mean anomaly from the previous 12 months (ie assuming persistence of the anomaly from seasonal climatology). These last 4 months are shown in each panel in green.

Can you predict the AMOC?

Join the experts and respond to the RAPID Challenge 2015 

On 15th October scientists and engineers from the National Oceanography Centre (NOC) leave Southampton for a 6-week expedition on RRS Discovery (IV) to gather the most recent data from the RAPID array of moorings across the Atlantic from Florida to Morocco. The moorings, with sensors that measure temperature, salinity and currents from the sea floor to near the surface, monitor the Atlantic Meridional Overturning Circulation (AMOC) at 26°N.  

The AMOC with the moorings of the RAPID array at 26 North

The AMOC transports heat northwards in the Atlantic, and is important for the climate of Northwest Europe.  Recent studies indicate that it is decreasing in strength, and the decline in heat transport has been implicated in the formation of a 'cold blob' in the North Atlantic.   

Understanding of the AMOC has advanced greatly since the start of RAPID in 2004.  We now know much more about its variability, and the new knowledge allows ocean and climate scientists to make improved predictions about how the AMOC will change. 

Data is collected from the RAPID array of moorings once every 18 months, and ten years of data is now available to ocean and climate scientists from the RAPID website.  Earlier this year the  team that looks after the RAPID array challenged other ocean and climate experts to ‘predict’ what the new data will show once it has been gathered and analysed.  The response from a team at Oxford University is the first post on this blog.  If other experts join in the challenge, we will report on their thinking as well.

Now the RAPID Challenge has been opened to the wider community.  If you read this, and want to have a go at your own ocean prediction, you are very welcome to download the data, carry out your own analysis, and let us have your own estimate.

The RAPID Challenge web pages at www.rapid.ac.uk/challenge/ provide some of the scientific background information.  It also gives access to the time-series of AMOC data so far, and allows you to submit your own estimate of what the new data for the 18 months from April 2014 to September 2015 will show.

The deadline for submission is 31st December 2015.  In early 2016 a preliminary analysis of the new data will be available.  The estimates that come closest will be published on the RAPID website and awarded a Discovery mug.

Prediction from Oxford University

What will the RAPID team find when they recover their ocean moorings this autumn?

By Helen Johnson, David Marshall, Helen Pillar and So Takao, University of Oxford

Since 2004 oceanographers from the National Oceanography Centre in Southampton and their US colleagues have been using data from ocean moorings on the eastern and western sides of the Atlantic Ocean at 26^oN to monitor the strength of the Atlantic meridional overturning circulation (AMOC).  This has resulted in a remarkable and unprecedented 10-year time-series of this key climate index (black line in the figure).  

The AMOC is closely related to ocean heat transport in the Atlantic and therefore of great importance for the climate of Western Europe as well as the entire globe.  The observations have revealed large variations in the AMOC on all time-scales.  There is an apparent decline in AMOC strength over the ten years, and significant wind-driven weakening in several recent winters.

This autumn the team will collect a further 18 months of data from their ocean moorings.  But what will this latest batch of data tell us about the strength of the AMOC?

At the University of Oxford, we have been working to reconstruct the time-series of AMOC variability, based on our knowledge of how winds, heat and freshwater fluxes over the Atlantic have changed over the last few decades, combined with our understanding of how sensitive the AMOC is to variations in these quantities. 

Our reconstructed AMOC time series (orange line) successfully reproduces most of the interannual variability in the observations. These short-timescale fluctuations are dominated by wind forcing. However, the decadal trend seen in the observations is not well captured by our reconstruction.  This longer-timescale variability results from the response of the ocean to heat fluxes over the subpolar North Atlantic over the last two decades at least, and as yet ocean models are unable to accurately represent the ocean’s adjustment on such timescales.

Our reconstructed AMOC time series extends 15 months beyond the end of the observations available to date.  We have reasonable confidence in that portion of the variability which is wind-driven (blue line) and therefore make two predictions:

1. The observations will show a mean AMOC over this period which has been roughly equal to that over the previous few years (a small increase of 0.3 ± 0.2 Sv over the 2009-2014 mean).
2. The RAPID data recovered in the autumn this year won't reveal any evidence of a large "dip" over the 2014-2015 winter; in contrast we expect to find that the AMOC reached a maximum in November-January.

Our predictions will be validated when the RAPID team publish their updated AMOC time-series early in 2016!  And as RAPID data continue to accrue, alongside observations from higher latitudes such as those made by the OSNAP programme, we will learn more about the climatically-important longer-term AMOC changes which are currently inaccessible via our reconstruction.  Watch this space!

How the predictions were made

We use an ocean model and its adjoint to determine the sensitivity of the AMOC to surface wind, heat and freshwater forcing over the entire globe and the preceding 15 years. We then project observed forcing anomalies onto these sensitivity patterns; only those forcing anomalies which project strongly in space and time onto the sensitivity fields will generate variability in the AMOC.  Since NCEP II reanalysis atmospheric forcing data is available until June 2015, our reconstructed AMOC time series extends 15 months beyond the end of the currently available observed AMOC time-series.