Minutes of 4th 36°N consortium meeting

NOCS , 21-22 June 2006

Present: Ric Williams, Elaine McDonagh, Harry Bryden, Richard Sanders, Susan Leadbetter, Rhiannon Mather, Brian King, Ute Schuster, Paula Mcleod, Tim Lesworth, Marie-Jose Messias, Andy Watson, Tim Jickells, Sinhue Torres, Alex Gardiner
Apologies: George Wolff, Alex Baker, Pete Brown, Carol Robinson

1. Science progress reports
A series of science updates were provided for three main themes: property distributions (Paula/Elaine/Richard/Marie-Jo/Rhiannon), circulation estimates (Paula/Elaine), changes in properties and circulation (Harry/Susan/Paula). Requirements for combining the carbon and nutrient measurements with the inverse model were also discussed and are reported along with a timescale for processing the remaining data in the following sections.

The presentations and any preliminary results will be made available on a password-protected site: http://www.bodc.ac.uk/36n/restricted/. Consequently, only key points will be presented here:

1.1 Property Distributions
Paula: Presented water mass characteristics and hydrography progress.

Marie-Jo: Results show the North Atlantic Deep Waters are well ventilated in the Western Basin compared to the Eastern Basin. Presented results of Transient Tracer Distributions (TTDs) analysis. Initial results indicate broad TTDs and substantial mixing. SF6 and CFC-11 and CFC-12 data calibrated and merged with hydrographic data. In the process of identifying outliners using properties/properties graphs and individual bottle blank corrections.

Richard/Rhiannon: 1235 Organic C and N Samples in total, analysed 344 so far. Presented profiles and sections calculated so far. Nitrogen data more scattered and less reproducible but this is because two potential sources of error enter the calculation. Organic Nitrogen contribution to the total nitrogen pool is greatest at the surface and decreases with depth. Organic Carbon and Nitrogen data are of the expected concentrations and comparable with other authors findings. All 36N analyses should be completed by February 2007.

Ute: Alkalinity and tCO2 complete and calibrated for 36N. In the process of comparing to 24N data. Comparing anthropogenic CO2 calculated according to two different methods (TROCA and Delta C*).

1.2 Changes in properties and circulation
Susan: Presented results on watermass property changes across 36N. Found warming surface at 800m (0.8°C at 270m) and cooling and freshening of deep waters - up to 0.2°C. Possible explanations are that this could be caused by heave or watermass changes. To warm upper 200m by 0.7°C requires ~18Wm-2. To cool 1000-2000m by 0.2°C requires ~ -25Wm-2. Observed regional heat fluxes can be up to 30Wm-2. Further investigation into surface heat flux and windstress variability is needed.
Harry: Showed mid-ocean geostrophic transport profiles at 25N over time from 1957 to 2004 to asses transport variability associated with Meridional Overturning Circulation.

1.3 Atmospheric inputs of nutrients
Tim L.: Presented results on the deposition of soluble NH4 and NO3 (coarse and fine). Calculated deposition rate of Ca (desert dust indicator) is lower but of similar orders of magnitude to on-shore calculations. He is hoping to have completed TN and TOC analysis by September this year.

1.4 Circulation Estimates
Paula/Elaine: Careful consideration of different solutions to circulation estimates leads to best initial estimate to be bottom-tracked referenced geostrophic transport field. Eddies are prominent features and extend all the way through the water column.

1.4.1 Diagnostics – Initial field and inverse estimates
In the following table, the vertical column shows the initial field choice for the southern crossing of the Gulf Stream (relying on thermal wind calculations from the density gradients plus a reference velocity from independent ADCP data), then three different inverse calculations estimates: (i) default choice, (ii) requiring the Gulf Stream transport is 60 Sv and (iii) also requiring Ekman transport is -2 Sv and transport of LDW in eastern basin is 1.9 Sv.

Table 1 Transport estimates for 2005 section with southern Gulf Stream crossing


Initial field


i ) SV=0

 ii) SV=0


iii) SV=0





Size (Sv)





Depth (m)





Western Boundary Transport

Gulf Stream (Sv)










Heat Flux

Total (PW)










Horizontal/ recirculating











Flow through Discovery gap = 1.9Sv (Saunders, 1987. “Flow through Discovery Gap.”)
Gulf Stream transport (area above DWBC) along EN214 line ~60Sv (Pickart, 1994. “Interaction of the Gulf Stream and DWBC….”)
Mean Ekman transport across 36oN = -2±2Sv (Hellerman and Rosenstein, 1983)

1.4.2 Changes in circulation- 1981 vs 2005
Also looked at changes in circulation compared to 1981, both based on northern crossings of the Gulf Stream. Unfortunately, along the northern crossing, there is much more variability in the position of the Gulf Stream, a more vigorous eddy field and the 2005 section did not fully complete the section. If one accepts these important caveats, the 2005 section suggests an increase in overturning at 36N, at odds with Harry’s estimate at 24N.

Table 2: Transport estimates for 1981 and 2005 based on northern Gulf Stream crossing





Size (Sv)

19.1 (17.8)


Depth (m)

2670 (1300)


Western Boundary Transport

Gulf Stream (Sv)






Heat Flux

Total (PW)






Horiz./ recirc.



2. Inverse model requirements
A discussion led by Ric and Elaine. Ric stressed the importance of providing error bounds on parameters for the inversion and presented three approaches to the inversion: single section (obtain transport and property flux), two sections (examine budgets and storage), and global inverse. The main inversion issues are: the importance of eddies and how solutions change if they are smoothed out, seasonality, as well as air-sea inputs/transfers for N and C budgets, and mixing.

The issue of spatial and temporal constraints on uncertainties was raised and it was agreed to focus on annual means.
The following timetable for the inverse model was proposed:
Circulation estimate – done?
Meld in nutrients –early autumn
Meld in C fields – done? Need to include seasonality and air-sea flux.
ACTION ALL: Provide initial field estimates and uncertainties to Elaine.

2.1 Reliability of inversion
The southern crossing was viewed as being more reliable (Table 1) and will be the primary focus, rather than the northern crossing (Table 2).

2.2 Aim to meld nutrient and carbon data
Primary aim is to repeat Table 1 for (i) nutrients including inorganic and dissolved organics for nitrogen and phosphorus; and (ii) repeat for carbon. Need to identify relative importance of overturning versus gyre circulation.

Subsequently, need to consider how transport estimates compare to those made for 26N. On the basis of that comparison, need to consider how the budgets are sustained between 26N and 36N. To solve these issues, need to consider annual fluxes of nutrients and CO2, as well as storage and mixing.

3. Data management
Alex: Availability of data was discussed. All data needs to be made available by the end of the grant following NERC rules. Prior to then selected data might be made available on a request basis. The data will only be made available if the appropriate investigator provides permission. Final data to be submitted to Alex ASAP, no need to wait for all data to be completed.

4. Timetable for next meeting
Next meeting is scheduled for 6 and 7 September 2006 at UEA focusing on initial field estimates and initial model outputs. The meeting will be Wednesday afternoon and Thursday morning.

Summary list of actions:

ACTION: All to confirm to Alex whether attending meeting in September and then accommodation to be booked.

ACTION: All to send outstanding submissions to Elaine for the cruise report.

ACTION: All provide initial field estimates and uncertainties to Elaine.

ACTION: Alex to email people to get estimated time of completion for datasets.

ACTION: Alex to provide a statement about data availability and up date the web when agreed.

ACTION: All to think about proposed themes for next meeting.