Minutes of 5th 36°N consortium meeting

NOCS , 18-19 December 2006

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

1. Science progress reports
A series of science updates were provided for three main themes: heat content changes and transport, nutrient transport and CO2 storage and transport.

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 Heat content changes and transport
Paula: Comparison of 2005 section to 1981 Rintoul & Wunsch section shows apparent increase in the strength of the overturning from 16 Sv to 19 Sv, with the following estimates from the preferred solution:

  • Overturning transport = 18+/- 2Sv @1200m
  • Total heat flux = 1.2 +/- 0.1PW
  • Freshwater input north of 36oN = 0.8 +/- 0.1 Sv

Susan: Patterns of heat content change in the North Atlantic from 1981 to 2005 show reversing temperature trends, with upper waters (<800m) controlled by heave and intermediate waters (800-2500m) controlled by water mass change. Also a clear signal of tropical and subtropical warming and subpolar cooling.

1.2 Nutrient transport
Richard: Explained how, by combining data from AMT, 24N and 36N with an improved modeling study based on Roussenov et al. (2006), he believed export production is sustained in the Southern Gyre by DOP with Nitrogen fixation.

Rhiannon: Dissolved organic carbon distributions at 36N vary from 34 to 83 uM (TOC varies from 34 to 87 uM) with a coefficient of variation of only 4%, indicating good quality data. Comparing 36N data to AMT data suggests that the North and South Gyres of the Atlantic Ocean operate differently. Nitrogen Fixation in the N. Hemisphere forces phosphate concentrations low, resulting in raised levels of APA and faster turnover of the DOP pool. Nitrate concentrations remain low throughout the surface Atlantic, giving rise to a consistent level of LAP. Equatorial upwelling produces higher enzyme activities and faster turnover of the organic pools.

Tim L.: Has analysed 36N and AMT17 filters for NH3, has begun isotope analysis of NH4 and is in a queue for TN and TOC analysis. Presented results showing higher nutrient inputs at boundaries and is presently working on defining source regions and affected regions as well as estimating the frequency of wind blowing from these regions.

1.3 CO2 storage and transport
Pete: Provided update on anthropogenic C02 and a comparison of delta C* and Troca techniques across 1992, 1998 and 2004 sections; using Multiple Linear Regression to quantify the changes between sections for several independent chemical and hydrographic parameters, with the residual of coefficients due to anthropogenic carbon.

2. Inverse model questions
  • At which latitude does the ocean heat transport rapidly weaken between 36N and Iceland? What are the contributions of air/sea fluxes and storage?
  • How is the loss of Nitrate and Phosphate compensated for? What is the role of organic nutrients? Do we expect DOP to close the P budget and Nitrogen fixation to close the N budget? Air/sea fluxes appear to be small so are we looking at source/sinks versus storage?
  • How is Carbon changing in the subtropical North Atlantic: by transport or air/sea fluxes? What are the controlling mechanisms?
  • How connected are the budgets in N, C and heat?

3. Data management
Alex: All data needs to be made available by the end of the grant following NERC rules. Please submit any worked up data to BODC; this is especially important if your Post Doc is coming to an end. Final data to be submitted to Alex ASAP; there is no need to wait for all data to be completed. A final dataset comprising the basic quantities is expected to be ready by the end of January, through Elaine. All primary data expected to be worked up by end of June (He and Tri work will start in February or March and take 3.5 years).

4. Timeline and timetable for next meeting

Dec 2006/Jan 2007   36N inversion
Estimate of T/S changes
Air-sea fluxes of N, P …
Property estimates of N, DON, P, DOP, C, DOC
May/June 2007   26N + 36N inversion
Combine with N and P budget
July 2007   Combine with C budget
Autumn 2007 synthesis   How consistent are T, S, N, P and C changes?

There is no meeting scheduled at the moment, but it is likely that there will be one more 36N meeting.

Summary list of actions:

ACTION: All think about dissemination at meetings, e.g. Oceans Sciences, Florida, Spring 2008. Look into travel grants.

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

ACTION: All to think about inversion model questions.

ACTION: All to submit worked up data to BODC.