Minutes of 8th 36°N consortium meeting

NOCS, 03 December 2008

Present: Ric Williams (Liv), Pete Brown, Ute Schuster, Andy Watson (UEA), Elaine McDonagh, Brian King, Richard Sanders, Harry Bryden (NOCS).
Apologies: Tim Jickells, Rhianon Mather and Kay Thorne.

Work in progress for combined 36N and 26N inversion - Elaine McDonagh

Update on inversion for 36N section.

Report on combined section inversion showing property fluxes, decomposition into overturning and horizontal components, together with error analysis.

Broadly plausible estimates for heat and freshwater fluxes: 1.2 PW at 36N and 1.3 PW at 24N, -1.5 Sv of freshwater at 36N and -1.2 Sv at 24N.

For the nitrogen transport, need to incorporate DON from Florida Strait.

Phosphate budget is nearly closed within error bars. Unlikely to be significantly altered by incorporating atmospheric inputs or DOP.

Nitrate budget reveals divergence over domain. This mismatch between nitrate and phosphate budgets reflects non-Redfield ratio of concentrations. Loss of nitrate is at odds with prevailing view of Rintoul and Wunsch, as well as view of nitrogen fixation occurring here. Inputs of surface N and P are too small to alter the budgets significantly. Partition of nitrate flux at 26N has large opposing contributions from overturning and horizontal transfer, surprising that the horizontal contribution is larger at 26N than at 36N.

Analysis of N/P ratio reveals some large local deviations from Redfield ratio in deep waters, particularly along 26N. Perhaps input N and P fields need further quality checking for 26N.

For total N budget, need to accommodate DON from Florida Strait. Certainly see a large northwards DON flux linked to the Gulf Stream and possible that there is an overall outflux of DON, which partly opposes nitrate influx. DIC transports directed southwards, while anthropogenic C transport directed northwards. Large divergence of anthropogenic C fluxes over domain implies large air-sea uptake. At 26N, the northwards anthropogenic C flux is smaller than expected, perhaps a consequence of large southward horizontal contribution offsetting much of northwards overturning contribution. Concerns about the DIC and nutrient budget might both relate to the size of northward horizontal tracer flux at 26N.

Uncertainty analysis presented by Brian where uncertainty depends on i) overall volume flux error x property variance, ii) local volume flux error x property value, iii) volume flux variance x error in property and iv) product of volume flux error and property error. Error in property can represent either observational error or how representative the measurement is compared with ,say, an annual mean. 

Requests - Elaine McDonagh and Brian King

  1. Advice about seasonal variation of parameters to meld with Ekman transport
  2. Advice about error of property or how representative the measurement is for the error analysis
  3. Advice/feedback about the property fluxes and budgets

Data report - Kay Thorne

A written report was provided for the meeting by Kay Thorne. This has been circulated by email.

Progress reports - Ric Williams and Pete Brown

Progress reports were provided for nutrient (Ric) and carbon (Pete) work.

Papers - Ric Williams

In spring we will need to prepare a final report, describing our outputs and reporting papers. Below is a list but please update. Note: * - indicates using 36 North data.


Leadbetter et al. (2007), GRL, - report on property changes along 36N *
Lozier et al. (2008), Science, - heat content change
McDonagh et al. (2008), - submitted report on single 36N inversion *


Williams et al. (2006), GBC, - nutrient stream work
Roussenov et al. (2006), GBC, - don/dop modelling
Reynolds et al. (2007), GBC, - N2 fixation *
Mather et al. (2008), Nature Geosciences, - DOP cycling *
Torres et al. (2008), submitted to GBC, - DON/DOP distributions + modelling *


Brown et al. - 26N Anthropogenic carbon *
Tim, Tim & Alex - atmospheric inputs *
Eric and Xie -plan for DOC paper *

Next meeting

To be arranged after the spring cruise, potentially April or May, will be announced nearer that time.