The role of Eighteen Degree Water in establishing the North Atlantic

nutrient reservoir:Adding nutrient analyses to the

CLIMODE hydrography and moorings

Jaime PalterSusan LozierDick Barber

Downstream impact of EDW on the nutrient reservoir is visible in example profiles from

BATS and WOCE sections

32°N 64°W 7/1960 Hydro S

32°N 64°W 7/1989 BATS

30°N 52°W 7/1997 16°N 52°W

7/1997

[NO-

3]T

3

A wedge of uniform density, low-nitrate EDW resides below the euphotic zone.

At the edge of the EDW PV minima, there is a clear front in nitrate.

The PV-nitrate relationship, at the PV minima, along the WOCE sections and for the time series data

Annual mean chlorophyll

Biological impact

The annual mean chlorophyll minimum coincides with the EDW location, shown to depress and weaken the nutricline.PV on = 26.5

Summary of EDW impact on nutrient reservoir

• The formation and advection of EDW pycnostad cause the depression of the NA subtropical pycnocline and nutricline

• The pycnostad itself is relatively low in nutrients, so above the depressed nutricline the inserted EDW is a low concentration "nutristad."

• The presence of the depleted EDW damps biological response to the vertical processes that flux sub-euphotic zone water upward.• As EDW transits the gyre, its low nutrient signature is destroyed by mixing processes and as a result of the sinking and remineralization of organic matter

Objectives of CLIMODE collaboration

1) Directly evaluate the hypothesis that EDW is depleted in nutrients during formation and exported with a low nutrient signature

2) Quantify the flux of nutrients to the surface by entrainment of thermocline water into the ML during convection

3) Illuminate the time scale of biological depletion of inorganic nutrients during EDW formation

4) Observe the downstream decay of the nutrient signature as it transits the gyre.

Field Measurements and Data Analysis

1) Ship-based nutrient sampling (nitrate, nitrite, silicate, phosphate, and ammonium)

• ODF accompanying 1st cruise—Jan/Feb 2006

• Freezing samples on Feb/Mar cruises 2007

• Collect and freeze samples for Sigman Lab to conduct nitrogen fixation study

2) Nutrient time series at the moored profiler stations

• Deploying Satlantic’s ISUS nitrate sensor at 50m—Nov 2005

• Recovering, servicing and redeploying—Nov 2006

3) Analyze BATS nutrient data, in conjunction with float data, to look at the destruction of EDW during gyre transit

Mooring configuration

50m Profiler stop

ISUS sensor

Relative time scales in the nitrate conservation equation

Local time rate of change

Advection Vertical diffusion

Horizontal diffusion

Sources &

sinks

Rdz

NkNkNu

t

NvHH

2

22

Rdz

NkNkNu

t

NvHH

2

22= - +

= + + +

Peclet number, the ratio of the vertical diffusive time scale to the horizontal advective time scale, UD2/Lkv 103. The ratio of the remineralization time scale to the advective time scale, UN /LR 1 – 5, reflecting an important competition between these processes.

Nitrate and CFC age are both significantly correlated to PV.