CFC PIs: Fine and Smethie
The CFC data from earlier occupations of sections in the North Atlantic have been used recently
to estimate: formation rates for NADW components (Smethie and Fine, 2001; Rhein et al., 2002,
Kieke et al., submitted), describe pathways for the deep circulation (Smethie et al., 2000),
describe the zonal circulation along 66°W section (Joyce et al. 2001), and estimate propagation
and storage rates for NADW components (Fine et al., 2002). We propose to expand on this work
by comparing formation rates and deep circulation pathways from earlier occupations to those
from the 2003-04 timeframe. The recent occupations for CFCs include our laboratory’s
measurements on A20 (52°W), A22 (66°W), and 24.5°N (with British), NOAA/PMEL (Bullister)
measurements on A16, German measurements on TTO Repeat (Toste and Wallace) and Subpolar
work (Rhein and co-workers) (Fig. x). We have plans to collaborate with Bullister, Cunningham,
Rhein, Toste and Wallace.
Throughout most of the North Atlantic the full water column is ventilated with CFCs. Ventilation
involves both the physical pathways of the flow as well as the timescales for which the water is
isolated from contact with the atmosphere. Previous hydrographic/tracer programs have provided
snapshots of ocean ventilation (e.g. Smethie, 1993; Doney and Jenkins, 1994; Rhein et al., 1995;
Warner et al., 1996; Orsi et al., 1999; Fine et al., 2001). An important remaining challenge is to
measure and understand how this ventilation changes over time. Observing the temporal
penetration of transient tracers offers the most direct measure of ocean ventilation. Pathways of
recently ventilated water carrying signals of perturbations imprinted at the ocean surface, such as
anthropogenic carbon, are clearly revealed (e.g., Weiss et al., 1985), and inventory calculations
yield integral water mass formation rates (e.g., Smethie and Fine, 2001). Furthermore, the ages,
temporal evolution of the transient tracers, and especially the tracer-tracer relationships, contain
significant information about ventilation processes, especially the relative roles of mixing and
advection (e.g., Smethie et al., 2000), which are not easily diagnosed from steady-state tracers
(Robbins et al., 2000).

Objectives:
How have North Atlantic Deep Water (NADW) component formation rates changed? The
CFC inventory of a subsurface water mass is directly related to the formation rate of that water
mass integrated over the time interval of CFC input by
ICFC = R CS dt

Where ICFC is the CFC inventory, R is the water mass formation rate, CS is the source water CFC
concentration and t is time. The total CFC inventory is determined by integrating a map of the
water column CFC inventories. As an example, the CFC-11 inventory map for DSOW
determined from the 1997 WOCE survey is shown in Fig. xx. The total inventory is 8.3 million
moles, which corresponds to an average formation rate of 2.2 Sv from about 1970 – 1997 (90+%
of CFCs in the ocean in 1997 entered after 1970). Rates of formation for the components of
NADW have been calculated from CFC-11 inventories from data collected around 1990 and from
the WOCE survey of the North Atlantic in 1997 (Smethie and Fine, 2001; Rhein et al., 2002;
LeBel et al., 2003). The 2003-04 data will provide a comparison to rates calculated from these
earlier surveys and thus evidence for variability in the formation rate of NADW over the last
decade. This will be an international collaborative effort between United States and German
scientists with W. Smethie taking the lead for the US component and M. Rhein the lead on the
German component.
2) What are the pathways and rates by which interior regions of the North Atlantic are
being filled in with CFCs, and are there temporal variations? The large-scale interior deep
circulation affects the propagation of climate anomalies from the high latitude source regions as
they provide a pathway into the interior. High CFC concentrations and CFC maxima are observed
in the interior far offshore of the DWBC and south of the Gulf Stream/DWBC crossover
recirculation region in the 1997 and 2003 occupations of the A20 and A22 lines (Fig. xxx). Most
prominent are the CFC maxima associated with UNADW (~1500 m), e.g. 24.5°N, 52°W. We do
not understand the pathway for this well ventilated water to reach the interior. Could there be
recirculation from the DWBC south of the Gulf Stream/DWBC crossover or could there be export
of the water from the subpolar region to the subtropical region seaward of the DWBC? We
propose to use the CFC distributions with inverse model absolute velocities from A. Macdonald
and deep float data to determine the circulation patterns. Comparisons of the deep contours of
CFC-11 (Fig. xxx) between occupations also provide an opportunity to assess the rates at which
interior regions are filling with waters of high latitude origin. R. Fine will take the lead in this
analysis.
3) Has there been a slowing down in the subduction rates of upper thermocline waters in
the North Atlantic? R. Fine will take the lead on estimating subduction rates using available
CFC data from the North Atlantic thermocline, she will be working with T. Joyce. An important
challenge is to measure and understand how ventilation changes over time. The North Atlantic
Oscillation (NAO) is one of the dominant modes of atmospheric variability in the North Atlantic
sector. Variations in the NAO index and associated wind fields imply strong changes in the airsea fluxes. These variations will impact the large scale circulation and the local thermodynamic
response of the mixed layer. Subduction and Ekman pumping rates will be estimated for upper
thermocline waters including Salinity Maximum Water (SMW) (e.g., O’Connor et al., accepted)
and correlated to other properties and major atmospheric indices (e.g., NAO, NTA, TNA, etc.).
Subduction rates will be estimated using the inverse CFC age gradients (e.g., Jenkins 1987, 1997,
O’Connor et al., 2002). Concurrent with the general reductions between 1997 and 2003 in net
heat and freshwater fluxes across 66°W and zonal circulation of the Gulf Stream recirculation
(see Joyce, Toole, Macdonald), preliminary calculations suggest there is a slowing down in the
subduction rate of SMW. Some of the interest in SMW comes from their equatorward subduction
in subtropical circulation cells (e.g., Schott et al., 2004) and from their secular trends (Curry et
al., 2003).
4) To what extent has the extreme low temperature low salinity vintage of LSW that formed
during the high NAO index years of 1988-1994 spread into the subtropical Atlantic? This
water was also tagged with a high oxygen and CFC content. Its presence in the DWBC south of
Cape Cod was observed from 1994 - 1996 (Smethie and Pickart, 2001), and off Abaco from 1994
- 1997 (Molinari et al., 1998). We observed it at the southern ends of the A20 and A22 repeat
lines taken in 2003 by its decrease in salinity and increase in oxygen and CFC content relative to
the 1997 lines (Smethie et. al., 2004; Joyce, 2004). We propose to do a careful comparison of the
2003 and 1997 lines and to include the 24°N line as well to determine where this vintage of LSW
is reaching the interior region of the subtropical North Atlantic. This will be a collaborative
effort between W. Smethie and T. Joyce and closely coordinated with the deep interior circulation
analysis (see 2 above) lead by R. Fine.
A byproduct of the data originators, Smethie and Fine, participation in the proposed analysis is
that CFC ages will be calculated and analyzed as to appropriateness of different age applications.
Ages are used in estimating CO2 inventories and other physical (ventilation, subduction, effective
spreading) and biogeochemical quantities (AOUR, denitrification rates, DOC usage rates), and
for models.
Fig. x
Map of CLIVAR cruises in
the North Atlantic Ocean
carried out in 2003-2004
Fig. xx. CFC-11 inventory in DSOW from the
WOCE 1997 survey
Fig. xxx. Sections of CFC-11 along A20/52W occupied in 1997 and 2003. High CFC11 concentrations are in read, concentrations less than 0.02 pmol/kg are in dark blue.