by Harry L. Bryden et. al.
1 December 2005
Its heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change. A transatlantic section along latitude 25° N has been used as a baseline for estimating the overturning circulation and associated heat transport.2-4
Here we analyze a new 25° N transatlantic section and compare it with four previous sections taken over the past five decades. The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004.
Whereas the northward transport in the Gulf Stream across 25° N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth.
In 2004, more of the northward
Gulf Stream flow was recirculating
back southward in
the thermocline within the
subtropical gyre, and less was returning southward at depth.
Thus, any indication of a slowdown in the Atlantic overturning circulation has profound implications for climate change.
In March 2004 we deployed an array of
moored instruments along 25° N to begin to monitor the overturning
circulation 7 and in April-May we took a transatlantic
hydrographic section along 25°N to provide an initial calibration
for the time-series array measurements8.
In 1998, the 25° N section was again
occupied,11 so the section of 2004 marked the fifth
complete transatlantic section along 25° N. Here we analyze the new
2004 section and the 1998 section using methods similar to those
previously developed for the 1957, 1981 and 1992 sections 2,4
and examine the structure of the overturning circulation for all
The 1957 and 1992 sections were effectively along 24.5° N over the entire width of the Atlantic. The 1981, 1998 and 2004 sections angled southwestward from the African continental shelf at about 28° N to join the standard 24.5° N section at about 23.5° W.
To take advantage of the continuous
electromagnetic cable monitoring of Gulf Stream transport through
the Florida Straits12, the 1998 and 2004 sections angled
northwestward at about 73° W to complete the section along 26.5° N.
A reference level of 3,200 dbar (decibar) is used for station pairs east of the western boundary region where current meter observations suggest 1,000 dbar to be more suitable.4,13 The transition between the two reference levels is identified from the distribution of dissolved oxygen concentration that marks the eastern edge of the boundary region 4 and ranges from 68.38 W to 70.68 W.
The concept behind the analysis is to estimate the annual average over-turning, so the annual averaged wind-driven surface Ekman transport and the annual averaged Gulf Stream transport through Florida Straits must be balanced by the overall southward geostrophic transport across the mid-ocean section.
Thus a uniform reference level velocity is added everywhere along the section to force the mid-ocean geostrophic transport to balance the Gulf Stream plus Ekman transport.
This approach assumes that the
large-scale baroclinic interior flow does not vary on seasonal or
shorter timescales; theoretical arguments and modeling results
support such an assumption.14,15
Sporadic estimates of Gulf Stream transport back to the 1960s16-19 and cable estimates of transport since 2000 (ref. 20) show no evidence of changes in annual averaged transport through.
In the SOC and NCEP wind stress climatologies,21,22 the mean Ekman transport at 25.5° N is 3.8 Sv (SOC) or 3.6 Sv (NCEP) and the variability in annual averaged Ekman transport is 0.6 Sv.
There is a small net southward transport
across 25° N associated with the 0.8 Sv Bering Straits through-flow
from the Pacific which is diminished by a net evaporation of order
0.1 Sv over the Atlantic north of 25° N (ref. 23) but this is
smaller than the uncertainty in the calculations.
The difference of 2 Sv is due to the flow through the northwest Providence channel25 that joins the Gulf Stream flow north of 24.5° N to make up the 32.2 Sv measured by cable at 26.5° N.
In summary, the southward mid-ocean
geostrophic transport equals 35.6 Sv for the 1957, 1981 and 1992
sections and 37.6 Sv for the 1998 and 2004 sections. From the
observed variability, we estimate that the uncertainty in forcing
the southward mid-ocean geostrophic transport to equal a constant
value for each of the five sections is only ±2 Sv.
The strength of the flows has changed, however. In the main thermocline, the southward flow is much stronger between 100 and 600 m depth in 2004 (Fig. 2b) so that the mid-ocean southward transport above 1,000 m depth has increased from 13 Sv in 1957 to nearly 23 Sv in 2004 (Table 1).
In the deep waters the southward transport between 1,000 and 3,000 m that is associated with upper North Atlantic Deep Water (NADW) originating in the Labrador Sea has remained reasonably constant, varying between 9 and 12 Sv; below 3,000 m, however, the southward transport of lower NADW originating in the Greenland-Iceland Norwegian Sea has steadily decreased from 15 Sv in 1957 to 7 Sv in 2004 (Table 1).
Not only has the lower-NADW transport
decreased but the bottom part of the flow is gone: in 1998 and 2004
the flow passes through zero at about 4,800 m depth, whereas in
earlier sections the southward flow extended down to 5,200 m (Fig.
Lower NADWs defined to have temperatures
between 1.8 and 2.5° C exhibit a consistent decrease in southward
transport from 16 Sv in 1957 to 7 Sv in 1998 and 2004.
The variability near the western boundary is evident in Supplementary Fig. S3, so we can argue that the upper 1,000 m transport depends critically on the nature of the western end station: for example, whether it is inside or outside an eddy.
Careful consideration of the errors in geostrophic transports derived from transoceanic sections and simulated in ocean circulation models led Ganachaud to the conclusion that there is an error of ± 6 Sv in overall upper and lower layer transports.26
Although there is little error in overall transport owing to the constraint of basin-scale mass conservation (as discussed above), there is an uncertainty of ± 6 Sv in upper layer transport that is due to sampling in or out of eddies, and because there must be compensation by deep flows, there is also an uncertainty in deep transport of ± 6 Sv.
The increased southward thermocline transport of 8 Sv and the 9 Sv decrease in lower-NADW transport in the 2004 section are close to this expected uncertainty.
First, the increased southward thermocline transport is a result of substantially warmer waters in the thermocline near the Bahamas. Temperatures between 400 and 800 m depths are 1 to 2°C 8 warmer in 2004 and the 14°C 8, 12° C 8 and 9.5° C 8 isotherms are 75 m deeper in 2004 than they were in 1957, 1981 or 1992.
This warming is not restricted just to the end-stations but extends eastward from the Bahamas over several hundred kilometers. These deeper isotherms and warmer thermocline waters near the western boundary lead to a steeper slope of the thermocline across the basin and to larger overall southward geostrophic currents in the thermocline relative to a deeper reference level.
The result is larger southward mid-ocean
flow above 1,000 m depth and the smaller overall northward transport
when the Gulf Stream and Ekman transports above 1,000 m depth are
added (Table 1). Smaller net northward transport across 25° N is
consonant with a reported reduction in overall northward flow
through the subpolar gyre based on satellite measurements27.
Instead, the observed structure shows that the upper NADW trans-port from 1,000 to 3,000 m depth has not changed and effectively only the lower NADW transport has decreased. In the deep water, this change is visually evident in an upward slope of the 3°C 8 isotherm to the west in the 1998 and 2004 sections that was not evident in the earlier sections.
This slope leads to reduced southward flow of lower NADW beneath the southward core of upper NADW at about 1,800 dbar (Fig. 2b).
Thus, there have been subtle changes in
the structure of the deep-water circulation, resulting only in
decreased lower NADW transport. Such a reduction in lower-NADW
transport is consonant with observations of the cessation of lower-NADW
formation in the Norwegian-Greenland Sea 28 and the
general freshening and weakening of the flow of lower NADW coming
over the northern sills.29,30
But the warmer waters near the western boundary in the 1998 and 2004 section leading to an increase in southward mid-ocean recirculation in the thermocline and the reduction in deep water flow only in lower NADW represent strong arguments that the observed changes are robust.
The decrease in net northward flow of
warm upper waters and decrease in net southward flow of cold deep
waters across the 25° N section result in a reduction of the
northward heat transport across 25° N from 1.3-1.4 PW (1 PW = 1015
W) for the 1957, 1981 and 1992 sections to 1.1 PW for the 1998 and
The 2004 transatlantic hydrographic section along 25° N was supported by the Natural Environment Research Council as part of the Core Strategic Research Program ‘Ocean Variability and Climate’ at Southampton Oceanography Centre.
Analysis of the five sections along 25° N was also supported by NERC as part of the Rapid Program.
Comments on an earlier draft by J. Hirschi, W. Johns, S. Josey, C. Meinen, G. Parrilla, P. Rhines, P. Saunders, J. Toole, P. Velez and R. Wood led to substantial improvement.