Supplementary Material
Figure Captions
Figure S1.
Cumulative mid-ocean geostrophic transport for the 1957, 1981, 1992,
1998 and 2004 hydrographic sections along 25ºN. Transport is accumulated from east
to west and bottom to top such that the value at any point equals the sum of all
transport to the east and below that point. Cumulative transport contours are at 5 Sv
intervals, positive northward, all with the same colour scale. Contours of 1.6, 2.5, 5.5,
9.5 and 15ºC potential temperature are overlaid to identify the water masses of Table
S1. Topography is plotted as the maximum depth for each station pair across the
section. Blank regions are present at the eastern boundary since transports are only
computed to the lowest common depth of the station pair and cumulative transports
are assigned the longitude of the westernmost station of the pair.
Cumulative transport of the lower waters in all years is weak over most of the
eastern basin and into the western basin while strong flows are associated with the
deep western boundary current and its offshore recirculations; these change over
time. In the western region, southward transport of lower North Atlantic Deep water
accumulates to 10-15 Sv steadily between 5000m and 3000m in 1957, 1981 and 1992
while in 1998 and 2004 smaller southward flow is concentrated in a narrower region
adjacent to the western boundary and does not exceed 10 Sv. It is clear this change is
associated with the deep western boundary current rather than anomalous flow in
the mid-ocean. In the upper North Atlantic Deep Water, comparable transport
accumulations in all sections are seen near the western boundary although over a
much narrower region in 1998 and 2004.
Figure S2. Cumulative mid-ocean geostrophic transport in the upper 1000 m for the
1957, 1981, 1992, 1998 and 2004 hydrographic sections along 25ºN. Transport is
accumulated from east to west and upwards from zero at 1000m to the surface.
1
Cumulative transport contours are at 5 Sv intervals, positive northward, all with the
same colour scale. Contours of 9.5, 12.5, 15.5 and 24.5ºC potential temperature are
overlaid to illustrate the thermocline slope. Topography is plotted as the maximum
depth of each station pair across the section. Blank regions are present at the eastern
boundary since transports are only computed to the lowest common depth of the
station pair if this is less than the reference depth and cumulative transports are
assigned the longitude of the westernmost station of the pair.
In the eastern basin, cumulative transport is almost everywhere southward or
smaller than 5 Sv except in 1957 where the northward transport is slightly above 5Sv.
The largest southward flows are in the upper 500m towards the west of the sections.
Southward transports are stronger and the extent of southward transport penetrates
further eastward with time. Particularly strong southward flows are seen west of
67ºW in the upper 400m in 2004 and this feature is also present in the 1998 section
(west of 70ºW) but not in earlier years.
Figures S3. Accumulated meridional transport (Sv) above 1000 m depth as a
function of zonal distance from the African coast for each of the 5 sections in 1957,
1981, 1992, 1998 and 2004. Geostrophic velocity referenced to zero velocity at 3200
dbar over the mid-ocean section east of the western boundary region and to zero
velocity at 1000 dbar in the western boundary region is estimated for each station
pair and then adjusted with a uniform reference level velocity to ensure overall mass
transport. The geostrophic velocities are then integrated vertically from 1000 m
depth up to the surface and accumulated westward from 0 Sv at the African coast. In
order to display clearly the transport distribution for each section, the transports are
sequentially displaced by 20 Sv (the origin for the 1957 section is at 0 Sv, for 1981 at
20 Sv, for 1992 at 40 Sv, for 1998 at 60 Sv and for 2004 at 80 Sv). The best least squares
fit line to the accumulated transport over the region from 0 to 5800 km from the
African coast is indicated for each section.
2