Oceanic Mass Transport in the Atlantic at 16°N and 26.5°N
National Oceanography Centre, Southampton UNIVERSITY OF SOUTHAMPTON AND NATURAL ENVIRONMENT RESEARCH COUNCIL
Uta Neumann(1,*), Johannes Karstensen (1), Katja Lorbacher (1), Martin Visbeck (1), Torsten Kanzow (2) and Uwe Send (3) SPP 1257 – Project: TRANSOCEAN Combined Oceanic-Geodetic Analysis of Global and Regional Ocean Mass-, and Freshwater Transport Divergences
(1) IFM-GEOMAR, Leibniz Institute of Marine Sciences (at Kiel University), Kiel, Germany (2) National Oceanography Centre, Southampton, Great Britain (3) Scripps Institution of Oceanography, UCSD, La Jolla, USA *
[email protected], fax: +49 431 600-4102
Background An important element of the global cli-
Zonally averaged oceanic mass transport can be decomposed into two different con-
mate system is the meridional over-
tributions related to the zonal wind stress acting on the ocean surface and to the
turning circulation (MOC). Its strength
geostrophic driven flow. The variations of the latter can be divided in two parts again:
is directly associated with the meridional
➢
heat transport. According to model stud-
a baroclinic component, which can be estimated by the density structure of the water column derived from hydrographic mooring data
ies the MOC shows variability on interan-
and
nual to decadal time scales, but observa-
➢
tional proof is still sparse at present.
a barotropic component, which is invisible to hydrography and difficult to separate from the baroclinic field in mooring data, but is often associ-
Within the last three to seven years the
ated with bottom pressure fluctuations.
fluctuations of deep meridional flow has been observed by two mooring arrays in
Fig. 1: Scheme of meridional overturning circulation with sinking regions in high latitudes in the Atlantic Ocean.
the Atlantic Ocean at 16°N and 26.5°N.
Variability of the baroclinic flow is expected to be linked to the MOC and disturbs the gravity field measurements of GRACE amongst others.
Observations
Model
Two mooring arrays are operating in the tropical and subtropical North Atlantic Ocean
A comparison of MOC associated flow
in order to monitor MOC variability:
estimated from different models show
➢
➢
At 16°N the Meridional Overturning Variability Experiment (MOVE) is designed to
that their variability is in the same or-
provide a multi-year time series of meridional North Atlantic Deep Water (NADW)
der of magnitude (fig. 5), although the
fluctuations since February 2000 in the western trough of the Atlantic Ocean.
mean
Since June 2006 the section has been extended to the eastern basin.
ORCA025-KAB001. The variability lies
At 26.5°N the U.K. RAPID (Rapid Climate Change) programme estimates the me-
within the error of meridional transport
ridional basin-wide flow of the whole water column since April 2005 including flow
estimations (Bryden et al., 2005) that
through the Florida Strait.
are based on hydrographic sections, Both arrays use the “End Point” approach to determine
underestimated
from which the authors concluded a
and ocean bottom pressure
to the high intraseasonal flow variabili-
(OBP) measurements.
ty of the observed flow (see fig. 3).
The flow contribution over
The influence of a changing MOC on
the continental slope is mea-
ocean bottom pressure and sea sur-
sured directly.
face height signals has been analysed Fig. 6: Deviation of sea surface elevation (in m) from the global global mean in ORCA025-KAB001 in the North Atlantic Region: Snapshot in February 2002. Areas of deep convection are associated with minima in sea level. Courtesy of K. Lorbacher
Fig. 3: Absolute transport fluctuations below 1180 dbar (MOVE) / 1110 dbar (RAPID), combining geostrophic transports and the continental slope contribution. The light gray and bold coloured curves represent 2 and 3 day low-pass filtered time series.
-15.9 Sv for RAPID and MOVE, resp.
slowing of Atlantic MOC by about 30% sults of RAPID do not support this due
by Lorbacher [2007] using the global ocean model ORCA025-KAB001.
Next Steps
longer time scales, Long-term flow averages -18.5 Sv and
Fig. 5: MOC associated transport estimated from model data (curves) and hydrographic section (stars after Bryden et al., 2005) at 26.5°N. Courtesy of A. Biastoch
slowing of Atlantic MOC by about 25%
high variability on monthly as well as a none-coherent behaviour.
ORCA025-KAB001 ORCA025-G70 FLAME HS4
from moored dynamic height
tuations of both meridional sections show
●
by
within the last 5 decades. But the re-
geostrophic
Time series of deep meridional flow fluc-
●
is
transport
the
Fig. 2: Mooring (squares) and bottom pressure sensors (triangle) along the MOVE (southern) and RAPID (northern) array. The white broken line indicates the further extension of the MOVE array over the whole Atlantic.
flow
➔
Comparing variability of and estimating the cross correlation between me-
Monthly OBP variability of 0.02 dbar is ob-
ridional mass transport at two meridional sections (16°N and 26.5°N) by
served in MOVE (Fig. 4) and RAPID (not
means of different models and observational data based on expectations of
shown) moorings, while GRACE data show a
continuing simultaneous monitoring
●
seasonal cycle of 0.02 to 0.05 dbar (Fig. 4) ●
new GRACE products show slight
➔
im-
cally forced ones
provements, but the annual cycle is still unrealistic [Böning, 2007] Fig. 4: Time series of bottom pressure fluctuations at 16°N from in-situ (MOVE PIES, monthly means, blue curve) and GRACE products (GRACE: do2-50, 750 km Gauss filter).
Try separating meridionally large-scale (climate-relevant) signals from lo-
➔
Comparison of horizontal and meridional transports with GRACE and other satellite derived data based indices
Courtesy of A. Macrander
Related contributions to GSTM + DFG-SPP Symposium 2007: ● Böning, C., A.Macrander, R. Timmermann, O. Boebel and J. Schröter (2007): Global Validation of GRACE Gravity Measurements by in-situ and modelled Ocean Bottom Pressure (16) – Session E. Oral presentation Monday, 15.10.2007 14:25 ● Lorbacher, K. and J. Dengg (2007): Manifestation of long-term trends of the Thermohaline Ocean Circulation in Sea Surface Height and Ocean Bottom Pressure Fields? Results from a Model Process Study (51) – Session E. Poster presentation Monday, 15.10.2007
References: Weblinks: ● ● Kanzow,T., U. Send, M. S. McCartney (2007): On the variability of the deep meridional GRACE: transports in the tropical North-Atlantic, submitted http://www.gfz-potsdam.de/grace ● ● Cunningham, S.A., T. Kanzow, D Rayner, M.O. Baringer, W.E. Johns, J. Marotzke, H.R. MOVE: Longworth, E.M. Grant, J.J.-M. Hirshi, L.M. Beal, C.S. Meinen, H.L.Bryden (2007): Temporal ftp://ftp.ifremer.fr/ifremer/oceansites/MOVE/ ● Variability of the Atlantic Meridional Overturning Circulation at 26.5°N, Science, 317, 935, RAPID: doi: 10.1126/science.1141304 http://www.noc.soton.ac.uk/rapid.php ● Bryden, H.L., H.R. Longworth and S.A. Cunningham (2005): Slowing of the Atlantic Overturning circulation at 25°N, Nature, 438, doi: 10.1038/nature04385 ● Church, J.A. (2007): A Change in Circulation?, Science, 308, 935, doi: 10.1126/science.1147796
