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Secular Variability of Warm Water Transformation in the North Atlantic

Principal Investigators: Michael McCartney and Ruth Curry

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Final Summary Report CICOR Cooperative Agreement 1998-2002

Analyses were undertaken of low frequency ocean signals that are coordinated with North Atlantic Oscillation (NAO) atmospheric patterns . These investigations have explored aspects of the ocean's natural variability (e . g. water mass properties, strength of gyre and overturning circulations) in the context of NAO timescales.

NAO-correlated variability of subpolar deep water (LSw) has atime-delayed impact on the subtropical circulation and mid-depth water properties . Analyses of the Labrador Basin, Bermuda, and Abaco time series , indicate a 5-6 year lag for the subpolar signal to influence the interior subtropical gyre (Bermuda); and 8-10 years to propagate via the DwBC to the Latitude of Abaco (26° N). The subpolar variability is manifested as shifts in T-S properties and alteration of the vertical density structure and potential vorticity characteristics of the mid-depth subtropical waters.

The North Atlantic subpolar and subtropical gyre circulations have exhibited fluctuations in baroclinic transport strength over the observational record (~50 years} that are organized around the NAO time structures . An index of the gyre strength was constructed from time series of potential energy anomaly (PEA) at the gyre centers -- an oceanic analogue to the SLP-based atmospheric NAO index. The analysis explored the mechanics of the individual gyre PEA histories, and demonstrated that they are representative of basin-scale shifts in PEA distributions. The transport fluctuation that results from these shifts reflects not only wind- and buoyancydriven thermocline  variability, but also changes in deeper layers related to deep convection, advective -diffusive movement of ocean waters, the growth and decay of mid-depth geostrophic shear, and dynamics of the deep inertial gyre system . The time-scales and coordination of these baroclinic adjustments sometimes lead to the development of large ocean signals : the eastward baroclinic transport in the upper 2000 db of the Gulf StreamlNorth Atlantic Current system varied by ~30% from a minimum of 48 megatons/sec circa 1970 (following a persistent low NAO phase) to a maximum of ~68 megatons /sec circa 1995 (at the culmination of a persistently high NAO phase}. To first order, the ocean PEA signals reflect an integration of the atmospheric forcing history . The resulting fluctuation of gyre transport strength is synchronous with the
warrn/cold propagating SST anomalies , which are surface manifestations ofdeeper -reaching upper ocean temperature content anomalies. The enhanced or diminished gyre circulation appears to contribute to decadal alterations in the distribution of upper ocean heat and salt -- with the potential to influence the overlying atmosphere and the strength of the overturning circulation.

Warm and saline waters entering the Nordic Seas from the south are primarily supplied by upper  ocean subtropical waters carried to the eastern North Atlantic by the North Atlantic Current. The progressive cooling and freshening by winter convection along this advection pathway sets the inflow characteristics . The Mediterranean Outflow Waters, which were previously hypothesized to directly influence the Nordic Seas Inflow, plays a diminutive role through its contribution to defining the T-S relationship of the interior subtropical gyre -- from which the North Atlantic Current draws its waters. Rather than directly feeding the Nordic Seas inflow, the eastern boundary undercurrent wholly expels its transport of MOW into the eastern edge of the subtropical gyre, resulting in the plume of high salinity subtropical waters.

The following people collaborated with the Principal Investigators on this research :
C. Mauritzen (WHOI)
R. Molinari (AOML)
J. Paillet (Ifremer)
M. Arhan (Ifremer)
J. Hurrell (NCAR)
H. Bezdek (AOML)

Major journal publications
Curry, R.G., M.S . McCartney, and T.M. Joyce, 1998. Oceanic Transport of Subpolar Climate Signals to Mid-Depth Subtropical Waters. Nature, 391: 575-577.

Molinari, R.L., R.A. Fine, W. D. Wilson, R.G. Curry, J. Abell, and M.S. McCartney, 1998. The Arrival of Recently Formed Labrador Sea Water in the Deep Western Boundary Current at 26.5°N. Geophysical Research Letters , 25: 2249-2252.

Paillet, J., M. Arhan, and M. S. McCartney, 1998. The spreading of Labrador Sea Water in the eastern North Atlantic. Journal of Geophysical Research, 103: 10223 -1023 9.
McCartney, M. S., 1997 . Is the Ocean at the Helm ? Nature, 3 88: 521-522.

McCartney, M. S. and C. Mauritzen, 2000. On the origin of the warm inflow to the Nordic Seas. Progress in Oceanography, submitted.

Curry, R.G. and M. S. McCartney, 2000. Ocean gyre circulation changes associated with the North Atlantic Oscillation. Journal of Physical Oceanography, submitted.

Books and other one-time publications
McCartney, M.5., 1997. The North Atlantic Atmosphere-Ocean Oscillation. In: 1997 U.S. WOCE Report, U.S. WOCE Implementation Report Number 9, U.S. WOCE Office, College Station, TX. pp 55-60.

Last updated: August 19, 2008

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