Journal of Climate: Vol. 13, No. 14, pp. 2550-2569.
The Relation between Decadal Variability of Subtropical Mode
Water and the North Atlantic Oscillation
Terrence M. Joyce
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
Clara Deser
Climate and Global Dynamics Division, National Center for Atmospheric
Research, Boulder, Colorado
Michael A. Spall
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
(Manuscript received 14 October 1998, in final form 30 September 1999)
The Bermuda station “S” time series has been used to define the
variability of subtropical mode water (STMW) from 1954 to 1995. This record,
which shows decadal variability at a nominal period of about 12–14 yr,
has been used as a baseline for seeking correlation with large-scale
atmospheric forcing and with decadal north–south excursions of the Gulf
Stream position defined by the subsurface temperature at 200-m depth. A common
time period of 1954–89 inclusive, defined by the data sources, shows a
high degree of correlation among the STMW potential vorticity (PV), Gulf Stream
position, and large-scale atmospheric forcing (buoyancy flux, SST, and sea
level pressure). Two pentads with anomalously small and large STMW PV were
further studied and composites were made to define a revised North Atlantic
Oscillation (NAO) index associated with the decadal forcing. During years of low
PV at Bermuda, the NAO index is low, the Gulf Stream is in a southerly position,
and the zero wind stress curl latitude is shifted south as are the composite
extratropical winter storm tracks, in comparison to the period of high PV at
Bermuda. Because the NAO, Gulf Stream separation latitude, and STMW PV variations
are in phase with maximum annually averaged correlation at zero year time lag,
the authors hypothesize that all must be either coupled with one another or with
some other phenomenon that determines the covariability. A mechanism is proposed
that could link all of the above together. It relies on the fact that during
periods of high STMW PV, associated with a northerly Gulf Stream and a high NAO,
one finds enhanced production of mode water in the subpolar gyre and Labrador Sea.
Export of the enhanced Labrador Sea Water (LSW) component into the North Atlantic
via the Deep Western Boundary Current can influence the separation point of the
Gulf Stream in the upper ocean once the signal propagates from the source region
to the crossover point with the Gulf Stream. If the SST signal produced by the
100-km shift of the Gulf Stream along a substantial (1000 km) length of its path
as it leaves the coast can influence the NAO, a negative feedback oscillation may
develop with a timescale proportional to the time delay between the change of phase
of the air–sea forcing in the Labrador Basin and the LSW transient at the
crossover point. Both a simple mechanistic model as well as a three-layer numerical
model are used to examine this feedback, which could produce decadal oscillations
given a moderately strong coupling.
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Hongjun Zhang:
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