Observed impact of Atlantic SST anomalies on the North Atlantic Oscillation
2002, J. of Climate, vol 15, No 6, 606-623.
Abstract:
The large scale patterns of co-variability
between monthly sea surface temperature (SST)
and 500 mb height anomalies (Z500) in the Atlantic
sector are investigated as a function of time lag
in the NCEP/NCAR Reanalysis (1958-1997).
In agreement with previous studies, the dominant signal
is the atmospheric forcing of SST anomalies, but
statistically significant covariances are also found when SST
leads Z500 by several months. In winter,
a Pan - Atlantic SST pattern
precedes the North Atlantic Oscillation (NAO)
by up to 6 months. Such long lead time covariance
is interpreted in the framework of the stochastic
climate model, reflecting the forcing of the NAO
by persistent Atlantic SST anomalies.
A separate analysis
of midlatitudes (20 - 70 N) and tropical
(20 S - 20 N) SST anomalies reveals that the
bulk of the NAO signal comes from the midlatitudes.
A dipolar anomaly, with warm SST south east of Newfoundland
and cold SST to the northeast and
southeast, precedes a positive phase of the NAO,
and it should provide a prediction of up to 15 % of its
monthly variance several months in advance.
Since the `forcing' SST pattern projects significantly
onto the tripole pattern generated by the NAO,
our results indicate a positive feedback between
the SST tripole and the NAO, with a strength of up to
25 m/K at 500 mb or 2-3 mb/K at sea level.
Additionally, a warming of the tropical
Atlantic (20 S - 20 N), roughly symmetric about the equator,
induces a negative NAO phase in early winter. This tropical forcing of
the NAO is nearly uncorrelated with, and weaker than that resulting
from the midlatitudes, and is associated with shorter lead times
and reduced predictive skill.
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