The influence of the ocean and the stratosphere on climate persistence in the North Atlantic region

Has Part:

Paper I: Geophysical Research Letters 35, Breiteig, T., Extra-tropical synoptic cyclones and downward propagating anomalies in the Northern Annular Mode, L07809. Copyright 2008 The American Geophysical Union. Full text not available in BORA due to publisher restrictions. The published version is available at: http://dx.doi.org/10.1029/2007GL032972

Paper III: Breiteig, T., 2009, Tropospheric precursors of downward propagating anomalies in the Northern Annular Mode. Full text not available in BORA.

Paper IV: Breiteig, T.; Stephenson, D. B.; Kvamstø, N. G., 2009, Local interactions between the North Atlantic Ocean and the NAO. Full text not available in BORA.

Paper V: Breiteig, T., 2009, The association between a weakening AMOC and the ENSO and NAO interannual variability. Full text not available in BORA.

Abstract:

The North Atlantic Oscillation (NAO) is generally regarded to largely be a product
of processes internal to the troposphere. This thesis investigates various aspects of how
the NAO and related climatical fields are influenced by two of the tropospheric boundary
components; the stratosphere above and the ocean below. The thesis consists of five
manuscripts, three considering the role of the stratosphere, and two considering the role of
the ocean.
Data provided by the NCEP/NCAR re-analyses is used to investigate how anomalies in
the stratospheric circulation relate to subsequent anomalies in various important climatic
fields. The mean storm tracks are found to shift meridionally, although not symmetrically
between the Pacific and the Atlantic regions (paper I). Following events of anomalously
weak stratospheric circulation, a statistically significant shift in the storm track is found
only over the Atlantic. The spatial temperature distribution is found to go through several
spatially and temporally distinct stages (paper II). In key regions, including northern
Asia, Europe and the oceanic areas off the east coast of North America, the probability of
cold air outbreaks increases by 50 – 80% at various stages relative to the negative stratospheric
circulation anomaly peak. The strongest cold air outbreak appears in Northern
Asia several weeks before the stratospheric anomaly peaks. This cooling is associated with
cold, northwesterly winds due to a strong high pressure anomaly centered above Northern
Scandinavia. In a separate study (paper III), it is shown that this high pressure anomaly
is a typical precursor of negative stratospheric circulation anomalies. It is found that it
is associated with enhanced vertical wave propagation – which is known to favour negative
stratospheric circulation anomalies. When the Scandinavian high precedes negative
stratospheric circulation anomalies, the stratospheric anomalies become more intense, and
stronger subsequent surface climate signals arise compared to events without a preceding
Scandinavian high.
The oceanic influence on the NAO is investigated by using both re-analysed and modeled
data. The feedback by North Atlantic Sea Surface Temperatures (SSTs) on the NAO
is quantified by applying lagged regression models on daily, re-analysed data (paper IV).
It is found that interactions between the SSTs and the NAO on daily time scale provide
predictive information about the NAO in excess of what is provided by the NAO alone.
SSTs along the coast of Northern America, coinciding partly with the Gulf Stream, are
especially important, and are in particular found to help account for decadal NAO variability.
The Gulf Stream SSTs are found to provide more prediction skill to the NAO than
the well-known SST Tripole is able to.
The relation between a continuously weakening Atlantic meridional overturning circulation
and interannual variability in the NAO is investigated by using data produced by
the Bergen Climate Model (BCM) (paper V). It is found that the NAO develops a pronounced
biennial cycle in the phase of a continuously weakening overturning circulation.
A corresponding biennial signal is found also in El Niño-Southern Oscillation (ENSO). Although
several hypotheses are tested, the exact processes involved in the NAO and ENSO
responses are yet to be disclosed.