NAO may be related to or forced by the lower-frequency SST in the Pacific and
Atlantic.
In particular, the linkage between the NAO and the AMO (Peings and
Magnusdottir 2014b; Omrani et al. 2014) has been widely explored, since the
AMO has been recognized as an important driver of Northern Hemisphere climate
variability (e.g., Kerr 2000; Enfield et al. 2001; Zhang et al. 2007; Sun et al. 2012,
2015). A warm AMO phase usually accompanies the occurrence of more frequent
negative NAO events (e.g., Ting et al. 2011; Kavvada et al. 2013; Peings and
Magnusdottir 2014b).
Arctic sea ice exhibits multi-decadal variability that is likely related to the AMO
at least for the Arctic sea-ice variations in the Atlantic sector. Arctic sea-ice decline
during the satellite era is likely a consequence of both multi-decadal variation and
anthropogenic forcing. Most of the studies addressing its climatic impacts focus on
the reduction of autumn and winter Arctic sea ice. A negative feedback between the
Arctic sea ice and the AO/NAO has been suggested (Deser et al. 2010; Honda et al.
2009), but there are also modelling studies that did not show negative AO/NAO
response to the reduction in autumn Arctic sea ice.
Because the AO/NAM is a dominant mode of the internal variation of the NH
atmosphere, the sea-ice reduction does not always induce the negative phase of
AO/NAM. However, because the preferred waveguide of the winter climatological
jet stream is located near the anomalous turbulent heat flux around the Barents Sea
these dynamical processes often occur in association with the sea-ice reduction. As a
result, the probability of a negative phase of the AO/NAM tends to increase
(Nakamura et al. 2015).
NAO may be seen as an Atlantic–European manifestation of the hemispheric
Arctic Oscillation (AO) or Northern Annular Mode (NAM) (e.g., Thompson and
Wallace 1998). The AO has a similar structure to the NAO, but is zonally more
symmetric. Many studies have been performed to distinguish the two mode patterns.
Kodera and Kuroda (2004) showed that NAO and AO represent two independent
teleconnection patterns for sea level pressure: one is an SLP seesaw between the
Azores and Iceland and the other one between the Polar and Mediterranean regions.
Wang et al. (2005) have further shown that AO affects interannual variability of
winter surface air temperature over the Euro-Asian and African continents, whereas
NAO is more regional, with the major effect on the surface air temperature in the
western North Africa. This discrepancy can be reflected in other atmospheric variables such as sea level pressure and geopotential height fields as well. Christiansen
(2002) and Wang et al. (2005) demonstrated that the AO-related signal could
penetrate deeply into the stratosphere while the NAO one is largely a tropospheric
phenomenon. Wang and Ikeda (2000) identified the leading Sea-Ice Area (SIA)
mode as AO related and the second SIA mode as NAO-related, which served to
differentiate AO and NAO by their atmosphere–ice interaction mechanism.
Over the NAE sector, a robust SLP response to ENSO appears largest in
wintertime. The response to El Niño events appears as a projection onto a negative
phase of the NAO (Mann et al. 2000; Brönnimann et al. 2007). The signal manifests
as a dipole over Europe with anomalously high SLP over northern Europe, with a
68
J. M. Sánchez-Laulhé et al.
Atlantic.
In particular, the linkage between the NAO and the AMO (Peings and
Magnusdottir 2014b; Omrani et al. 2014) has been widely explored, since the
AMO has been recognized as an important driver of Northern Hemisphere climate
variability (e.g., Kerr 2000; Enfield et al. 2001; Zhang et al. 2007; Sun et al. 2012,
2015). A warm AMO phase usually accompanies the occurrence of more frequent
negative NAO events (e.g., Ting et al. 2011; Kavvada et al. 2013; Peings and
Magnusdottir 2014b).
Arctic sea ice exhibits multi-decadal variability that is likely related to the AMO
at least for the Arctic sea-ice variations in the Atlantic sector. Arctic sea-ice decline
during the satellite era is likely a consequence of both multi-decadal variation and
anthropogenic forcing. Most of the studies addressing its climatic impacts focus on
the reduction of autumn and winter Arctic sea ice. A negative feedback between the
Arctic sea ice and the AO/NAO has been suggested (Deser et al. 2010; Honda et al.
2009), but there are also modelling studies that did not show negative AO/NAO
response to the reduction in autumn Arctic sea ice.
Because the AO/NAM is a dominant mode of the internal variation of the NH
atmosphere, the sea-ice reduction does not always induce the negative phase of
AO/NAM. However, because the preferred waveguide of the winter climatological
jet stream is located near the anomalous turbulent heat flux around the Barents Sea
these dynamical processes often occur in association with the sea-ice reduction. As a
result, the probability of a negative phase of the AO/NAM tends to increase
(Nakamura et al. 2015).
NAO may be seen as an Atlantic–European manifestation of the hemispheric
Arctic Oscillation (AO) or Northern Annular Mode (NAM) (e.g., Thompson and
Wallace 1998). The AO has a similar structure to the NAO, but is zonally more
symmetric. Many studies have been performed to distinguish the two mode patterns.
Kodera and Kuroda (2004) showed that NAO and AO represent two independent
teleconnection patterns for sea level pressure: one is an SLP seesaw between the
Azores and Iceland and the other one between the Polar and Mediterranean regions.
Wang et al. (2005) have further shown that AO affects interannual variability of
winter surface air temperature over the Euro-Asian and African continents, whereas
NAO is more regional, with the major effect on the surface air temperature in the
western North Africa. This discrepancy can be reflected in other atmospheric variables such as sea level pressure and geopotential height fields as well. Christiansen
(2002) and Wang et al. (2005) demonstrated that the AO-related signal could
penetrate deeply into the stratosphere while the NAO one is largely a tropospheric
phenomenon. Wang and Ikeda (2000) identified the leading Sea-Ice Area (SIA)
mode as AO related and the second SIA mode as NAO-related, which served to
differentiate AO and NAO by their atmosphere–ice interaction mechanism.
Over the NAE sector, a robust SLP response to ENSO appears largest in
wintertime. The response to El Niño events appears as a projection onto a negative
phase of the NAO (Mann et al. 2000; Brönnimann et al. 2007). The signal manifests
as a dipole over Europe with anomalously high SLP over northern Europe, with a
68
J. M. Sánchez-Laulhé et al.