Geoscience Reference
In-Depth Information
lot, with spatial and temporal characteristics that have been defined
progressively, though it is difficult to know whether they are a natural
manifestation or a sign of change, since the signals are interlinked and
our time perspective too short. Our knowledge of modes of variability
has increased spectacularly during the last decades. Numerous types
of oscillation have been identified and characterized by their signature
in terms of anomaly patterns in surface temperature such as the North
Atlantic Oscillation, the Arctic Oscillation, the Atlantic Multidecadal
Oscillation, the Pacific Decadal Oscillation, the Southern Annual Mode,
etc. Each of these modes affects the ocean but is this a question of
noise, of transitory disturbances or of a durable impact exercised by the
atmosphere on the ocean? And can the persistence of certain modes be
explained by a feedback action of the ocean on the atmosphere?
This last hypothesis in particular has been studied for North
Atlantic Oscillation (NAO) in the North Atlantic. The NAO is
characterized by an anomaly in pressure difference between Iceland
and the Azores. Its positive phase corresponds to a decrease of low
pressure for Iceland and a reinforcement of high pressure for the
Azores, which accelerates the flux that crosses the Atlantic to the east
toward northern Europe. Therefore, there is a disturbed situation with
precipitation in the north of Europe and on the contrary dry conditions
in the south. The negative phase corresponds to an anomaly of inverse
pressure, so to a decrease of the pressure gradient and a weakening of
the flux from the west, which is oriented more zonally: it rains in the
south of Europe. Contrary to El Niño, there is no dominant frequency
in the NAO spectrum. However, we note decadal periods where a
particular phase of the anomaly dominates [HUR 95]. The persistence
of the NAO has been explored by several groups who have examined
how an initial hot anomaly in temperature off Florida could find itself
buried in the thermocline over winter to surge back, in the axis of the
Gulf Stream the following year, contributing to the reactivation of the
atmospheric anomaly, but in a slightly different region. This oceanic
resilience, which can explain the multiannual persistence of the
atmospheric anomaly, warns us to be careful regarding the evolution of
climate change: what is buried in the ocean finishes by returning to the
surface, with the ocean's characteristic tempo, but also its inexorable
force.
