A systematic study of the impact of freshwater pulses with respect to different geographical locations
Résumé
The first comparative and systematic climate
model study of the sensitivity of the climate response under
Last Glacial Maximum (LGM) conditions to freshwater
perturbations at various locations that are known to have
received significant amounts of freshwater during the LGM
(21 kyr BP) climate conditions is presented. A series of ten
regions representative of those receiving most of the meltwater from decaying ice-sheets during the deglaciation is
defined, comprising the border of LGM ice-sheets, outlets of
rivers draining part of the melting ice-sheets and iceberg melt
zones. The effect of several given freshwater fluxes applied
separately in each of these regions on regional and global
climate is subsequently tested. The climate response is then
analysed both for the atmosphere and oceans. Amongst the
regions defined, it is found that the area close by and
dynamically upstream to the main deep water formation zone
in the North Atlantic are most sensitive to freshwater pulses,
as is expected. However, some important differences
between Arctic freshwater forcing and Nordic Seas forcing
are found, the former having a longer term response linked to
sea-ice formation and advection whereas the latter exhibits
more direct influence of direct freshening of the deep water
formation sites. Combining the common surface temperature
response for each respective zone, we fingerprint the particular surface temperature response obtained by adding
freshwater in a particular location. This is done to examine if
a surface climate response can be used to determine the
origin of a meltwater flux, which is relevant for the interpretation of proxy data. We show that it is indeed possible to
generally classify the fingerprints by their origin in terms of
sea-ice modification and modification of deep-water formation. Whilst the latter is not an unambiguous characterization of each zone, it nonetheless provides important clues
on the physical mechanisms at work. In particular, it is shown
that in order to obtain a consistent see-saw temperature
pattern, addition of freshwater in the Northern Hemisphere at
sites dynamically close to the deep water formation zones is
needed. Finally a preliminary data—model comparison for
the time of the Heinrich event 1 suggests that those sites are
indeed the most favourable to explain the pattern of climate
variability recorded in proxy data for this period. More
importantly, this model—data comparison enables us to
clearly reject a substantial fraction of the zones tested as
potential source for large freshwater entering the ocean at
that time.
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