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- Author or Editor: J. D. Opsteegh x
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Abstract
The variability in the subpolar Southern Hemisphere is studied with a coupled atmosphere–ocean–sea-ice model (the ECBilt). After having reached an approximate statistical equilibrium in coupled mode without flux corrections, a subsequent 1000-yr integration is performed and analyzed. A singular value decomposition of austral winter SST anomalies and 800-hPa geopotential height in the Antarctic Circumpolar Current region reveals a mode of covariability that resembles the observed Antarctic circumpolar wave. Subsequent analysis of this mode shows that it is basically an oscillation in the subsurface of the ocean. Additional experiments suggest that it is generated by the advective resonance mechanism: the oscillation is excited by the dominant modes of variability in the atmosphere, whereas the timescale is set by the ratio of the horizontal scale of these atmospheric modes and the advection velocity of the mean oceanic currents. The atmospheric response mainly consists of a local temperature adjustment to the SST anomaly, which reduces the damping of the SST anomalies. Salinity, wind stress, and sea-ice anomalies do modify the structure and intensity of the mode without playing an essential role.
Abstract
The variability in the subpolar Southern Hemisphere is studied with a coupled atmosphere–ocean–sea-ice model (the ECBilt). After having reached an approximate statistical equilibrium in coupled mode without flux corrections, a subsequent 1000-yr integration is performed and analyzed. A singular value decomposition of austral winter SST anomalies and 800-hPa geopotential height in the Antarctic Circumpolar Current region reveals a mode of covariability that resembles the observed Antarctic circumpolar wave. Subsequent analysis of this mode shows that it is basically an oscillation in the subsurface of the ocean. Additional experiments suggest that it is generated by the advective resonance mechanism: the oscillation is excited by the dominant modes of variability in the atmosphere, whereas the timescale is set by the ratio of the horizontal scale of these atmospheric modes and the advection velocity of the mean oceanic currents. The atmospheric response mainly consists of a local temperature adjustment to the SST anomaly, which reduces the damping of the SST anomalies. Salinity, wind stress, and sea-ice anomalies do modify the structure and intensity of the mode without playing an essential role.
Abstract
North Atlantic decadal climate variability is studied with a coupled atmosphere–ocean–sea ice model (ECBILT). After having reached an approximate statistical equilibrium in coupled mode without applying flux corrections, a subsequent 1000-yr integration is performed and analyzed. Compared to the current climate, the surface temperatures are 2°C warmer in the Tropics to almost 8°C warmer in the polar regions.
The covariability between the atmosphere and ocean is explored by performing a singular value decomposition (SVD) of boreal winter SST anomalies and 800-hPa geopotential height anomalies. The first SVD pair shows a red variance spectrum in SST and a white spectrum in 800-hPa height. The second mode shows a peak in both spectra at a timescale of about 16–18 yr. The geopotential height pattern is the model’s equivalent of the North Atlantic oscillation (NAO) pattern; the SST anomaly pattern is a north–south oriented dipole.
Additional experiments have revealed that the decadal oscillation in ECBILT is basically an oscillation in the subsurface of the ocean. The oscillation is excited by anomalies in the atmospheric NAO pattern, both through anomalous surface heat fluxes and anomalous Ekman transports. The atmospheric response to the SST anomaly enhances the oscillation and slightly modifies it, but is not essential. The atmospheric response consists primarily of a local surface air temperature adjustment to the SST anomaly. An important element in the physical mechanism of the oscillation is the geostrophic response of the ocean circulation to the forced temperature anomalies creating surface salinity anomalies through anomalous horizontal advection. These salinity anomalies influence the convective activity in the area of the temperature anomaly such as to break down the subsurface temperature anomaly. Both temperature and salinity anomalies slowly propagate eastward at a rate consistent with the mean current.
Abstract
North Atlantic decadal climate variability is studied with a coupled atmosphere–ocean–sea ice model (ECBILT). After having reached an approximate statistical equilibrium in coupled mode without applying flux corrections, a subsequent 1000-yr integration is performed and analyzed. Compared to the current climate, the surface temperatures are 2°C warmer in the Tropics to almost 8°C warmer in the polar regions.
The covariability between the atmosphere and ocean is explored by performing a singular value decomposition (SVD) of boreal winter SST anomalies and 800-hPa geopotential height anomalies. The first SVD pair shows a red variance spectrum in SST and a white spectrum in 800-hPa height. The second mode shows a peak in both spectra at a timescale of about 16–18 yr. The geopotential height pattern is the model’s equivalent of the North Atlantic oscillation (NAO) pattern; the SST anomaly pattern is a north–south oriented dipole.
Additional experiments have revealed that the decadal oscillation in ECBILT is basically an oscillation in the subsurface of the ocean. The oscillation is excited by anomalies in the atmospheric NAO pattern, both through anomalous surface heat fluxes and anomalous Ekman transports. The atmospheric response to the SST anomaly enhances the oscillation and slightly modifies it, but is not essential. The atmospheric response consists primarily of a local surface air temperature adjustment to the SST anomaly. An important element in the physical mechanism of the oscillation is the geostrophic response of the ocean circulation to the forced temperature anomalies creating surface salinity anomalies through anomalous horizontal advection. These salinity anomalies influence the convective activity in the area of the temperature anomaly such as to break down the subsurface temperature anomaly. Both temperature and salinity anomalies slowly propagate eastward at a rate consistent with the mean current.
Abstract
The mean state and variability of deep convection in the ocean influence the North Atlantic climate. Using an ensemble experiment with a coupled atmosphere–ocean–sea ice model, it is shown that cooling and subdued warming areas can occur over the North Atlantic Ocean and adjacent landmasses under global warming. Different “present-day” convection patterns in the Greenland–Iceland–Norway (GIN) Sea result in different future surface-air temperature changes. At higher latitudes, the more effective positive sea ice feedback increases the likelihood of changes in convection causing a regional cooling that is larger than the warming brought about by the enhanced greenhouse effect. The modeled freshening of deep ocean layers in the North Atlantic in a time period preceding a reorganization of GIN Sea convection is consistent with recent observations. Low-frequency internal variability in the ocean model has relatively little impact on the response patterns.
Abstract
The mean state and variability of deep convection in the ocean influence the North Atlantic climate. Using an ensemble experiment with a coupled atmosphere–ocean–sea ice model, it is shown that cooling and subdued warming areas can occur over the North Atlantic Ocean and adjacent landmasses under global warming. Different “present-day” convection patterns in the Greenland–Iceland–Norway (GIN) Sea result in different future surface-air temperature changes. At higher latitudes, the more effective positive sea ice feedback increases the likelihood of changes in convection causing a regional cooling that is larger than the warming brought about by the enhanced greenhouse effect. The modeled freshening of deep ocean layers in the North Atlantic in a time period preceding a reorganization of GIN Sea convection is consistent with recent observations. Low-frequency internal variability in the ocean model has relatively little impact on the response patterns.
Abstract
Statistical analysis of the wind speeds, generated by a climate model of intermediate complexity, indicates the existence of areas where the extreme value distribution of extratropical winds is double populated, the second population becoming dominant for return periods of order 103 yr. Meteorological analysis of the second population shows that it is caused when extratropical cyclones merge in an extremely strong westerly jet stream such that conditions are generated that are favorable for occurrence of strong diabatic feedbacks. Doubling of the greenhouse gas concentrations changes the areas of second population and increases its frequency. If these model results apply to the real world, then in the exit areas of the jet stream the extreme wind speed with centennial-to-millennial return periods is considerably larger than extreme value analysis of observational records implies.
Abstract
Statistical analysis of the wind speeds, generated by a climate model of intermediate complexity, indicates the existence of areas where the extreme value distribution of extratropical winds is double populated, the second population becoming dominant for return periods of order 103 yr. Meteorological analysis of the second population shows that it is caused when extratropical cyclones merge in an extremely strong westerly jet stream such that conditions are generated that are favorable for occurrence of strong diabatic feedbacks. Doubling of the greenhouse gas concentrations changes the areas of second population and increases its frequency. If these model results apply to the real world, then in the exit areas of the jet stream the extreme wind speed with centennial-to-millennial return periods is considerably larger than extreme value analysis of observational records implies.
