scholarly journals A Model Investigation of Aerosol-Induced Changes in Boreal Winter Extratropical Circulation

2011 ◽  
Vol 24 (23) ◽  
pp. 6077-6091 ◽  
Author(s):  
Y. Ming ◽  
V. Ramaswamy ◽  
Gang Chen
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Boreal Winter ◽  
Anthropogenic Aerosols ◽  
The North ◽  
The North Atlantic ◽  
Extratropical Circulation ◽  
Circulation Changes

Abstract The authors examine the key characteristics of the boreal winter extratropical circulation changes in response to anthropogenic aerosols, simulated with a coupled atmosphere–slab ocean general circulation model. The zonal-mean response features a pronounced equatorward shift of the Northern Hemisphere subtropical jet owing to the midlatitude aerosol cooling. The circulation changes also show strong zonal asymmetry. In particular, the cooling is more concentrated over the North Pacific than over the North Atlantic despite similar regional forcings. With the help of an idealized model, the authors demonstrate that the zonally asymmetrical response is linked tightly to the stationary Rossby waves excited by the anomalous diabatic heating over the tropical east Pacific. The altered wave pattern leads to a southeastward shift of the Aleutian low (and associated changes in winds and precipitation), while leaving the North Atlantic circulation relatively unchanged. Despite the rich circulation changes, the variations in the extratropical meridional latent heat transport are controlled strongly by the dependence of atmospheric moisture content on temperature. This suggests that one can project reliably the changes in extratropical zonal-mean precipitation solely from the global-mean temperature change, even without a good knowledge of the detailed circulation changes caused by aerosols. On the other hand, such knowledge is indispensable for understanding zonally asymmetrical (regional) precipitation changes.

scholarly journals Modelling the wintertime response to upper tropospheric and lower stratospheric ozone anomalies over the North Atlantic and Europe

2003 ◽  
Vol 21 (10) ◽  
pp. 2107-2118 ◽  
Author(s):  
I. Kirchner ◽  
D. Peters
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Initial Conditions ◽  
Stratospheric Ozone ◽  
Circulation Model ◽  
Sensitivity Study ◽  
Boreal Winter ◽  
Radiative Processes ◽  
The North ◽  
The North Atlantic

Abstract. During boreal winter months, mean longitude-dependent ozone changes in the upper troposphere and lower stratosphere are mainly caused by different ozone transport by planetary waves. The response to radiative perturbation induced by these ozone changes near the tropopause on the circulation is unclear. This response is investigated with the ECHAM4 general circulation model in a sensitivity study. In the simulation two different mean January realizations of the ozone field are implemented in ECHAM4. Both ozone fields are estimated on the basis of the observed mean January planetary wave structure of the 1980s. The first field represents a 14-year average (reference, 1979–1992) and the second one represents the mean ozone field change (anomaly, 1988–92) in boreal extra-tropics during the end of the 1980s. The model runs were carried out pairwise, with identical initial conditions for both ozone fields. Five statistically independent experiments were performed, forced with the observed sea surface temperatures for the period 1988 to 1992. The results support the hypothesis that the zonally asymmetric ozone changes of the 80s triggered a systematic alteration of the circulation over the North Atlantic – European region. It is suggested that this feedback process is important for the understanding of the decadal coupling between troposphere and stratosphere, as well as between subtropics and extra-tropics in winter.Key words. Meteorology and atmospheric dynamics (general circulation; radiative processes; synoptic-scale meteorology)

scholarly journals Temperature Anomalies in the Northeastern North Atlantic: Subpolar and Subtropical Precursors on Multiannual Time Scales

2007 ◽  
Vol 20 (10) ◽  
pp. 1976-1990
Author(s):  
Martina M. Junge ◽  
Klaus Fraedrich
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Time Scale ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
State Variables ◽  
Decadal Time Scale ◽  
The North ◽  
The North Atlantic

Abstract An adjoint ocean general circulation model of the North Atlantic is employed to calculate sensitivities of temperature in the northeastern North Atlantic with respect to atmospheric nonlocal fluxes and ocean state variables at prior times, up to 7 yr. Maximum sensitivities cross the Atlantic from east to west within 3 to 4 yr. On this interannual time scale, advection of temperature perturbations by the climatological flow is suggested as the prime mechanism responsible for SST perturbations in the northeastern North Atlantic. The pathway of sensitivities lies preferentially beneath the surface and can be understood in terms of the reemergence mechanism. This provides the link between local forcing, mainly by heat flux in winter, and resurfacing of perturbations at remote locations. On the multiannual to decadal time scale, the western subpolar gyre plays a key role: negative temperature sensitivities that evolve in parallel with positive salinity sensitivities in the Labrador and Irminger Seas give rise to pressure gradients and velocity perturbations that have effects on SST by modifying the oceanic heat transport into the northeastern North Atlantic. Together with additional influence from the Tropics and the subtropical gyre on time scales of 5 yr and beyond, these sensitivities combine to make a plethora of time scales that play a role in shaping SST perturbations in the North Atlantic.

scholarly journals Marine productivity response to Heinrich events: a model-data comparison

2012 ◽  
Vol 8 (5) ◽  
pp. 1581-1598 ◽  
Author(s):  
V. Mariotti ◽  
L. Bopp ◽  
A. Tagliabue ◽  
M. Kageyama ◽  
D. Swingedouw
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Global Ocean ◽  
Biogeochemical Model ◽  
Heinrich Events ◽  
The North ◽  
Heinrich Event ◽  
The North Atlantic ◽  
Marine Productivity

Abstract. Marine sediments records suggest large changes in marine productivity during glacial periods, with abrupt variations especially during the Heinrich events. Here, we study the response of marine biogeochemistry to such an event by using a biogeochemical model of the global ocean (PISCES) coupled to an ocean-atmosphere general circulation model (IPSL-CM4). We conduct a 400-yr-long transient simulation under glacial climate conditions with a freshwater forcing of 0.1 Sv applied to the North Atlantic to mimic a Heinrich event, alongside a glacial control simulation. To evaluate our numerical results, we have compiled the available marine productivity records covering Heinrich events. We find that simulated primary productivity and organic carbon export decrease globally (by 16% for both) during a Heinrich event, albeit with large regional variations. In our experiments, the North Atlantic displays a significant decrease, whereas the Southern Ocean shows an increase, in agreement with paleo-productivity reconstructions. In the Equatorial Pacific, the model simulates an increase in organic matter export production but decreased biogenic silica export. This antagonistic behaviour results from changes in relative uptake of carbon and silicic acid by diatoms. Reasonable agreement between model and data for the large-scale response to Heinrich events gives confidence in models used to predict future centennial changes in marine production. In addition, our model allows us to investigate the mechanisms behind the observed changes in the response to Heinrich events.

The Effect of a Large Freshwater Perturbation on the Glacial North Atlantic Ocean Using a Coupled General Circulation Model

2006 ◽  
Vol 19 (17) ◽  
pp. 4436-4447 ◽  
Author(s):  
C. D. Hewitt ◽  
A. J. Broccoli ◽  
M. Crucifix ◽  
J. M. Gregory ◽  
J. F. B. Mitchell ◽  
...  
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Quasi Equilibrium ◽  
Last Glacial ◽  
Proxy Records ◽  
The North ◽  
The North Atlantic ◽  
The Last Glacial

Abstract The commonly held view of the conditions in the North Atlantic at the last glacial maximum, based on the interpretation of proxy records, is of large-scale cooling compared to today, limited deep convection, and extensive sea ice, all associated with a southward displaced and weakened overturning thermohaline circulation (THC) in the North Atlantic. Not all studies support that view; in particular, the “strength of the overturning circulation” is contentious and is a quantity that is difficult to determine even for the present day. Quasi-equilibrium simulations with coupled climate models forced by glacial boundary conditions have produced differing results, as have inferences made from proxy records. Most studies suggest the weaker circulation, some suggest little or no change, and a few suggest a stronger circulation. Here results are presented from a three-dimensional climate model, the Hadley Centre Coupled Model version 3 (HadCM3), of the coupled atmosphere–ocean–sea ice system suggesting, in a qualitative sense, that these diverging views could all have occurred at different times during the last glacial period, with different modes existing at different times. One mode might have been characterized by an active THC associated with moderate temperatures in the North Atlantic and a modest expanse of sea ice. The other mode, perhaps forced by large inputs of meltwater from the continental ice sheets into the northern North Atlantic, might have been characterized by a sluggish THC associated with very cold conditions around the North Atlantic and a large areal cover of sea ice. The authors’ model simulation of such a mode, forced by a large input of freshwater, bears several of the characteristics of the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) Project’s reconstruction of glacial sea surface temperature and sea ice extent.

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