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 Estimating the Decadal Predictability of a Coupled AOGCM

2004 ◽  
Vol 17 (22) ◽  
pp. 4463-4472 ◽  
Author(s):  
Holger Pohlmann ◽  
Michael Botzet ◽  
Mojib Latif ◽  
Andreas Roesch ◽  
Martin Wild ◽  
...  
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
General Circulation ◽  
Circulation Model ◽  
Tropospheric Temperature ◽  
Oceanic Circulation ◽  
Max Planck ◽  
The North ◽  
The North Atlantic ◽  
Decadal Predictability

Abstract On seasonal time scales, ENSO prediction has become feasible in an operational framework in recent years. On decadal to multidecadal time scales, the variability of the oceanic circulation is assumed to provide a potential for climate prediction. To investigate the decadal predictability of the coupled atmosphere–ocean general circulation model (AOGCM) European Centre-Hamburg model version 5/Max Planck Institute Ocean Model (ECHAM5/MPI-OM), a 500-yr-long control integration and “perfect model” predictability experiments are analyzed. The results show that the sea surface temperatures (SSTs) of the North Atlantic, Nordic Seas, and Southern Ocean exhibit predictability on multidecadal time scales. Over the ocean, the predictability of surface air temperature (SAT) is very similar to that of SST. Over land, there is little evidence of decadal predictability of SAT except for some small maritime-influenced regions of Europe. The AOGCM produces predictable signals in lower-tropospheric temperature and precipitation over the North Atlantic, but not in sea level pressure.

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.

Anomalies of Meridional Overturning: Mechanisms in the North Atlantic

2005 ◽  
Vol 35 (8) ◽  
pp. 1455-1472 ◽  
Author(s):  
Armin Köhl
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
General Circulation ◽  
Numerical Models ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract Optimal observations are used to investigate the overturning streamfunction in the North Atlantic at 30°N and 900-m depth. Those observations are designed to impact the meridional overturning circulation (MOC) in numerical models maximally when assimilated and therefore establish the most efficient observation network for studying changes in the MOC. They are also ideally suited for studying the related physical mechanisms in a general circulation model. Optimal observations are evaluated here in the framework of a global 1° model over a 10-yr period. Hydrographic observations useful to monitor the MOC are primarily located along the western boundary north of 30°N and along the eastern boundary south of 30°N. Additional locations are in the Labrador, Irminger, and Iberian Seas. On time scales of less than a year, variations in MOC are mainly wind driven and are made up through changes in Ekman transport and coastal up- and downwelling. Only a small fraction is buoyancy driven and constitutes a slow response, acting on time scales of a few years, to primarily wintertime anomalies in the Labrador and Irminger Seas. Those anomalies are communicated southward along the west coast by internal Kelvin waves at the depth level of Labrador Sea Water. They primarily set the conditions at the northern edge of the MOC anomaly. The southern edge is mainly altered through Rossby waves of the advective type, which originate from temperature and salinity anomalies in the Canary Basin. Those anomalies are amplified on their way westward in the baroclinic unstable region of the subtropical gyre. The exact meridional location of the maximum MOC response is therefore set by the ratio of the strength of these two signals.

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.

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