Planetary Waves, Cyclogenesis, and the Irregular Breakdown of Zonal Motion over the North Atlantic

2009 ◽  
Vol 137 (11) ◽  
pp. 3907-3917 ◽  
Author(s):  
Paul J. Roebber
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Low Frequency ◽  
Zonal Flow ◽  
Forecast Model ◽  
The North ◽  
Medium Range Forecast ◽  
Probabilistic Forecasts ◽  
The North Atlantic ◽  
Flow Transitions

Abstract Numerous observational and modeling studies have suggested the importance of cyclogenesis to the breakdown of the zonal flow and the maintenance of atmospheric blocks, whereas other studies have shown that low-frequency dynamics are sufficient to produce and maintain these patterns. Experiments with a simple model of the general circulation and empirical evidence obtained from reanalysis and cyclone tracking data are used to develop a conceptual understanding of this irregular response. The model’s results and observational data are consistent with the idea that the North Atlantic flow response to cyclone forcing is preconditioned by the state of the hemispheric circulation. A characteristic hemispheric flow configuration reminiscent of the shedding of a potential vorticity (PV) filament and tightening of the PV gradient is particularly responsive to cyclogenesis, with the likelihood of below 10th percentile North Atlantic zonal flow under these conditions increased by a factor of 2.37 for each cyclone event. A second characteristic pattern, reminiscent of the PV wave-breaking anticyclonic roll up, responds in an opposite way to cyclogenesis, with a decrease in the likelihood of below 10th percentile zonal flow by a factor of 0.67. This perspective is connected to the decades of research on blocking and opens an opportunity for the development of medium-range forecast model postprocessors that might provide probabilistic forecasts of North Atlantic flow transitions.

scholarly journals Direct temporal cascade of temperature variance in eddy-permitting simulations of multidecadal variability

2020 ◽  
Author(s):  
Antoine Hochet ◽  
Thierry Huck ◽  
Olivier Arzel ◽  
Florian Sevellec ◽  
Alain Colin de Verdiere ◽  
...  
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Numerical Models ◽  
Low Frequency ◽  
Circulation Model ◽  
Temperature Variance ◽  
The North ◽  
Basin Scale ◽  
The North Atlantic ◽  
Meso Scale

<p>The North Atlantic is characterized by basin-scale multidecadal fluctuations of the sea surface temperature with periods ranging from 20 to 70 years.<br>One candidate for such a variability is a large-scale baroclinic instability of the North Atlantic Current. Because of the long time scales involved, most of the studies devoted to this problem are based on low resolution numerical models leaving aside the effect of explicit meso-scale eddies.   <br>How high-frequency motions associated with the meso-scale eddy field affect the basin-scale low-frequency variabiliy is the central question of this study.</p><p>This issue is addressed using an idealized configuration of an Ocean General Circulation Model at eddy-permitting resolution (20 km). A new diagnostic allowing the calculation of nonlinear fluxes of temperature variance in frequency space is presented. Using this diagnostic, we show that the primary effect of meso-scale eddies is to damp low frequency  temperature variance and to transfer it to high frequencies.</p>

scholarly journals Direct Temporal Cascade of Temperature Variance in Eddy-Permitting Simulations of Multidecadal Variability

2020 ◽  
Vol 33 (21) ◽  
pp. 9409-9425
Author(s):  
Antoine Hochet ◽  
Thierry Huck ◽  
Olivier Arzel ◽  
Florian Sévellec ◽  
Alain Colin de Verdière ◽  
...  
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Numerical Models ◽  
Mesoscale Eddy ◽  
Low Frequency ◽  
Mesoscale Eddies ◽  
Temperature Variance ◽  
The North ◽  
Basin Scale ◽  
The North Atlantic

AbstractThe North Atlantic is characterized by basin-scale multidecadal fluctuations of the sea surface temperature with periods ranging from 20 to 70 years. One candidate for such a variability is a large-scale baroclinic instability of the temperature gradients across the Atlantic associated with the North Atlantic Current. Because of the long time scales involved, most of the studies devoted to this problem are based on low-resolution numerical models leaving aside the effect of explicit mesoscale eddies. How high-frequency motions associated with the mesoscale eddy field affect the basin-scale low-frequency variability is the central question of this study. This issue is addressed using an idealized configuration of an ocean general circulation model at eddy-permitting resolution (20 km). A new diagnostic allowing the calculation of nonlinear fluxes of temperature variance in frequency space is presented. Using this diagnostic, we show that the primary effect of mesoscale eddies is to damp low-frequency temperature variance and to transfer it to high frequencies.

scholarly journals Transport and storage of anthropogenic C in the North Atlantic Subpolar Ocean

2018 ◽  
Vol 15 (14) ◽  
pp. 4661-4682 ◽  
Author(s):  
Virginie Racapé ◽  
Patricia Zunino ◽  
Herlé Mercier ◽  
Pascale Lherminier ◽  
Laurent Bopp ◽  
...  
Keyword(s):  
Interannual Variability ◽  
North Atlantic ◽  
General Circulation ◽  
Meridional Overturning Circulation ◽  
Intermediate Water ◽  
Data Set ◽  
The North ◽  
Storage Rate ◽  
The North Atlantic ◽  
And Storage

Abstract. The North Atlantic Ocean is a major sink region for atmospheric CO2 and contributes to the storage of anthropogenic carbon (Cant). While there is general agreement that the intensity of the meridional overturning circulation (MOC) modulates uptake, transport and storage of Cant in the North Atlantic Subpolar Ocean, processes controlling their recent variability and evolution over the 21st century remain uncertain. This study investigates the relationship between transport, air–sea flux and storage rate of Cant in the North Atlantic Subpolar Ocean over the past 53 years. Its relies on the combined analysis of a multiannual in situ data set and outputs from a global biogeochemical ocean general circulation model (NEMO–PISCES) at 1∕2∘ spatial resolution forced by an atmospheric reanalysis. Despite an underestimation of Cant transport and an overestimation of anthropogenic air–sea CO2 flux in the model, the interannual variability of the regional Cant storage rate and its driving processes were well simulated by the model. Analysis of the multi-decadal simulation revealed that the MOC intensity variability was the major driver of the Cant transport variability at 25 and 36∘ N, but not at OVIDE. At the subpolar OVIDE section, the interannual variability of Cant transport was controlled by the accumulation of Cant in the MOC upper limb. At multi-decadal timescales, long-term changes in the North Atlantic storage rate of Cant were driven by the increase in air–sea fluxes of anthropogenic CO2. North Atlantic Central Water played a key role for storing Cant in the upper layer of the subtropical region and for supplying Cant to Intermediate Water and North Atlantic Deep Water. The transfer of Cant from surface to deep waters occurred mainly north of the OVIDE section. Most of the Cant transferred to the deep ocean was stored in the subpolar region, while the remainder was exported to the subtropical gyre within the lower MOC.

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.

scholarly journals Modeling dust emission response to MIS 3 millennial climate variations from the perspective of East European loess deposits

2013 ◽  
Vol 9 (1) ◽  
pp. 143-185 ◽  
Author(s):  
A. Sima ◽  
M. Kageyama ◽  
D.-D. Rousseau ◽  
G. Ramstein ◽  
Y. Balkanski ◽  
...  
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Reference Site ◽  
Dust Emission ◽  
Dust Storms ◽  
Surface Model ◽  
Climate Signal ◽  
The North ◽  
Loess Deposits ◽  
The North Atlantic

Abstract. European loess sequences of the last glacial period (~ 100–15 kyr BP) show periods of strong dust accumulation alternating with episodes of reduced sedimentation, favoring soil development. In the western part of the loess belt centered around 50° N, these variations appear to have been caused by the North Atlantic rapid climate changes: the Dansgaard-Oeschger (DO) and Heinrich (H) events. It has been recently suggested that the North-Atlantic climate signal can be detected further east, in loess deposits from Stayky (50° 05.65' N, 30° 53.92' E), Ukraine. Here we use climate and dust emission modeling to investigate this data interpretation. We focus on the areas north and northeast of the Carpathians, where loess deposits can be found, and the corresponding main dust sources must have been located as well. The simulations, performed with the LMDZ atmospheric general circulation model and the ORCHIDEE land-surface model, represent a Greenland stadial, a DO interstadial and an H event respectively. Placed in Marine Isotope Stage 3 (~ 60–25 kyr BP) conditions, they only differ by the surface conditions imposed in the North Atlantic between 30° and 63° N. The main source for the loess deposits in the studied area is identified as a dust deflation band, with two very active spots located west–northwest from our reference site. Emissions only occur between February and June. Differences from one deflation spot to another, and from one climate state to another, are explained by analyzing the relevant meteorological and surface variables. Over most of the source region, the annual emission fluxes in the "interstadial" experiment are 30 to 50% lower than the "stadial" values; they would only be about 20% lower if the inhibition of dust uplift by the vegetation were not taken into account. Assuming that lower emissions result in reduced dust deposition leads us to the conclusion that the loess-paleosol stratigraphic succession in the Stayky area reflects indeed North-Atlantic millennial variations. In the main deflation areas of Western Europe, the vegetation effect alone determined most of the ~ 50% stadial-interstadial flux differences. Even if its impact in Eastern Europe is less pronounced, this effect remains a key factor in modulating aeolian emissions at millennial timescale. Conditions favorable to initiating particularly strong dust storms within a few hundred kilometers upwind from our reference site, simulated in the month of April of the "H event" experiment, support the identification of H events as layers of particularly coarse sedimentation in some very detailed profiles.

scholarly journals Contributions of Atmospheric Stochastic Forcing and Intrinsic Ocean Modes to North Atlantic Ocean Interdecadal Variability

2020 ◽  
Vol 33 (6) ◽  
pp. 2351-2370 ◽  
Author(s):  
Olivier Arzel ◽  
Thierry Huck
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
General Circulation ◽  
Thermal Structure ◽  
Ocean Dynamics ◽  
Dimensionless Number ◽  
Stochastic Forcing ◽  
The North ◽  
Wide Range ◽  
The North Atlantic

AbstractAtmospheric stochastic forcing associated with the North Atlantic Oscillation (NAO) and intrinsic ocean modes associated with the large-scale baroclinic instability of the North Atlantic Current (NAC) are recognized as two strong paradigms for the existence of the Atlantic multidecadal oscillation (AMO). The degree to which each of these factors contribute to the low-frequency variability of the North Atlantic is the central question in this paper. This issue is addressed here using an ocean general circulation model run under a wide range of background conditions extending from a supercritical regime where the oceanic variability spontaneously develops in the absence of any atmospheric noise forcing to a damped regime where the variability requires some noise to appear. The answer to the question is captured by a single dimensionless number Γ measuring the ratio between the oceanic and atmospheric contributions, as inferred from the buoyancy variance budget of the western subpolar region. Using this diagnostic, about two-thirds of the sea surface temperature (SST) variance in the damped regime is shown to originate from atmospheric stochastic forcing whereas heat content is dominated by internal ocean dynamics. Stochastic wind stress forcing is shown to substantially increase the role played by damped ocean modes in the variability. The thermal structure of the variability is shown to differ fundamentally between the supercritical and damped regimes, with abrupt modifications around the transition between the two regimes. Ocean circulation changes are further shown to be unimportant for setting the pattern of SST variability in the damped regime but are fundamental for a preferred time scale to emerge.

scholarly journals North Atlantic 20th century multidecadal variability in coupled climate models: sea surface temperature and ocean overturning circulation

Ocean Science ◽  
2011 ◽  
Vol 7 (3) ◽  
pp. 389-404 ◽  
Author(s):  
I. Medhaug ◽  
T. Furevik
Keyword(s):  
Surface Temperature ◽  
20Th Century ◽  
North Atlantic ◽  
General Circulation ◽  
General Circulation Models ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Atlantic Sector ◽  
The North ◽  
The North Atlantic

Abstract. Output from a total of 24 state-of-the-art Atmosphere-Ocean General Circulation Models is analyzed. The models were integrated with observed forcing for the period 1850–2000 as part of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. All models show enhanced variability at multi-decadal time scales in the North Atlantic sector similar to the observations, but with a large intermodel spread in amplitudes and frequencies for both the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The models, in general, are able to reproduce the observed geographical patterns of warm and cold episodes, but not the phasing such as the early warming (1930s–1950s) and the following colder period (1960s–1980s). This indicates that the observed 20th century extreme in temperatures are due to primarily a fortuitous phasing of intrinsic climate variability and not dominated by external forcing. Most models show a realistic structure in the overturning circulation, where more than half of the available models have a mean overturning transport within the observed estimated range of 13–24 Sverdrup. Associated with a stronger than normal AMOC, the surface temperature is increased and the sea ice extent slightly reduced in the North Atlantic. Individual models show potential for decadal prediction based on the relationship between the AMO and AMOC, but the models strongly disagree both in phasing and strength of the covariability. This makes it difficult to identify common mechanisms and to assess the applicability for predictions.

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