scholarly journals Application of the Gaussian anamorphosis to assimilation in a 3-D coupled physical-ecosystem model of the North Atlantic with the EnKF: a twin experiment

2009 ◽  
Vol 6 (1) ◽  
pp. 617-652 ◽  
Author(s):  
E. Simon ◽  
L. Bertino
Keyword(s):  
North Atlantic ◽  
Coupled Model ◽  
Surface Concentration ◽  
Ecosystem Model ◽  
Concentration Data ◽  
Linear Change ◽  
The North ◽  
Change Of Variables ◽  
Non Linear ◽  
Non Gaussian

Abstract. We consider the application of the Ensemble Kalman Filter (EnKF) to a coupled ocean ecosystem model (HYCOM-NORWECOM). Such models, especially the ecosystem models, are characterized by strongly non-linear interactions active in ocean blooms and present important limitations for the use of data assimilation methods based on linear statistical analysis. Besides the non-linearity of the model, one is confronted with physical/biological limitations, the analysis state having to be consistent with the model, especially with the constraints of positiveness of some variables. Furthermore the non-Gaussian distributions of the biogeochemical variables break an important assumption of the linear analysis, leading to a loss of optimality of the filter. We present an extension of the EnKF dealing with these limitations by introducing a non-linear change of variables (anamorphosis function) in order to execute the analysis step in a Gaussian space. We present also the initial results of the application of this non-Gaussian extension of the EnKF to the assimilation of simulated chlorophyll surface concentration data in a North Atlantic configuration of the HYCOM NORWECOM coupled model.

scholarly journals Application of the Gaussian anamorphosis to assimilation in a 3-D coupled physical-ecosystem model of the North Atlantic with the EnKF: a twin experiment

Ocean Science ◽  
2009 ◽  
Vol 5 (4) ◽  
pp. 495-510 ◽  
Author(s):  
E. Simon ◽  
L. Bertino
Keyword(s):  
North Atlantic ◽  
Coupled Model ◽  
Surface Concentration ◽  
Ecosystem Model ◽  
Concentration Data ◽  
Linear Change ◽  
The North ◽  
Change Of Variables ◽  
Non Linear ◽  
Non Gaussian

Abstract. We consider the application of the Ensemble Kalman Filter (EnKF) to a coupled ocean ecosystem model (HYCOM-NORWECOM). Such models, especially the ecosystem models, are characterized by strongly non-linear interactions active in ocean blooms and present important difficulties for the use of data assimilation methods based on linear statistical analysis. Besides the non-linearity of the model, one is confronted with the model constraints, the analysis state having to be consistent with the model, especially with respect to the constraints that some of the variables have to be positive. Furthermore the non-Gaussian distributions of the biogeochemical variables break an important assumption of the linear analysis, leading to a loss of optimality of the filter. We present an extension of the EnKF dealing with these difficulties by introducing a non-linear change of variables (anamorphosis function) in order to execute the analysis step in a Gaussian space, namely a space where the distributions of the transformed variables are Gaussian. We present also the initial results of the application of this non-Gaussian extension of the EnKF to the assimilation of simulated chlorophyll surface concentration data in a North Atlantic configuration of the HYCOM-NORWECOM coupled model.

ENSO influence on the North Atlantic European climate: a non-linear and non-stationary approach

2015 ◽  
Vol 47 (7-8) ◽  
pp. 2071-2084 ◽  
Author(s):  
Jorge López-Parages ◽  
Belén Rodríguez-Fonseca ◽  
Dietmar Dommenget ◽  
Claudia Frauen
Keyword(s):  
North Atlantic ◽  
The North ◽  
European Climate ◽  
Stationary Approach ◽  
Non Linear ◽  
The North Atlantic

scholarly journals Atlantic Meridional Overturning Circulation (AMOC) in CMIP5 Models: RCP and Historical Simulations

2013 ◽  
Vol 26 (18) ◽  
pp. 7187-7197 ◽  
Author(s):  
Wei Cheng ◽  
John C. H. Chiang ◽  
Dongxiao Zhang
Keyword(s):  
North Atlantic ◽  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
First Century ◽  
Model Intercomparison ◽  
The North ◽  
Intercomparison Project ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Twenty First Century

Abstract The Atlantic meridional overturning circulation (AMOC) simulated by 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the historical (1850–2005) and future climate is examined. The historical simulations of the AMOC mean state are more closely matched to observations than those of phase 3 of the Coupled Model Intercomparison Project (CMIP3). Similarly to CMIP3, all models predict a weakening of the AMOC in the twenty-first century, though the degree of weakening varies considerably among the models. Under the representative concentration pathway 4.5 (RCP4.5) scenario, the weakening by year 2100 is 5%–40% of the individual model's historical mean state; under RCP8.5, the weakening increases to 15%–60% over the same period. RCP4.5 leads to the stabilization of the AMOC in the second half of the twenty-first century and a slower (then weakening rate) but steady recovery thereafter, while RCP8.5 gives rise to a continuous weakening of the AMOC throughout the twenty-first century. In the CMIP5 historical simulations, all but one model exhibit a weak downward trend [ranging from −0.1 to −1.8 Sverdrup (Sv) century−1; 1 Sv ≡ 106 m3 s−1] over the twentieth century. Additionally, the multimodel ensemble–mean AMOC exhibits multidecadal variability with a ~60-yr periodicity and a peak-to-peak amplitude of ~1 Sv; all individual models project consistently onto this multidecadal mode. This multidecadal variability is significantly correlated with similar variations in the net surface shortwave radiative flux in the North Atlantic and with surface freshwater flux variations in the subpolar latitudes. Potential drivers for the twentieth-century multimodel AMOC variability, including external climate forcing and the North Atlantic Oscillation (NAO), and the implication of these results on the North Atlantic SST variability are discussed.

scholarly journals Projections of Cold Air Outbreaks In CMIP6 Earth System Models

2021 ◽  
Author(s):  
Erik T. Smith ◽  
Scott Sheridan
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
Cold Air ◽  
Complex Processes ◽  
The North ◽  
Meridional Overturning ◽  
Earth System Models ◽  
Cold Air Outbreaks

Abstract Historical and future simulated temperature data from five climate models in the Coupled Model Intercomparing Project Phase 6 (CMIP6) are used to understand how climate change might alter cold air outbreaks (CAOs) in the future. Three different Shared Socioeconomic Pathways (SSPs), SSP 1 – 2.6, SSP 2 – 4.5, and SSP 5 – 8.5 are examined to identify potential fluctuations in CAOs across the globe between 2015 and 2054. Though CAOs may remain persistent or even increase in some regions through 2040, all five climate models show CAOs disappearing by 2054 based on current climate percentiles. Climate models were able to accurately simulate the spatial distribution and trends of historical CAOs, but there were large errors in the simulated interannual frequency of CAOs in the North Atlantic and North Pacific. Fluctuations in complex processes, such as Atlantic Meridional Overturning Circulation, may be contributing to each model’s inability to simulate historical CAOs in these regions.

scholarly journals Understanding Bjerknes Compensation in Meridional Heat Transports and the Role of Freshwater in a Warming Climate

2018 ◽  
Vol 31 (12) ◽  
pp. 4791-4806 ◽  
Author(s):  
Qianzi Yang ◽  
Yingying Zhao ◽  
Qin Wen ◽  
Jie Yao ◽  
Haijun Yang
Keyword(s):  
Global Warming ◽  
North Atlantic ◽  
Coupled Model ◽  
Phase Changes ◽  
Box Model ◽  
Freshwater Flux ◽  
The North ◽  
Warming Climate ◽  
Transient Period ◽  
The North Atlantic

The Bjerknes compensation (BJC) under global warming is studied using a simple box model and a coupled Earth system model. The BJC states the out-of-phase changes in the meridional atmosphere and ocean heat transports. Results suggest that the BJC can occur during the transient period of global warming. During the transient period, the sea ice melting in the high latitudes can cause a significant weakening of the Atlantic meridional overturning circulation (AMOC), resulting in a cooling in the North Atlantic. The meridional contrast of sea surface temperature would be enhanced, and this can eventually enhance the Hadley cell and storm-track activities in the Northern Hemisphere. Accompanied by changes in both ocean and atmosphere circulations, the northward ocean heat transport in the Atlantic is decreased while the northward atmosphere heat transport is increased, and the BJC occurs in the Northern Hemisphere. Once the freshwater influx into the North Atlantic Ocean stops, or the ocean even loses freshwater because of strong heating in the high latitudes, the AMOC would recover. Both the atmosphere and ocean heat transports would be enhanced, and they can eventually recover to the state of the control run, leading to the BJC to become invalid. The above processes are clearly demonstrated in the coupled model CO2 experiment. Since it is difficult to separate the freshwater effect from the heating effect in the coupled model, a simple box model is used to understand the BJC mechanism and freshwater’s role under global warming. In a warming climate, the freshwater flux into the ocean can cool the global surface temperature, mitigating the temperature rise. Box model experiments indicate clearly that it is the freshwater flux into the North Atlantic that causes out-of-phase changes in the atmosphere and ocean heat transports, which eventually plays a stabilizing role in global climate change.

scholarly journals CMIP5 Projections of Arctic Amplification, of the North American/North Atlantic Circulation, and of Their Relationship

2015 ◽  
Vol 28 (13) ◽  
pp. 5254-5271 ◽  
Author(s):  
Elizabeth A. Barnes ◽  
Lorenzo M. Polvani
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Coupled Model ◽  
Arctic Region ◽  
The Arctic ◽  
Cmip5 Models ◽  
Arctic Amplification ◽  
Arctic Warming ◽  
The North ◽  
Sequence Of Events

Abstract Recent studies have hypothesized that Arctic amplification, the enhanced warming of the Arctic region compared to the rest of the globe, will cause changes in midlatitude weather over the twenty-first century. This study exploits the recently completed phase 5 of the Coupled Model Intercomparison Project (CMIP5) and examines 27 state-of-the-art climate models to determine if their projected changes in the midlatitude circulation are consistent with the hypothesized impact of Arctic amplification over North America and the North Atlantic. Under the largest future greenhouse forcing (RCP8.5), it is found that every model, in every season, exhibits Arctic amplification by 2100. At the same time, the projected circulation responses are either opposite in sign to those hypothesized or too widely spread among the models to discern any robust change. However, in a few seasons and for some of the circulation metrics examined, correlations are found between the model spread in Arctic amplification and the model spread in the projected circulation changes. Therefore, while the CMIP5 models offer some evidence that future Arctic warming may be able to modulate some aspects of the midlatitude circulation response in some seasons, the analysis herein leads to the conclusion that the net circulation response in the future is unlikely to be determined solely—or even primarily—by Arctic warming according to the sequence of events recently hypothesized.

The Control of Polar Haloclines by Along-Isopycnal Diffusion in Climate Models

2009 ◽  
Vol 22 (3) ◽  
pp. 486-498 ◽  
Author(s):  
Willem P. Sijp ◽  
Matthew H. England
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Multiple Equilibria ◽  
Climate Models ◽  
Coupled Model ◽  
Surface Salinity ◽  
The North ◽  
Winter Convection ◽  
Anomalous Surface ◽  
Strong Control

Abstract Increasing the value of along-isopycnal diffusivity in a coupled model is shown to lead to enhanced stability of North Atlantic Deep Water (NADW) formation with respect to freshwater (FW) perturbations. This is because the North Atlantic (NA) surface salinity budget is dominated by upward salt fluxes resulting from winter convection for low values of along-isopycnal diffusivity, whereas along-isopycnal diffusion exerts a strong control on NA surface salinity at higher diffusivity values. Shutdown of wintertime convection in response to a FW pulse allows the development of a halocline responsible for the suppression of deep sinking. In contrast to convection, isopycnal salt diffusion proves a more robust mechanism for preventing the formation of a halocline, as surface freshening leads only to a flattening of isopycnals, leaving at least some diffusive removal of anomalous surface FW in place. As a result, multiple equilibria are altogether absent for sufficiently high values of isopycnal diffusivity. Furthermore, the surface salinity budget of the North Pacific is also dominated by along-isopycnal diffusion when diffusivity values are sufficiently high, leading to a breakdown of the permanent halocline there and the associated onset of deep-water formation.

Modelling Wave-Current-Turbulence Interactions for Tidal Energy Applications

2020 ◽  
Author(s):  
Vengatesan Venugopal ◽  
Arne Vögler
Keyword(s):  
North Atlantic ◽  
Tidal Current ◽  
Current Model ◽  
Coupled Model ◽  
Wave Model ◽  
Tidal Energy ◽  
The North ◽  
Wave Parameters ◽  
The North Atlantic ◽  
Coupled Wave

Abstract This paper presents the nature of turbulence parameters produced from 3-dimensional numerical simulations using an ocean scale wave-tidal current model applied to tidal energy sites in the Orkney waters in the United Kingdom. The MIKE 21/3 coupled wave-current model is chosen for this study. The numerical modelling study is conducted in two stages. First, a North Atlantic Ocean large-scale wave model is employed to simulate wave parameters. Spatial and temporal wind speeds extracted from the European Centre for Medium Range Weather Forecast (ECMWF) is utilised to drive the North Atlantic wave model. Secondly, the wave parameters produced from the North Atlantic model are used as boundary conditions to run a coupled wave-tidal current model. A turbulence model representing the turbulence and eddy viscosity within the coupled model is chosen and the turbulence kinetic energy (TKE) due to wave-current interactions are computed. The coupled model is calibrated with Acoustic Doppler and Current Profiler (ADCP) measurements deployed close to a tidal energy site in the Inner Sound of the Pentland Firth. The model output parameters such as the current speed, TKE, horizontal and vertical eddy viscosities, significant wave height, peak wave period and wave directions are presented, and, their characteristics are discussed in detail.

scholarly journals Erratum to: ENSO influence on the North Atlantic European climate: a non-linear and non-stationary approach

2016 ◽  
Vol 48 (1-2) ◽  
pp. 707-707
Author(s):  
Jorge López-Parages ◽  
Belén Rodríguez-Fonseca ◽  
Dietmar Dommenget ◽  
Claudia Frauen
Keyword(s):  
North Atlantic ◽  
The North ◽  
European Climate ◽  
Stationary Approach ◽  
Non Linear ◽  
The North Atlantic
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