scholarly journals Simulating Southern Hemisphere extra-tropical climate variability with an idealized coupled atmosphere-ocean model

2011 ◽  
Vol 4 (3) ◽  
pp. 1907-1940
Author(s):  
H. Kurzke ◽  
M. V. Kurgansky ◽  
K. Dethloff ◽  
D. Handorf ◽  
D. Olbers ◽  
...  
Keyword(s):  
Climate Variability ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Low Frequency ◽  
Coupled Model ◽  
Circulation Model ◽  
Ocean Model ◽  
Decadal Climate Variability ◽  
Basic Model ◽  
Physics Model

Abstract. The design and implementation of a simplified coupled atmosphere-ocean model over mid and high Southern Hemisphere latitudes are described. The development of the model is motivated by the clear indications of important low-frequency variability of extratropical origin in atmosphere-only models and the crucial role of atmosphere-ocean interaction in altering and shaping the climate variability on decadal and multidecadal time-scales. The basic model consists of an idealized quasi-geostrophic model of Southern Hemisphere's wintertime atmospheric circulation coupled to a general ocean circulation model with simplified physics. Model spin-up is described, some basic descriptors of the model climatology are discussed, and it is argued that the model exhibits skill in reproducing essential features of decadal and multi-decadal climate variability in the extratropical Southern Hemisphere. Notably, 1000 yr long coupled model simulations reveal sea surface temperature fluctuations on the timescale of several decades in the Antarctic Circumpolar Current region.

scholarly journals Simulating Southern Hemisphere extra-tropical climate variability with an idealised coupled atmosphere-ocean model

2012 ◽  
Vol 5 (5) ◽  
pp. 1161-1175 ◽  
Author(s):  
H. Kurzke ◽  
M. V. Kurgansky ◽  
K. Dethloff ◽  
D. Handorf ◽  
S. Erxleben ◽  
...  
Keyword(s):  
Climate Variability ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Coupled Model ◽  
Circulation Model ◽  
Horizontal Resolution ◽  
Global Ocean ◽  
Simplified Model ◽  
Cmip5 Models ◽  
Decadal Climate Variability

Abstract. A quasi-geostrophic model of Southern Hemisphere's wintertime atmospheric circulation with horizontal resolution T21 has been coupled to a global ocean circulation model with a resolution of 2° × 2° and simplified physics. This simplified coupled model reproduces qualitatively some features of the first and the second EOF of atmospheric 833 hPa geopotential height in accordance with NCEP data. The variability patterns of the simplified coupled model have been compared with variability patterns simulated by four complex state-of-the-art coupled CMIP5 models. The first EOF of the simplified model is too zonal and does not reproduce the right position of the centre of action over the Pacific Ocean and its extension to the tropics. The agreement in the second EOF between the simplified and the CMIP5 models is better. The total variance of the simplified model is weaker than the observational variance and those of the CMIP5 models. The transport properties of the Southern Ocean circulation are in qualitative accord with observations. The simplified model exhibits skill in reproducing essential features of decadal and multi-decadal climate variability in the extratropical Southern Hemisphere. Notably, 800 yr long coupled model simulations reveal sea surface temperature fluctuations on the timescale of several decades in the Antarctic Circumpolar Current region.

scholarly journals On the Total, Mean, and Eddy Heat and Freshwater Transports in the Southern Hemisphere of a ⅛° × ⅛° Global Ocean Model

2007 ◽  
Vol 37 (2) ◽  
pp. 277-295 ◽  
Author(s):  
A. J. Meijers ◽  
N. L. Bindoff ◽  
J. L. Roberts
Keyword(s):  
Southern Hemisphere ◽  
Ocean Circulation ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Drake Passage ◽  
Ocean Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Eddy Transport

Abstract The large-scale volume, heat, and freshwater ocean transports in the Southern Hemisphere are investigated using time-averaged output from a seasonless, high-resolution general circulation model. The ocean circulation is realistic, and property transports are comparable to observations. The Antarctic Circumpolar Current (ACC) carries 144 Sv (Sv ≡ 106 m3 s−1) of water eastward across Drake Passage, increasing to 155 Sv south of Australia because of the Indonesian Throughflow (ITF). There is a clear Indo-Pacific gyre around Australia exchanging −10 Sv, 0.9 PW of heat, and 0.2 Sv of freshwater through the ITF, and there is a 9-Sv leakage from the Tasman Sea to the Indian Ocean. The transport of heat and freshwater by eddies is localized to the upper 1000 m of the water column and specific regions, such as western boundary currents, confluences, and the subantarctic front (SAF). Eddy transport of heat and freshwater is negligible in gyre interiors and south of the SAF but is vital across the northern edge of the ACC, in particular at the Agulhas Retroflection where eddies accomplish almost 100% of the net ocean heat and 60% of the southward freshwater transport. The eddy transport is almost zero across the latitude of Drake Passage while in a quasi-Lagrangian frame eddy transports are significant across the ACC but surprisingly are still smaller than the mean transport of heat. Mean and eddy property transport divergences are found to be strongly compensating in areas of high eddy activity. This is caused by increased baroclinic instability in strong mean flows, which induces an opposing eddy transport. This relationship is observed to be stronger in the case of horizontal heat transport than in corresponding horizontal freshwater transports.

Quantifying the Predictive Skill in Long-Range Forecasting. Part II: Model Error in Coarse-Grained Markov Models with Application to Ocean-Circulation Regimes

2012 ◽  
Vol 25 (6) ◽  
pp. 1814-1826 ◽  
Author(s):  
Dimitrios Giannakis ◽  
Andrew J. Majda
Keyword(s):  
Long Range ◽  
Ocean Circulation ◽  
Climate Models ◽  
Markov Models ◽  
Low Frequency ◽  
Model Error ◽  
Ocean Model ◽  
Coarse Grained ◽  
Predictive Skill ◽  
Relaxation To Equilibrium

Abstract An information-theoretic framework is developed to assess the predictive skill and model error in imperfect climate models for long-range forecasting. Here, of key importance is a climate equilibrium consistency test for detecting false predictive skill, as well as an analogous criterion describing model error during relaxation to equilibrium. Climate equilibrium consistency enforces the requirement that long-range forecasting models should reproduce the climatology of prediction observables with high fidelity. If a model meets both climate consistency and the analogous criterion describing model error during relaxation to equilibrium, then relative entropy can be used as an unbiased superensemble measure of the model’s skill in long-range coarse-grained forecasts. As an application, the authors investigate the error in modeling regime transitions in a 1.5-layer ocean model as a Markov process and identify models that are strongly persistent but their predictive skill is false. The general techniques developed here are also useful for estimating predictive skill with model error for Markov models of low-frequency atmospheric regimes.

scholarly journals Central Mediterranean Sea forecast: effects of high-resolution atmospheric forcings

2006 ◽  
Vol 3 (3) ◽  
pp. 637-669 ◽  
Author(s):  
S. Natale ◽  
R. Sorgente ◽  
S. Gaberšek ◽  
A. Ribotti ◽  
A. Olita
Keyword(s):  
High Resolution ◽  
Ocean Circulation ◽  
Regional Scale ◽  
Circulation Model ◽  
Surface Current ◽  
Ocean Model ◽  
Central Mediterranean ◽  
Resolution Model ◽  
Atmospheric Forcings ◽  
Very High

Abstract. Ocean forecasts over the Central Mediterranean, produced by a near real time regional scale system, have been evaluated in order to assess their predictability. The ocean circulation model has been forced at the surface by a medium, high or very high resolution atmospheric forcing. The simulated ocean parameters have been compared with satellite data and they were found to be generally in good agreement. High and very high resolution atmospheric forcings have been able to form noticeable, although short-lived, surface current structures, due to their ability to detect transient atmospheric disturbances. The existence of the current structures has not been directly assessed due to lack of measurements. The ocean model in the slave mode was not able to develop dynamics different from the driving coarse resolution model which provides the boundary conditions.

scholarly journals Multi-grid algorithm for passive tracer transport in the NEMO ocean circulation model: a case study with the NEMO OGCM (version 3.6)

2020 ◽  
Vol 13 (11) ◽  
pp. 5465-5483
Author(s):  
Clément Bricaud ◽  
Julien Le Sommer ◽  
Gurvan Madec ◽  
Christophe Calone ◽  
Julie Deshayes ◽  
...  
Keyword(s):  
Spatial Resolution ◽  
Ocean Circulation ◽  
Circulation Model ◽  
Ocean Model ◽  
Ocean Dynamics ◽  
Global Ocean ◽  
Passive Tracer ◽  
Tracer Transport ◽  
Ocean Circulation Model ◽  
Grid Algorithm

Abstract. Ocean biogeochemical models are key tools for both scientific and operational applications. Nevertheless the cost of these models is often expensive because of the large number of biogeochemical tracers. This has motivated the development of multi-grid approaches where ocean dynamics and tracer transport are computed on grids of different spatial resolution. However, existing multi-grid approaches to tracer transport in ocean modelling do not allow the computation of ocean dynamics and tracer transport simultaneously. This paper describes a new multi-grid approach developed for accelerating the computation of passive tracer transport in the Nucleus for European Modelling of the Ocean (NEMO) ocean circulation model. In practice, passive tracer transport is computed at runtime on a grid with coarser spatial resolution than the hydrodynamics, which reduces the CPU cost of computing the evolution of tracers. We describe the multi-grid algorithm, its practical implementation in the NEMO ocean model, and discuss its performance on the basis of a series of sensitivity experiments with global ocean model configurations. Our experiments confirm that the spatial resolution of hydrodynamical fields can be coarsened by a factor of 3 in both horizontal directions without significantly affecting the resolved passive tracer fields. Overall, the proposed algorithm yields a reduction by a factor of 7 of the overhead associated with running a full biogeochemical model like PISCES (with 24 passive tracers). Propositions for further reducing this cost without affecting the resolved solution are discussed.

Zonal Wave 3 Pattern in the Southern Hemisphere generated by tropical convection

2021 ◽  
Author(s):  
Rishav Goyal ◽  
Martin Jucker ◽  
Alex Sen Gupta ◽  
Harry Hendon ◽  
Matthew England
Keyword(s):  
Sea Ice ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
General Circulation ◽  
Deep Convection ◽  
Circulation Model ◽  
Hadley Cell ◽  
Wave Source ◽  
The Tropics

Abstract A distinctive feature of the Southern Hemisphere (SH) extratropical atmospheric circulation is the quasi-stationary zonal wave 3 (ZW3) pattern, characterized by three high and three low-pressure centers around the SH extratropics. This feature is present in both the mean atmospheric circulation and its variability on daily, seasonal and interannual timescales. While the ZW3 pattern has significant impacts on meridional heat transport and Antarctic sea ice extent, the reason for its existence remains uncertain, although it has long been assumed to be linked to the existence of three major land masses in the SH extratropics. Here we use an atmospheric general circulation model to show that the stationery ZW3 pattern is instead driven by zonal asymmetric deep atmospheric convection in the tropics, with little to no role played by the orography or land masses in the extratropics. Localized regions of deep convection in the tropics form a local Hadley cell which in turn creates a wave source in the subtropics that excites a poleward and eastward propagating wave train which forms stationary waves in the SH high latitudes. Our findings suggest that changes in tropical deep convection, either due to natural variability or climate change, will impact the zonal wave 3 pattern, with implications for Southern Hemisphere climate, ocean circulation, and sea-ice.

Evaluating Effects of Observational Data Assimilation in General Ocean Circulation Model by Ensemble Kalman Filtering: Numerical Experiments with Synthetic Observations

2021 ◽  
pp. 50-66
Author(s):  
V. N. Stepanov ◽  
◽  
Yu. D. Resnyanskii ◽  
B. S. Strukov ◽  
A. A. Zelen’ko ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Observational Data ◽  
Ocean Circulation ◽  
Numerical Experiments ◽  
Turbulent Viscosity ◽  
Bottom Topography ◽  
Synthetic Data ◽  
Circulation Model ◽  
Ocean Model ◽  
Global Ocean

The quality of simulation of model fields is analyzed depending on the assimilation of various types of data using the PDAF software product assimilating synthetic data into the NEMO global ocean model. Several numerical experiments are performed to simulate the ocean–sea ice system. Initially, free model was run with different values of the coefficients of horizontal turbulent viscosity and diffusion, but with the same atmospheric forcing. The model output obtained with higher values of these coefficients was used to determine the first guess fields in subsequent experiments with data assimilation, while the model results with lower values of the coefficients were assumed to be true states, and a part of these results was used as synthetic observations. The results are analyzed that are assimilation of various types of observational data using the Kalman filter included through the PDAF to the NEMO model with real bottom topography. It is shown that a degree of improving model fields in the process of data assimilation is highly dependent on the structure of data at the input of the assimilation procedure.

scholarly journals Changes in Ocean Heat Content Caused by Wave-Induced Mixing in a High-Resolution Ocean Model

2018 ◽  
Vol 48 (5) ◽  
pp. 1139-1150 ◽  
Author(s):  
Lachlan Stoney ◽  
Kevin J. E. Walsh ◽  
Steven Thomas ◽  
Paul Spence ◽  
Alexander V. Babanin
Keyword(s):  
High Resolution ◽  
Ocean Circulation ◽  
Heat Content ◽  
Circulation Model ◽  
Additional Term ◽  
Ocean Model ◽  
Ocean Heat Content ◽  
Sea Surface ◽  
Wave Heights ◽  
Wave Induced

Abstract A parameterization of turbulent mixing from unbroken surface waves is included in a 16-yr simulation within a high-resolution ocean circulation model (MOM5). This “surface wave mixing” (SWM) derives from the wave orbital motion and is parameterized as an additional term in a k-epsilon model. We show that SWM leads to significant changes in sea surface temperatures but smaller changes in ocean heat content, and show the extent to which these changes can reduce pre-existing model biases with respect to observed data. Specifically, SWM leads to a widespread improvement in sea surface temperature in both hemispheres in summer and winter, while for ocean heat content the improvements are less clear. In addition, we show that introducing SWM can lead to an accumulation of wave-induced ocean heat content between years. While it has been well established that secular positive trends exist in global wave heights, we find that such trends are relatively unimportant in driving the accumulation of wave-induced ocean heat content. Rather, in response to the new source of mixing, the simulated ocean climate evolves toward a new equilibrium with greater total ocean heat content.

scholarly journals Simulating the Role of Subtropical Stratocumulus Clouds in Driving Pacific Climate Variability

2014 ◽  
Vol 27 (13) ◽  
pp. 5119-5131 ◽  
Author(s):  
Katinka Bellomo ◽  
Amy Clement ◽  
Thorsten Mauritsen ◽  
Gaby Rädel ◽  
Bjorn Stevens
Keyword(s):  
Climate Variability ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Cloud Feedback ◽  
Decadal Climate Variability ◽  
Cloud Feedbacks ◽  
Southeast Pacific ◽  
Low Cloud ◽  
Pacific Climate Variability

This study examines the influence of the northeast and southeast Pacific subtropical stratocumulus cloud regions on the modes of Pacific climate variability simulated by an atmospheric general circulation model (ECHAM6) coupled to a slab ocean. The sensitivity of cloud liquid water to underlying SST is changed in the radiation module of the atmospheric model to increase the strength of positive low-cloud feedback in the two regions. Enhanced low-cloud feedback increases the persistence and variance of the leading modes of climate variability at decadal and longer time scales. Additional integrations show that the southeast Pacific influences climate variability in the equatorial ENSO region, whereas the effects of the northeast Pacific remain confined to the North Pacific. The results herein suggest that a positive feedback among SST, cloud cover, and large-scale atmospheric circulation can explain decadal climate variability in the Pacific Ocean. In particular, cloud feedbacks over the subtropical stratocumulus regions set the time scale of climate variability. A proper representation of low-level cloud feedbacks in the subtropical stratocumulus regions could therefore improve the simulation of Pacific climate variability.

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