scholarly journals Mechanisms of Internal Atlantic Multidecadal Variability in HadGEM3-GC3.1 at Two Different Resolutions

2021 ◽  
pp. 1-60
Author(s):  
J. I. Robson ◽  
L. J. Wilcox ◽  
N. Dunstone
Keyword(s):  
Ocean Circulation ◽  
Low Frequency ◽  
Heat Fluxes ◽  
The Arctic ◽  
Western Boundary ◽  
Atlantic Multidecadal Variability ◽  
Surface Heat Fluxes ◽  
Ocean Salinity ◽  
Circulation Changes ◽  
Sst Anomalies

Abstract This study broadly characterises and compares the key processes governing internal AMV in two resolutions of HadGEM3-GC3.1: N216ORCA025, corresponding to ~ 60km in the atmosphere and 0.25° in the ocean, and N96ORCA1 (~ 135km / 1°). Both models simulate AMV with a timescale of 60-80 years, which is related to low frequency ocean and atmosphere circulation changes. In both models, ocean heat transport convergence dominates polar and subpolar AMV, whereas surface heat fluxes associated with cloud changes drive subtropicalAMV. However, details of the ocean circulation changes differ between the models. In N216 subpolar subsurface density anomalies propagate into the subtropics along the western boundary, consistent with the more coherent circulation changes and widespread development of SST anomalies. In contrast, N96 subsurface density anomalies persist in the subpolar latitudes for longer, so circulation anomalies and the development of SST anomalies are more centred there. The drivers of subsurface density anomalies also differ between models. In N216, the NAO is the dominant driver, while upper-ocean salinity-controlled density anomalies that originate from the Arctic appear to be the dominant driver in N96. These results further highlight that internal AMV mechanisms are model dependent and motivate further work to better understand and constrain the differences.

scholarly journals Role of the Gulf Stream and Kuroshio–Oyashio Systems in Large-Scale Atmosphere–Ocean Interaction: A Review

2010 ◽  
Vol 23 (12) ◽  
pp. 3249-3281 ◽  
Author(s):  
Young-Oh Kwon ◽  
Michael A. Alexander ◽  
Nicholas A. Bond ◽  
Claude Frankignoul ◽  
Hisashi Nakamura ◽  
...  
Keyword(s):  
Climate Variability ◽  
Atmospheric Circulation ◽  
Gulf Stream ◽  
Large Scale ◽  
Climate Model ◽  
Low Frequency ◽  
Heat Fluxes ◽  
Western Boundary ◽  
Basin Scale ◽  
Sst Anomalies

Abstract Ocean–atmosphere interaction over the Northern Hemisphere western boundary current (WBC) regions (i.e., the Gulf Stream, Kuroshio, Oyashio, and their extensions) is reviewed with an emphasis on their role in basin-scale climate variability. SST anomalies exhibit considerable variance on interannual to decadal time scales in these regions. Low-frequency SST variability is primarily driven by basin-scale wind stress curl variability via the oceanic Rossby wave adjustment of the gyre-scale circulation that modulates the latitude and strength of the WBC-related oceanic fronts. Rectification of the variability by mesoscale eddies, reemergence of the anomalies from the preceding winter, and tropical remote forcing also play important roles in driving and maintaining the low-frequency variability in these regions. In the Gulf Stream region, interaction with the deep western boundary current also likely influences the low-frequency variability. Surface heat fluxes damp the low-frequency SST anomalies over the WBC regions; thus, heat fluxes originate with heat anomalies in the ocean and have the potential to drive the overlying atmospheric circulation. While recent observational studies demonstrate a local atmospheric boundary layer response to WBC changes, the latter’s influence on the large-scale atmospheric circulation is still unclear. Nevertheless, heat and moisture fluxes from the WBCs into the atmosphere influence the mean state of the atmospheric circulation, including anchoring the latitude of the storm tracks to the WBCs. Furthermore, many climate models suggest that the large-scale atmospheric response to SST anomalies driven by ocean dynamics in WBC regions can be important in generating decadal climate variability. As a step toward bridging climate model results and observations, the degree of realism of the WBC in current climate model simulations is assessed. Finally, outstanding issues concerning ocean–atmosphere interaction in WBC regions and its impact on climate variability are discussed.

scholarly journals Scale Dependence of Midlatitude Air–Sea Interaction

2017 ◽  
Vol 30 (20) ◽  
pp. 8207-8221 ◽  
Author(s):  
Stuart P. Bishop ◽  
R. Justin Small ◽  
Frank O. Bryan ◽  
Robert A. Tomas
Keyword(s):  
Climate Models ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Balance Model ◽  
Western Boundary ◽  
Surface Heat Fluxes ◽  
Functional Relationships ◽  
Sst Variability ◽  
Temporal Smoothing ◽  
Circumpolar Current

Abstract It has traditionally been thought that midlatitude sea surface temperature (SST) variability is predominantly driven by variations in air–sea surface heat fluxes (SHFs) associated with synoptic weather variability. Here it is shown that in regions marked by the highest climatological SST gradients and SHF loss to the atmosphere, the variability in SST and SHF at monthly and longer time scales is driven by internal ocean processes, termed here “oceanic weather.” This is shown within the context of an energy balance model of coupled air–sea interaction that includes both stochastic forcing for the atmosphere and ocean. The functional form of the lagged correlation between SST and SHF allows us to discriminate between variability that is driven by atmospheric versus oceanic weather. Observations show that the lagged functional relationship of SST–SHF and SST tendency–SHF correlation is indicative of ocean-driven SST variability in the western boundary currents (WBCs) and the Antarctic Circumpolar Current (ACC). By applying spatial and temporal smoothing, thereby dampening the signature SST anomalies generated by eddy stirring, it is shown that the oceanic influence on SST variability increases with time scale but decreases with increasing spatial scale. The scale at which SST variability in the WBCs and the ACC transitions from ocean to atmosphere driven occurs at scales less than 500 km. This transition scale highlights the need to resolve mesoscale eddies in coupled climate models to adequately simulate the variability of air–sea interaction. Away from strong SST fronts the lagged functional relationships are indicative of the traditional paradigm of atmospherically driven SST variability.

scholarly journals Long Rossby Wave Basin-Crossing Time and the Resonance of Low-Frequency Basin Modes

2002 ◽  
Vol 32 (9) ◽  
pp. 2652-2665 ◽  
Author(s):  
François Primeau
Keyword(s):  
Ocean Circulation ◽  
Rossby Wave ◽  
Low Frequency ◽  
Western Boundary ◽  
Singular Case ◽  
Long Wave ◽  
Eastern Boundary ◽  
Wave Basin ◽  
Crossing Time ◽  
Basin Geometry

Abstract The ability of long-wave low-frequency basin modes to be resonantly excited depends on the efficiency with which energy fluxed onto the western boundary can be transmitted back to the eastern boundary. This efficiency is greatly reduced for basins in which the long Rossby wave basin-crossing time is latitude dependent. In the singular case where the basin-crossing time is independent of latitude, the amplitude of resonantly excited long-wave basin modes grows without bound except for the effects of friction. The speed of long Rossby waves is independent of latitude for quasigeostrophic dynamics, and the rectangular basin geometry often used for theoretical studies of the wind-driven ocean circulation is such a singular case for quasigeostrophic dynamics. For more realistic basin geometries, where only a fraction of the energy incident on the western boundary can be transmitted back to the eastern boundary, the modes have a finite decay rate that in the limit of weak friction is independent of the choice of frictional parameters. Explicit eigenmode computations for a basin geometry similar to the North Pacific but closed along the equator yield basin modes sufficiently weakly damped that they could be resonantly excited.

A laboratory model of the wind-driven ocean circulation

1969 ◽  
Vol 38 (2) ◽  
pp. 255-271 ◽  
Author(s):  
R. C. Beardsley
Keyword(s):  
Boundary Layer ◽  
Ocean Circulation ◽  
Low Frequency ◽  
Azimuthal Velocity ◽  
Laboratory Model ◽  
Western Boundary ◽  
Bottom Slope ◽  
Flow Oscillation ◽  
Two Dimensional Flow ◽  
Topographic Rossby Waves

A simple laboratory model for the wind-driven ocean circulation is re-studied experimentally and theoretically. Introduced by Pedlosky & Greenspan (1967), the model consists of a rotating cylinder with sloping bottom, the fluid inside being driven by the steady relative rotation of the cylinder's lid. A linear theory is developed to illustrate the modification in the interior and Stewartson boundary layers caused by variation of the bottom slope from 0 to O(1); Stommel's (1948) model is obtained when the bottom slope tan α [Lt ] E¼, and the Munk & Carrier (1950) model is obtained for E¼ [Lt ] tan α [Lt ] 1 (E is the Ekman number). Measurements of the interior cross-contour ‘Sverdrup’ velocity agree well with theory when the Ekman-layer Reynolds number RE is ≈ 1 or less. The western boundarylayer azimuthal velocity agrees reasonably well with theory, although the observed variation with depth and bottom slope were not predicted. The western boundary layer shows downstream intensification when RE is increased from ≈ 1 until topographic Rossby waves appear in the transition region between western boundary layer and interior. The motion becomes unstable when a critical value of RE is reached, independent of the bottom slope, and a low-frequency two-dimensional flow oscillation is observed. A brief comparison is made with previous wind-driven ocean circulation studies.

Sea Surface Temperature Variability along the Path of the Antarctic Circumpolar Current

2006 ◽  
Vol 36 (7) ◽  
pp. 1317-1331 ◽  
Author(s):  
Ariane Verdy ◽  
John Marshall ◽  
Arnaud Czaja
Keyword(s):  
Antarctic Circumpolar Current ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Heat Advection ◽  
Surface Heat Fluxes ◽  
Sst Variability ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Sst Anomalies

Abstract The spatial and temporal distributions of sea surface temperature (SST) anomalies in the Antarctic Circumpolar Current (ACC) are investigated, using monthly data from the NCEP–NCAR reanalysis for the period 1980–2004. Patterns of atmospheric forcing are identified in observations of sea level pressure and air–sea heat fluxes. It is found that a significant fraction of SST variability in the ACC can be understood as a linear response to surface forcing by the Southern Annular Mode (SAM) and remote forcing by ENSO. The physical mechanisms rely on the interplay between atmospheric variability and mean advection by the ACC. SAM and ENSO drive a low-level anomalous circulation pattern localized over the South Pacific Ocean, inducing surface heat fluxes and Ekman heat advection anomalies. A simple model of SST propagating in the ACC, forced with heat fluxes estimated from the reanalysis, suggests that surface heat fluxes and Ekman heat advection are equally important in driving the observed SST variability. Further diagnostics indicate that SST anomalies, generated mainly upstream of Drake Passage, are subsequently advected by the ACC and damped after a couple of years. It is suggested that SST variability along the path of the ACC is largely a passive response of the oceanic mixed layer to atmospheric forcing.

GFDL's CM2 Global Coupled Climate Models. Part III: Tropical Pacific Climate and ENSO

2006 ◽  
Vol 19 (5) ◽  
pp. 698-722 ◽  
Author(s):  
Andrew T. Wittenberg ◽  
Anthony Rosati ◽  
Ngar-Cheung Lau ◽  
Jeffrey J. Ploshay
Keyword(s):  
Thermal Structure ◽  
Tropical Pacific ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Trade Winds ◽  
Equatorial Undercurrent ◽  
Surface Heat Fluxes ◽  
Zonal Winds ◽  
Sst Anomalies

Abstract Multicentury integrations from two global coupled ocean–atmosphere–land–ice models [Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), developed at the Geophysical Fluid Dynamics Laboratory] are described in terms of their tropical Pacific climate and El Niño–Southern Oscillation (ENSO). The integrations are run without flux adjustments and provide generally realistic simulations of tropical Pacific climate. The observed annual-mean trade winds and precipitation, sea surface temperature, surface heat fluxes, surface currents, Equatorial Undercurrent, and subsurface thermal structure are well captured by the models. Some biases are evident, including a cold SST bias along the equator, a warm bias along the coast of South America, and a westward extension of the trade winds relative to observations. Along the equator, the models exhibit a robust, westward-propagating annual cycle of SST and zonal winds. During boreal spring, excessive rainfall south of the equator is linked to an unrealistic reversal of the simulated meridional winds in the east, and a stronger-than-observed semiannual signal is evident in the zonal winds and Equatorial Undercurrent. Both CM2.0 and CM2.1 have a robust ENSO with multidecadal fluctuations in amplitude, an irregular period between 2 and 5 yr, and a distribution of SST anomalies that is skewed toward warm events as observed. The evolution of subsurface temperature and current anomalies is also quite realistic. However, the simulated SST anomalies are too strong, too weakly damped by surface heat fluxes, and not as clearly phase locked to the end of the calendar year as in observations. The simulated patterns of tropical Pacific SST, wind stress, and precipitation variability are displaced 20°–30° west of the observed patterns, as are the simulated ENSO teleconnections to wintertime 200-hPa heights over Canada and the northeastern Pacific Ocean. Despite this, the impacts of ENSO on summertime and wintertime precipitation outside the tropical Pacific appear to be well simulated. Impacts of the annual-mean biases on the simulated variability are discussed.

scholarly journals EDITORIAL

2013 ◽  
Vol 31 (2) ◽  
pp. 207
Author(s):  
Jose Antonio Moreira Lima
Keyword(s):  
Data Assimilation ◽  
Atlantic Ocean ◽  
Ocean Circulation ◽  
Large Scale ◽  
Early Stage ◽  
Specific Area ◽  
Heat Fluxes ◽  
Surface Heat Fluxes ◽  
Forecast Models ◽  
Ocean Forecasting

This issue presents a set of papers related to the development of ocean forecasting models with data assimilation skills for the South Atlantic Ocean, more specifically for the Metarea V maritime region whose western border is delimited by the Brazilian shelf. This work has been done with the collaboration of many Brazilian researchers under the Oceanographic Modeling and Observation Network (REMO) research group. The evolution from an early stage of running ocean models with mean climatological forcings aiming at the study of specific oceanographic processes to the present stage of running operational ocean forecast models with synoptic forcings and data assimilation had a strong contribution from researchers with a meteorological background, who brought their expertise on numerical weather forecasting.The papers present distinct topics associated with an ocean forecasting system, such as a detailed description of network design and implementation of the ocean circulation models, a proposed approach of nesting distinct models starting from a large scale Atlantic Ocean grid to regional high-resolution local grids, data assimilation methods, synoptic sea surface fields obtained from remote sensing, surface heat fluxes, and planning observational measurement programs for assimilation and model evaluation.We hope that these papers contribute towards developing this specific area of operational oceanic forecasting within the Brazilian scientific and ocean technology communities. We still have a steady way to follow in order to consolidate and improve the propo-sed initiatives, but the first steps were already given and sound results are now available. In the near future, we foresee continuous improvement of oceanic models and data assimilation methods as well as collaboration with interested researchers from Brazilian and foreign institutions. Jose Antonio Moreira LimaInvited Editor  Este volume apresenta um conjunto de artigos relacionados com o tema previsão oceânica de curto prazo para o Oceano Atlântico Sul, mais especificamente para a região marítima Metarea V, através de modelos numéricos de circulação com assimilação de dados observacionais. Este trabalho está sendo desenvolvido a partir da cooperação de diversos pesquisadores brasileiros colaboradores da Rede de Modelagem e Observação Oceanográfica (REMO). No estudo dos processos oceanográficos, a evolução do estágio de rodar modelos oceânicos utilizando forçantes climatológicas médias para o estágio atual de rodar modelos operacionais com forçantes sinóticas e assimilação de dados teve uma forte contribuição de pesquisadores oriundos da área de meteorologia, que trouxeram seu conhecimento aplicado dos modelos de previsão do tempo.Os artigos abordam diversos tópicos associados com um sistema de previsão oceânica, tais como uma descrição detalhada do projeto e implementação dos modelos de circulação oceânica; aninhamento escalonado de modelos com escalas distintas, a partir de malha computacional do Oceano Atlântico, para malhas regionais com alta resolução espacial; métodos de assimilação de campos e dados observados; campos sinóticos da superfície do mar através sensoriamento remoto; fluxos de calor de superfície; e planejamento de observações para assimilação e avaliação dos modelos.Desejamos que estes artigos contribuam para desenvolvimento desta área específica de previsão oceânica operacional junto às comunidades científica e de tecnologia oceânica brasileira. Temos ainda um extenso caminho pela frente para consolidar e aperfeiçoar as iniciativas propostas, mas os primeiros passos foram dados e bons resultados já estão disponíveis. Para o futuro, vislumbramos aprimoramento contínuo dos modelos oceânicos e métodos de assimilação de dados, assim como a colaboração com pesquisadores interessados de instituições brasileiras ou estrangeiras. Jose Antonio Moreira LimaEditor Convidado 

scholarly journals Mechanisms of Decadal Variability of the Wind-Driven Ocean Circulation

2005 ◽  
Vol 35 (4) ◽  
pp. 512-531 ◽  
Author(s):  
Andrew Mc C. Hogg ◽  
Peter D. Killworth ◽  
Jeffrey R. Blundell ◽  
William K. Dewar
Keyword(s):  
Flow Field ◽  
Ocean Circulation ◽  
Potential Vorticity ◽  
Decadal Variability ◽  
Low Frequency ◽  
Ocean Basin ◽  
Western Boundary ◽  
Boundary Current ◽  
Low Frequency Oscillations ◽  
Low Frequency Variability

Abstract Eddy-resolving quasigeostrophic simulations of wind-driven circulation in a large ocean basin are presented. The results show that strong modes of low-frequency variability arise in many parameter regimes and that the strength of these modes depends upon the presence of inertial recirculations in the flow field. The inertial recirculations arise through advection of anomalous potential vorticity by the western boundary current and are barotropized by the effect of baroclinic eddies in the flow. The mechanism of low-frequency oscillations is explored with reference to previous studies, and it is found that the observed mode can be linked to the gyre mode but is strongly modified by the effect of eddies.

scholarly journals Increased Surface Ocean Heating by Colored Detrital Matter (CDM) Linked to Greater Northern Hemisphere Ice Formation in the GFDL CM2Mc ESM

2016 ◽  
Vol 29 (24) ◽  
pp. 9063-9076 ◽  
Author(s):  
Grace E. Kim ◽  
Anand Gnanadesikan ◽  
Marie-Aude Pradal
Keyword(s):  
Arctic Ocean ◽  
Light Attenuation ◽  
Ocean Model ◽  
Heat Fluxes ◽  
Shortwave Radiation ◽  
The Arctic ◽  
Ice Formation ◽  
Surface Heat Fluxes ◽  
Northern Latitudes ◽  
Modular Ocean Model

Abstract Recent observations of Arctic Ocean optical properties have found that colored dissolved organic matter (CDOM) is of primary importance in determining the nonwater absorption coefficient of light in this region. Although CDOM is an important optical constituent in the Arctic Ocean, it is not included in most of the current generation of Earth system models (ESMs). In this study, model runs were conducted with and without light attenuation by colored detrital matter (CDM), the combined optical contribution of CDOM and nonalgal particles. The fully coupled GFDL CM2 with Modular Ocean Model version 4p1 (MOM4p1) at coarse resolution (CM2Mc) ESM was used to examine the differences in heating and ice formation in the high northern latitudes. The annual cycle of sea surface temperature (SST) is amplified in the model run where the optical attenuation by CDM is included. Annually averaged integrated ice mass is 5% greater and total ice extent is 6% greater owing to colder wintertime SSTs. Differences in ocean heating (i.e., temperature tendency) between the two model runs are well represented by the combined changes in heating by penetrating shortwave radiation, mixing, and surface heat fluxes in the upper 100 m. Shortwave radiation is attenuated closer to the surface, which reduces heating below 10 m during summer months. Mixing entrains colder waters into the mixed layer during the autumn and winter months. Increased cloudiness and ice thickness in the model run with CDM reduces incoming shortwave radiation.

ENSO related SST anomalies and relation with surface heat fluxes over south Pacific and Atlantic

2016 ◽  
Vol 49 (1-2) ◽  
pp. 391-401 ◽  
Author(s):  
S. Chatterjee ◽  
M. Nuncio ◽  
K. Satheesan
Keyword(s):  
South Pacific ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Surface Heat Fluxes ◽  
Sst Anomalies
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