scholarly journals Mapping Ocean Observations in a Dynamical Framework: A 2004–06 Ocean Atlas

2010 ◽  
Vol 40 (6) ◽  
pp. 1201-1221 ◽  
Author(s):  
Gaël Forget
Keyword(s):  
Ocean Circulation ◽  
Seasonal Cycle ◽  
Field Experiments ◽  
Salt Content ◽  
Heat Content ◽  
Global Ocean ◽  
Reference Level ◽  
Altimeter Data ◽  
Dynamic Topography ◽  
Ongoing Research

Abstract This paper exploits a new observational atlas for the near-global ocean for the best-observed 3-yr period from December 2003 through November 2006. The atlas consists of mapped observations and derived quantities. Together they form a full representation of the ocean state and its seasonal cycle. The mapped observations are primarily altimeter data, satellite SST, and Argo profiles. GCM interpolation is used to synthesize these datasets, and the resulting atlas is a fairly close fit to each one of them. For observed quantities especially, the atlas is a practical means to evaluate free-running GCM simulations and to put field experiments into a broader context. The atlas-derived quantities include the middepth dynamic topography, as well as ocean fluxes of heat and salt–freshwater. The atlas is publicly available online (www.ecco-group.org). This paper provides insight into two oceanographic problems that are the subject of vigorous ongoing research. First, regarding ocean circulation estimates, it can be inferred that the RMS uncertainty in modern surface dynamic topography (SDT) estimates is only on the order of 3.5 cm at scales beyond 300 km. In that context, it is found that assumptions of “reference-level” dynamic topography may yield significant errors (of order 2.2 cm or more) in SDT estimates using in situ data. Second, in the perspective of mode water investigations, it is estimated that ocean fluxes (advection plus mixing) largely contribute to the seasonal fluctuation in heat content and freshwater/salt content. Hence, representing the seasonal cycle as a simple interplay of air–sea flux and ocean storage would not yield a meaningful approximation. For the salt–freshwater seasonal cycle especially, contributions from ocean fluxes usually exceed direct air–sea flux contributions.

Internal salt content: a useful framework for understanding the oceanic branch of the water cycle

2021 ◽  
Author(s):  
Christopher Bladwell ◽  
Ryan M. Holmes ◽  
Jan D. Zika
Keyword(s):  
Fresh Water ◽  
Ocean Circulation ◽  
Water Cycle ◽  
Salt Content ◽  
Ocean Model ◽  
Global Ocean ◽  
Freshwater Forcing ◽  
Freshwater Fluxes ◽  
Ocean Salinity ◽  
Isopycnal Mixing

AbstractThe global water cycle is dominated by an atmospheric branch which transfers fresh water away from subtropical regions and an oceanic branch which returns that fresh water from subpolar and tropical regions. Salt content is commonly used to understand the oceanic branch because surface freshwater fluxes leave an imprint on ocean salinity. However, freshwater fluxes do not actually change the amount of salt in the ocean and – in the mean – no salt is transported meridionally by ocean circulation. To study the processes which determine ocean salinity we introduce a new variable: “internal salt” and its counterpart “internal fresh water”. Precise budgets for internal salt in salinity coordinates relate meridional and diahaline transport to surface freshwater forcing, ocean circulation and mixing, and reveal the pathway of fresh water in the ocean. We apply this framework to a 1° global ocean model. We find that in order for fresh water to be exported from the ocean’s tropical and subpolar regions to the subtropics, salt must be mixed across the salinity surfaces that bound those regions. In the tropics, this mixing is achieved by parameterized vertical mixing, along-isopycnal mixing, and numerical mixing associated with truncation errors in the model’s advection scheme, while along-isopycnal mixing dominates at high latitudes. We analyze the internal freshwater budgets of the Indo-Pacific and Atlantic Ocean basins and identify the transport pathways between them which redistribute fresh water added through precipitation, balancing asymmetries in freshwater forcing between the basins.

scholarly journals Impact of Multiple Altimeter Data and Mean Dynamic Topography in a Global Analysis and Forecasting System

2019 ◽  
Vol 36 (7) ◽  
pp. 1255-1266 ◽  
Author(s):  
Mathieu Hamon ◽  
Eric Greiner ◽  
Pierre-Yves Le Traon ◽  
Elisabeth Remy
Keyword(s):  
Data Assimilation ◽  
Sea Surface ◽  
Global Ocean ◽  
Altimeter Data ◽  
Dynamic Topography ◽  
Mean Dynamic Topography ◽  
Ocean Data Assimilation ◽  
The Mean ◽  
Data Assimilation System ◽  
Assimilation System

AbstractSatellite altimetry is one of the main sources of information used to constrain global ocean analysis and forecasting systems. In addition to in situ vertical temperature and salinity profiles and sea surface temperature (SST) data, sea level anomalies (SLA) from multiple altimeters are assimilated through the knowledge of a surface reference, the mean dynamic topography (MDT). The quality of analyses and forecasts mainly depends on the availability of SLA observations and on the accuracy of the MDT. A series of observing system evaluations (OSEs) were conducted to assess the relative importance of the number of assimilated altimeters and the accuracy of the MDT in a Mercator Ocean global 1/4° ocean data assimilation system. Dedicated tools were used to quantify impacts on analyzed and forecast sea surface height and temperature/salinity in deeper layers. The study shows that a constellation of four altimeters associated with a precise MDT is required to adequately describe and predict upper-ocean circulation in a global 1/4° ocean data assimilation system. Compared to a one-altimeter configuration, a four-altimeter configuration reduces the mean forecast error by about 30%, but the reduction can reach more than 80% in western boundary current (WBC) regions. The use of the most recent MDT updates improves the accuracy of analyses and forecasts to the same extent as assimilating a fourth altimeter.

scholarly journals Water Mass Characteristics and Distribution Adjacent to Larsen C Ice Shelf, Antarctica

2020 ◽  
Author(s):  
Katherine Hutchinson ◽  
Julie Deshayes ◽  
Jean-Baptiste Sallee ◽  
Julian Dowdeswell ◽  
Casimir de Lavergne ◽  
...  
Keyword(s):  
Continental Shelf ◽  
Ocean Circulation ◽  
Deep Water ◽  
Water Mass ◽  
Heat Content ◽  
Weddell Sea ◽  
Global Ocean ◽  
Spatial And Temporal Variability ◽  
Ice Shelf ◽  
Shed Light

<p>The physical oceanographic environment, water mass mixing and transformation in the area adjacent to Larsen C Ice Shelf (LCIS) are investigated using hydrographic data collected during the Weddell Sea Expedition 2019. The results shed light on the ocean conditions adjacent to a thinning LCIS, on a continental shelf that is a source region for the globally important water mass, Weddell Sea Deep Water (WSDW). Modified Weddell Deep Water (MWDW), a comparatively warmer water mass of circumpolar origin, is identified on the continental shelf and is observed to mix with local shelf waters, such as Ice Shelf Water (ISW), which is a precursor of WSDW. Oxygen measurements enable the use of a linear mixing model to quantify contributions from source waters revealing high levels of mixing in the area, with much spatial and temporal variability. Heat content anomalies indicate an introduction of heat, presumed to be associated with MWDW, into the area via Jason Trough. Furthermore, candidate parent sources for ISW are identified in the region, indicating the potential for the circulation of continental shelf waters into the ice shelf cavity. This highlights the possibility that offshore climate signals are conveyed under LCIS. ISW is observed within Jason Trough, likely exiting the sub-ice shelf cavity en route to the Slope Current. This onshore-offshore flux of water masses links the region of the Weddell Sea adjacent to northern LCIS to global ocean circulation and Bottom Water characteristics via its contribution to ISW and hence WSDW properties. </p><p>What remains to be clarified is whether MWDW found in Jason Trough has a direct impact on basal melting and thus thinning of LCIS. More observations are required to investigate this, in particular direct observations of ocean circulation in Jason Trough and underneath LCIS. Modelling experiments could also shed light on this, and so preliminary results based on NEMO global simulations explicitly representing the circulation in under-ice shelf seas, will be presented. </p>

scholarly journals An ocean modelling and assimilation guide to using GOCE geoid products

Ocean Science ◽  
2011 ◽  
Vol 7 (1) ◽  
pp. 151-164 ◽  
Author(s):  
K. Haines ◽  
J. A. Johannessen ◽  
P. Knudsen ◽  
D. Lea ◽  
M.-H. Rio ◽  
...  
Keyword(s):  
Sea Level ◽  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Lessons Learned ◽  
Altimeter Data ◽  
Dynamic Topography ◽  
Satellite Altimeter ◽  
Level Information

Abstract. We review the procedures and challenges that must be considered when using geoid data derived from the Gravity and steady-state Ocean Circulation Explorer (GOCE) mission in order to constrain the circulation and water mass representation in an ocean general circulation model. It covers the combination of the geoid information with time-mean sea level information derived from satellite altimeter data, to construct a mean dynamic topography (MDT), and considers how this complements the time-varying sea level anomaly, also available from the satellite altimeter. We particularly consider the compatibility of these different fields in their spatial scale content, their temporal representation, and in their error covariances. These considerations are very important when the resulting data are to be used to estimate ocean circulation and its corresponding errors. We describe the further steps needed for assimilating the resulting dynamic topography information into an ocean circulation model using three different operational forecasting and data assimilation systems. We look at methods used for assimilating altimeter anomaly data in the absence of a suitable geoid, and then discuss different approaches which have been tried for assimilating the additional geoid information. We review the problems that have been encountered and the lessons learned in order the help future users. Finally we present some results from the use of GRACE geoid information in the operational oceanography community and discuss the future potential gains that may be obtained from a new GOCE geoid.

Surface circulation in the Ligurian Sea: an assessment of satellite radar altimeter-derived geostrophic currents.

2021 ◽  
Author(s):  
Paola Picco ◽  
Stefano Vignudelli ◽  
Luca Repetti ◽  
Maurizio Demarte
Keyword(s):  
Ocean Circulation ◽  
Altimeter Data ◽  
Dynamic Topography ◽  
Dynamic Features ◽  
Ligurian Sea ◽  
Radar Altimetry ◽  
Geostrophic Currents ◽  
Temporal Sampling ◽  
Satellite Radar ◽  
Satellite Radar Altimetry

<p>Recent improvements of satellite altimeter observations allow to approach investigations on the surface ocean circulation even in those regions where the slope associated to dynamic structures is reduced. The capability to detect the main dynamic features and their variability from satellite radar altimetry in the Ligurian Sea (Western Mediterranean) is here assessed.</p><p>Altimeter data from X-TRACK products recently released are used for this study: the time series of satellite-based- currents along the track n.044, which crosses the Ligurian Sea from the Corsica Channel to the Ligurian coast, is analysed. The temporal sampling is about 10 days and the along-track resolution is 7 km. Geostrophic currents computed from satellite radar altimetry are checked for consistency against the dynamic topography obtained from concurrent CTD casts collected during recent oceanographic campaigns carried out by the Italian Hydrographic Institute along the track. A more detailed assessment of the computed current velocities is based on the analysis of long-term ADCP measurements from a fixed mooring deployed from 2004 to 2006 in the Central Ligurian Sea (43°47.77’ N; 9°02.85’ E) 40 nm from the coast, quite close to the altimeter track. An RD&I 300 kHz upward-looking ADCP sampled the upper layer at 8 m vertical resolution. Currents in the upper layer (0-100 m) are almost barotropic with the variability due to the wind confined to the upper few meters. In order to define an appropriate metrics to compare currents from different measuring systems, EOF analysis of ADCP profiles have proved to be a good tool to filter out the high frequency and wind driven currents, thus enhancing the contribution of the geostrophic component.</p>

scholarly journals Climatic Consequences of a Pine Island Glacier Collapse

2015 ◽  
Vol 28 (23) ◽  
pp. 9221-9234 ◽  
Author(s):  
J. A. M. Green ◽  
A. Schmittner
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Deep Water ◽  
Heat Content ◽  
Meridional Overturning Circulation ◽  
Ocean Heat Content ◽  
Global Ocean ◽  
Intermediate Water ◽  
The Impact ◽  
Global Surface

Abstract An intermediate-complexity climate model is used to simulate the impact of an accelerated Pine Island Glacier mass loss on the large-scale ocean circulation and climate. Simulations are performed for preindustrial conditions using hosing levels consistent with present-day observations of 3000 m3 s−1, at an accelerated rate of 6000 m3 s−1, and at a total collapse rate of 100 000 m3 s−1, and in all experiments the hosing lasted 100 years. It is shown that even a modest input of meltwater from the glacier can introduce an initial cooling over the upper part of the Southern Ocean due to increased stratification and ice cover, leading to a reduced upward heat flux from Circumpolar Deep Water. This causes global ocean heat content to increase and global surface air temperatures to decrease. The Atlantic meridional overturning circulation (AMOC) increases, presumably owing to changes in the density difference between Antarctic Intermediate Water and North Atlantic Deep Water. Simulations with a simultaneous hosing and increases of atmospheric CO2 concentrations show smaller effects of the hosing on global surface air temperature and ocean heat content, which the authors attribute to the melting of Southern Ocean sea ice. The sensitivity of the AMOC to the hosing is also reduced as the warming by the atmosphere completely dominates the perturbations.

scholarly journals Assessment of the ocean circulation in the Azores region as predicted by a numerical model assimilating altimeter data from Topex/Poseidon and ERS-1 satellites

1997 ◽  
Vol 15 (10) ◽  
pp. 1354-1368 ◽  
Author(s):  
T. Mailly ◽  
E. Blayo ◽  
J. Verron
Keyword(s):  
Numerical Model ◽  
Ocean Circulation ◽  
Wave Number ◽  
Altimeter Data ◽  
First Year ◽  
Dynamic Topography ◽  
Sea Level Changes ◽  
The North ◽  
Circulation Patterns ◽  
Changes Over Time

Abstract. Two years of altimetric data from Topex/Poseidon (October 1992–September 1994) and ERS-1 (October 1992–December 1993) were assimilated into a numerical model of the North Atlantic. The results of these simulations are analysed in the Azores region to assess the performance of our model in this particular region. Maps of instantaneous dynamic topography and transports show that the model performs well in reproducing the velocities and transports of the Azores Front. Drifter data from the Semaphore experiment are also used to study the correlation between the drifter velocities and the corresponding model velocities. Some interesting oceanographic results are also obtained by examining the seasonal and interannual variability of the circulation and the influence of bathymetry on the variability of the Azores Front. Thus, on the basis of our two year experiment, it is possible to confirm the circulation patterns proposed by previous studies regarding the seasonal variations in the origin of the Azores Current. Moreover, it is shown that the Azores Current is quite narrow in the first year of assimilation (1992–1993), but becomes much wider in the second year (1993–1994). The role of the bathymetry appears important in this area since the mesoscale activity is shown to be strongly related to the presence of topographic slopes. Finally, spectral analyses of sea-level changes over time and space are used to identify two types of wave already noticed in other studies: a wave with (300 km)–1 wave number and (120 days)–1 frequency, which is characteristic of mesoscale undulation, and a wave with (600 km)–1 wave number and (250 days)–1 frequency which probably corresponds to a Rossby wave generated in the east of the basin.

On the Interannual Variability of the Indonesian Throughflow and Its Linkage with ENSO

2005 ◽  
Vol 18 (9) ◽  
pp. 1435-1444 ◽  
Author(s):  
Matthew H. England ◽  
Fei Huang
Keyword(s):  
Interannual Variability ◽  
Heat Transport ◽  
Ocean Circulation ◽  
Large Scale ◽  
Southern Oscillation ◽  
Heat Content ◽  
Global Ocean ◽  
Enso Cycle ◽  
Indonesian Throughflow ◽  
Tropical Ocean

Abstract The Indonesian Throughflow (ITF) variability is assessed using a retrospective analysis of the global ocean based on the Simple Ocean Data Assimilation (SODA) experiment spanning the period 1950–99. A comparison between the 1983–95 observed ITF, and the simulated ITF suggests a reasonably accurate reconstruction of ocean circulation in the vicinity of the ITF during the available measurement record. A wavelet analysis shows that once the seasonal cycle is removed, the dominant variation of the ITF anomaly is an interannual oscillation with a period of about 4–7 yr. This interannual variability is significantly correlated with the El Niño–Southern Oscillation (ENSO) pattern, with the ITF lagging the ENSO cycle by 8–9 months. This suggests that large-scale tropical ocean–atmosphere interaction plays an important role in the interannual variability of the ITF. Regional upper-ocean heat content variability might also play a role in controlling interannual fluctuations of the ITF transport via geostrophic flows, though it could equally be ITF variations that establish heat content anomalies downstream of the Indonesian archipelago. The model heat transport associated with the ITF is in good agreement with the limited observational record available. Resultant variability in annual mean ITF heat transport is in the range 0.4–1.2 PW, which is significantly correlated with ITF and ENSO indices.

scholarly journals A near-global eddy-resolving OGCM for climate studies

2016 ◽  
Author(s):  
X. Zhang ◽  
P. R. Oke ◽  
M. Feng ◽  
M. A. Chamberlain ◽  
J. A. Church ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Heat Content ◽  
Circulation Model ◽  
Surface Heat ◽  
Global Ocean ◽  
Biogeochemical Model ◽  
Relaxation Rates ◽  
Ocean Forecasting ◽  
Climate Studies

Abstract. Eddy-resolving global ocean models are highly desired for spatially-improved climate studies, but this is challenging because they require careful configuration and substantial computational resources. Model drift, partially related to insufficient model spin-up, imperfect model physics or bias in surface forcing, can be problematic, leading to contamination of climate change signals. In this study, we adapt a near-global eddy-resolving ocean general circulation model, originally developed for short-range ocean forecasting, for climate studies. The Ocean Forecasting Australia Model version 3 (OFAM3) is spun up for 20 years, with repeated year 1979 forcing and adaptive relaxation (Newtonian nudging) of temperature and salinity in the deep ocean to an observation-based climatology. In addition, surface heat fluxes from the JRA-55 atmospheric reanalysis are adjusted during the spin-up experiment to minimise excessive net heat uptake in the ocean. In the historical experiment, spanning 1979–2014, a non-adaptive relaxation is applied by repeating the same relaxation rates derived from the last five years of the spin-up experiment, and the surface heat flux adjustment diagnosed during the spinup experiment is also maintained. We demonstrate that the historical experiment driven by the JRA-55 reanalysis does not have significant drifts (e.g., as shown by simulated global ocean heat content), and also provides an eddy-resolving simulation of the global ocean circulation over the period 1979–2014. Decadal changes, such as the strengthening of the subtropical gyre circulation, are also reasonably simulated. A biogeochemical model is coupled with OFAM3 to produce patterns of primary productivity and carbon fluxes that are consistent with observations. Experiences gained from our numerical experiments will be helpful to other modelling groups who are interested in running global eddy-resolving OGCMs for climate studies.

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