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Research at St Andrews

SAVEX - South Atlantic Variability Experiment

Project: Standard

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Description

SAVEX was the UK funded part of a larger international project, MEOP, and was developed from a previous international project, SEaOS. It was based on instrumentation (CTD-SRDLs) developed designed and built by the Sea Mammal Research Unit which made it feasible to collect detailed ocean data using marine mammals as observation platforms. The specific objectives of SAVEX are detailed under Key findings, below. However, the impact of the project is much broader than these. To date, over 300 CTD-SRDLs have been deployed in the polar oceans by researches in 10 countries. Over 300,000 profiles have been collected and made freely available via the GTS and World Ocean Database. The approach allowed previously un- or under- sampled regions of the polar oceans to be observed at times when little or no data were available. When the animal platform data was combined with that from other sources, it has facilitated improved modeling and understanding of important local and global ocean processes. A recently published paper (Fedak, 2013) summarizes the ways in which these data have impacted many areas of ocean science. A submitted paper (Roquet et al) reports on the consequences for global ocean circulation models.

Key findings

Objective #1:
The seasonal variability of the ACC will be investigated with special emphasis on relation between the frontal positions and atmospheric conditions and we will investigate the driving force of the strong variability observed.

FINDINGS:
We investigate the impact of the southward shift of the wind field on the Southern Ocean circulation system. A southward shift of the frontal mean position was not observed, but instead an increased variability, which we ascribe to an increased eddy activity and their polward movement across the Antarctic Circumpolar Current (unpublished). These observational findings are currently checked with output from eddy-resolving ocean circulation models. This is ongoing work.

Objective #2:
These data will also be combined with data from other sources to form a more complete, detailed database of high temporal and spatial resolution for delayed mode characterisation of upper ocean structure. With this, we will better describe the seasonal dynamics and physical properties of water masses in the Antarctic Circumpolar Current. We will use this database to describe the short-term variability of circulation patterns around the island of South Georgia over an extended time, with special reference to the seasonal dynamics of the Southern ACC Front and its implications for krill transport to this productive region.

FINDINGS:
We investigated the physical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage region on time scales down to intraseasonal, within the 1969 - 2009 period by combine all available data sources. Both SAMW and AAIW experience substantial interannual to interdecadal variability.The two water masses have also experienced a substantial lightening since the start of the record. Examination of the mechanisms underpinning water mass property variability shows that SAMW characteristics are controlled predominantly by a combination of air-sea turbulent heat fluxes, cross-frontal Ekman transport of Antarctic surface waters and the evaporation-precipitation balance, whilst AAIW properties reflect air-sea turbulent heat fluxes and sea ice formation in the Bellingshausen Sea. We also investiagted the seasonal progression of upper-ocean water mass properties and stratification at the southern boundary of the Antarctic Circumpolar Current.

Close et al., JC, 2013
Meredith et al., DSR II, 2011
Charrasin et al., OCeanObs'09, 2010



Objective #3:
We will then compare the observed hydrographic fields with those obtained from general ocean circulation models. The model frontal positions will be compared with those inferred from our in-situ temperature and salinity fields. Subsequently, we will investigate the coupling of oceanfronts in the ACC and their seasonal and interannual variability as predicted by models. We will also compare the standard deviation of the observed frontal locations to the bottom depth and bottom slope of the topography to confirm or disprove the role of isobaths in controlling these fronts. We will then investigate how the frontal locations of the models are affected by bathymetry. This will increase our understanding of the influence of bathymetry in controlling the splitting and steering of the frontal jets.

FINDINGS:
SAVEX data was utelized to improve the assimilating ECCO model. The model output was improved by more than 35% in areas where SAVEX data were collected and by about 5% in the whole SOuthern Ocean area. We are also still investigating the seasonal and interannual variability of the Southern Ocean frontal system using our observational data together with output from eddy-resolving ocean circulation models (see above). This works shows that most fronts are topograhically controlled (unpublished and still ongoing work)

Roquet et al., submitted

Objective #4:
Due to severe weather and ice conditions for most of the year, hydrographic data from the Southern Ocean are extremely sparse, in spite of the fact that these areas play a crucial role in upper ocean processes. The Southern Ocean contains some of the most important sites of global intermediate and deepwater formation. The seals on which we deploy CTD-SRDLs (Satellite Relay Data Loggers) regularly migrate into these regions during their summer and winter feeding trips from South Georgia. The data from these seals will provide 1-3 CTD profiles per seal-day in near real-time, year-round, producing data from rarely sampled regions. We will make these available for inclusion in operational ocean circulation and climate models. We will be major contributors to the datasets gathered during the International Polar Year, as well as providing analytical techniques for their analysis.

FINDINGS:
Fedak, DSR, 2012
Roquet et al. submitted
AcronymSAVEX South Atlantic Variability Experim
StatusFinished
Effective start/end date1/12/0730/11/10

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