Sebastiaan Swart

An altimetry-based gravest empirical mode south of Africa: 1. Development and validation

[1] Hydrographic transects of the Antarctic Circumpolar Current (ACC) south of Africa are projected into baroclinic stream function space parameterized by pressure and dynamic height. This produces a two-dimensional gravest empirical mode (GEM) that captures more than 97% of the total density and temperature variance in the ACC domain. Weekly maps of absolute dynamic topography data, derived from satellite altimetry, are combined with the GEM to obtain a 16 year time series of temperature and salinity fields. The time series of thermohaline fields are compared with independent in situ observations. The residuals decrease sharply below the thermocline and through the entire water column the mean root-mean-square (RMS) error is 0.15C, 0.02, and 0.02 kg m �3 for temperature, salinity, and density, respectively. The positions of ACC fronts are followed in time using satellite altimetry data. These locations correspond to both the observed and GEM-based positions. The available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar. This is compared with velocity measurements from repeat hydrographic transects at the GoodHope line. The net accumulated transports of the ACC, derived from these different methods are within 1–3 Sv of each other. Similarly, GEM-produced cross-sectional velocities at 300 dbar compare closely to the observed data, with the RMS difference not exceeding 0.03 m s �1 . The continuous time series of thermohaline fields, described here, are further exploited to understand the dynamic nature of the ACC fronts in the region, and which is given by Swart and Speich (2010).

Transport and variability of the Antarctic Circumpolar Current south of Africa

Data from five CTD and 18 XBT sections are used to estimate the baroclinic transport ( referenced to 2500 dbar) of the ACC south of Africa. Surface dynamic height is derived from XBT data by establishing an empirical relationship between vertically integrated temperature and surface dynamic height calculated from CTD data. This temperature-derived dynamic height data compare closely with dynamic heights calculated from CTD data ( average RMS difference = 0.05 dyn m). A second empirical relationship between surface dynamic height and cumulative baroclinic transport is defined, allowing us to study a more extensive time series of baroclinic transport derived from upper ocean temperature sections. From 18 XBT transects of the ACC, the average baroclinic transport, relative to 2500 dbar, is estimated at 90 +/- 2.4 Sv. This estimate is comparable to baroclinic transport values calculated from CTD data. We then extend the baroclinic transport time-series by applying an empirical relationship between dynamic height and cumulative baroclinic transport to weekly maps of absolute dynamic topography derived from satellite altimetry, between 14 October 1992 and 23 May 2007. The estimated mean baroclinic transport of the ACC, obtained this way, is 84.7 +/- 3.0 Sv. These transports agree well with simultaneous in-situ estimates ( RMS difference in net transport = 5.2 Sv). This suggests that sea level anomalies largely reflect baroclinic transport changes above 2500 dbar.

An altimetry-based gravest empirical mode south of Africa: 2. Dynamic nature of the Antarctic Circumpolar Current fronts

[1] Swart et al. (2010) applied altimetry data to the gravest empirical mode south of Africa to yield a 16 year time series of temperature and salinity sections. In this study we use these thermohaline sections to derive weekly estimates of heat content (HC) and salt content (SC) at the GoodHope meridional transect of the Antarctic Circumpolar Current (ACC). These estimates compare favorably to observed data. The resulting 16 year time series of HC and SC estimates are used to explain the subsurface thermohaline variability at each ACC front and frontal zone. The variability at the Subantarctic Zone (SAZ) is principally driven by the presence of Agulhas Rings, which occur in this region approximately 2.7 times per annum and are responsible for the longest and highest scales of observed variability. The variability of the SAZ is responsible for over 50% and 60% of the total ACC HC and SC variability, respectively. Poleward of the SAZ, the variability is largely determined by the influence of the local topography on the fronts of the region and can be explained by the conservation of potential vorticity. Wavelet analysis is conducted on the time series of meridionally integrated HC and SC in each ACC front and frontal zone, revealing a consistent seasonal mode that becomes more dominant toward the southern limit of the ACC. The lower-frequency signals are compared with two dominant modes of variability in the Southern Ocean. The Southern Annular Mode correlates well with the HC and SC anomaly estimates at the Antarctic Polar Front, while the Southern Oscillation Index appears to have connections to the variability found in the very southern domains of the ACC.

Intraseasonal variability linked to sampling alias in air‐sea CO2 fluxes in the Southern Ocean

The Southern Ocean (SO) contributes most of the uncertainty in contemporary estimates of the mean annual flux of carbon dioxide CO2 between the ocean and the atmosphere. Attempts to reduce this uncertainty have aimed at resolving the seasonal cycle of the fugacity of CO2 (fCO2). We use hourly CO2 flux and driver observations collected by the combined deployment of ocean gliders to show that resolving the seasonal cycle is not sufficient to reduce the uncertainty of the flux of CO2 to below the threshold required to reveal climatic trends in CO2 fluxes. This was done by iteratively subsampling the hourly CO2 data set at various time intervals. We show that because of storm-linked intraseasonal variability in the spring-late summer, sampling intervals longer than 2 days alias the seasonal mean flux estimate above the required threshold. Moreover, the regional nature and long-term trends in storm characteristics may be an important influence in the future role of the SO in the carbon-climate system.

Submesoscale processes promote seasonal restratification in the Subantarctic Ocean

Traditionally, the mechanism driving the seasonal restratification of the Southern Ocean mixed layer (ML) is thought to be the onset of springtime warming. Recent developments in numerical modeling and North Atlantic observations have shown that submesoscale ML eddies (MLE) can drive a restratifying flux to shoal the deep winter ML prior to solar heating at high latitudes. The impact of submesoscale processes on the intraseasonal variability of the Subantarctic ML is still relatively unknown. We compare 5 months of glider data in the Subantarctic Zone to simulations of a 1-D mixing model to show that the magnitude of restratification of the ML cannot be explained by heat, freshwater, and momentum fluxes alone. During early spring, we estimate that periodic increases in the vertical buoyancy flux by MLEs caused small increases in stratification, despite predominantly down-front winds that promote the destruction of stratification. The timing of seasonal restratification was consistent between 1-D model estimates and the observations. However, during up-front winds, the strength of springtime stratification increased over twofold compared to the 1-D model, with a rapid shoaling of the MLD from >200 m to <100 m within a few days. The ML stratification is further modified under a negative Ekman buoyancy flux during down-front winds, resulting in the destruction of ML stratification and deepening of the MLD. These results propose the importance of submesoscale buoyancy fluxes enhancing seasonal restratification and mixing of the Subantarctic ML.

Using Optical Sensors on Gliders to Estimate Phytoplankton Carbon Concentrations and Chlorophyll-to-Carbon Ratios in the Southern Ocean

One approach to deriving phytoplankton carbon biomass estimates (Cphyto) at appropriate scales is through optical products. This study uses a high-resolution glider data set in the Sub-Antarctic Zone (SAZ) of the Southern Ocean to compare four different methods of deriving Cphyto from particulate backscattering and fluorescence-derived chlorophyll (chl-a). A comparison of the methods showed that at low (<0.5 mg m-3) chlorophyll concentrations (e.g. early spring and at depth), all four methods produced similar estimates of Cphyto, whereas when chlorophyll concentrations were elevated one method derived higher concentrations of Cphyto than the others. The use of methods derived from particulate backscattering rather than fluorescence can account for cellular adjustments in chl-a:Cphyto that are not driven by biomass alone. A comparison of the glider chl-a:Cphyto ratios from the different optical methods with ratios from laboratory cultures and cruise data found that some optical methods of deriving Cphyto performed better in the SAZ than others and that regionally derived methods may be unsuitable for application to the Southern Ocean. A comparison of the glider chl-a:Cphyto ratios with output from a complex biogeochemical model shows that although a ratio of 0.02 mg chl-a mg C-1 is an acceptable mean for SAZ phytoplankton (in spring-summer), the model misrepresents the seasonal cycle (with decreasing ratios from spring to summer and low sub-seasonal variability). As such, it is recommended that models expand their allowance for variable chl-a:Cphyto ratios that not only account for phytoplankton acclimation to low light conditions in spring but also to higher optimal chl-a:Cphyto ratios with increasing growth rates in summer.

Wind forced variability of the Antarctic Circumpolar Current south of Africa between 1993 and 2010

The variability of the Antarctic Circumpolar Current (ACC) system is largely linked to the atmospheric forcing. The objective of this work is to assess the link between local wind forcing mechanisms and the variability of the upper-ocean temperature and the dynamics of the different fronts in the ACC region south of South Africa. To accomplish this, in situ and satellite-derived observations are used between 1993 and 2010. The main finding of this work is that meridional changes in the westerlies linked with the Southern Annular Mode (SAM) drive temperature anomalies in the Ekman layer and changes in the Subantarctic Front (SAF) and Antarctic Polar Front (APF) transports through Ekman dynamics. The development of easterly anomalies between 35°S and 45°S during positive SAM is linked to reduced (increased) SAF (APF) transports and a warmer mixed layer in the ACC. The link between the changes in the wind stress and the SAF and APF transport variations occurs through the development of Ekman pumping anomalies near the frontal boundaries, driving an opposite response on the SAF and APF transports. The observed wind-driven changes in the frontal transports suggest small changes to the net ACC transport. In addition, observations indicate that the SAF and APF locations in this region are not linked to the local wind forcing, emphasizing the importance of other factors (e.g., baroclinic instabilities generated by bottom topography) to changes in the frontal location. Results obtained here highlight the importance of repeat XBT temperature sections and their combined analysis with other in situ and remote sensing observations.

Decadal‐scale thermohaline variability in the Atlantic sector of the Southern Ocean

An enhanced Altimetry Gravest Empirical Mode (AGEM), including both adiabatic and diabatic trends, is developed for the Antarctic Circumpolar Current (ACC) south of Africa using updated hydrographic CTD sections, Argo data, and satellite altimetry. This AGEM has improved accuracy compared to traditional climatologies and other proxy methods. The AGEM for the Atlantic Southern Ocean offers an ideal technique to investigate the thermohaline variability over the past two decades in a key region for water mass exchanges and transformation. In order to assess and attribute changes in the hydrography of the region, we separate the changes into adiabatic and diabatic components. Integrated over the upper 2000 dbar of the ACC south of Africa, results show mean adiabatic changes of 0.16 ± 0.11°C decade−1 and 0.006 ± 0.014 decade−1, and diabatic differences of −0.044 ± 0.13°C decade−1 and −0.01 ± 0.017 decade−1 for temperature and salinity, respectively. The trends of the resultant AGEM, that include both adiabatic and diabatic variability (termed AD-AGEM), show a significant increase in the heat content of the upper 2000 dbar of the ACC with a mean warming of 0.12 ± 0.087°C decade−1. This study focuses on the Antarctic Intermediate Water (AAIW) mass where negative diabatic trends dominate positive adiabatic differences in the Subantarctic Zone (SAZ), with results indicating a cooling (−0.17°C decade−1) and freshening (−0.032 decade−1) of AAIW in this area, whereas south of the SAZ positive adiabatic and diabatic trends together create a cumulative warming (0.31°C decade−1) and salinification (0.014 decade−1) of AAIW.

Submesoscale cyclones in the Agulhas current

Gliders were deployed for the first time in the Agulhas Current region to investigate processes of interactions between western boundary currents and shelf waters. Continuous observations from the gliders in water depths of 100–1000 m and over a period of 1 month provide the first high-resolution observations of the Agulhas Currents inshore front. The observations collected in a nonmeandering Agulhas Current show the presence of submesoscale cyclonic eddies, generated at the inshore boundary of the Agulhas Current. The submesoscale cyclones are often associated with warm water plumes, which extend from their western edge and exhibit strong northeastward currents. These features are a result of shear instabilities and extract their energy from the mean Agulhas Current jet.

Ocean predictive skill assessments in the South Atlantic: Crowd-sourcing of student-based discovery

Autonomous Underwater Gliders have over a decade long history of successful regional deployments serving scientific, societal and security needs in application areas ranging from pole to pole and including the full range of water depths from shallow coastal seas to the deep ocean. Glider deployments covering the basin scale are much fewer, but are a growing capacity as demonstrated by the Woods Hole to Bermuda line that crosses the Gulf Stream, the Atlantic Crossing line that follows the Gulf Stream, and the basin circling flights now being conducted as part of the Challenger Glider Mission. The next step in the evolution of the global Challenger mission is to enable an ensemble of modelers from different institutions and agencies to participate in a meaningful way. This process with be formalized in 2014 by leveraging the data management tools of the U.S. Integrated Ocean Observing System (IOOS) and the education tools of the U.S. National Science Foundations (NSF) Ocean Observing Initiative (OOI). The Education Visualization (EV) tools developed by the OOIs Education and Public Engagement (EPE) Implementing Organization (IO) are currently being configured through the cyber OOI net to display real time OOI glider data with intuitive interactive browser-based tools, reducing the barriers for student participation in sea exploration and discovery. Through U.S. IOOS, forecast ocean data will be harvested from the ephemeral ocean snapshots produced by an ensemble of ocean models along the same glider tracks as Challenger. The parallel observed and forecast datasets, both evolving in real time, will be accessible through the same OOI EV tools, enabling student participation in a crowd-sourced ocean predictive skill experiment. The result will satisfy one of the important goals of the Challenger mission by enabling students to assess of the quality of the ensemble of available global scale ocean models. Student research team projects that use the new model data comparison capabilities will be conducted during the summer of 2014. Students will compare an ensemble of the global ocean models along the high velocity transport pathways by gliders on basin-scale missions, such as one that traverses the northern side of the South Atlantic gyre along the Brazilian shelfbreak. The lasting impact of the Challenger mission will be a global fleet available to respond to events, an assessment of the ocean models along the fastest ocean transport pathways, and the establishment of a network of gliderports for global response.