ALBERT

Description: Observations indicate a significant intensification of the Southern Hemisphere westerlies during the last decades. A continuation of this trend is projected by climate scenarios for the 21st century. The response of the Antarctic Circumpolar Current (ACC) and Southern Ocean carbon sink to changes in wind stress and surface buoyancy fluxes is under debate. Here we utilize the Argo network of profiling floats and historical data to assess the changes in temperature, salinity and density across the ACC during the last four decades. We find coherent hemispheric-scale warming and freshening trends extending to depths of more than 1000m. The trends are partly related to changes in water mass properties, consistent with the effect of anthropogenic changes in heat and freshwater fluxes suggested by climate models. However, there is no increase in the tilt of density surfaces across the ACC, in contrast to coarse-resolution model studies. The result implies ACC transport and meridional overturning - and therefore the Southern Ocean carbon sink - are insensitive to decadal changes in wind stress, suggesting wind-driven Ekman transport is compensated by eddy fluxes, as simulated by models with explicit eddies.

Description: The importance of the Southern Ocean to the global climate system and the uniqueness of its ecosystems are well known. The region is remote and logistically difficult to access and thus is one of the least sampled regions on the planet. Design and implementation of an observing system that encompasses physical, biogeochemical and ecological processes is therefore a formidable challenge. Building on meetings held in Hobart in 2006, Bremen in 2007 and St Petersburg in 2008 we present a draft plan for a Southern Ocean Observing System (SOOS). This document examines: (i) why sustained observations are needed in the Southern Ocean and what science/policy questions they address; (ii) what mix of observations are required to address these questions; (iii) what is presently done and possible and (iv) a vision for the future. SOOS is a key component of the SCAR Pan Antarctic Observing System (PantOS) and is co-sponsored by the Scientific Committee on Antarctic Research (SCAR), the Scientific Committee on Oceanic Research (SCOR), the Census of Antarctic Marine Life (CAML), the Partnership for Observation of the Global Oceans (POGO) and the Global Ocean Observing System (GOOS). Other organisations, in particular the National Oceanic and Atmospheric Administration (NOAA), have provided significant funding.

Description: Due to the low accessibility of the region, most of the seasonally ice-covered Southern Ocean remains unobserved during winter. Here we show that southern elephant seals (Mirounga leonina), equipped with oceanographic sensors, can measure winter hydrography with unprecedented temporal and spatial resolution. Seals provided a 30-fold increase in hydrographic profiles from the sea ice zone, allowing the major fronts to be mapped south of 60°S. Sea-ice formation rates in the East Antarctic sector were estimated from salinity increase in the upper water column and compared to simulations with a circumpolar finite-element sea-ice ocean model. From the observations, peak freezing rates of 2.5-3 cm/d were estimated for the period from late April to early May during the rapid northward expansion of the ice cover. While the pack ice becomes more compact, sea ice formation declines to 1 cm/d in June/July, and virtually stops by the end of August, when the maximum ice extent is reached. Modeled and observed freezing rates agree remarkably well, so that the model can be used to complete the seasonal cycle. By sampling the ocean during winter, elephant seals fill a blind spot in our sampling coverage, bringing us closer to a truly global ocean observing system.

Description: Over the past decade, Argo floats have provided an unprecedented number of profiles of the global oceans (to 2000m depth), far surpassing the number collected historically fromship-based hydrography. The original design of the Argomission specified nominal 3 x 3 degree spacing, with 10 daysampling interval, of the oceans between 60 °N and 60 °S,excluding the high latitudes and marginal seas. The exclusionof the high latitudes was due to the inability of early floats tosample under sea-ice. Technological advances in float designin recent years now give us this capability. Advancements havecome through re-design of hardware (i.e. armoured ice floats),software (ice-avoidance algorithm and open-water test) andcommunications (Iridium), allowing the transmission of storedwinter profiles. Observing circulation in seasonally ice-coveredseas is challenging. To date, most observations have beenmade during ice-free summer periods and consequently thewinter circulation beneath the sea-ice is not well understood.Despite this, Argo has already made a significant contribution tohigh latitude research with successful deployments of floats inthe polar oceans of both hemispheres. As of December 2008,over 100 floats had been deployed above 60 °N and over 200below 60 °S. Approximately 60% of these floats are still active(the failure rate of early floats was high as the ice-capabletechnology was being developed and tested). Mortality rates ofnewer ice floats are now equivalent to those deployed in lessdemanding conditions. In fact, a number of floats deployed inthe Weddell Sea have survived for 7 years (surpassing 225profiles) equal to some of the longest-lived floats deployedglobally. The high latitudes are important deep water massformation regions. The Southern Ocean connects the globalocean basins and regulates the meridional overturningcirculation. The exposed Arctic Ocean will have importantconsequences for ocean and atmospheric circulation, moistureand heat fluxes. Therefore, both polar regions play a critical rolein setting the rate and nature of global climate variabilitythrough their moderation of the earth's heat, freshwater andcarbon budgets. Recent studies have shown that certainregions at high latitudes are warming more rapidly than theglobal average. Some of the most important climate changesignals are seen near ice shelves and within the sea ice zone.In the Arctic, reductions in sea-ice extent and changes infreshwater fluxes, deep water mass properties and convectionhave been observed. Similarly strong reductions in sea-icecoverage are occurring near the Antarctic Peninsula while smallincreases appear in the Ross Sea. At the same time decreasingsalinity on the Ross Sea shelf is thought to be linked toincreased glacial melt. The Argo network has been crucial fordocumenting the recent changes in the open ocean; robust andlarge-scale freshening of the Southern Ocean has beenobserved from Argo and historical hydrographic data. Butsampling at these higher latitudes is less systematic than forthe rest of the globe. Therefore, observations of high latitudeoceans in both hemispheres should be a top priority. Inconsidering sampling strategies for the high latitudes werecommend extending the Argo network beyond 60 °S and60 °N through the deployment of ice-capable floats at thenominal density (3 x 3 degrees). In addition, regional arrays ofacoustically-tracked floats will provide a more focused effort onbasin scales. An established array of sound-sources (RAFOS)and acoustically-tracked floats in the Weddell Sea is alreadyyielding valuable information on ocean circulation and structurebeneath the sea-ice. A similar array should be established tosample the Ross Sea gyre. In the Arctic, an array of lowfrequency (〈 100 Hz) sound sources would be required toprovide basin-wide geo-location for profiling floats. Now that wehave come to review the past decade of progress within Argo,we find there is considerable support and justification for theofficial extension of the Argo array into the seasonally icecoveredseas. Sustained, comprehensive observation of thepolar oceans is required to adequately monitor global climatechange signals. This can only be achieved in a broad-scale andcost-effective way by using autonomous platforms like Argoprofiling floats. It is thus imperative that a commitment is madeto enhance and maintain a profiling float array in the highlatitudes. The extension of the core Argo array beyond 60degrees in both hemispheres will ensure that it remains one ofthe most important and truly global components of the oceanobserving system.