Italian field activities in the Pacific Sector of the ACC
Austral Summer 2014-2015
30th Italian Expedition in Antarctica (PNRA XXX)
The absence of continental barriers across the latitude band of Drake Passage permits an Antarctic Circumpolar Current (ACC) to flow and act as the primary driver for inter-basin exchanges. Due to its important climate impact, the ACC band is an area of high interest, especially since global ocean temperatures have been shown to increase in the last. Hydrographic observations are thus important as they aid the description and understanding of the processes which occur within the Southern Ocean and across the ACC.
The oceanographic Italian field activities in the Southern Ocean during Austral summer 2014-15 will be carried on in the framework of the MORSea project (http://morsea.uniparthenope.it/, founded by the Italian PNRA - National Program for Antarctic Research) and they will be focused on the in situ monitoring of the Pacific Sector of the ACC - between New Zealand and the Ross Sea - by means of XBT, floats, and drifters. Two Italian scientists of University of Napoli “Parthenope” will be host onboard of the South Korean R/V Araon in the framework of the collaboration between Italian PNRA and KOPRI (Korea Polar Research Institute).
Three main actions will be undertaken during the cruise:
• collection of in situ temperature data in the layer 0-800m;
• release of 10 SVP2 surface drifters;
• deployment of 10 ARVOR and PROVOR floats.
Main focus of the cruise is the continuation of the about twenty year-long time series of temperature data along the endurance line New Zealand - Antarctica. Since 1994 in situ temperature data of the surface layer of the southern ocean have been collected in the framework of the Italian CLIMA and SOChIC projects. Up to 2013, 25 transects between New Zealand and the Ross Sea have been realized by the R/V Italica mainly in January and February (figure 1) collecting SST, SSS, XBT and XCTD data.
During Austral summer 2014/2015 field activities will increase and preserve the importance of the large dataset collected since 1994 and will represent also a significant contribution to the Italian deployment of lagrangian platforms in the latitude band of the ACC with a grand total of 20 drifting platforms (10 floats and 10 surface drifters) provided by the ARGO-Italy program (http://argoitaly.ogs.trieste.it/).
Figure 1. Transects of XBT casts relating to the CLIMA/SOChIC/MORSea programmes (black dots) from 1994 to 2013. Underlying bathymetry (meters below sea level) is indicated by color shading.
Temperature data are usually collected using Sippican T7 probes providing temperature profiles with a vertical resolution of 65 cm and a maximum nominal depth of 760 m. A regular 20 km sampling rate is adopted with increased sampling frequency over the main frontal regions of the ACC. Transects are usually performed in 5 days providing a synoptic picture of the thermal structure of the upper SO in this sector.
Surface data related to temperature, salinity, chlorophyll and – more recently – other bio-geochemical tracers, as well as atmospheric data and CO2 atmospheric partial pressure, are usually collected by the R/V Italica along the entire transect while drifters and floats are released in correspondence of the main ACC fronts.
Since 2009 the location of ACC fronts in this sector is also monitored on the basis of satellite altimetry data (SSH), in order to provide indications to sea-going staff on the position of fronts and also to study the variability of ACC fronts during the entire year and observe their modification during any eventual eddy shedding event.
In our data set the Polar Front (PF) positions (figure 2) are characterized by the minimum (northest position) value of 60.2°S reached during November 1994. The mean position of the PF is slightly southern (61.7°S) than previous estimation (61.5°S) from Budillon and Rintoul , while the standard deviation (0.9°) remains unchanged. Figure 2 also shows the presence of isolated latitude band associated to the PF_SSH values not linked to any in situ front position. These isolated signals, often characterized by a very small latitudinal extension, can be due to the presence of water masses with the same physical properties of PF water in an unusual northern position. The explanation for the northern migration of water with PF characteristics can be linked to the presence of eddies detaching from the PF and moving northward to the Sub Antarctic Front.
Figure 2. Positions of the PF during the CLIMA/SOChIC/MORSea cruises from 1994 to 2010. Black line represents the mean latitude of front from in situ data, gray vertical bars represent the along track-latitude bands where fronts are detected using altimetry values (from Cotroneo et al. 2013).