Description: Immediately north of the circumpolar subantarctic front, deep {400-600 m) well-mixed Jayers are found in late winter. Spring and summer heating isolates {but does not completely erase) these layers beneath the seasonal thermocline as thermostads. The zone in which this active renewal is found is several hundred kilometers wide, but the associated thermostad can be traced much further north-on the order of 2000 km. The thermostads and the often associated dissolved oxygen maxima can be found as far north as the south equatorial current regions of each southem hemisphere subtropical gyre. This water mass formation and spreading process is equivalent to that occurring east and south of the Gulf Stream and Kuroshio currents, where the thermostads are called Subtropical Mode Water {STMW). In light of the association of these southern ocean thermostads with the circumpolar subantarctic front, rather than the subtropical fronts {westem boundary currents such as the Agulhas current) the name Subantarctic Mode Waier (SAMW) is suggested. In common with STMW. SAMW contributes substantial volumetric modes to the central water masses. indicating SAMW tobe the renewal agent ofthe high oxygen parts ofthe main thermoc!ine water of the southern hemisphere subtropical gyres. Finally, it is noted that the specific types of SAMW formed in the southeast Pacific and Scotia Sea areas are identical in temperature and salinity to the South Pacific and South Atlantic varieties of Antarctic Intermediate Water {AAIW). The renewal process for AAIW is hence indicated as taking place 11orth of the polar front zone, in the southeast Pacific and Scotia Sea parts of the subantarctic zone. The actual process is late winter convective overturning of the somewhat warmer and more saline waters advecting into the region from the west along the subantarctic front. The low salinity of AATW is due to the pronounced excess of precipitation over evaporation in the subantarctic zone. This process is quite different from the traditional concept of circumpolar cross-polar-frontal mixing of Antarctic Surface Water with Subantarctic Surface Water.

Description: Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.