Subantarctic Mode and Antarctic Intermediate Water during the Present and Last Glacial Maximum

• Main Author: Hartin, Corinne A.
• Other Authors: Rana A. Fine, Amy C. Clement, Larry C. Peterson, Igor Kamenkovich, J.R. Toggweiler
• Format: Other/Unknown Material
• Language: unknown
• Published: Scholarly Repository 2012
• Subjects: SAMW; AAIW; CFCs; CCSM; formation rates; Antarctic; Austral; Pacific; Antarc*
• Online Access: https://scholarlyrepository.miami.edu/oa_dissertations/866; https://scholarlyrepository.miami.edu/cgi/viewcontent.cgi?article=1873&context=oa_dissertations

The formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) significantly contributes to the total uptake and storage of anthropogenic gases, such as CO2 and chlorofluorocarbons (CFCs), within the world's oceans. SAMW and AAIW formation rates in the South Pacific are quantified based on CFC-12 inventories using data from WOCE, CLIVAR, and data collected in the austral winter of 2005. This thesis documents the first wintertime observations of CFC-11 and CFC-12 saturations with respect to the 2005 atmosphere in the formation region of the southeast Pacific for SAMW and AAIW. CFC-12 inventories are 16.0x106 moles for SAMW and 8.7x106 moles for AAIW, corresponding to formation rates of 7.3 Sv ± 2.1 Sv for SAMW and 5.8 Sv ± 1.7 Sv for AAIW circulating within the South Pacific. Inter-ocean transports of SAMW from the South Pacific to the South Atlantic are estimated to be 4.4 Sv ± 0.6 Sv. Thus, the total formation of SAMW in the South Pacific is approximately 11.7 Sv ± 2.2 Sv. These formation rates represent the average formation rates over the major period of CFC input, from 1970 to 2005. The CFC-12 inventory maps provide direct evidence for two areas of formation of SAMW, one in the southeast Pacific and one in the central Pacific. These CFC-derived rates provide a baseline with which to compare past and future formation rates of SAMW and AAIW. Average formation rates for SAMW and AAIW in the South Pacific were calculated from the National Center for Atmospheric Research Community Climate System Model version 4 (NCAR-CCSM4), using CFC-12 inventories. These inventories and rates are compared to those calculated earlier from observations within the South Pacific. CCSM4 accurately simulates the southeast Pacific as the main region of formation for both SAMW and AAIW. Model formation rates in the South Pacific for SAMW are 3.4 Sv, about one-third of the observational rate. Shallow mixed layer depth and insufficient meridional transport of high CFC waters in CCM4 are probable reasons for lower formation rates for SAMW. However, for AAIW in CCSM4, a rate of 8.1 Sv is slightly higher than the observational rate including error. Increased inventories in CCSM4, particularly in the southwest and central Pacific, and higher surface inventories are the main reasons for greater formation rates of AAIW. This comparison of formation rates in observations and model is useful for understanding the uptake and transport of anthropogenic CO2, by the model, as CFCs are used for calculating anthropogenic CO2 inventories. Lastly, it is important to understand how SAMW and AAIW have changed under differing climatic periods as compared to the present. Formation rates are calculated for SAMW and AAIW between the Last Glacial Maximum (LGM; 21,000 years ago) and preindustrial (PI) period in the South Pacific, using the NCAR Community Climate System Model version 3 (CCSM3). Water mass formation rates are computed from thermodynamic (surface fluxes) and dynamic methods. The thermodynamic method does not account for contributions from mixing, while the dynamic method measures the total rate at which water enters the permanent thermocline. Formation rates from surface fluxes tend to destroy the surface water formation of G-SAMW and G-AAIW. However, using the dynamic method, SAMW and AAIW over the South Pacific exhibit increased subduction rates from 4.3 Sv during the PI to 8.9 Sv during the LGM for SAMW, and 4.3 Sv during the PI to 8.3 Sv during the LGM for AAIW. Calculations of interior mixing between the surface and the permanent thermocline are responsible for the formation and transport of these water masses into the interior subtropical gyre. Despite a few model limitations, these results agree well with the limited paleo evidence available within the South Pacific. The model results provide a better understanding of the processes controlling the properties and formation of SAMW and AAIW under differing climatic conditions. This diagnosis is timely because of the inconclusiveness about modern and future changes in mode and intermediate water formation, and its potential role in the sequestration of atmospheric CO2.