Freshening and Warming of Antarctic Intermediate Water Along a Transect from New Zealand to French Polynesia Using a Historical and Modern Data Comparison

Global warming is a driver of sometimes subtle shifts in climate patterns. One such area is deep ocean water formation, where alterations in processes regulating water density may occur (e.g., precipitation, evaporation, ice melt). At the Antarctic Polar Front, modelling results have shown ice melt and precipitation increases that can influence the conservative characteristics of the Antarctic Intermediate Water mass (AAIW) as it is formed. AAIW should retain signatures of temperature and salinity at the time of its formation as it sinks and moves throughout the South Pacific Ocean during thermohaline circulation. In March and April of 2018, the expectation of preserved temperature and salinity change was tested aboard the Sea Education Association vessel SSV Robert C. Seamans. During a semester abroad, this undergraduate research project sought to calculate modern freshening and warming rates of the AAIW from 1970 to 2018. These calculations would enable us to confirm the continuity of, or detect change in, previously determined freshening and warming rates that did not include data beyond 2008 (Durack & Wijffels, 2010, https://doi.org/10.1175/2010JCLI3377.1). Throughout a six-week cruise from Lyttelton, NZ, to Pape’ete, French Polynesia, 17 CTD casts were collected. AAIW was identified by its signature temperature, salinity, and depth ranges. The 2018 CTD data were compared to historical cast data from the World Ocean Database to derive updated AAIW freshening and warming rates since 1970. For the sampled region of the South Pacific, decadal comparisons of representative CTD casts yielded a calculated freshening rate of -0.0023 psu/yr and a warming rate of 0.008 ˚C/yr, with a cumulative decrease of 0.109 psu and increase of 0.855˚C over 48 years. The warming of the AAIW agrees with surface warming trends at its zone of formation while the calculated rate of freshening was comparable to rates calculated for less recent time series’, suggesting that the rate of change has not abated or been altered over this time period.