A comparison of North Pacific and North Atlantic subtropical mode waters in a climatologically-forced model

Subtropical mode water (STMW), a water mass with homogeneous temperature and density and low potential vorticity, is formed in the subtropical gyres of both the North Pacific and North Atlantic Oceans. In the North Atlantic, this water mass is known as Eighteen Degree Water (EDW), while the corresponding water mass in the Pacific is identified as North Pacific Subtropical Mode Water (NPSTMW). This analysis compares properties of EDW with NPSTMW as well as the intrinsic oceanic variability of both, within the framework of a high resolution model with climatological atmospheric forcing. Interannual variability is evident in both volume and characteristics of EDW and NPSTMW, but the magnitude of variability is small, indicating that observed variability is largely forced by atmospheric changes. Volume of EDW is greater than volume of NPSTMW, but volume variability is similar in the two basins, and is dominated by the seasonal cycle in both cases. The most significant differences are found in the average ages of EDW compared to NPSTMW. Although the volume of STMW formed in each basin is similar, different circulation patterns lead to a higher average age in the North Atlantic, as EDW is more likely to be advected away from the formation region and remain subducted in the North Atlantic, while NPSTMW is more likely to be reventilated the following winter in the North Pacific.