Modal composition of the central water in the North Atlantic subtropical gyre

Abstract

The modal composition of the Central Water in the North Atlantic subtropical gyre is notclearly defined, as there are some uncertainties related to mode identification, as wellas modes which are not well documented. This study shows that eastern North AtlanticCentral Water (eastern NACW) in the subtropical gyre is composed of three modes:5The North Atlantic Subpolar Mode Water (NASPMWσt=27.1 to 27.3), the MadeiraMode Water (MMWσt=26.4 to 26.6), and the mode water with aσtnear 27.0, which iscurrently not well documented. We confirmed this mode based on the similarities foundbetween it and the mode waters already reported. The similarities were determinedfrom comparative analyses of the temperature/salinity standard curves and the gradi-10ents of the potential density anomalies of two concurrent data sets from two subtropicaltime-series stations (Bermuda Atlantic Time-series Study, BATS, in the west, and Eu-ropean Station for Time-series in the Ocean Canary Islands, ESTOC, in the east). Inorder to establish the outcropping regions, the corresponding pycnostads were deter-mined using another climatologic data set (World Ocean Database, WOD2005). In15this case, the pycnostads were located based on the presence of standard deviationminima from the average density anomalies. Finally, we confirmed that the pycnostadscorresponded to the temperature values related to the modes by overlaying the char-acteristic modal isotherm of each of the modes in the geographic distribution of thepycnostads. Sea surface temperature data (SST) from the Ocean Pathfinder Program20(OPP) were used to estimate the isotherms. The results showed a clear correspon-dence between the modal isotherms and the pycnostads, for both the modes that havealready been documented and the mode confirmed in this study.