Investigation of variability of the North Atlantic Subtropical Mode Water using profiling float data and numerical model output

Abstract

The basic characteristics of the North Atlantic Subtropical Mode Waters (STMW) are well documented in the literature from one-time hydrographic sections or from long-term measurements at one location. From July 1997 to March 1998, 71 vertical profiling floats were deployed in the Western Subtropical North Atlantic region to provide a broader spatial and temporal coverage of STMW. In this study, the STMW properties are estimated from a total of 5352 float temperature profiles in an area covering 30°W–80°W and 20°N–45°N, from January 1998 to December 2000. These STMW properties are compared to those derived from numerical simulation output using the Miami Isopycnic Coordinate Ocean Model (MICOM). The model is configured to the North Atlantic Ocean with a 1-deg horizontal resolution and 20 vertical layers. The model temperature profiles are collocated with the observed float profiles for a direct comparison. The values of the mean STMW temperature derived from both the float data and model output are roughly equal (18.1 °C for floats and 18.0 °C for the model). The model is found to capture well the observed annual cycle of the STMW volume. Given good agreement between float and model profiles, we use a longer MICOM simulation to investigate decadal variability of the STMW renewal rate in terms of the annual subduction rate and its relationship to the large-scale atmospheric circulation pattern changes measured by the North Atlantic Oscillation index. Good correlation is found between the model annual subduction rate anomaly at the Panulirus Station and the NAO winter index anomaly on decadal time scales with NAO leading by 2–3 years.