Recent inter-seasonal variability of EDW in the North Atlantic using Volume Index

Abstract The inter-annual, intra-seasonal, and seasonal variability of Eighteen Degree Water (EDW) volume was investigated in this paper using a new metric called the Volume Index (VI), based on the vertical integration of ocean’s layer probability to have a temperature in the mode water range over a given time range t. It has been applied to BOA-Argo datasets from 225,792 existing temperature profiles for the period 2004-2019 over the Subtropical North Atlantic region and sought to capture the variability of EDW volume. Monthly North Atlantic Oscillation (NAO) data from CRU and ENSO data from PSL-NOAA have also been used and compared to the trend of the EDW volume variability to understand its cause. We have analyzed the total volume of EDW during winter (JFM) and summertime (JAS) and have compared their signal and the major results of this paper demonstrate the total volume of EDW is more important in summer (JAS) than in winter (JFM). Our study has shown the EDW total volume fluctuates around 60Svy reaching its maximum at 72Svy. In addition, the EDW total volume fluctuates around 65Svy with values varying between 55-72Svy in summer, while it is around 52Svy with values varying between 41-106Svy in winter. The Inter-annual variability brought out a slight increase from 2004 to 2008, a signal oxalate around a stable from 2008 to 2014, and a remarkable downward trend during 2014- 2017. Volume Index displayed a good stratification of EDW in the ocean. NAO correlates well with the noticeable downward trend in EDW volume during 2015-2017. Intra-seasonal variability also showed the same downward trend in both winter and summertime. We equally have observed the weak signal of the EDW volume during the winter season of 2011, which is also well correlated with El Niño 2009-2011. Finally, the metric VI, the method of Volume Index proved to be an efficient analysis tool to capture the variability of EDW volume with a statistical uncertainty of around 0.1.