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 metric called the Volume Index (VI), based on the vertical integration of the oceans layer probability to have a temperature in the Mode Water range over a given time range t. Our main objective was to quantify and compare the total volume of EDW in the winter and summer. This method 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 compared their signals, and the major results of this paper demonstrate that the total volume of EDW is more important in summer (JAS) than in winter (JFM). We found that EDW total volume illustrated a significant anti-correlation with NAO in the summer. Our study has shown the EDW total volume fluctuates around 60 Svy, reaching its maximum at 72 Svy. In addition, the EDW total volume fluctuates around 65 Svy with values varying between 55 and 72 Svy in summer, while it is around 52 Svy with values varying between 41 and 106 Svy in winter. The interannual variability brought out a slight increase from 2004 to 2008, a signal oxalate around a stable level from 2008 to 2014, and a remarkable downward trend during 2014-2017. The Volume Index (VI) 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 also 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 VI proved to be an efficient analysis tool to capture the variability of EDW volume with a statistical uncertainty of around 0.1.

Introduction

Subtropical Mode Water (STMW)—specifically, Eighteen Degree Water (EDW) in the North Atlantic—is a thick, horizontally extensive layer of nearly uniform temperature and salinity, typically found just south of the Gulf Stream within the subtropical gyre. Formed by wintertime convective mixing due to surface cooling, EDW plays a key role in climate regulation by influencing ocean-atmosphere heat and CO? exchange.

This study analyzes the seasonal and interannual variability of EDW volume from 2004 to 2019 using a simple Volume Index. Instead of mapping or interpolation, the method is based on vertical integration of in-situ Argo temperature profiles within the 17–19°C range, with a minimum thickness criterion (>20m). The resulting metric, V^(t)\hat{V}(t)V^(t), captures variability rather than absolute volume.

Key findings:

• EDW forms and thickens during winter, with maximum thickness (~260m) south of the Gulf Stream.

• The Volume Index reveals seasonal and low-frequency variability of EDW.

• Most EDW layers occur between 200–600m depth, below the seasonal thermocline and above the main thermocline.

• EDW thickness is highly variable in space and time, peaking in areas of deep winter convection.

• EDW variability correlates with large-scale climate drivers such as North Atlantic Oscillation (NAO) and ENSO (via Niño 3.4 index).

Data sources include:

• BOA-Argo dataset (2004–2019): high-quality, gridded monthly ocean profiles.

• Niño 3.4 SST anomalies (2004–2021) for ENSO phases.

• NAO index (2004–2021) to assess atmospheric pressure patterns and their impact on EDW.

The Volume Index approach successfully tracks the temporal variability of EDW volume using Argo data without relying on interpolation or gridded models, offering a practical and efficient tool for monitoring ocean heat content and its role in climate dynamics.

Conclusion

The properties, characteristics and volume of EDW are a topic that has been widely studied by many authors [1{3, 5, 21, 23, 24]. Also, several works have been carried out on the variability of the EDW volume [6, 23, 25, 33]. In this paper, we explored the interannual, seasonal and intra-seasonal variability of the EDW using the Volume Index method. Our main objective was to quantify and compare the total volume of EDW in winter and summer. To compute and study the inter-annual and seasonal variability of EDW volume, Volume Index has been assessed focusing on the vertical integration of the oceans layer probability. This method is a metric based on a simple analysis of Argo in-situ vertical profiles of temperature, it is the vertical integral of the oceans layer probability of EDW. We have used the monthly objectively mapped BOA-Argo data from 2004 to 2019, with a 1? x 1? horizontal grid and 49 vertical levels from the surface to 1950 m depth. We analyzed the total volume of EDW in winter (JFM) and summer (JAS) and compared their signal. The main results of this paper demonstrate that the total volume of EDW is greater (more voluminous) in summer (JAS) than in winter (JFM). Our study showed that the total volume of the EDW fluctuates around 60Svy reaching its maximum at 72Svy. Furthermore, the total volume of the EDW fluctuates around 65Svy with values varying between 55-72Svy in summer, while it is around 52Svy with values varying between 41-106Svy in winter. It is important to note the higher number of profiles present in winter biases the volume value for the year 2004. The results of this article are in good agreement with the studies of Worthington and Forget [4, 6], who estimated the total volume of EDW to be approximately 75-80 Svy. Worthington et al. [4] published the first estimates of the volume of EDW, Kwon and Riser [20] estimated the winter formation rate at 12.47 Svy, Maze et al. [23] estimated the seasonal cycle of the mixed layer using the potential vorticity, Forget et al. [6] evaluated the seasonal cycle of the increase in winter rate of EDW volume at 8.6 Svy and total EDW volume at 75 Svy, Billheimer et al. [33] studied the restratification and destruction process of EDW and Li et al. [6] studied the extremes of EDW formation, showing that Ekman transport is an indicator of extreme EDW events. However, their work did not give an accurate estimate and difference in the total volume of EDW in winter and summer.We analyzed and evaluated the total volume of EDW in winter and summer using the VI method and showed that the volume of EDW is larger in summer than in winter. Billheimer’s work studied the mechanism of the subducted EDW layer, showing that the process of EDW destruction rate is highest in early summer, where EDW penetration is maximum, this which leads the total interior volume of the EDW to be larger in summer than in winter, as we demonstrated in our study. Interannual variability showed a slight increase from 2004 to 2008, a signal that oscillated around stability from 2008 to 2014, and a remarkable reduction from 2014 to 2019 as studied by Stevens et al. [25]. The volume index showed good stratification of EDWs in the ocean. NAO and El Ñino 2014-2016 were correlated with the reduction in EDW volume from 2015 to 2019. Intra-seasonal variability also showed a gradual reduction in EDW volume, from 2014 to 2019 in both winter and summer. It also showed a good correlation with NAO. We observed the weak signal of the EDW volume in winter, which was also well correlated with the NAO. The total volume of EDW illustrated a significant anti-correlation with NAO in summer. Finally, the metric, the Volume Index method, was found to be an effective analysis tool for capturing EDW volume variability, with a statistical uncertainty of approximately 0.1. Therefore, we believe that this work has brought new insights for a better understanding of the EDW volume.

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Copyright

Copyright © 2025 Sonfack Brice Rousvel, Komkoua Mbienda A. J , Guy Merlin Guenang, Kenfack-Sadem Christian, Mpame Guilène, Nzeukou Armand, Vondou Appolinaire Derbetini . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.