[A-OS15] Chemical and Biological Oceanography

[AOS15-02] Impact of North Atlantic subtropical mode water thickness variation on new production in oceanic surface layer

*Hatsumi Nishikawa¹, Eitarou Oka¹, Shusaku Sugimoto², Fumiaki Kobashi³, Masao Ishii⁴ (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Graduate School of Science, Tohoku University, 3.Department of Maritime Systems Engineering, Tokyo University of Marine Science and Technology, 4.Meteorological Research Institute, Japan Meteorological Agency)

Keywords:Subtropical Mode Water, North Atlantic, nutricline, new production

Subtropical Mode Water (STMW) is a vertically uniform water mass that exists on the low-latitude side of the western boundary current in each ocean basin. The thickness of STMW in the North Pacific varies on a decadal timescale. When the STMW is thicker, it lowers temperature in the overlying layers by uplifting the isotherms there (Kobashi et al., 2021, 2023). In addition, it weakens the intensity of typhoons passing northward in the region south of Japan by reducing upper ocean heat content (Oka et al., 2023). Such STMW thickness variation is also expected to affect new production in the euphotic layer by changing the nutricline depth.
STMW in the North Atlantic is called Eighteen Degree Water (EDW) after its temperature. In this study, using Argo float data in 2012-2023 and shipboard observation data at Bermuda Atlantic Time-series Study in 1989-2022, we examined thermal variation in the surface layer associated with the EDW thickness variation. Like in the North Pacific, when the EDW existing around 200-500 m depths was thicker, the isotherms in the overlying layers were uplifted, leading to a decrease in temperature centered at 50-100 m in the warm season. Furthermore, the nutricline existing around 100-150 m depths was also uplifted, leading to an increase of subsurface nutrient concentrations around those depths. In association with the nutrient concentration increase, chlorophyll concentration increased (decreased) at depths less (greater) than 100 m, at which the chlorophyll maximum lies, and the same tendency was found in the primary production.