Huge water masses in the ocean interior control biological productivity near the surface!?

Normally, seawater temperature decreases with depth from the sea surface. However, in some locations, water temperature and salinity in the ocean interior are vertically uniform over hundreds of meters. Such vertically uniform water masses are called “mode water”. Mode water forms in winter when the sea surface cools by atmosphere and deep convection occurs. After spring, it becomes trapped within the ocean interior and is spread from its formation area by ocean currents (Mov. 1). Mode water exists in various ocean basins around the world (for example, south of the Kuroshio and its extension in the North Pacific, and south of the Gulf Stream in the North Atlantic).

Mov. 1. Formation and spreading of subtropical mode water

Long-term observations show that the mode water thickness (i.e., volume) varies annually. Recent studies report that the thicker (thinner) mode water in the North Pacific has a stronger (weaker) uplifting effect on the near-surface water temperature structure while moving through the ocean interior, causing the surface layer to become colder (warmer) (Kobashi et al., 2021, 2023, Oka et al., 2023). Furthermore, it has also been found that the lower (higher) sea water temperature associated with thicker (thinner) mode water weakens (strengthens) typhoons (Fig. 1, Oka et al., 2023, Press release).

Fig. 1. Relationship between subtropical mode water and typhoon intensity （Oka et al., 2023, Press release）

The uplifting effect of mode water is expected to influence not only the water temperature structure near the sea surface but also the structure of nutrients. The ocean contains many nutrients such as nitrate and phosphate, and various biological species utilize these substances to sustain their life. In this study, we analyzed ocean observation data for the North Atlantic, including water temperature, salinity, and nutrients, to investigate whether the uplifting effect of North Atlantic mode water could also impact the marine ecosystem.

Nutrients in the ocean generally increase with depth below the sea surface (Fig. 2, left). Especially, within the euphotic layer where light reaches, extending from the surface to approximately 120 meters deep, nutrients are actively consumed by phytoplankton photosynthesis, leading to a state of nutrient exhaustion. We have revealed that thick (thin) mode water enhances (weakens) the lifting effect on the nutrient structure, causing nutrients within the euphotic layer to increase (decrease) (Fig. 2, right). Furthermore, during spring to early summer, we also found that primary production (i.e., phytoplankton producing organic matter from inorganic substances through photosynthesis) within the euphotic layer was more active when the mode water was thicker (Fig. 3). It is thought that one reason for increasing nutrients within the euphotic layer was the uplifting effect of the mode water.

Fig. 2. (Left) Average vertical structure of nutrients (nitrate) for each month. Cooler colors indicate lower nutrients. The layer enclosed by the thick pink line, at depths of approximately 200–500 m, shows the average distribution of mode water. White circles indicate the depth of the euphotic layer, and black stars denote the depth of the mixed layer. (Right) Nutrient difference between when mode water is thick and when it is thin (thick minus thin). Warmer (cooler) colors indicate higher (lower) nutrients while the mode water is thicker (thinner). When the mode water is thicker, nutrients are more abundant due to the uplifting effect above the mode water (approx. 100–200 m depths).

Fig. 3. Primary production within the euphotic layer while the mode water is thick (pink) and thin (light blue), and the difference between them (black line: thick – thin). Black double circles indicate statistically significant differences.

Mode water, a huge water mass in the ocean interior, has been shown to affect meteorological phenomena and marine ecosystems through the uplifting effect.  By considering the “uplifting effect of mode water,” improvements in the accuracy of forecasts for meteorology and fisheries are expected.

For more details:
Nishikawa, H., E. Oka, S. Sugimoto, F. Kobashi, M. Ishii, and NR. Bates (2025): Impact of Eighteen Degree Water thickness variation on the thermal and biogeochemical structure in the euphotic layer. Journal of Oceanography, 81, 235-246, doi:10.1007/s10872-025-00751-1.

(Hatsumi Nishikawa@A01-2, ECHOES. September 2025)