Variability and trends in subtropical gyres derived from 25-year satellite chlorophyll-a observations

Previous studies, based on satellite observations, suggested that global warming caused all subtropical gyres to expand over time (Polovina et al., 2008; Leonelli et al., 2022). This raised major concerns about the potential increase in ocean desertification and its impact on the Earth’s climate system. Here, the longest satellite chlorophyll-a (chl-a) concentration dataset currently available is used to analyze the evolution of the most oligotrophic areas in the five major subtropical gyres on Earth at a fine spatial-temporal resolution over 25 years (January 1998 - December 2022). These gyres include the North and South Atlantic SubTropical Gyres (NASTG and SASTG, respectively), the North and South Pacific SubTropical Gyres (NPSTG and SPSTG, respectively), and the Indian Ocean SubTropical Gyre (IOSTG). Different thresholds of chlorophyll-a concentrations are used to defined three subregions within each gyre: the oligotrophic area, whose chl-a is less than or equal to 0.1 mg m^-3, and the ultra- and hyper- oligotrophic areas where chl-a does not exceed 0.07 and 0.04 mg m^-3, respectively. Our results indicate that ultraoligotrophic conditions prevail in all five systems, and that oligotrophic and ultraoligotrophic zones in all gyres combined have reduced and expanded, respectively, at the same rate (0.3%/yr) since 1998, while the hyperoligotrophic area has increased globally at an annual rate of 3.4%. Results also reveal that the most affected gyres are the NASTG, the NPSTG and the IOSTG, where the hyperoligotrophic subregions have expanded at an annual rate of 17.8, 22.8 and 8.6%, respectively, and their combined area in 2022 was about 5 times larger than it was in 1998. No statistically significant (p>0.05) trends were detected in the SASTG and SPSTG, although an increasing tendency in their hyperoligotrophic subregions is observed. Altogether, the results suggest that, despite no significant variation in the overall size of subtropical gyres being observed in 25 years, their hyperoligotrophic cores are expanding and would lead to reduced productivity in these systems.

 

Polovina, J.J., Howell, E.A., and Abecassis, M. (2008). Ocean’s least productive waters are expanding. Geophysical Research Letters, 25, L03618, doi:10.1029/2007GL031745

Leonelli, F.E., Bellacicco, M., Pitarch, J., Organielli, E., Buongiorno Nardelli, B., de Toma, V., Cammarota, C., Marullo, S., and Santoleri, R. (2022). Ultra-oligotrophic Waters Expansion in the North Atlantic Subtropical Gyre Revealed by 21 Years of Satellite Observations. Geophysical Research Letters, 49, e2021GL096965, doi:10.1029/2021GL096965