05 Sep 2022
05 Sep 2022
Status: this preprint is open for discussion.

Global variability of high nutrient low chlorophyll regions using neural networks and wavelet coherence analysis

Gotzon Basterretxea1, Joan S. Font-Muñoz1, Ismael Hernández-Carrasco2, and Sergio Sañudo-Wilhelmy3 Gotzon Basterretxea et al.
  • 1Department of Marine Ecology, Instituto Mediterráneo de Estudios Avanzados, IMEDEA (UIB-CSIC), Miquel Marqués 21, 07190 Esporles, Illes Balears, Spain
  • 2Department of Oceanography and Global Change, Instituto Mediterráneo de Estudios Avanzados, IMEDEA (UIB-CSIC), Miquel Marqués 21, 07190 Esporles, Illes Balears, Spain
  • 3Department of Biological Sciences and Department of Earth Sciences, University of Southern California, Marine Biology and Biological Oceanography, Los Angeles, California 90089-0371, United States

Abstract. We examine 20-years of monthly global ocean color data and modelling outputs of nutrients using self-organizing map analysis (SOM) to identify characteristic spatial and temporal patterns of High Nutrient Low Chlorophyll (HNLC) regions and their association with different climate modes. Analyzing the properties of the probability distribution function of the global nitrate to chlorophyll ratio (NO3:Chl), we estimate that NO3:Chl>17 (mmol NO3/mg Chl) is a good indicator of the distribution limit of this unproductive biome that extends over ~25 % of the ocean. Trends in satellite-derived surface chlorophyll (0.6±0.4 to 2±0.4 % yr-1) suggest that HNLC regions in polar and subpolar areas have experienced an increase in phytoplankton biomass over the last decades. However, much of this variation is produced by a foremost climate-driven transition occurring after the year 2010, which resulted in a reduction in the extension of polar HNLC regions and an increase in their productivity. Chlorophyll variations at HNLC regions respond to all three major climate variability signals (Sea Surface Temperature, SST; El Niño Southern Oscillation, ENSO; and Meridional Overturning Circulation, MOC) and their annual and semiannual variabilities are coherent with seasonal temperature variations. At larger scales, ENSO driven variability (2–4 yr) and decadal-scale processes of heat uptake and redistribution by ocean circulation influence the HNLC extension. Our results are indicative of the long-term changes in phytoplankton biomass and productivity in the ocean and suggest global coupling in the functioning of distant biogeochemical regions.

Gotzon Basterretxea et al.

Status: open (until 31 Oct 2022)

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  • CC1: 'Comment on egusphere-2022-827', Yonggang Liu, 21 Sep 2022 reply

Gotzon Basterretxea et al.

Gotzon Basterretxea et al.


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Short summary
We examines the patterns of variability of high nutrient low chlorophyll regions (HNLC) identifies their response to major climate drivers of ocean variability. HNLC areas are ocean regions where primary production should be potentially high but phytoplankton biomass remains relatively low and constant despite the perennial nutrient availability for growth.