Satellite remote sensing of ocean color, as characterized by Rrs(l), provides critical information to support global and regional-scale research and applications on biological and biogeochemical constituents of the world’s oceans, including assessment of large-scale changes in the distribution of marine phytoplankton due to natural or anthropogenically-induced variations in the Earth’s climate24,25. The capability to track such climate-driven impacts is paramount, as ocean phytoplankton are responsible for roughly 50% of global net primary production, and rapid variations in phytoplankton populations can dramatically alter ocean ecosystems and the services those ecosystems provide, including impacts to food security and global biogoechemical cycles26,27. In open oceans, temporal variations in the ocean color signal over a fixed region typically demonstrate a clear seasonal cycle due to natural variations in light and nutrient availability that drive phytoplankton productivity, with deviations from that climatological cycle attributable to regional or global disturbances to the environment that impact phytoplankton growth.
Analysis of the MODIS Aqua OC data record for two key wavelengths in in the blue and green spectral region (Rrs at 443nm and 547nm, respectively), and two OC-derived phytoplankton biomass metrics, bbp(443) (directly related to phytoplankton carbon biomass) and Chla, for the NH and SH regions, demonstrates the distinct seasonal cycles in phytoplankton growth that follow the variation in solar illumination for the period of 2002–2021 (Fig. 1). In 2022, however, the seasonal cycle in Rrs(l) for the SH (red lines in Fig. 1) indicates a strong negative deviation from that 20-year record. A similar discrepancy is visible in the bbp(443) seasonal cycle in SH, while bbp(443) and Rrs(l) in the NH are consistent with the historical norm. In contrast, the Chla seasonal cycle for 2022 is in-family with the climatological record for the SH region and is slightly elevated over much of the year for the NH, consistent with expectations due to the persistent El Nino conditions prevailing in 2022.1
The same anomalous behavior in ocean color measurements is visible across several other satellite sensors operational in 2022, as shown here for Rrs in the green spectral range, for the SPSG region of the SH (Fig. 2). This cross-sensor comparison demonstrates that the year 2022 was consistently anomalous relative to the climatological record, with deviations typically around 3 standard deviations below the long-term mean for each sensor. We note that, while there is some temporal cross-calibration applied between MODIS-Terra and MODIS-Aqua28, all other sensors are fully independent with respect to calibration, thus ruling out the possibility that observed anomalies in the SH (Fig. 1) are due to systematic error in the MODIS-Aqua calibration related to uncorrected radiometric degradation. We note also that similar deviations are visible in datasets independent of the NASA atmospheric correction process. Seasonal cycles in the EUMETSAT-derived OLCI Rrs, while flatter relative to the NASA-derived OLCI Rrs of Fig. 2(e,f), clearly demonstrate the year 2022 to be out of family and with a negative deviation relative to their historical record (Fig. 2g,h),.
The observed rapid change in SH ocean color parameters in 2022 coincided with the eruptions of Hunga Tonga-Hunga Ha'apai, which had an unprecedented impact on the concentration and distribution of atmospheric aerosols. The annual progression of stratospheric aerosol, as measured by OMPS over 2022, differs substantially from the previous 10 years of measurements (Fig. 3). Near the equator and extending well into the SH, the stratospheric AOT reported at 675 nm increased dramatically, reaching a peak in June-July of 2022 that was 6–7 times the historical average. This positive anomaly in the SH stratospheric aerosol follows closely with the progression of negative Rrs(λ) anomalies observed in NASA and ESA/EUMETSAT OC data (Figs. 1 and 2).
The similarity in ocean color (Figs. 1 and 2) and aerosol trends (Fig. 3) over the year 2022 suggests that there is likely a causal link between the unusual distribution of aerosols from the Tonga eruptions and the OC SH anomalies, drawing two possible hypotheses. The first hypothesis is that we are observing a biological response of the phytoplankton community to the eruption, as has been observed in previous volcanic events and linked to transport of volcanic ash, which typically contains nutrients such as iron needed for photosynthesis, to nutrient deplete regions of the oceans29,30,31. What we observe in the ocean color trends for 2022 in the SH is a marked decrease in backscatter paired with no change in chlorophyll concentration, which could be interpreted as a physiological response of the phytoplankton community that resulted in a decrease in phytoplankton biomass (Cphy). The second hypothesis is that the anomalies observed in the satellite ocean color record are the result of an error in the atmospheric correction process due to the unusual aerosol conditions following the eruption. To evaluate these hypotheses, an independent dataset of phytoplankton biomass collected in situ by the BGC-Argo fleet was assessed. The BGC-Argo measurements of bbp from within the SPSG indicate no change in 2022 relative to previous years (2016–2022). This supports the assertion in Franz et al. that the anomalies observed in the satellite ocean color record are likely the result of error in the atmospheric correction process rather than an indication of a marked decrease in phytoplankton biomass in 20221.
The NASA AC algorithm assumes that aerosols are primarily scattering, with only weak absorption, and that those aerosols are located in the troposphere9. When first injected, the additional aerosols from the Tonga eruptions are believed to have been moderately absorbing32, but after aging and transport the sustained anomalous aerosol population in the stratosphere is found to be weakly absorbing7, consistent with the NASA AC algorithm assumptions. The AC algorithm also assumes that the aerosol (and most of the atmosphere) is below the stratospheric ozone layer, and therefore a correction for ozone absorption can be applied before attempting to separate the atmosphere and ocean contributions from the total signal reaching the satellite sensor9. The additional aerosols from the Tonga eruptions, however, were injected into the stratosphere and thus co-mingled with the ozone, breaking an inherent assumption in the NASA AC algorithm.
To evaluate the second hypothesis, we assess the impact of this unusual aerosol-ozone mixing on the AC process, and consequently on the OC products, through a sensitivity analysis that was conducted using a fully-coupled ocean-atmosphere vector radiative transfer simulation (Fig. 5). With atmospheric conditions modeled to be representative of the eruption, including stratospheric aerosol distributions following Taha et al.4, the total reflectance at the satellite sensor was simulated with and without the presence of ozone. These two simulation sets enable identification of the influence of stratospheric aerosol scattering effects on ozone absorption, highlighting the significance of the radiative interaction between these two components. If we apply the NASA standard correction for ozone absorption9, which assumes the aerosols are well below the ozone layer, to a signal that includes aerosol scattering contributions from within the ozone layer, the effect is to underestimate the correction, especially in the green-red region of the spectrum where ozone absorption is strongest. Once the remainder of the atmospheric contributions to the observed signal are subtracted, the result is an underestimation of the water-leaving signal with a spectral bias that roughly follows the ozone absorption spectrum. This spectral bias is further propagated into remote sensing reflectance and biogeochemical products, but due to the spectral characteristics of the bias and nature of the algorithms it is not affecting all products in the same way. While both bbp(443) and Chla are derived from Rrs(l), the Chla algorithm, as applied over relatively clear open-ocean waters, is based on spectral band differences and is thus less sensitive to changes in Rrs(l) that affect all bands in the same direction, as would be expected from (for example) an error in the AC algorithm due to over or underestimated aerosol contributions11. In contrast, bbp at 443 (and at all other wavelengths in the visible spectral range), is derived from the spectral fitting of a 3-parameter model to the Rrs(l) distribution, with fixed assumptions about pure seawater contributions, and is thus more sensitive to absolute values in Rrs(l)13.
Our findings also suggest a path forward to mitigate the impact of the Tonga eruptions and, potentially, other events that have contributed to unusual enhancements in stratospheric aerosol loads. Specifically, a series of sensitivity analyses could be performed to characterize the effect of statospheric aerosols as a function of concentration, altitude, and microphysical properties to produce a model or look-up table. Ancillary data from OMPS or similar sensors could then be used to compute a correction using that model, as a precursor to application of the standard AC algorithm; however, such a correction will take some time to develop and validate, and it will be limited by the availability, quality, and resolution of the ancillary aerosol information.