Contrasting hemispheric responses in seasonal surface ocean pH extremes

The oceans have played an important role in mitigating the effects of taking up 30% of anthropogenic CO₂ from the atmosphere, but not without effect. The ocean has already become more acidic. It is estimated that ocean pH has already decreased from 8.2 to 8.1 since the start of the industrial era (a 30% increase in [H⁺]; Royal Society 2005) and may decrease by a further 0.3 units in the coming century (Orr et al. 2004). The effects of increased acidification on marine life are complex, affecting marine organisms throughout trophic levels (Doney et al. 2009). Understanding the characteristics of regional pH changes are important to better define the potential impact on marine life. In this study, we use a combination of satellite and model assimilation data to map estimates of monthly surface ocean pH from 1985 to 2016. We apply two extreme event metrics to pH to assess the impact of long term trends and seasonal cycle variability on extremes. First, we define a metric for trend-driven extremes, where data is compared with the 5th percentile. Second, a metric for seasonally-driven extremes, where data is compared with 5th percentile of the monthly climatology plus the interannual variability. We find that the steady global decrease of pH of 0.05 units over the last 30 years (12% increase in [H⁺]) reflects strongly in trend-driven pH extreme events, which occur almost exclusively in the last decade of the time series. Seasonally-driven extremes show considerable interannual variability, particularly during summer in the Southern Hemisphere. This suggests that, while trend-driven pH extremes are increasing in the Southern Hemisphere, extremes around the seasonal cycle are not becoming more frequent. This is contrasted by the Northern Hemisphere, where seasonal pH extremes have increased from 2000 onwards. We test for possible mechanisms that may drive the variability and amplification of these seasonally-driven extremes.