Climatic modulation of recent trends in ocean acidification in the California Current System

Abstract

We reconstruct the evolution of ocean acidification in the California Current System (CalCS) from 1979 through 2012 using hindcast simulations with an eddy-resolving ocean biogeochemical model forced with observation-based variations of wind and fluxes of heat and freshwater. We find that domain-wide pH and arag W in the top 60 m of the water column decreased significantly over these three decades by about −0.02 decade −1 and −0.12 decade −1 , respectively. In the nearshore areas of northern California and Oregon, ocean acidification is reconstructed to have progressed much more rapidly, with rates up to 30% higher than the domain-wide trends. Furthermore, ocean acidification penetrated substantially into the thermocline, causing a significant domain-wide shoaling of the aragonite saturation depth of on average −33 m decade −1 and up to −50 m decade −1 in the nearshore area of northern California. This resulted in a coast-wide increase in nearly undersaturated waters and the appearance of waters with 1 arag W < , leading to a substantial reduction of habitat suitability.Averaged over the whole domain, the main driver of these trends is the oceanic uptake of anthropogenic CO 2 from the atmosphere. However, recent changes in the climatic forcing have substantially modulated these trends regionally. This is particularly evident in the nearshore regions, where the total trends in pH are up to 50% larger and trends in arag W and in the aragonite saturation depth are even twice to three times larger than the purely atmospheric CO 2 -driven trends. This modulation in the nearshore regions is a result of the recent marked increase in alongshore wind stress, which brought elevated levels of dissolved inorganic carbon to the surface via upwelling. Our results demonstrate that changes in the climatic forcing need to be taken into consideration in future projections of the progression of ocean acidification in coastal upwelling regions.

Response of O&lt;sub&gt;2&lt;/sub&gt; and pH to ENSO in the California Current System in a high-resolution global climate model

“…As Nam et al (2011) pointed out, the large variability in O 2 and pH during ENSO events seen in their study, which can be seen also in ours, suggests that the carbonate ion concentration ([CO 2− (Turi et al, 2016). If the pH drops below 7.75, which corresponds to arag < 1, waters become unfavorable for calcifying organisms to build and maintain their shell structure (e.g., Feely et al, 2008;Bednaršek et al, 2014;Bednaršek and Ohman, 2015).…”

“…For this analysis, we focus on surface values, as DIC and ALK were saved only in the surface layer in the simulation. To decompose pH into these four drivers, we use a Taylor expansion according to the following equation (after Lovenduski et al, 2007;Doney et al, 2009b;Turi et al, 2014Turi et al, , 2016:…”

“…Furthermore, Feely et al (2008 report an incidence of nearshore surface waters becoming corrosive during the early upwelling season (May/June) of 2007, with the saturation state of aragonite ( arag ) dropping below 1, corresponding to pH < 7.75 (aragonite is a mineral form of calcium carbonate, CaCO 3 , found in the shells of certain calcifying marine organisms). Turi et al (2016) used a regional ocean model to demonstrate that a recent intensification of upwellingfavorable winds is linked to a drop in coastal pH and arag . It currently remains unclear whether these observed and modeled changes in the CalCS ocean biogeochemistry are ongoing signals of anthropogenic climate change, and thus could continue into the future, or whether they are driven by natural fluctuations in the climate system (e.g., Bakun, 1990;Narayan et al, 2010;Bakun et al, 2015;Rykaczewski et al, 2015;Wang et al, 2015).…”

Twenty Years of Marine Carbon Cycle Observations at Devils Hole Bermuda Provide Insights into Seasonal Hypoxia, Coral Reef Calcification, and Ocean Acidification

“…Time-series of seawater CO 2 -carbonate chemistry in the coastal marine environment tend to be relatively short (i.e., less than 10 years) with observation over an annual cycle (e.g., Johnson et al, 2013), a few years of regular sampling (e.g., Oregon coast, Harris et al, 2013;Santa Monica Bay, Leinweber and Gruber, 2013;Bermuda, Yeakel et al, 2015) that extends to 8 years of sampling (Bay of Bengal, Sarma et al, 2015). Other approaches include assessments of change from repeat sampling multi-year intervals between sampling (e.g., North Sea, (Clargo et al, 2015); coastal margin of the USA, Wanninkhof et al, 2015), or model assessments of longer-term change (e.g., California Current, Hauri et al, 2013;Turi et al, 2016).…”

Contrasting Impact of Future CO2 Emission Scenarios on the Extent of CaCO3 Mineral Undersaturation in the Humboldt Current System

“…While a substantial amount of work has addressed ocean acidification in the California Current System (Feely et al, ; Gruber et al, ; Harris et al, ; Hauri et al, ; Turi et al, ), very little is known about the current state and future progression of ocean acidification in the HumCS, located off the west coast of South America, straddling Peru and Chile. The strong upwelling make this EBUS the most productive of the EBUS (Carr, ; Lachkar & Gruber, ), but also leads to high values of surface p CO 2 and low values of Ω (Friederich et al, ; Mayol et al, ).…”