Natural variability and anthropogenic change in equatorial Pacific surface ocean pCO₂ and pH

Abstract

The equatorial Pacific is a dynamic region that plays an important role in the global carbon cycle.This region is the largest oceanic source of carbon dioxide (CO 2 ) to the atmosphere, which varies interannually dependent on the El Niño-Southern Oscillation (ENSO) and other climatic and oceanic drivers. We present high-resolution observations of surface ocean CO 2 partial pressure (pCO 2 ) at four fixed locations in the Niño 3.4 area with data sets encompassing 10 ENSO warm and cold events from 1997 to 2011. The mooring observations confirm that ENSO controls much of the interannual variability in surface seawater pCO 2 with values ranging from 315 to 578 μatm. The mooring time series also capture the temporal variability necessary to make the first estimates of long-term pH trends in the equatorial Pacific, which suggests that the combination of ocean acidification and decadal variability creates conditions for high rates of pH change since the beginning of the mooring record. Anthropogenic CO 2 increases play a dominant role in significant observed seawater pCO 2 trends of +2.3 to +3.3 μatm yr À1 and pH trends of À0.0018 to À0.0026 yr À1 across the full time series in this region. However, increased upwelling driven by increased trade winds, a shallower thermocline, and increased frequency of La Niña events also contribute an average of 40% of the observed trends since 1998. These trends are higher than previous estimates based on underway observations and suggest that the equatorial Pacific is contributing a greater amount of CO 2 to the atmospheric CO 2 inventory over the last decade.

A high-frequency atmospheric and seawater <i>p</i>CO<sub>2</sub> data set from 14 open-ocean sites using a moored autonomous system

“…Variability of 100-150 µatm is also common in the equatorial Pacific during the extension of the warm water pool during El Niño events on timescales of months and the passing of tropical instability waves on timescales of weeks (e.g., Fig. 4 in Sutton et al, 2014). Sustained, long-term mooring time series also provide the opportunity to identify and remove the short-term variability from the time series and investigate long-term trends.…”

“…Sustained, long-term mooring time series also provide the opportunity to identify and remove the short-term variability from the time series and investigate long-term trends. For example, in a synthesis of equatorial Pacific mooring data, Sutton et al (2014) found that the uptake of anthropogenic CO 2 and an acceleration in equatorial upwelling since the shift in the Pacific Decadal Oscillation in 1998 has led to high rates of pCO 2 change of +2.3 to +3.3 µatm yr −1 in this region. This decadal shift in CO 2 outgassing is consistent with underway pCO 2 observations made in this region since 1982 .…”

A multi-decade record of high-quality <i>f</i>CO<sub>2</sub> data in version 3 of the Surface Ocean CO<sub>2</sub> Atlas (SOCAT)

“…Basin-specific rates in the f CO 2 increase vary from 1.2 to 2.1 µatm yr −1 for the years 1970 to 2007 , with higher rates of 2.3 to 3.3 µatm yr −1 at different mooring sites in the equatorial Pacific Ocean for the more recent period of 1997 to 2011 (Sutton et al, 2014a). The annual pH decreases at rates of −0.0013 yr −1 in the South Pacific Ocean (for 1998 to 2012) to −0.0026 yr −1 in the Irminger Sea (for 1982 to 2006) , while annual pH changes vary from −0.0018 to −0.0026 yr −1 for moorings in the equatorial Pacific Ocean for 1997 to 2011 (Sutton et al, 2014a). Here it is worth noting that such rates of change vary with the start date and period used for the calculation as a result of interannual to decadal variability (McKinley et al, 2011).…”

Variability and trends of carbon parameters at a time series in the eastern tropical Atlantic

“…In the equatorial Pacific, using pCO 2 measurements made at four moorings and the relation of Lee et al (2006) to calculate alkalinity, Sutton et al (2014) detected a decreasing pH trend associated with anthropogenic CO 2 absorption but also due to increased upwelling.…”