Thursday, May 14, 2015

Waiting to Exhale....

The surface ocean has takes up about a quarter of human-emitted carbon dioxide since the start of the industrial revolution. We know this, in part, because of the massive efforts to quantify the air-sea exchange of carbon dioxide.  The ocean takes up carbon because the partial pressure of carbon dioxide in the atmosphere over the ocean drives the gas into the water until the partial pressures across the air-water interface are equilibrated.  The same is true when the partial pressure of the gas in the water is "supersaturated" or higher than the equilibrated value with the atmosphere. In that situation, the gas is released or "exhaled" back to the atmosphere.  Two main processes influence whether or not carbon dioxide goes into the water from the atmosphere or is exhaled out: ocean circulation and biology.  Understanding how these processes change and alter this critical flux is imperative to understanding how the oceans will respond to climate change, and is a central focus of the Go-SHIP Repeat Hydrography Program.

Ocean circulation influences air-sea gas exchange in several ways: through the gas solubility relationship with ocean temperatures (warm water holds less gas), the interaction with the winds driving gas transfer (strong winds and waves drive supersaturated gas into the atmosphere more efficiently), and finally, through the currents supplying the surface with different water masses, carrying naturally varying amounts of dissolved carbon. For example, some ocean circulation patterns bring deep waters rich in carbon dioxide to the surface. These waters then interact with the atmosphere through wind.

Biology influences the surface carbon content and thus the transfer of CO2 between the ocean and atmosphere through the life cycle of phytoplankton.  Phytoplankton actively remove carbon dioxide from the water to form their organic carbon parts when they grow. When they die and sink through the water column, microbes and grazers respire the organic carbon back into carbon dioxide in the water.  Our chief scientist, Jessica, compares this process to a set of Lego blocks-- the individual blocks of "life" are put together at the surface during photosynthesis. These Lego sculptures are heavy, so they sink. At depth, usually away from sunlight, bacteria take these Lego sculptures apart again, returning the individual building blocks of carbon to the water column. This process is often called the biological pump, as it moves carbon dioxide from the surface into the deep ocean. The pump is most efficient at sequestering carbon dioxide from the atmosphere when remineralization-- or the demolition of our Lego sculptures-- happens far away from photosynthesis, and the two sets of blocks don't mix.

Both of the circulation and biological feedbacks play a role in the interannual variability of air-sea gas exchange that we observe in the ocean.  Looking at the Lamont-Doherty Earth Observatory surface ocean CO2 climatology (Figure below), the equatorial Pacific region is the largest outgassing source (positive net flux) on the planet.  Nearly 70% of interannual variability of global ocean uptake of carbon dioxide is due to changes in equatorial upwelling associated with El Nino and La Nina (also called ENSO) climate oscillations.

LDEO surface CO2 climatology based on 30 years worth of surface ocean CO2 observations or about 250,000 measurements (Takahashi et al., Proc. Natl. Acad. Sci., 94, 8292–8299, 1997). Notice the highest values exist around the equatorial Pacific. 

  The carbon group at the NOAA Pacific Marine Environmental lab in Seattle has participated in outfitting the TAO array with pCO2 sensors along the equator to get a better understanding of this variability. During El Nino years, the equatorial Pacific is thought to hold on to the carbon dioxide it would normally exhale through upwelling, reducing the air-sea flux.  This is because upwelling of higher CO2 waters is reduced during El Nino years due to a reduction in the upwelling favorable winds. El Nino effectively keeps ocean CO2 away from the atmosphere, so that it can't escape.

On our GO-SHIP research cruise on P16N, the underway pCO2 system on the Ron Brown is collecting surface carbon dioxide measurements that are being monitored by scientists on board from the NOAA Atlantic Oceanographic and Meteorological Laboratory. We have traveled across the central Pacific on our track, crossing the equatorial upwelling region in the process, and collecting data the whole way.  As you may have seen in our previous blog posts or in the news, this year is an El Nino year.

In 2006, the CLIVAR P16N cruise track occurred along the same track, and also used an underway pCO2 system. Unlike 2015, 2006 was a La Nina year. La Nina years are characterized by increased upwelling along the equator. In these climate phases, lots of deep ocean CO2 is brought to the surface by upwelling and returned to the atmosphere.

Underway surface gas concentrations from the P16N cruise track in 2015 (red) and 2006 (blue). The fCO2 from the TAO array (155 W, equatorial location) is plotted in green for 2005-2008. 
Both cruise tracks are plotted in the figure here, along with the TAO array's range from 2005 to 2008. In 2006 (blue), during the La Nina, the peak in the underway sampling of fCO2 occurred just south of the equator in a narrow band and was higher than in 2015.  In 2015 (red), the region of high fCO2 is broader, covering more latitudes north of the equator, and the maximum is lower.  This is consistent with what we know of El Nino's impact on air-sea gas exchange.  The equatorial Pacific is holding onto more carbon. In essence, it's waiting to exhale.


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