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.

Wednesday, May 13, 2015

Mixing in the Equatorial Pacific Ocean

Miguel Angel Jimenez-Urias is a 3rd year oceanography graduate student at the University of Washington. His work focuses on the topographic control of fronts at both small and large scales. While his work uses primarily numerical modeling and laboratory experiments, he tries to include observational methods in his projects to advance his academic and professional development.

To those who say the Equatorial Pacific is a calm, uninteresting part of the ocean: Think again. Over the last couple of weeks we have witnessed stark changes in weather, from calm clear skies and starry nights, to rainy days, strong winds and significant swells. In addition, a couple of days ago we crossed the equator, or if I may say for the physical oceanographers out there, the Coriolis singularity. We moved from spring to fall and I couldn’t help but wonder: Did we skip  summer or winter? I remember having this odd feeling that we were rotating “the other way around" while we were in the southern hemisphere,  And just like that, a pair of weeks sailing north on the Ronald H. Brown, we find ourselves rotating “the right way" again. Of course my feelings are pretty egocentric: Earth has and will continue to rotate in the same direction. Nothing has really changed. 

As a physical oceanographer, I can’t help but envision the large-scale ocean circulation nearing the equator: water masses from the Southern Pacific moving north, or the North Pacific intermediate waters moving south. A look at sections of salinity and potential temperature we have collected over our cruise reveal the movement of some of these water masses. As they near the equator, they also share my egocentric feeling: Earth is slowly moving the other way! This is a simple way of saying that they must adjust to the changes in physical forcing as they cross the equator.1 These forces have a strong impact on advection-- the lateral movement of large bodies of water north, south, east, or west in the oceans. 

While these physical changes are fun and even a little poetic for me to imagine, it's only a two-dimensional picture. Other forces can affect the pathways of the large-scale circulation either by modifying the way a water-mass feels rotation (friction), or by modifying the properties of the water-mass itself (mixing). The project that I am working on during this cruise is focused on the story of vertical mixing in the open ocean-- the third dimension. 

Unlike advection, turbulent vertical mixing does not care the planet moves in the “right way." That is, mixing is independent of our choice of reference frame.  It is actually more closely related to the thermodynamics of the ocean: that give and take between potential and kinetic energy. Mixing changes the (available) potential energy of the oceans by diffusing heat vertically. By slowly heating a water mass up, its density changes and it slowly migrates towards the surface. This can have a really big impact-- ocean heat transfer affects the large-scale circulation of the ocean and even climate! You could easily think of the oceans as a planetary-scale heat engine. In order to better understand how our planet works and even to more accurately forecast the weather, it's important to know where mixing (heat transfer) is happening, and what causes it. 

I came on board to the Ronald H. Brown to help an ongoing research effort that will try to answer some of these questions. On board we have a series of instruments called Chi-pods, that can help us to partially answer some of them. Each Chi-pod consists of a temperature sensor that allows us to read vertical temperature gradients in high definition. By studying how temperature changes with depth, we can estimate vertical mixing in the open ocean. 

Figure 2. Chipods attached to the rossete. Red circles: Sensors. Green circles: Pressure case containing accelerometer.
A picture of our Chi-pods can be seen in Figure 2, above. Two of them have their sensors at the top of the CTD, which we referred to as uplookers. The other two have their respective sensors at the base of the rosette with their sensors pointing down. We called them downlookers. At the downcast, the downlookers record more reliable data since the CTD has an associated wake much like the classical problem of flow past an object, which can contaminate the temperature record of the uplookers.

By making direct observations and getting vertical profiles of mixing estimates, we can immediately observe that mixing has an absolute maximum where water masses meet at the base of the thermocline, and a smaller local maximum near the bottom topography. This helps to partially answer one of our original questions:  Mixing happens near water mass boundaries. Remember, this is important, because mixing between water masses can help distribute heat, as well as nutrients, carbon and other important components, throughout the oceans. 

Figure 3. Typical vertical profile of Total Kinetic Energy dissipation rate epsilon (left) and temperature at the equatorial Pacific Ocean (16° 30’ N, 152° West). The spikes at the top left correspond to areas where temperature rapidly changes in the water column, signifying turbulent mixing.
Observations of turbulent mixing like these are still scarce and under-represented in field campaigns. Integrating Chipods into repeat hydrographic programs, like US GO-SHIP, represents a greater effort to increase the observational database of open ocean mixing, and hopefully in the future there will be more opportunities like this. A better understanding of turbulent mixing and heat transfer is a pressing need for both physical oceanographers and climate modelers alike. I am delighted to say that I have participated in this project, and I have gained a lot from this experience. I learned about observational techniques on turbulent mixing, which will help complementing my understanding of the effects of small-scale processes into the large-scale ocean circulation-- something I hope to study in the near future.

1A simple two water-mass laboratory experiment would show that the water mass moving south in the Northern hemisphere would need to lean on its right against topography in order to break the angular momentum conservation restriction, the true nature of the Coriolis deflection. Upon crossing the equator, it would now lean on its left, since Earth is rotating the other way. A similar argument could be applied to a water mass moving north from the southern hemisphere.