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. 

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