When the sea is completely still, its surface can look like polished glass, mirroring everything above it. It can seem almost as though another world exists beneath the water, perfectly opposite to the one we see above. Those quiet days are among my favorite moments on the ocean.

During these calm periods, sunlight warms the uppermost part of the sea, producing a thin layer of warmer, more buoyant water that may be only a few feet deep. Much like oil resting above vinegar in a bottle of salad dressing, this warmer layer sits on top of the cooler water below, forming a boundary between the atmosphere and the deeper ocean.

For years, scientists have generally believed that when the ocean surface is calm, this warm layer remains fairly stable and experiences little interaction with the colder water beneath it. New observations, however, indicate that the reality is much more complicated. Even beneath an apparently motionless surface, the warm upper layer can swirl and become turbulent, moving energy downward into deeper parts of the ocean.

The ocean plays a fundamental role in the planetary processes that control Earth's climate and weather. Its surface is especially important because it serves as the meeting point where energy moves between the atmosphere and the sea. Even relatively minor changes in the uppermost ocean can alter how much heat and momentum are transferred between the water, the atmosphere, and deeper layers.

My research has concentrated on this poorly understood part of the system. As a graduate student in the MIT-WHOI Joint Program, I am investigating how the ocean's upper layers respond to daily solar heating and how those small changes can influence interactions between the atmosphere and ocean across enormous distances and extended periods.

Traditionally, observing the uppermost few feet of the sea has been extremely difficult with conventional oceanographic equipment. New underwater instruments and autonomous vehicles now allow my research team to investigate the chaotic mixing that occurs at much smaller scales. A clearer understanding of how daily warming influences the transfer of energy between air and sea could ultimately improve our knowledge of climate processes and many other systems driven by ocean-atmosphere interactions.

The Ocean in a Cup

In certain ways, the ocean behaves like a cup of coffee.

Imagine pouring milk into your morning coffee. The milk creates swirling motion and turbulence as it spreads through the drink. Stir the coffee with a spoon and the turbulence becomes even stronger. Generally, greater mixing means greater turbulence.

Oceanographers are not concerned with milk and coffee, of course. Instead, they study how heat and momentum move into the ocean, a process known as a flux. Using the coffee comparison, the quantity of milk entering the cup represents the flux of milk.

Although watching milk enter coffee is easy to understand, the transfer of heat and momentum between the atmosphere and ocean is far more complicated. Turbulent ocean fluxes are affected by many variables, including atmospheric humidity, sunlight, longwave radiation released by Earth, wind strength, and the temperature difference between air and seawater.

These variables also interact with one another. Leaving even one of them out of the calculation can therefore lead to inaccurate estimates of turbulent energy exchange.

In a 2013 study published in the Journal of Climate, WHOI oceanographer Carol Anne Clayson and I examined how much error can enter turbulent flux calculations when one particular factor is overlooked: the ocean's daily warming caused by sunlight.

The ocean surface extends across thousands of miles, covering an area larger than many continents. Because of this enormous scale, even a relatively small calculation error can become significant when applied across the entire ocean. An inaccurate estimate could change calculated ocean-atmosphere energy exchange by several percent, potentially influencing our understanding of numerous important processes in both the atmosphere and the sea.

The situation becomes even more complicated because daily solar heating is not uniform around the planet. Some regions experience almost no noticeable warming, while other areas can undergo substantial surface heating during exceptionally calm days.

Where and how strongly the ocean warms can influence currents and weather patterns. Solar heating at the surface increases evaporation, which can contribute to thunderstorm formation. It can also influence large-scale climate patterns such as the El Niño-Southern Oscillation, or ENSO.

Previously Impossible Measurements

Each day, sunlight warms the upper ocean through a layer that may extend tens of feet below the surface, with the strongest warming occurring closest to the top. Much of this change happens within only a few hours around the warmest part of the day.

Instead of imagining your coffee as one uniform liquid, picture it divided into temperature layers. The hottest liquid is at the top, while the coldest remains at the bottom.

On a calm day with almost no wind, the ocean can behave like this layered cup of coffee without a spoon. Nothing appears to be stirring the water strongly enough to make its temperature uniform throughout the layers. Consequently, scientists have traditionally assumed that the warm surface layer remains relatively stable and does not mix substantially with the colder water below.

The key to my research is a new source of previously unavailable information. These observations make it possible to examine the movement of heat and momentum into and out of the ocean's uppermost layer, as well as the factors responsible for mixing between surface and deeper waters.

The turbulent movements involved occur on extremely small spatial scales, sometimes just centimeters. They can be present under many different conditions, including those remarkably calm days when the ocean surface looks like a mirror.

Two recent field experiments supported by NASA and the National Science Foundation provided an opportunity to investigate these processes. A team of scientists led by WHOI physical oceanographer Lou St. Laurent launched autonomous gliders and profiling instruments from a research ship to examine motion at the smallest scales.

Profilers are relatively conventional scientific instruments capable of recording more than 500 turbulence measurements every second. However, they have to be deployed from a ship each time a new profile is collected. This makes repeated observations slow and labor-intensive, while measurements within the uppermost few feet of the ocean are particularly difficult.

The newer autonomous gliders offer a major advantage. They travel vertically through the upper ocean and can rise all the way to the surface. Once released, they can continue collecting measurements for as long as a week without requiring constant intervention.

During two separate 14-day field campaigns, the gliders gathered profiles of the upper ocean more than 4,000 times. Specially designed instruments attached to the vehicles allowed researchers to measure turbulence with accuracy comparable to that of traditional profiling equipment.

A Look Into the Hidden World

Thousands of measurements collected across only a few feet of water give scientists an opportunity to identify patterns in the ocean's smallest movements. The results suggest that the traditional picture of a calm surface with almost no mixing underneath is far too simple.

I am investigating what mechanisms may generate this mixing during daylight hours. Our existing understanding of the upper ocean under calm conditions does not appear to explain everything that is happening.

So far, the observations suggest that surface waves may play an important role, along with another type of motion known as internal waves. Surface waves are the familiar waves that eventually break along beaches, while internal waves move within the ocean itself.

Returning to the salad dressing comparison, internal waves can be thought of as the movements and undulations occurring along the boundary between oil and vinegar. These hidden motions may help explain why water beneath an apparently smooth ocean surface can still be highly dynamic.

My graduate research is only beginning to uncover what is happening within these layers, but the evidence is already challenging some long-standing assumptions about calm ocean conditions.

When evening arrives and the sun disappears, the thin layer of warm surface water gradually vanishes. In general, the atmosphere becomes cooler than the ocean after sunset, allowing the water closest to the surface to lose heat.

As this surface water cools, it becomes denser and begins to sink. The descending water mixes with deeper layers, eventually producing a much more uniform layer near the surface. In effect, the ocean resets itself, preparing for another cycle of warming the following day.

And then, as you sit down with your morning coffee, the entire process begins again.

The next time you look across a perfectly calm sea, remember that the smooth surface may conceal a surprisingly active world underneath. Just as turbulence moves through your coffee as milk enters the cup, heat and momentum are constantly being transferred and redistributed in the thin, hidden layers beneath the ocean's shimmering surface.