Hidden Upwelling Systems Revealed

Upwelling is the upward movement of deeper, colder water to the surface. The water that rises to the surface through upwelling is often rich in nutrients. These nutrients “fertilize” surface waters, encouraging the growth of plant life including tiny algae and plants that fuel the marine food web, known as "phytoplankton."

Upwelling in many regions of the global ocean – such as along the equator and eastern boundaries of ocean basins – has long been recognized. Why? In these areas, upwelling's distinct signatures reach the ocean surface: low water temperatures and high concentrations of chlorophyll, the green pigment found in many species of phytoplankton.

You've probably heard of a notorious disruption in upwelling that occurs along the west coast of the Americas: El Niño. This climate-related event has impacted Peruvian fisheries for centuries.

Believe it or not, satellite images of chlorophyll let you actually "see" Earth's equator in the Pacific Ocean!

In contrast, this study shows that intense upwelling systems also exist along the major western boundary currents (WBCs) around the global ocean. Despite the fact that WBCs are essential branches of global ocean circulation, their contribution to upwelling have been largely unrecognized in the literature.

Generally, measuring how the ocean moves horizontally is straightforward. One data source is a  global array of simple instruments  that ride along with currents at the sea surface and transmit their positions to satellites. Think of a message in bottle... but with GPS.

On the other hand, because of its small magnitude, measuring vertical ocean flow – whether water is moving up or down – is almost impossible. As a result, studies of the ocean's vertical motion are limited, especially well below the sea surface.

That's where  Estimating the Circulation and Climate of the Ocean  (ECCO) comes in! Its solutions combine state-of-the-art ocean circulation models with global ocean data sets. Like the real ocean, ECCO model solutions obey the laws of physics. Unlike the real ocean, ECCO allows easy sampling at full ocean depths. It give us access to quantities that are otherwise difficult to measure.

Although we used several sources to study upwelling in WBCs, the following maps and data plots are based on ECCO monthly products. The first five – Kuroshio, Gulf Stream, Agulhas, East Australian, and Brazil currents – are WBCs. For the sake of comparison, the last example – Peruvian Upwelling – represents conditions in a typical eastern boundary current.

For each region, we looked at data from "slices" of the ocean, known as cross sections. Using a loaf of bread as an analogy, its cross section is where you'd spread butter. Lines on the top of the loaf show where it's sliced.

In our study, we're looking at how vertical velocity is distributed within the ocean. For each WBC, we chose a cross section approximately perpendicular to the local coastline.

The next set of interactive globes will focus on ocean pressure around our WBC regions. To familiarize you with how pressure affects ocean flow, we start at the ocean surface. Over large regions of the ocean, the sea surface has highs and lows.

What does this mean for ocean motion? Pressure is oriented from high to low. It is balanced by the Coriolis force – which is related to Earth's spin – in the opposite direction. In response, water flows horizontally around highs and lows at the ocean surface.

What is happening deeper in the ocean? Away from the sea surface, pressure-related balance also affects water flow. The process is tied to the layering of seawater’s density.

In the figure above, density is constant along the black lines. The slopes of these lines – or the subsurface density structure – affects the ocean’s pressure field. Similar to how geostrophic balance affects horizontal flow at the sea surface, thermal wind balance affects vertical shear below the sea surface.

Now we're going to look at potential density, which tells us about the stability of ocean layering. In a stable ocean, potential density must decrease upward. On the other hand, where there are horizontal differences in potential density, vertical shear will develop. This is thermal wind balance at work!

With these types of relationships in mind, we looked at subsurface density structures in areas around our WBCs of interest. In each case, upwelling along the WBC is supported by a potential density field that increases toward the pole along a line of longitude.

Let's think more about how water moves both horizontally and vertically within an idealized three-dimensional volume: a triangular prism with each apex labeled as A, B, or C. An important thing to remember is that there is no vertical motion out of the top or bottom of the volume.

In the slider below, the left image shows the density structure of two faces, A-B and B-C, including clear differences in their density gradients. The slope of density lines is much steeper along the B-C face than along the A-B face.

What's the result? Because of the density structure and thermal wind balance, there is a smaller difference in horizontal velocity between Layers 1 and 2 on the left (A-B) face than the right (B-C) face. This is can be seen by the difference in arrow sizes between Layer 1 and Layer 2 going into the volume (A-B face) versus the difference in arrow sizes between Layer 1 and Layer 2 coming out of the volume (B-C face).

Now, let's look at the right image in the slider (i.e., move the arrow icons to the left). Because of mass conservation, the total flux of water going into Layer 1 through the A-B face should equal that coming out of Layer 1 through the B-C face. But in this case, there's more water leaving horizontally out of the B-C face. How is water mass conserved in Layer 1? Upwelling from Layer 2!

How do we calculate these types of mass budgets in the real-world ocean?  ECCO ! As mentioned earlier, its model solutions obey the laws of physics. Moreover, ECCO provides access to quantities that are difficult to measure in the real-world ocean.

Unlike their eastern boundary current counterparts, vertical motion in WBC regions are generally weak close to the surface but are strong below the surface. Also, direct detection of surface upwelling in WBCs is hampered by strong horizontal currents and swirling eddies.

Our work demonstrates the existence of intense upwelling in five WBCs. In addition to being an overlooked branch of the global ocean circulation, these deep-reaching upwelling systems can rise up near the surface. So, they can transport nutrients, carbon, and heat inside the ocean. Overall, these systems are an important yet unexplored route through which oceanic biological, chemical, and physical processes – and ultimately the climate system – can be affected.

Our work points to WBC upwelling being a very important branch of ocean circulation. It carries lot of water from very deep ocean, potentially delivering a lot of nutrients and carbon to the surface. So, it not only has important implications for ocean circulation but also for marine life and the global carbon cycle. This study is just the beginning... I hope scientists in different disciplines take advantage of the wealth of data that already exists — such as ECCO — to further investigate these previously "hidden" upwelling. –– Xingfeng Liang, Study co-author

Liao, F., Liang, X., Li, Y., and Spall, M. (2022). Hidden Upwelling Systems Associated With Major Western Boundary Currents, J. Geophys. Res. Oceans, 173(3),  https://doi.org/10.1029/2021JC017649 

 Read more about this work  in the American Geophysical Union's news magazine, Eos.