Undersea Topography

[Deidamia Bassiti]

Usingunderwater robots in the waters surrounding Antarctica, scientists at Caltech have shown that the intersection of strong currents with the slope of landmasses rising from the ocean floor makes a significant contribution to the mixing of different waters in the Southern Ocean. A study on the research was published online in the journal Nature Geoscience on October 30.

To understand the important role of the seafloor in mixing ocean water, imagine a liquid in a blender. Mixing of the liquid does not occur evenly throughout the blender; rather, the liquid blends more rapidly closer to the spinning blades at the bottom than it does at the top. Nevertheless, the strength and speed of the blades determines the degree to which material is mixed throughout the container.

Similarly, in the ocean, global water properties may depend on very localized mixing processes. Researchers are interested in understanding where and how this mixing occurs, as it governs the large-scale circulation of the ocean and its ability to sequester carbon dioxide. (The ocean stores atmospheric carbon dioxide by absorbing it in the surface waters and then, pushing it into the deep ocean at a rate controlled by ocean mixing. The carbon remains in the deep ocean for hundreds to thousands of years before it returns to the surface again.)

"Most global ocean observations acquire measurements in the open ocean or in the top layers of the water, while our research shows that important mixing processes may be occurring in the deep ocean in thin layers over sloping topography," says senior author Andrew Thompson, professor of environmental science and engineering at Caltech.

Thompson and his colleagues deployed two autonomous underwater drones, or "gliders," for a period of eight months over the course of a year and a half in the Southern Ocean, which encircles Antarctica. The team concentrated on the region around Drake Passage, the 1,000-kilometer-wide waterway between Antarctica and South America.

"There is growing evidence that topography plays a bigger role in oceanographic mixing than we had previously suspected," says lead author Xiaozhou Ruan, a Caltech graduate student. "While this boundary region represents a small fraction of the ocean, the interaction between water and continental topography plays an outsized role in mixing."

Such mixing has been predicted by high-resolution ocean circulation models, but this is the first time it has been observed directly over a period of many months. Documenting these physical processes and improving our understanding of where and how they arise may improve our ability to simulate the changes in ocean circulation and in Earth's climate in the past and in the future, Thompson and his colleagues say.

Next, the team plans to deploy multiple gliders in the Bellingshausen Sea, located to the west of the Antarctic Peninsula, where ocean processes contribute to the high melting rates of the floating Antarctic ice shelves that buttress the West Antarctic Ice Sheet.

The study is titled "Contribution of topographically generated submesoscale turbulence to Southern Ocean overturning." Co-authors include Mar Flexas, senior research scientist at Caltech, and Janet Sprintall of Scripps Institute of Oceanography at UC San Diego. This research was supported by the National Science Foundation and the David and Lucile Packard Foundation.

GEBCO produces and makes available a range of bathymetric data sets and products. This includes a global bathymetric grid; gazetteer of undersea feature names, a Web Map Service and printable maps of ocean bathymetry.

Seabed 2030 is a collaborative project between the Nippon Foundation and GEBCO. It aims to bring together all available bathymetric data to produce the definitive map of the world ocean floor by 2030 and make it available to all.

The deepest depth measured so far in the oceans is around 10,920m in the Challenger Deep, part of the Mariana Trench, in the Northwest Pacific Ocean. For comparison, Mount Everest is around 8,850m high.

Since 2004, the Nippon Foundation/GEBCO Training Program has trained students from around the world in seafloor mapping through a one-year postgraduate course held at the University of New Hampshire, USA.

GEBCO's aim is to provide the most authoritative publicly-available bathymetry of the world's oceans. It operates under the joint auspices of the International Hydrographic Organization (IHO) and the Intergovernmental Oceanographic Commission (IOC) (of UNESCO).

We examined the topography of the sea floor from the perspective of plate tectonics in Chapter 10, but here we are going to take another look at the important features from an oceanographic perspective. The topography of the northern Atlantic Ocean is shown in Figure 18.2. The important features are the extensive continental shelves less than 250 m deep (pink); the vast deep ocean plains between 4,000 and 6,000 m deep (light and dark blue); the mid-Atlantic ridge, in many areas shallower than 3,000 m; and the deep ocean trench north of Puerto Rico (8,600 m).

A topographic profile of the Pacific Ocean floor between Japan and British Columbia is shown in Figure 18.3. Be careful when interpreting this diagram (and others like it), because in order to show the various features clearly the vertical axis is exaggerated, in this case by about 200 times. The floor of the Pacific, like those of the other oceans, is actually very flat, even in areas with seamounts or deep trenches. The vast sediment-covered abyssal plains of the oceans are much flatter than any similar-sized areas on the continents.

The continental shelf and slope offshore from Nova Scotia is shown in Figure 18.4. In this passive-margin area (no subduction zone), the shelf is over 150 km wide. On the Pacific coast of Canada, the shelf is less than half as wide. Continental shelves are typically less than 200 m in depth; 200 m is also the limit of the photic zone, the maximum depth to which sufficient light penetrates to allow photosynthesis to take place. As a result of that photosynthesis, the photic zone is oxygenated, and therefore suitable for animal life. Approximately 90% of marine life is restricted to the photic zone. The photic zone is also known as the epipelagic zone. The mesopelagic zone extends from 200 m to 1,000 m; the bathypelagic zone from 1,000 m to 4,000 m; and abyssalpelagic zone is deeper than 4,000 m. (Pelagic refers to the open ocean, and thus excludes areas that are near to the shores or the ocean floor.)

Although the temperature of the ocean surface varies widely, from a few degrees either side of freezing in polar regions to over 25C in the tropics, in most parts of the ocean, the water temperature is around 10C at 1,000 m depth and about 4C from 2,000 m depth all the way to the bottom.

The deepest parts of the ocean are within the subduction trenches, and the deepest of these is the Marianas Trench in the southwestern Pacific (near Guam) at 11,000 m (Figure 18.5). There are other trenches in the southwestern Pacific that are over 10,000 m deep; the Japan Trench is over 9,000 m deep; and the Puerto Rico and Chile-Peru Trenches are over 8,000 m deep. Trenches that are relatively shallow tend to be that way because they have significant sediment infill. There is no recognizable trench along the subduction zone of the Juan de Fuca Plate because it has been filled with sediments from the Fraser and Columbia Rivers (or their ancient equivalents).

Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations are expressed in terms of average sea surface height (SSH) relative to Earth's geoid.[1] The main purpose of measuring ocean surface topography is to understand the large-scale ocean circulation.

Unaveraged or instantaneous sea surface height (SSH) is most obviously affected by the tidal forces of the Moon and by the seasonal cycle of the Sun acting on Earth. Over timescales longer than a year, the patterns in SSH can be influenced by ocean circulation. Typically, SSH anomalies resulting from these forces differ from the mean by less than 1 m (3 ft) at the global scale.[2][3] Other influences include changing interannual patterns of temperature, salinity, waves, tides and winds. Ocean surface topography can be measured with high accuracy and precision at regional to global scale by satellite altimetry (e.g. TOPEX/Poseidon).

Slower and larger variations are due to changes in Earth's gravitational field (geoid) due to melting ice, rearrangement of continents, formation of sea mounts and other redistribution of rock. The combination of satellite gravimetry (e.g. GRACE and GRACE-FO) with altimetry can be used to determine sea level rise and properties such as ocean heat content.[4][5]

Ocean surface topography is used to map ocean currents, which move around the ocean's "hills" and "valleys" in predictable ways. A clockwise sense of rotation is found around "hills" in the northern hemisphere and "valleys" in the southern hemisphere. This is because of the Coriolis effect. Conversely, a counterclockwise sense of rotation is found around "valleys" in the northern hemisphere and "hills" in the southern hemisphere.[6]

Ocean surface topography is also used to understand how the ocean moves heat around the globe, a critical component of Earth's climate, and for monitoring changes in global sea level. The collection of the data is useful for the long-term information about the ocean and its currents. According to NASA science this data can also be used to provide understanding of weather, climate, navigation, fisheries management, and offshore operations. Observations made about the data are used to study the oceans tides, circulation, and the amount of heat the ocean contains. These observations can help predict short and long term effects of the weather and the earth's climate over time.

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