File:Coriolis-force-driven anticyclonic water-swirls and currents at the oceans' surfaces change Earth's gravitational field (32312280745).jpg

English: Like the ocean, gravity ebbs and flows

Sea Level News | January 13, 2017 | By Pat Brennan, NASA's Sea Level Portal

Visualization of ocean surface currents, based on output from an ocean circulation model called ECCO2 (Estimating the Circulation and Climate of the Ocean, Phase II). Such currents can leave their marks on Earth's gravitational field, a recent study shows. By Greg Shirah, NASA/Goddard Space Flight Center Scientific Visualization Studio.

Shifting ocean currents can leave “fingerprints” on Earth’s gravitational field, a novel analysis shows, adding to the list of hidden processes that subtly influence sea levels around the world.

Flowing and gyrating masses of warm or cold water can cause gravity—and sea level—to bump and dip in unexpected ways. The effect is most pronounced for short-term, or “high frequency,” changes in ocean circulation, on the order of days or weeks.

“The bottom line is, when ocean currents shift mass around rapidly, it causes changes in gravity, which in turn affects sea level,” said Nadya Vinogradova of the Cambridge Climate Institute in Massachusetts. “While traditionally neglected, best practices need to take into account those sea level changes.”

Scientists have long known of the gravitational fingerprints left on Earth’s oceans by melting glaciers, with effects quite the opposite of most people’s expectations. Sea level in the vicinity of a melting glacier goes down, not up. And gravity is the reason: As the glacier loses mass, its gravitational pull on nearby water weakens, and the water migrates away.

The result is a kind of depression, or fingerprint, with the lowest sea level closest to the melting object.

But when such fingerprints are caused by rapidly moving masses of ocean water, they are much more difficult to detect. To tease apart such effects, Vinogradova, a member of NASA’s Sea Level Change Team, and her fellow researchers began with a standard computer representation of Earth’s oceans known as a general circulation model.

Such models faithfully recreate temperature, salinity, currents and other factors that govern sea level, which can vary from region to region.

Gravity's ups and downs

The novel step in Vinogradova’s analysis, published in March of 2015, was to add “new physics” to the model, allowing it to account for bumps and dips in the gravitational field caused by shifting water masses. Scientists call these localized gravitational shifts “self attraction and loading.”

The idea itself isn’t new, Vinogradova said, but until now was considered far too heavy a burden in terms of computer time. So traditional methods of modeling such global changes left out the seawater flow’s effects on gravity.

“Computing gravitational effects caused by the ocean dynamics was considered prohibitively expensive, and thus excluded from most general circulation models,” she said.

The gravitational effects of ocean circulation also were considered too small to be important for modeling sea level.

On both counts, Vinogradova and her team found out otherwise.

Their technique involves the application of “spherical harmonics” to ocean circulation. As the name implies, this is a unified mathematical representation of a sphere, like Earth, that greatly simplifies calculations of changing conditions on the surface.

Changes in ocean mass are isolated and converted into mathematical functions, allowing the modelers to infer alterations in Earth’s crust, as well as gravitational shifts, that the mass changes cause.

“We were able to do it in a computationally efficient way, increasing our computing time by only six percent,” she said. “It’s no longer prohibitive, as we saw in the early 2000s, and the new physics is worth exploring.”

A modeling 'penalty'

The researchers also determined that computing these gravitational effects using traditional methods was safe—but only to a point. Earlier research teams assumed that over time, such effects would settle into equilibrium, balancing out and canceling anything of measurable significance.

But that was only true for ocean changes occurring on the scale of months, or longer, Vinogradova found.

“We see gravity-driven, dynamic sea level signals occurring within a week, meaning that a week is too short for the ocean to adjust and come to an equilibrium,” she said.

That brings a modeling “penalty” for short-term ocean phenomena.

So whether modelers benefit from accounting for these short-lived gravitational wrinkles depends greatly on where, and when, they are studying ocean circulation.

“Shallow regions benefit from this new physics, such as the coastal oceans, including the Arctic,” she said. “But there were vast areas in the ocean that were quite insensitive to the dynamic implementation of the gravitational effects within a climate model.”

Likewise, the effects become magnified at “sub-weekly” time scales.

“With this short a time scale, (modelers) should probably worry about changes in sea level induced by gravitational effects generated by very fast ocean processes,” she said. “A short period tide would be a prime candidate.”

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In physics, the Coriolis force is an inertial force (also called a fictitious force) that acts on objects that are in motion relative to a rotating reference frame. In a reference frame with clockwise rotation, the force acts to the left of the motion of the object. In one with anticlockwise rotation, the force acts to the right. Though recognized previously by others, the mathematical expression for the Coriolis force appeared in an 1835 paper by French scientist Gaspard-Gustave de Coriolis, in connection with the theory of water wheels. Early in the 20th century, the term Coriolis force began to be used in connection with meteorology. Deflection of an object due to the Coriolis force is called the 'Coriolis effect'. Newton's laws of motion describe the motion of an object in an inertial (non-accelerating) frame of reference. When Newton's laws are transformed to a rotating frame of reference, the Coriolis force and centrifugal force appear. Both forces are proportional to the mass of the object. The Coriolis force is proportional to the rotation rate and the centrifugal force is proportional to its square. The Coriolis force acts in a direction perpendicular to the rotation axis and to the velocity of the body in the rotating frame and is proportional to the object's speed in the rotating frame. The centrifugal force acts outwards in the radial direction and is proportional to the distance of the body from the axis of the rotating frame. These additional forces are termed inertial forces, fictitious forces or pseudo forces. They allow the application of Newton's laws to a rotating system. They are correction factors that do not exist in a non-accelerating or inertial reference frame.

A commonly encountered rotating reference frame is the Earth. The Coriolis effect is caused by the rotation of the Earth and the inertia of the mass experiencing the effect. Because the Earth completes only one rotation per day, the Coriolis force is quite small, and its effects generally become noticeable only for motions occurring over large distances and long periods of time, such as large-scale movement of air in the atmosphere or water in the ocean. Such motions are constrained by the surface of the Earth, so only the horizontal component of the Coriolis force is generally important. This force causes moving objects on the surface of the Earth to be deflected to the right (with respect to the direction of travel) in the Northern Hemisphere and to the left in the Southern Hemisphere. The horizontal deflection effect is greater near the poles and smallest at the equator, since the rate of change in the diameter of the circles of latitude when travelling north or south, increases the closer the object is to the poles.[3] Rather than flowing directly from areas of high pressure to low pressure, as they would in a non-rotating system, winds and currents tend to flow to the right of this direction north of the equator and to the left of this direction south of it. This effect is responsible for the rotation of large cyclones (see Coriolis effects in meteorology). To explain this intuitively, consider how an object that moves northwards from the equator has a tendency to maintain its greater speed at the equator (rotating around towards the right as you look at the sphere of the Earth), where the "horizontal diameter" is larger, and therefore tends to move towards the right as it passed northwards where the "horizontal diameter" of the Earth (the rings of latitude) is smaller, and the linear speed of local objects on the Earth's surface at that latitude is slower.