Regional Sea Level Change
A naïve view of the global ocean yields the familiar “bathtub” presumption: sea level, like bathtub water, rises and falls uniformly. The reality once again trades large-scale average for small-scale nuance. As we’ve seen, globally averaged sea level rises over decades, but regional factors cause far more variability over smaller time and spatial scales.
Among the most significant of these regional influences is the circulation of seawater itself. The interplay of two dominant forcings—density gradients and wind—account for much of this smaller scale variation, pushing sea levels higher in some places and lower in others. The differences in ocean-water density caused by warming or cooling, for example, create density gradients and associated currents. Pressure gradients are most effective as a driving force over longer time scales, wind over shorter time scales.
Meanwhile, the Earth’s rotation ensures that gaps over a large enough distance between higher and lower sea level—that is, higher and lower pressure—drive water along a curving path as seen from an earthbound perspective. This distance is called the “baroclinic Rossby radius of deformation,” ranging from more than 200 kilometers near the equator to under 10 kilometers at high latitudes. The apparent force that turns winds and ocean currents is known as the “Coriolis effect;” this curvature pushes ocean water in a counter-clockwise direction around high-pressure patches in the northern hemisphere, clockwise around low pressure (and the opposite in the southern hemisphere). The degree of tilt of surfaces of constant pressure is measured against the local horizontal, a manifestation of the Earth’s gravitational field.
Decades of observation reveal major variation in ocean circulation patterns across the globe from year to year and from decade to decade, bringing notable changes in regional sea levels. Most of these regional changes are associated with changing wind patterns. A useful illustration can be seen in the western Pacific. Easterly trade winds have intensified over two decades, leading to higher, wind-driven sea levels in the western tropical Pacific than in any other ocean region, according to ocean circulation modeling based on altimeter and tide-gauge data. Those levels were found to be about three times the global average—close to 10 millimeters per year [Merrifield and Maltrud, 2011].
Most changes in density are driven by temperature—or thermosteric—fluctuations, and some by increased or decreased salt content, or halosteric change; in fact, the two are often linked. For the latter half of the 20th century, the data show that these “steric” influences shift in rhythm with regional climatic effects, such as the El Niño-Southern Oscillation, demonstrating that the ocean and atmosphere are really a coupled system [Church et al., 2013].
The ENSO factor: El Niño and La Niña
Climatic influences over shorter timescales also modulate regional sea-level trends. One of the strongest of these signals comes from the El Niño Southern Oscillation, the alternating warmer and cooler phases of the tropical eastern Pacific. This regional climatic phenomenon can bring sharp swings in global sea level, though these appear only on the scale of years to decades; measurements so far do not clearly show a trend over multiple decades.
The key to these sea level effects is water storage on land. The warmer phase, El Niño, can raise sea level because of increased rainfall over the oceans; the cooler phase, La Niña, can lower sea level because of increased rainfall over land, where the water is temporarily stored before draining back into the sea. These effects are magnified in the tropics [Llovel et al., 2011].
A compelling example emerges from satellite altimeter and tide-gauge data from early 2010 to mid 2011. A strong La Niña developed during this period, and its timing corresponds to a global mean sea-level drop of five millimeters—despite a background sea level rise of about three millimeters per year during an 18-year satellite altimetry record [Boening et al., 2012].
The increased water storage, in this case, occurred in Australia, northern South America and Southeast Asia, and is closely tied to a shift from El Niño to La Niña conditions. This temporary transfer of water mass could not, however, ultimately overcome the long-term trend of rising sea level caused by melting ice and warming, expanding oceans. Global mean sea level rebounded quickly after the La Niña event was over.
The masses of ice around Earth’s poles exert their own gravitational pull—one that can alter the mass balance of the oceans. These gravitational changes have come to be known as sea level “fingerprints,” a particularly apt analogy. Their effects on sea level present a profile similar to that of a finger pressed into a pliant surface: a kind of cratering, with lower sea level at the center, higher sea level at the edges. The melting of ice sheets and glaciers, as we have seen, leaves behind a gravitational hollow of lowered sea level, as the water that had been pulled toward the ice mass, no longer captive to its gravitational attraction, migrates away; meanwhile, the additional water mass transferred from the melting ice to the ocean will, at a sufficient distance, raise sea levels. Maps of these variations even look a bit like a fingerprint, with concentric, irregular curves.
A total meltdown of the massive ice sheets, such as those in East or West Antarctica, would warp the ocean surface at a global scale [Gomez et al., 2010]. But even the more moderate, regional effects of real-world melting are significant and measurable. The melting of specific ice sheets or glaciers, in fact, exhibits distinctive patterns of geographic variation in sea level; the original source of the melt-water, in other words, leaves a unique gravitational fingerprint [Mitrovica et al., 2011], allowing it to be identified much as a detective identifies the fingerprints of a criminal.
One study that relied on GRACE measurements of ice loss for the planet’s three largest ocean-altering ice masses—the Greenland and Antarctic ice sheets, and Alaskan glaciers—sought to compute their combined effects on relative sea levels over the nine-year period, 2000 to 2008 [Bamber et al., 2010]. The study, which considered gravitational changes (as well as some other effects, such as the rotation of the Earth) showed that sea levels were driven highest in the Pacific and Indian oceans between about 20 degrees north latitude and 40 degrees south: a rise of 1.6 millimeters per year, or about 23 percent above the global “eustatic” mean sea level for the period of 1.4 millimeters per year (excluding other glaciers and thermal expansion). The lowest sea level rise with combined contributions from all three sources was Northern Europe’s 0.8 millimeters per year, about 45 percent below the global mean.
That would place Micronesia, the Solomon and Marshal Islands, the Maldives and small atolls, among other locations, in the zone of highest relative sea level rise; the same area has seen its sea level driven higher by thermal expansion as well.
The study also mapped gravitationally based, regional sea-level variations for the same nine-year period, linked to individual ice-mass contributions from each of the three large sources.
Postglacial rebound: the regional view
To make accurate estimates of sea-level rise, changes in gravitation at the local and regional level must be balanced against the gradual, and linear, recovery of ocean basins from the last ice age. The widening of these basins as they slowly spring upward can cause sea levels to drop. The equally linear nature of mass reduction from modern-day ice melt, however, requires researchers to tease apart the two signals—an essential step in parsing the contribution of ice to sea level rise. One modeling study [Wu et al., 2010] found significant rebound over Alaska and West Antarctica, and less pronounced effects over other regions. The study’s authors estimated mass losses from 2002 to 2008 of about 104 gigatons per year in Greenland, 101 in the Alaska-Yukon region, and 64 in West Antarctica (though with a somewhat wide range of uncertainties—plus or minus 23 gigatons per year for Greenland and Alaska-Yukon, and 32 for West Antarctica).
The authors recommended adding 0.66 millimeters per year to GRACE-based global sea-level rise estimates to compensate for the drop in sea level as rebounding ocean basins widened.