Ocean Currents & Sea Level

Sea levels can be impacted by changes in ocean currents which are most often driven by changes in climate/weather patterns, or by relatively slow changes in geography of land masses (plate tectonics, volcanism, etc.).

Our previous example for the West Coast of the U.S. shows a correlation of higher sea levels with strong El Niño events and lower sea levels for La Niña events. The NASA SEA LEVEL VIEWER that we have already visited (see Figure 8) shows that satellite imagery has been able to detect variations in sea level height around the globe. These variations are often attributed to stronger or weaker circulation of atmospheric cells. In some years/seasons, very strong trade winds help push water westward across ocean basins producing higher sea levels on the east coasts of continents (in the viewer, warmer colors - yellow, orange, yellow indicate higher sea levels). When this is particularly strong and substantial, this often coincides with La Niña events. In contrast, weak trade winds allow water masses to move back eastward causing sea levels to fall on the west side of ocean basins, and sea levels to rise on the east side of ocean basins. This is what happens during El Niño events. So, you might ask, what causes climate oscillations that might impact ocean circulation in seemingly repetitive ways? We will discuss this topic more shortly.

Geographic changes also have the potential to modify climate-ocean circulation and interaction patterns. We will not belabor this point, but, for example, at the end of the Miocene (5 million years ago), the equatorial seaway that connected the Atlantic and Pacific Oceans closed. As the Isthmus of Panama emerged from the seafloor due to tectonic activity, water from the Atlantic was no longer free to exchange with the colder waters of the Pacific. This resulted in changes to the circulation of water masses in the Gulf of Mexico and Caribbean Sea. Ultimately, this event allowed for formation and intensification of the Gulf Stream which moves substantial amounts of warm water produced in the Gulf of Mexico region to the North Atlantic. This seemingly simple geographic change resulted in pronounced changes in ocean circulation and likely helped to drive global climate change which ultimately led up to the development and growth of glaciers not only in Europe, Iceland and Greenland, but also globally during the Pleistocene Glaciations. See this NASA image and this WHOI (Woods Hole Oceanographic Institute) feature for more info.

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Figure 4.22: Ocean temperature distribution and sea-surface elevation for a transect across the eastern Pacific from April 1997. El Niño Southern Oscillation (ENSO) La Niña events can contribute to pronounced sea level change as warm and cool water masses move back and forth across the Pacific. Bulges of water can contribute to bulges and depressions on the ocean's surface. In this case, notice the red uplifted bulge of warm water located off the coast of South America. As ENSO cycles abate, this warm water and bulge will move back to the west and produce somewhat lower sea levels along the South American coast.

Credit: NASA: EO: What is El Niño?

Now, we can argue about the degree to which some of the former processes are impacted/amplified/exacerbated by human activities. However, there is growing evidence that human activities related to natural resource extraction and use is beginning to have very noticeable impacts on ground subsidence both on land, and under the sea. USGS scientists are using an exciting new technique called InSAR to measure the spatial extent and magnitude of surface deformation that results from fluid extraction and other natural hazards. InSAR data produces extremely high-quality visualizations that demonstrate subsidence patterns that show strong correlations between human activities (i.e., ground water withdrawal, oil/gas extraction, organic soil depletion, etc.) and ground level subsidence. See the USGS document: Measuring Human-Induced Land Subsidence from Space. Although this study focuses primarily on land-based subsidence, numerous research studies in the Gulf Coast of Texas, Louisiana, and elsewhere have demonstrated similar patterns of subsidence over areas where extensive removal of subsurface materials have produced subsidence and associated sea level rise (i.e., drowning of coastal wetlands and even some upland areas that are transitioning to wetlands).

The question can still be raised, does this activity impact global sea levels? The jury is still out, but where there is substantial drilling in near shore areas the potential for impact is significant. Off-shore drilling is unlikely to produce as pronounced impacts, but given the volume of extraction globally... the science is not yet there on this one.