Ocean Surface actually fell
because of La Niña in 2011

          Sea level has been rising slowly for at least 130 years, but last year the sea level actually fell. Looking at all possible influences, a team led by Carmen Boening1 at the Jet Propulsion Laboratory in Pasadena, CA, concluded that the very strong La Niña of 2010–2012 caused the drop. In general, either cooling of the ocean or a loss of water would cause sea level to drop. The team insists that loss of water was responsible. The atmosphere transported enough water from the sea to the continents that sea level actually fell by one-half centimeter (5 mm) worldwide.

          The El Niño-Southern Oscillation (ENSO) is the largest year-to-year climate signal on Earth2, according to Dr. Michael McPhaden of the NOAA Pacific Marine Laboratory in Seattle. El Niño is the warm phase of this oscillation, and has been shown to cause temporary rises in sea level. The cold phase, popularly known as La Niña, may similarly cause sea level to fall for a while.

          As you may see on the figure at right, sea level began to fall in the middle of 2010 after a long and fairly steady rise. In that year, the last El Niño (a strong one) morphed into a strong La Niña that continued until 2012. Boening’s team was able to separate the effect of ocean temperature on sea level from the effect of a gain or loss of water, by combining measurements of ocean temperature taken by buoys, and satellite observations of ocean mass and rainfall. The satellites observed increasing amounts of water stored on land in Australia, Southeast Asia, and northern South America by the end of 2010. These regions were coming out of a drought and were receiving more precipitation than in earlier years.

Continues

      Above: Change in sea level (cm) since 1994 over the last twenty years. Sea level is arbitrarily set to zero in 1994. Sea level has been rising one centimeter every 3 years, but fell in 2010–11.

      The team concluded that ocean temperatures affected sea level very little in this case, but that enough water was transferred from the sea to these three land areas, that sea level dropped by one-half centimeter.

          From the figure, you can see that sea level was already recovering in 2011 after the sharp drop in 2010, even though La Niña continued through 2011. Because the excess water was running off the land in streams and rivers, such a drop in sea level is only temporary. The century-long rise of global sea level is back on track.

 CITATIONS                                                                            Top

1. The 2011 La Niña: So strong, the oceans fell” by Carmen Boening and four others (2012). Geophysical Research Letters, vol. 39, L19602, doi:10.1029/2012GL053055; 4 October 2012.

2. “ENSO as an integrating concept in Earth science” by Michael J. McPhaden, S. E. Zebiak, and M. H. Glantz (2006). Science, vol. 314, p. 1740–1745, doi:10.1126/science.1132588.

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