That’s right, there is new evidence that demonstrates that the amount of rain that fell on the terrestrial biosphere during the 2011 La Nina event was enough to register a decrease in global mean sea level (GMSL). And while this may not come as news to people living in Queensland it certainly highlights the magnitude of influence that the El Niño (and his opposite La Niña) Southern Oscillation has on the global water cycle.
In an article published in Geophysical Research Letters and highlighted in Science Editor’s Choice, Boening et al., report that GMSL dropped 5mm between the beginning of 2010 and mid 2011. This is a significant amount if you consider that GMSL has been rising 1.7mm per year since ~1880, and accelerated to 3mm per year over the past 18 years (measured by satellite altimeters, and programs such as GRACE). These findings do not in any way undermine the sea-level rise predicted with continued warming. The strength of the 2011 La Niña only demonstrates the extreme nature of events that are anticipated under current climate change projections. So how does a La Nina event cause sea-level to drop?
What we are more accustomed to hearing is that the significant warming trend is melting ice sheets and glaciers thereby increasing the volume of water in the ocean. The volume of water in the ocean further increases due to thermal expansion….caused by the significant warming trend. However other variables are superimposed on this warming trend, including precipitation patterns that are driven by large-scale ocean-atmosphere interactions, such as the ENSO and PDO cycles. Boening et al. are able to demonstrate that the decline in sea-level coincides with the 2010-2011 transition into a La Niña climate state. A transition that saw a significant increase in terrestrial water storage, particularly over Australia (hello Queensland floods), northern South America, and Southeast Asia. However this storage is only temporary, as run-off will return the water from whence it came, the ocean (hello sea-leve rise).