El Niño on its way – what is sea level doing?

Many signs are pointing to a strong El Niño forming in the Pacific Ocean. It is interesting to see what that means for sea level in this area. The sea level in the western tropical Pacific has been rising up to four times faster than the global mean and scientists have yet to agree on whether the  remarkable rise can be blamed on natural variability or is partly human-made.
ssha_trend_1993_2014

The reasons were relatively strong trade winds in the equatorial Pacific leading to an accumulation of warm water in the western Pacific. Now, with the advent of a potentially record-strong El Niño, winds have weakened and the warm water is spilling back to the eastern part of the Pacific where sea level rise has been relatively weak for the past 20 years.

duacs_global_nrt_msla_merged_h_compare

Sea level anomalies for August 1997, 2013 and 2015 from http://bulletin.aviso.oceanobs.com/html/produits/aviso/welcome_uk.php3

Check out this website for an overview of past and present sea level anomalies. A look at trend patterns (upper figure) does not yet show a strong signal when including 2014 in the analysis. This is not surprising as El Niño only started to show its characteristic footprint in early 2015, peaking in the coming winter season. It will be interesting to look at trends in a couple of months. Until then, I pass the time by looking at snapshots of past and present sea level anomalies. Apart from the anomalies becoming more positive, the similarity of the anomalies in August 1997, when a record-strong El Niño was to come, and August 2015 are striking and in sharp contrast to 2013 anomalies. It has been debated for quite some time now, whether the remarkable sea level rise in the western tropical Pacific is caused by natural climate variability or is at least partly of anthropogenic origin. The CMIP5 climate models do not show such strong trends in their unforced simulations implying that the signal is forced. But neither do they reproduce the signal in the forced simulations, implying that it’s natural. It seems there is still a lot of potential for improvement in this particular field of research.

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