Higher Sea Levels at Hawaii Caused by Strong El Nino and Weak Trade Winds

Hawaii experienced record-high sea levels during 2017, which followed the 2015 strong El Nino and coincided with weak trade winds in the tropical northeastern Pacific. The record sea levels were associated with a combination of processes, an important contributing factor of which was the persistent high sea level (similar to 10 cm above normal) over a large region stretching between Hawaii and Mexico. High sea levels at Mexico are known to occur during strong El Nino as the coastal thermocline deepens. Planetary wave theory predicts that these coastal anomalies propagate westward into the basin interior; however, high sea levels at Hawaii do not occur consistently following strong El Nino events. In particular, Hawaii sea levels remained near normal following the previous strong El Nino of 1997. The processes controlling whether Hawaii sea levels rise after El Nino have so far remained unknown. Atmosphere-forced ocean model experiments show that anomalous surface cooling, controlled by variable trade winds, impacts sea level via mixed layer density, explaining much of the difference in Hawaiian sea level response after the two recent strong El Nino events. In climate model projections with greenhouse warming, more frequent weak trade winds following El Nino events are expected, suggesting that the occurrence of high sea levels at Hawaii will increase as oceanic anomalies more often traverse the basin.