Role of forced and internal variability in Pacific sea-level trends

Abstract:

Satellite observations show that sea-level rise (SLR) is not uniform, in particular in the Pacific Ocean. During the last 20 years, the tropical western Pacific has been rising at a rate of ~9 mm/year (three times faster than the global rate of ~3 mm/year). Conversely, sea-level over the eastern tropical Pacific has slightly decreased. These spatially varying trends have been generally attributed to decadal fluctuations in atmospheric circulation associated with the Pacific Decadal Oscillation (PDO). However, controversy remains over whether the accelerated SLR over the tropical western Pacific is man-made. We explore this issue using the large ensemble of historical simulations performed with the Community Earth System Model (CESM-LE). The simulated sea-surface height (SSH) trends exhibit large spread among the 30 ensemble members. A large fraction (40%) of the spread is explained by two modes associated the PDO. Using CESM-LE we develop a methodology to estimate SSH trends associated with observed trends in atmospheric circulation. The resulting circulation-adjusted SSH trends exhibit spatial patterns in remarkable agreement with the satellite-derived sea-level trends (pattern correlation = 0.8). Furthermore, the observed atmospheric circulation trends are within the range of unforced trends simulated by CESM-LE. Together, these results suggest that the accelerated SLR in the western northern tropical Pacific is likely the response to unforced multi-decadal variability in the Walker circulation and the Aleutian Low.