Recent Pacific sea level trends: internally or externally forced?

Abstract:

A prominent sea level rise pattern in the Pacific Ocean – large increases of 12 mm/yr in the West and near-stagnant levels in the East – has been observed with satellite altimetry since the 1990s. A significant fraction of this dipole trend pattern has been attributed to the Pacific Decadal Oscillation (PDO), a large-scale mode of internal climate variability. However, the PDO cannot fully account for the Western Pacific Ocean trend pattern. Whether or not the remaining non-PDO pattern can be linked to an anthropogenic influence has been discussed in several recent studies. Here, we use a large ensemble of CMIP5 20^th century simulations (more than 150 runs from 27 different GCM setups) to further investigate the role and contribution of the PDO and other modes of equatorial variability to Pacific sea level change patterns. Specifically, we focus on 20-year trend patterns since 1950, and attempt to isolate and attribute an externally forced sea level pattern. Rather than averaging over the large CMIP5 model ensemble to dampen non-synchronized internal variability, we remove sea level trend contributions from internal variability to test if the remaining patterns become more consistent across the models, and thus support the notion of a detectable common externally forced sea level change since 1990. Our results also indicate that the representation of the PDO and its link to large-scale regional sea level variations can differ significantly across the CMIP5 ensemble. While some CMIP5 models can replicate observed connections between PDO-related and equatorial climate modes and Pacific sea level patterns, others show large differences.