Model Assessment of Decadal Variability and Trends in the Tropical Pacific Ocean

Abstract

In this report, global coupled ocean?atmosphere models are used to explore possible mechanisms for observed decadal variability and trends in Pacific Ocean SSTs over the past century. The leading mode of internally generated decadal (>7 yr) variability in the model resembles the observed decadal variability in terms of pattern and amplitude. In the model, the pattern and time evolution of tropical winds and oceanic heat content are similar for the decadal and ENSO timescales, suggesting that the decadal variability has a similar ?delayed oscillator? mechanism to that on the ENSO timescale. The westward phase propagation of the heat content anomalies, however, is slower and centered slightly farther from the equator (?12° vs 9°N) for the decadal variability. Cool SST anomalies in the midlatitude North Pacific during the warm tropical phase of the decadal variability are induced in the model largely by oceanic advection anomalies. An index of observed SST over a broad triangular region of the tropical and subtropical Pacific indicates a warming rate of +0.41°C (100 yr)?1 since 1900, +1.2°C (100 yr)?1 since 1949, and +2.9°C (100 yr)?1 since 1971. All three warming trends are highly unusual in terms of their duration, with occurrence rates of less than 0.5% in a 2000-yr simulation of internal climate variability using a low-resolution model. The most unusual is the trend since 1900 (96-yr duration): the longest simulated duration of a trend of this magnitude is 85 yr. This suggests that the observed trends are not entirely attributable to natural (internal) variability alone, although natural variability could potentially account for much of the observed trends. To quantitatively explore the possible role of greenhouse gases and aerosols in the observed warming trends, two simulations (using different initial conditions) of twentieth-century climate change due to these two radiative forcings were analyzed. These simulate an accelerated warming trend [?2°C (100 yr)?1] in the triangular Pacific region beginning around the 1960s and suggest that nearly all of the recent warming in the region could be attributable to such a thermal forcing. In summary, the authors? model results indicate that the observed warming trend in the eastern tropical Pacific is not likely to be solely attributable to internal (natural) climate variability. Instead, it is likely that a sustained thermal forcing, such as the increase of greenhouse gases in the atmosphere, has been at least partly responsible for the observed warming.