High-resolution simulation of eastern Pacific climate with a regional coupled model

Eastern Pacific climate is characterized by a northward-displaced ITCZ and a pronounced annual cycle on the equator; both of these features are not driven by variations in solar radiation. While readily observed, these features are difficult for state-of-the-art global climate models to simulate, severely limiting the models' skill in predicting El Niño and in projecting climate change.

To understand better those eastern Pacific regional air-sea processes that shape the basin-wide climate, a regional coupled ocean-atmospheric model (ROAM) is developed by coupling a full-physics regional atmospheric model with the Modular Ocean Model version 2. Resolution is 0.5º in the horizontal, with 28 and vertical atmospheric levels and 30 ocean levels. In collaboration with the Kyousei-7 Project at Frontier Research Center for Global Change, the IPRC-ROAM has been implemented on the Earth Simulator and integrated for seven years, from 1997 to 2003.

The model captures the salient features of eastern Pacific climate that have been so difficult to model, including the northward-displaced ITCZ and the equatorial annual cycle. Just as in observations, the model ITCZ stays north of the equator most of the year except for a brief period in March–April when the equatorial sea surface temperatures reach their annual maximum. During June–December, when the equatorial cold tongue develops, a temperature front forms north of the equator with meandering tropical instability waves, the turbulent waves with time scales of 20–30 days and wavelengths of the order of 1000 km for which this region is known.

Its relatively high resolution enables IPRC-ROAM to resolve the narrow mountains of the Americas. In particular, it reproduces over the Gulf of Papagayo a strong wind jet in winter that rushes through a major gap in the Central American mountain range. The strong upwelling caused by this wind jet forces a thermocline dome that maintains a cool region in the tropical ocean during summer, just as the one actually observed west of Costa Rica.