East Pacific climate is characterized by strong gradients in SST, surface wind and cloud fields and is strongly influence by the shape and orography of the American continents. Many of these features are inadequately represented in global climate models (at a typical resolution of 2-3 degree in horizontal) and poorly observed by traditional measurements. We analyze new satellite observations to uncover new phenomena (e.g., an intraseasonal oscillation of low clouds in the Southeast Pacific) and use high-resolution regional atmospheric model (RAM) to better represent these high-gradient features. The RAM developed at IPRC simulates with some success the low cloud deck over the Southeast Pacific and the associated temperature inversion. It also captures the wind acceleration across the equatorial front and other boundary layer adjustment observed in the EPIC transects. The presentation highlights results from this combined satellite observations/RAM simulation approach.
The Andes are a steep and narrow mountain range; at the equator, it rises from near the sea level to 3.5 km high in less than 100 km. The IPRC RAM is used to study the effect of the steeply rising Andes on eastern Pacific climate. During the equatorial cold season (August- October), the removal of the Andes results in a reduction in stratocumulus cloudiness and surface solar radiation. During the warm season (March-May), on the other hand, the removal of the mountains causes the southern ITCZ to stay a few weeks longer south of the equator. Thus, both effects of the Andes are to strengthen the equatorial asymmetry of eastern Pacific climate and help keep the Pacific ITCZ north of the equator.
The eastern Pacific ITCZ is displaced south of the maximum over the eastern Pacific warm pool SST in January-March, casting doubt on whether SST controls atmospheric convection there. Our model experiments show that the orographic effect of Central American cordillera is the cause of this displaced ITCZ. The direct effect of these mountains is to cause downdraft on the Pacific side, with an additional indirect effect through the sea surface cooling associated with the strong gap winds, both unfavorable for convection. While these gap winds are strong in boreal winter, satellite observations reveal a delayed effect of these gap winds on summer ITCZ convection through ocean memory. This delayed effect on the summer ITCZ is captured by the RAM simulation with observed SST prescribed.