Processes controlling the mean tropical Pacific precipitation pattern: I. The Andes and the eastern Pacific ITCZ

Ken Takahashi and David S. Battisti, 2007

Journal of Climate 20, 14, 3434-3451.

Submitted to Journal of Climate: May 11, 2006. Revised: October 5, 2006. Accepted: October 24, 2006

We address the question of why the ocean off Peru is colder than that at the same latitudes in the northern hemisphere, where strong convective rainfall (the ITCZ) is found. The presence of the Andes is important, as they force drier air over the ocean, which cools it by evaporation and prevents deep convection to occur off Peru. Even if the Rockies were as big as the Andes, this would not change because they are too far to the west.

Mean rainfall (mm/day, colors) and sea surface temperature (°C, contours) distribution. Topography higher than 1000 m is shown brown.

Abstract

The question of why the intertropical convergence zone (ITCZ) is generally north of the equator in the eastern half of the Pacific is addressed. Supported by idealized numerical experiments, it is argued that the longitudinal offset between the north and south Pacific anticyclones can drive this north-south asymmetry, even in the absence of coastal upwelling and feedbacks involving low clouds.

The south Pacific anticyclone, which is mostly forced by the presence of the Andes, forces the north-south asymmetry in the coupled ocean-atmosphere system (including the asymmetry in the ITCZ) in the far eastern Pacific, which then propagates westward; the anticyclonic circulation in the north Pacific, which in our model is realistically driven by the orography of the Himalayas and Rockies, produces a forcing comparable to that in the south Pacific that acts against the asymmetry but is not able to reverse it. In our experiments, the presence of the Andes cools the southeast Pacific by evaporation, mainly due to dry vertical and meridional advection by the subtropical anticyclone. The asymmetry instigated by the Andes is further amplified by coupled ocean-atmosphere feedbacks, as described by Xie and Philander (1994).

Sensitivity experiments indicate that convection in the Amazon region has little effect on the asymmetry. The presence of a strong east Pacific equatorial cold tongue and, to a lesser extent, the seasonality of insolation have a weak symmetrizing influence on the ITCZ.

Resumen

¿Por qué la zona de convergencia intertropical (ZCIT, o ITCZ por sus siglas en inglés) en el Pacífico oriental se encuentra generalmente al norte del ecuador? Experimentos numéricos idealizados apoyan la idea de que el desplazamiento longitudinal entre los anticiclones del Pacífico sur y norte promueve esta asimetría, aún en la ausencia de factores como el afloramiento costero y procesos retroalimentivo asociadas a nubes bajas.

El anticiclón del Pacífico sur, producido principalmente por la presencia de los Andes, fuerza la asimetría norte-sur en el sistema oceano-atmósfera (incluyendo la asimetría en la ZCIT) cerca a la costa americana, la cual luego se propaga hacia el oeste; la circulación anticiclónica en el Pacífico norte, la cual en nuestro modelo es realísticamente producida por las montañas Rocosas e Himalayas, produce un forzamiento similar al del Pacífico sur que actua contra la asimetría pero no es capaz de revertirla. En nuestros experimentos, la presencia de los Andes enfría el Pacífico suroriental por evaporación, principalmente a través de la advección vertical y meridional de aire seco asociada al anticiclón subtropical. La asimetría instigada por los Andes es amplificada por procesos que involucran el acoplamiento entre el oceano y la atmósfera, como describen Xie y Philander (1994).

Experimentos de sensitividad indican que la convección sobre la Amazonía tienen poco efecto sobre la asimetría. La presencia de una intensa lengua fría ecuatorial y, en un menor grado, la estacionalidad de la insolación tienen una débil influencia simetrizadora en la ZCIT.

Oct, 2006.