Westward Propagation of Latitudinal Asymmetry in a Coupled Ocean–Atmosphere Model

Shang-Ping Xie Graduate School of Environmental Earth Science, Hokkaido University, Japan

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Abstract

Hemispheric asymmetries of continental geometry have long been speculated to be the cause of the Northern Hemisphere position of the intertropical convergence zone over the central and eastern Pacific. It is unknown, however, how the effects of continental asymmetries are transmitted to and felt by the central Pacific thousands of kilometers away. This paper proposes a transmitter mechanism by investigating the response of a coupled ocean–atmospheric model to a symmetry-breaking force by the American continents. The model treats land forcing implicitly as an eastern boundary condition. In the absence of oceanic feedback, the model response to the eastern boundary forcing is tightly trapped and confined to a small longitudinal extent off the coast, whereas the climate over the interior ocean is symmetric about the equator. Ocean–atmosphere coupling greatly enhances the transmissibility of the effects of the land forcing, establishing large latitudinal asymmetry over a great zonal extent. A westward propagating coupled instability is found to be responsible, which is antisymmetric about the equator and is caused by a wind–evaporation–SST feedback proposed previously by Xie and Philander. The solution to an initial value problem shows that a coupled ocean–atmosphere wave front generated by the land forcing amplifies as it moves westward, leaving behind a latitudinally asymmetric steady state.

Abstract

Hemispheric asymmetries of continental geometry have long been speculated to be the cause of the Northern Hemisphere position of the intertropical convergence zone over the central and eastern Pacific. It is unknown, however, how the effects of continental asymmetries are transmitted to and felt by the central Pacific thousands of kilometers away. This paper proposes a transmitter mechanism by investigating the response of a coupled ocean–atmospheric model to a symmetry-breaking force by the American continents. The model treats land forcing implicitly as an eastern boundary condition. In the absence of oceanic feedback, the model response to the eastern boundary forcing is tightly trapped and confined to a small longitudinal extent off the coast, whereas the climate over the interior ocean is symmetric about the equator. Ocean–atmosphere coupling greatly enhances the transmissibility of the effects of the land forcing, establishing large latitudinal asymmetry over a great zonal extent. A westward propagating coupled instability is found to be responsible, which is antisymmetric about the equator and is caused by a wind–evaporation–SST feedback proposed previously by Xie and Philander. The solution to an initial value problem shows that a coupled ocean–atmosphere wave front generated by the land forcing amplifies as it moves westward, leaving behind a latitudinally asymmetric steady state.

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