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Ocean-Atmosphere Interaction and the Tropical Climatology. Part I: The Dangers of Flux Correction

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  • 1 Department of Atmospheric Sciences, University of California Los Angeles, Los Angeles, California
  • | 2 Institute for Marine and Atmospheric Research Utrecht, University of Utrecht, Utrecht, the Netherlands
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Abstract

This sequence of papers examines the role of dynamical feedbacks between the ocean and the atmosphere in determining features of the tropical climatology. A stripped-down, intermediate, coupled ocean-atmosphere model is used to provide a prototype problem for the Pacific basin. Here the authors contrast the fully coupled case with the case where flux correction is used to construct the climatology. In the fully coupled case, the climatology is determined largely by feedback mechanisms within the ocean basin: winds driven by gradients of sea surface temperature (SST) within the basin interact with the ocean circulation to maintain SST gradients. For all realistic cases, these lead to a unique steady solution for the tropical climatology. In the flux-corrected case, the artificially constructed climatology becomes unstable at sufficiently large coupling, leading to multiple steady states as found in a number of coupled models. Using continuation methods, we show that there is a topological change in the bifurcation structure as flux correction is relaxed toward a fully coupled case; this change is characterized as an imperfection and must occur generically for all flux-corrected cases. The cold branch of steady solutions is governed by mechanisms similar to the fully coupled case. The warm branch, however, is spurious and disappears. The dynamics of this and consequences for coupled models are discussed. Multiple steady states can be ruled out as a mechanism for El Niño in favor of oscillatory mechanisms. The important role that coupled feedbacks are suggested to play in establishing tropical climatology is referred to as “the climatological version of the Bjerknes hypothesis."

Abstract

This sequence of papers examines the role of dynamical feedbacks between the ocean and the atmosphere in determining features of the tropical climatology. A stripped-down, intermediate, coupled ocean-atmosphere model is used to provide a prototype problem for the Pacific basin. Here the authors contrast the fully coupled case with the case where flux correction is used to construct the climatology. In the fully coupled case, the climatology is determined largely by feedback mechanisms within the ocean basin: winds driven by gradients of sea surface temperature (SST) within the basin interact with the ocean circulation to maintain SST gradients. For all realistic cases, these lead to a unique steady solution for the tropical climatology. In the flux-corrected case, the artificially constructed climatology becomes unstable at sufficiently large coupling, leading to multiple steady states as found in a number of coupled models. Using continuation methods, we show that there is a topological change in the bifurcation structure as flux correction is relaxed toward a fully coupled case; this change is characterized as an imperfection and must occur generically for all flux-corrected cases. The cold branch of steady solutions is governed by mechanisms similar to the fully coupled case. The warm branch, however, is spurious and disappears. The dynamics of this and consequences for coupled models are discussed. Multiple steady states can be ruled out as a mechanism for El Niño in favor of oscillatory mechanisms. The important role that coupled feedbacks are suggested to play in establishing tropical climatology is referred to as “the climatological version of the Bjerknes hypothesis."

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