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Linking the Pacific Meridional Mode to ENSO: Utilization of a Noise Filter

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  • 1 Center for Ocean–Land–Atmosphere Studies, Calverton, Maryland
  • | 2 Texas A&M University, College Station, Texas
  • | 3 International Research Institute for Climate and Society, Columbia University, Palisades, New York
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Abstract

A novel noise filter is used to effectively reduce internal atmospheric variability in the air–sea fluxes of a coupled model. This procedure allows for a test of the impact of the internal atmospheric variability on ENSO through its effect on the Pacific meridional mode (MM). Three 100-yr coupled experiments are conducted, where the filter is utilized to suppress internal atmospheric variability in 1) both the surface wind stress and the heat flux (fully filtered run), 2) only the surface heat flux (filtered-flux run), and 3) only the surface wind stress (filtered-wind run). The fully filtered run indicates that suppressing internal atmospheric variability weakens the MM, which in turn results in substantially reduced ENSO variability. ENSO is no longer phase locked to the boreal winter. The filtered-flux and filtered-wind experiments reveal that different types of noise affect ENSO in different ways. The noise in the wind stress does not have a significant impact on the MM and its relationship to ENSO. This type of noise, however, tends to broaden the spectral peak of ENSO while shifting it toward lower frequencies. The noise in the heat flux, on the other hand, has a direct impact on the strength of the MM and consequently its ability to influence ENSO. Reducing the effect of heat flux noise yields substantially weakened MM activity and a weakened relationship to ENSO, which leads to altered seasonal phase-locking characteristics.

Corresponding author address: Li Zhang, Center for Ocean–Land–Atmosphere Studies, Suite 302, 4041 Powder Mill Rd., Calverton, MD 20705. Email: lzhang@cola.iges.org

Abstract

A novel noise filter is used to effectively reduce internal atmospheric variability in the air–sea fluxes of a coupled model. This procedure allows for a test of the impact of the internal atmospheric variability on ENSO through its effect on the Pacific meridional mode (MM). Three 100-yr coupled experiments are conducted, where the filter is utilized to suppress internal atmospheric variability in 1) both the surface wind stress and the heat flux (fully filtered run), 2) only the surface heat flux (filtered-flux run), and 3) only the surface wind stress (filtered-wind run). The fully filtered run indicates that suppressing internal atmospheric variability weakens the MM, which in turn results in substantially reduced ENSO variability. ENSO is no longer phase locked to the boreal winter. The filtered-flux and filtered-wind experiments reveal that different types of noise affect ENSO in different ways. The noise in the wind stress does not have a significant impact on the MM and its relationship to ENSO. This type of noise, however, tends to broaden the spectral peak of ENSO while shifting it toward lower frequencies. The noise in the heat flux, on the other hand, has a direct impact on the strength of the MM and consequently its ability to influence ENSO. Reducing the effect of heat flux noise yields substantially weakened MM activity and a weakened relationship to ENSO, which leads to altered seasonal phase-locking characteristics.

Corresponding author address: Li Zhang, Center for Ocean–Land–Atmosphere Studies, Suite 302, 4041 Powder Mill Rd., Calverton, MD 20705. Email: lzhang@cola.iges.org

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