The Annual Warm to Cold Phase Transition in the Eastern Equatorial Pacific: Diagnosis of the Role of Stratus Cloud-Top Cooling

Sumant Nigam Department of Meteorology, Cooperative Institute for Climate Studies, University of Maryland, College Park, Maryland

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Abstract

The forcing of the March to May southerly surface-wind tendency along the equatorial South American coast, which leads to the annual transition of the eastern tropical Pacific basin’s climate from its peak warm phase in April, is explored through diagnostic modeling.

Modeling experiments with a high-resolution (18 σ-levels, Δθ = 2.5°, 30 zonal waves) steady-state global linear primitive equation model that produces a striking simulation of most aspects of the March to May change in the lower tropospheric circulation over the eastern tropical Pacific, including the notable southerly surface-wind tendency, have provided unique insight into the role of various physical processes. The model is forced by the 3D distribution of the residually diagnosed diabatic heating and the submonthly momentum and thermal transients, all obtained from the twice-daily 2.5° × 2.5° European Centre for Medium-Range Weather Forecasts uninitialized analyses for 1985–95. The principal findings are the following:

  • The initial southerly surface-wind tendency along the equatorial South American coast in April is forced by the March to May abatement in deep heating (p ≲ 900 mb) over the Amazon due to the northward migration of continental convection, and by the elevated Andean cooling.

  • The increased Northern Hemisphere deep heating due to the developing Central American monsoons and the eastern Pacific ITCZ also contributes to the generation of the initial coastal southerly wind tendency, but not more strongly than the March to May cooling over South America.

  • The March to May cooling of the lower troposphere (600–900 mb) over the southeastern tropical Pacific, which likely results from the longwave radiative cooling from the developing stratocumulus cloud tops, generates relatively strong southerly surface-wind tendencies over the eastern Pacific, particularly at the equatorial South American coast.

Based on the last finding, a new feedback mechanism can be envisioned for the rapid development of the coastal southerly surface-wind tendency and stratocumulus clouds—in which the lower tropospheric cooling over the southeastern tropical Pacific, due to longwave radiative cooling from the stratocumulus cloud tops, generates southerly surface winds, which in turn foster stratocumulus growth from the increased meridional cold advection and latent heat flux.

With respect to the role of stratus clouds in the coupled annual cycle evolution, the new feedback, based on the dynamic response of cloud-top longwave cooling, should proceed more rapidly than the feedback based on the thermodynamic impact of stratus shading on SST.

Corresponding author address: Dr. Sumant Nigam, Department of Meteorology, CICS, University of Maryland, College Park, MD 20742-2425.

Email: nigam@atmos.umd.edu

Abstract

The forcing of the March to May southerly surface-wind tendency along the equatorial South American coast, which leads to the annual transition of the eastern tropical Pacific basin’s climate from its peak warm phase in April, is explored through diagnostic modeling.

Modeling experiments with a high-resolution (18 σ-levels, Δθ = 2.5°, 30 zonal waves) steady-state global linear primitive equation model that produces a striking simulation of most aspects of the March to May change in the lower tropospheric circulation over the eastern tropical Pacific, including the notable southerly surface-wind tendency, have provided unique insight into the role of various physical processes. The model is forced by the 3D distribution of the residually diagnosed diabatic heating and the submonthly momentum and thermal transients, all obtained from the twice-daily 2.5° × 2.5° European Centre for Medium-Range Weather Forecasts uninitialized analyses for 1985–95. The principal findings are the following:

  • The initial southerly surface-wind tendency along the equatorial South American coast in April is forced by the March to May abatement in deep heating (p ≲ 900 mb) over the Amazon due to the northward migration of continental convection, and by the elevated Andean cooling.

  • The increased Northern Hemisphere deep heating due to the developing Central American monsoons and the eastern Pacific ITCZ also contributes to the generation of the initial coastal southerly wind tendency, but not more strongly than the March to May cooling over South America.

  • The March to May cooling of the lower troposphere (600–900 mb) over the southeastern tropical Pacific, which likely results from the longwave radiative cooling from the developing stratocumulus cloud tops, generates relatively strong southerly surface-wind tendencies over the eastern Pacific, particularly at the equatorial South American coast.

Based on the last finding, a new feedback mechanism can be envisioned for the rapid development of the coastal southerly surface-wind tendency and stratocumulus clouds—in which the lower tropospheric cooling over the southeastern tropical Pacific, due to longwave radiative cooling from the stratocumulus cloud tops, generates southerly surface winds, which in turn foster stratocumulus growth from the increased meridional cold advection and latent heat flux.

With respect to the role of stratus clouds in the coupled annual cycle evolution, the new feedback, based on the dynamic response of cloud-top longwave cooling, should proceed more rapidly than the feedback based on the thermodynamic impact of stratus shading on SST.

Corresponding author address: Dr. Sumant Nigam, Department of Meteorology, CICS, University of Maryland, College Park, MD 20742-2425.

Email: nigam@atmos.umd.edu

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