Origin of Low-Frequency (Intraseasonal) Oscilliations in the Tropical Atmosphere. Part II: Structure and Propagation of Mobile Wave-CISK Modes and Their Modification by Lower Boundary Forcings

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  • 1 Department of Meteorology, University of Maryland, College Park, Maryland
  • | 2 Laboratory for Atmosphere, NASA/Goddard Space Flight Center, Greenbelt, Maryland
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Abstract

An improved treatment of diabatic heating due to moist convection is introduced into the dynamical model used in Part I of this paper to further investigate the origin of intraseasonal oscillations in the tropics. The convective heating in the model is parameterized by a simple one-dimensional cloud model which takes into account the available moisture supply in the lower troposphere and the mean thermodynamic states for the entire troposphere. Consequently, the spatial distribution of convective heating in the model can be determined internally as a function of the sea surface temperature consistent with observed convection-SST relationship in the tropics. The periods of low-frequency oscillations excited in the numerical simulations range from 20 to 50 days depending primarily on the vertical distribution of heating through condensation-moisture-convergence feedback or “mobile wave-CISK” The “fast” wave (period around 20 days) is excited by deep convection which has heating maximum at or above the 500 mb level. The “slow” wave (period near 50 days) is excited by heating maximized in the lower troposphere between 500 and 700 mb. A crude parameterization of lower boundary forcing due to heat flux from the ocean surface is incorporated in the model. The boundary forcing tends to further destabilize the mobile wave-CISK modes. It is also found that the boundary forcing plays an important role in sustaining the propagation of intraseasonal oscillations around the globe, especially over the eastern part of ocean where SST is cold and deep convection is strongly inhibited.

Abstract

An improved treatment of diabatic heating due to moist convection is introduced into the dynamical model used in Part I of this paper to further investigate the origin of intraseasonal oscillations in the tropics. The convective heating in the model is parameterized by a simple one-dimensional cloud model which takes into account the available moisture supply in the lower troposphere and the mean thermodynamic states for the entire troposphere. Consequently, the spatial distribution of convective heating in the model can be determined internally as a function of the sea surface temperature consistent with observed convection-SST relationship in the tropics. The periods of low-frequency oscillations excited in the numerical simulations range from 20 to 50 days depending primarily on the vertical distribution of heating through condensation-moisture-convergence feedback or “mobile wave-CISK” The “fast” wave (period around 20 days) is excited by deep convection which has heating maximum at or above the 500 mb level. The “slow” wave (period near 50 days) is excited by heating maximized in the lower troposphere between 500 and 700 mb. A crude parameterization of lower boundary forcing due to heat flux from the ocean surface is incorporated in the model. The boundary forcing tends to further destabilize the mobile wave-CISK modes. It is also found that the boundary forcing plays an important role in sustaining the propagation of intraseasonal oscillations around the globe, especially over the eastern part of ocean where SST is cold and deep convection is strongly inhibited.

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