Role of local thermodynamic coupling in the life cycle of the intraseasonal oscillation in the Indo-Pacific warm pool
Abstract
Intraseasonal oscillations (ISOs) are important elements of the tropical Indo-Pacific climate with time-scales in the 20--80 day period range during which time they modulate higher frequency tropical weather. The ISO is poorly simulated and predicted by numerical models. This work presents a joint diagnostic and modeling study of the ISO that examines the hypothesis that local coupling between the ocean and the atmosphere is essential to the existence and evolution of the intraseasonal oscillation in the Indo-Pacific warm pool region. It is found that during the transition between the suppressed and active periods of the ISO, the ocean and the atmosphere undergo gradual, but large-scale and high-amplitude changes, especially the moistening of the lower troposphere by the anomalously warm sea surface temperature arising from minimal cloud coverage and weak winds during the suppressed phase and the large-scale subsidence that inhibits the formation of deep convection locally. The low-level moistening preconditions the atmosphere for deep convection. The regional and local vertical structure of ISO from numerical forecasts using the ECMWF coupled model during different phases of the oscillation as well as the skill of the numerical model in simulating the processes that occur during the transition from suppressed to active convection were studied. Results suggest that forecast skill of the tropospheric vertical structure associated with the ISO is greater for winter ISO cases than for summer events. Predictability of the convective period is poor when initialized before the transitional phase. However, when initialized within the transition period including lower tropospheric moistening, predictability increases substantially, although the model convective parameterization appears to trigger convection too quickly without allowing an adequate buildup of CAPE during the transition. The model tends to simulate a relatively more stable atmosphere compared to data, limiting the production of deep convective events associated with the ISO. Two different one-dimensional coupled models are used to analyze the role of local ocean-atmosphere coupling in generating ISO. In both models, the ocean component is a one-dimensional mixed layer model. In the first model the atmospheric component corresponds to the SCCM. Results suggest that convection in the model tends to be "overactive," inhibiting development of lower frequency oscillations in the atmosphere. In the second case, the atmospheric component is a semi-empirical one-dimensional model that allows reproducing the coupled ISO over long integration periods including only local mechanisms and without convective parameterization, In the semi-empirical scheme the rate of change of atmospheric variables is statistically related to changes in SST. The stable state of this simple model is a quasi-periodic oscillation with a time scale between 25 and 80 days that matches well the observed ISO. Further experimentation suggest that the dominant period of the oscillation depends on the characteristics of the ocean mixed layer, with a higher frequency oscillation for a shallow mixed layer and vice versa.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2007
- Bibcode:
- 2007PhDT.......153A