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[AAS03-05] An OLR-SST feedback on the climatological BSISO in the Indian Ocean
Keywords:BSISO, MJO, CISO
The Boreal Summer Intraseasonal Oscillation (BSISO), which is characterized by northeastward propagation of convective envelopes with a 30-60-day interval, is the leading intraseasonal mode in the tropics during boreal summer. The BSISO tends to be phase-locked to the annual cycle, and the daily climatological fields with a BSISO-like spatial pattern are referred to as the Climatological BSISO (CBSISO). The understanding of the CBSISO has implications for the prediction of local onset and retreats of the Asian Summer Monsoon (ASM) because the propagation of the CBSISO links to the life cycle of the ASM. However, it still remains open how the BSISO has a phase-locking and when the CBSISO starts. Here we discuss a possible mechanism of the BSISO’s phase-locking using 42-year records of the outgoing longwave radiation (OLR) data.
As a preparatory step, we compared the phase angle of the BSISO’s OLR time series for each year with that of the CBSISO with a focus on the Arabian Sea (AS) where the standard deviation of the CBSISO is larger than those over other tropical regions. Using this phase information, we classified the OLR and other data into three groups: group which is closely in phase with the CBSISO (in-phase group), group with the earlier phase (early-phase group), and group with the later phase (late-phase group).
Based on the composite analysis of the in-phase group, we propose the following explanation for the oscillatory behavior: The phase-locking of the BSISO begins with the positive OLR anomaly over the AS in early April. This dry condition warms the ocean (higher sea surface temperature, SST) through increased incoming shortwave radiation, which is followed by the wet phase of the CBSISO in the AS. Enhanced convection leads to a lower SST, which subsequently suppresses convection, and thus a dry condition over the AS region emerges again. This cycle appears to repeat till the July-August period. We also identified that the first wet phase of the CBSISO corresponds to the monsoon onset over the Bay of Bengal around early May, and the second wet phase of the CBSISO coincides with the Indian summer monsoon onset around early June.
These results suggested that the OLR-SST feedback triggered by the dry condition in early April plays an important role in the maintenance of the CBSISO and has implications for the predictability of the ASM beyond a month.
As a preparatory step, we compared the phase angle of the BSISO’s OLR time series for each year with that of the CBSISO with a focus on the Arabian Sea (AS) where the standard deviation of the CBSISO is larger than those over other tropical regions. Using this phase information, we classified the OLR and other data into three groups: group which is closely in phase with the CBSISO (in-phase group), group with the earlier phase (early-phase group), and group with the later phase (late-phase group).
Based on the composite analysis of the in-phase group, we propose the following explanation for the oscillatory behavior: The phase-locking of the BSISO begins with the positive OLR anomaly over the AS in early April. This dry condition warms the ocean (higher sea surface temperature, SST) through increased incoming shortwave radiation, which is followed by the wet phase of the CBSISO in the AS. Enhanced convection leads to a lower SST, which subsequently suppresses convection, and thus a dry condition over the AS region emerges again. This cycle appears to repeat till the July-August period. We also identified that the first wet phase of the CBSISO corresponds to the monsoon onset over the Bay of Bengal around early May, and the second wet phase of the CBSISO coincides with the Indian summer monsoon onset around early June.
These results suggested that the OLR-SST feedback triggered by the dry condition in early April plays an important role in the maintenance of the CBSISO and has implications for the predictability of the ASM beyond a month.