3:30 PM - 5:00 PM
[AAS05-P05] MJO decay over the Maritime Continent: seasonality and effects of background fields
Keywords:Madden-Julian Oscillation, Maritime Continent
Although many previous studies have shown that the eastward propagation of the Madden-Julian Oscillation (MJO) is occasionally obstructed by the “barrier” effects of the Maritime Continent (MC), its underlying physics has not been fully understood. It is meaningful to understand the differences between the cases that the MJO propagates eastward across the MC (P cases) and that it decays due to the barrier effects (D cases) since the teleconnection responses to the MJO are strongest when the MJO crosses the MC and reaches the western Pacific. To clarify the seasonality in the MJO decay, this study focused on the climatological intraseasonal differences of the major track of the MJO. We conducted composite analysis for 216 months (November to April from 1985/86 to 2020/21), using outgoing longwave radiation (OLR) and the Japanese 55-year Reanalysis (JRA-55). Specifically, a 20-100-day bandpass filter was used to extract the intraseasonal variability, and a 100-day low-pass filter was applied to separate the background fields.
As a result, while there is no significant difference in the total monthly frequency of MJOs through the analyzed months, the ratio of P to D cases (P/D ratio) varies from month to month. The ratio is around 1 in November and December; in contrast, it is apparently higher in January and February. To investigate the causes of the seasonal changes in the P/D ratio, we examined monthly variance of the 20-100 day bandpass filtered OLR and zonal winds at 850 hPa. The areas of maximum daily variance in the filtered OLR are observed along the equator in November and December, whereas along the 10°S in January and February. This result indicates that the major track of MJOs shifts southward in January and February.
Moreover, background flows may be related to the passage of the MJO over the MC. The differences in the 100-day low-pass filtered 850-hPa zonal wind are significantly different from the MC to the central tropical Pacific in November and December. This result suggests that the assist by the background low-level flow can be important for the MJO to overcome the barrier effects and pass over the MC during November and December. On the other hand, we concluded that the southward shift of the MJO track dominantly affects the eastward propagation of the MJO in January and February because there are no significant differences in the background low-level flow between the two cases.
As a result, while there is no significant difference in the total monthly frequency of MJOs through the analyzed months, the ratio of P to D cases (P/D ratio) varies from month to month. The ratio is around 1 in November and December; in contrast, it is apparently higher in January and February. To investigate the causes of the seasonal changes in the P/D ratio, we examined monthly variance of the 20-100 day bandpass filtered OLR and zonal winds at 850 hPa. The areas of maximum daily variance in the filtered OLR are observed along the equator in November and December, whereas along the 10°S in January and February. This result indicates that the major track of MJOs shifts southward in January and February.
Moreover, background flows may be related to the passage of the MJO over the MC. The differences in the 100-day low-pass filtered 850-hPa zonal wind are significantly different from the MC to the central tropical Pacific in November and December. This result suggests that the assist by the background low-level flow can be important for the MJO to overcome the barrier effects and pass over the MC during November and December. On the other hand, we concluded that the southward shift of the MJO track dominantly affects the eastward propagation of the MJO in January and February because there are no significant differences in the background low-level flow between the two cases.