The influence of the Madden Julian Oscillation (MJO) on rainfall in Sri Lanka (SL) is examined based on 30 years of daily station data from 1981-2010. Composites are constructed for each of the eight phases of the MJO defined with the Real-time Multivariate MJO (RMM) index, using daily rainfall data from 44 stations over SL for four climatic seasons and comparing to similar results from a satellite-based rainfall product. Composites of lower tropospheric wind and convective anomaly are also investigated in order to examine how the local rainfall anomalies are associated with large-scale circulations. The greatest impact of the MJO on rainfall over SL occurs in the Second Inter-Monsoon (SIM) and Southwest Monsoon (SWM) seasons. Enhanced rainfall generally occurs over SL during RMM phases 2 and 3 when the MJO convective envelop is located in the Indian Ocean and conversely suppressed rainfall in phases 6 and 7. This rainfall impact is due to the direct influence of the MJO’s tropical convective anomalies and associated low-level circulations in the vicinity of SL. In contrast, the MJO influence during the Northeast Monsoon (NEM) season is slightly less than during the SWM and SIM seasons as a result of the southward shift of the MJO convective envelop during boreal winter. Occurrence of extreme rainfall events is most frequent during phase 2 in First Inter-Monsoon (FIM) phases 2 and 3 in SWM, phases 1, 2 and 3 in SIM and phases 2 and 3 in NEM seasons. The analysis of this study provides a useful reference of when and where the MJO has significant impacts on rainfall as well as extreme rainfall events during four climatic seasons in SL. This information can be used along with accurately predicted MJO phase by dynamical or statistical models, to improve extended range forecasting in SL.
Man has lived in harmony with nature adapting to its changes. Science has enabled him to get a better understanding of nature and how it behaves. Scientists have studied how and why changes take place in natural phenomena such as weather and climate. Based on the results we have further adapted to the environment around us.
Outgoing long‐wave radiation (OLR), objectively analyzed daily winds (u, v) and geopotential height data for 5 northern winters are used to study the relationship between low‐level equatorward (meridional) winds and tropical convection on the 30–60 day time scale. The general tendency of equatorial 30–60 day OLR perturbations is to propagate eastward. Occasionally, these perturbations exhibit irregular movement. This study is limited to the periods when the eastward propagation is clearly defined. Correlation maps indicate that the low‐frequency modes are of global‐scale character with zonal wavenumber one. The low‐level 30–60 day northerly wind along the eastern edge of the Siberian high (anomaly) does not contribute to convection and monsoon activity in the Indonesian‐northern Australian region. However, 30–60 day northeasterly flows of subtropical Pacific origin play a significant rôle in monsoon activity. An active monsoon condition with strong upper‐level southerly outflow from the monsoon region and an intense westerly jet near Japan, is observed approximately 10 days prior to the maximum low‐level northerly currents from Siberia. Fluctuations with periods less than 30 days are considered as transient disturbances. The square of transient meridional winds shows pronounced spectral peaks in the 30–60 day period range. This is associated with an amplitude modulation of synoptic‐scale transient disturbances, which is most pronounced in both the NH and SH extratropics. Perhaps, transient disturbances, as a group, can make a contribution toward the enhancement and/or maintenance of extratropical low‐frequency perturbations.
Outgoing long-wave radiation (OLR), objectively analyzed daily winds (u, u) and geopotential height data for 5 northern winters are used to study the relationship between low-level equatorward (meridional) winds and tropical convection on the 30-60 day time scale. The general tendency of equatorial 30-60 day OLR perturbations is to propagate eastward. Occasionally, these perturbations exhibit irregular movement. This study is limited to the periods when the eastward propagation is clearly defined.Correlation maps indicate that the low-frequency modes are of global-scale character with zonal wavenumber one. The low-level 3&60 day northerly wind along the eastern edge of the Siberian high (anomaly) does not contribute to convection and monsoon activity in the Indonesian-northem Australian region. However, 30-60 day northeasterly flows of subtropical Pacific origin play a significant r6le in monsoon actibity. An active monsoon condition with strong upper-level southerly outflow from the monsoon region and an intense westerly jet near Japan, is observed approximately 10 days prior to the maximum low-level northerly currents from Siberia.Fluctuations with periods less than 30 days are considered as transient disturbances. The square of transient meridional winds shows pronounced spectral peaks in the 30--60 day period range. This is associated with an amplitude modulation of synoptic-scale transient disturbances, which is most pronounced in both the NH and SH extratropics. Perhaps, transient disturbances, as a group, can make a contribution toward the enhancement and/or maintenance of extratropical low-frequency perturbations.
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