Propagation Characteristics of BSISO Indices

Abstract

The spatial structure and propagation of tropical intraseasonal convection anomalies diagnosed with the outgoing longwave radiation-based Madden-Julian Oscillation index are examined in the boreal summer and winter seasons. It is shown that the outgoing longwave radiation-based Madden-Julian Oscillation index represents both northward and eastward propagation in summer and eastward propagation in winter in a manner consistent with Madden-Julian Oscillation and Boreal Summer Intraseasonal Oscillation propagation as diagnosed in many previous studies. The outgoing longwave radiation-based Madden-Julian Oscillation index and three other widely used indices for tracking the Boreal Summer Intraseasonal Oscillation are then compared in their lag correlation structure over selected reference areas, cross-correlation coefficients of the two principal component time series, and time-dependent phase angle composites. The outgoing longwave radiation anomalies from these different indices propagate differently according to these diagnostics. One of them exhibits little propagation at all, even though one would expect good propagation based on composites of the phases separately. This illustrates the general point that while composites of individual phases, presented in a sequence, are generally taken to imply that the phases tend to occur in that sequence in time, that need not be the case. It is suggested that propagation characteristics are relevant to the application of the indices and that smoother propagation is desirable. Plain Language SummaryThe tropical intraseasonal oscillation is a large-scale pattern of cloud and circulation in the equatorial regions recurring every 30 to 60 days. The pattern propagates both northward and eastward in the northern summer season, and brings a large amount of rain and disturbed weather along its path. To track its movement, scientists have developed simple indices using winds and the energy radiating from the Earth as seen from satellites. Here we examine how well these indices accurately track the oscillation by looking into a large number of historical events, and show that our diagnostics may further help improve these indices. Key Points:• Lag correlations of the reconstructed OLR anomalies reveal that OMI represents northward and eastward propagation in boreal summer • Intercomparison of the OLR anomalies from OMI, RMM, and two BSISO indices shows that they differ significantly in representing zonal and meridional propagation • Phase composite on the intraseasonal indices is not reliable in estimating propagation Supporting Information:• Supporting Information S1

Characterization of Intraseasonal Kelvin Waves in the Equatorial Pacific Ocean

“…To ensure that the IKW index appropriately represents the eastward propagation of intraseasonal SSH anomalies associated with IKWs, an additional lag correlation analysis is performed as in Wang et al (). The lag correlation is referenced to SSH anomalies averaged from 2°S–2°N and 160–150°W that are filtered to retain eastward propagating (wave numbers ≥ 0) and intraseasonal (30–120‐day band‐pass) filtered variability.…”

Intraseasonal Variability of the Diurnal Cycle of Precipitation in the Philippines

“…However, this index still captures about 3-6% of variance in precipitation. Its nature as a smoothed index probably results in explaining less of the variance in real-time variables without any filtering, but it likely captures propagating ISO events better than its real-time counterparts (S. Wang et al 2018). Had variance explained been calculated as percent of total intraseasonal variance explained by the index, this index would likely perform dramatically better.…”

Evidence of Specific MJO Phase Occurrence with Summertime California Central Valley Extreme Hot Weather

“…Both negative and positive OLR (convection and subsidence) over the Indian Ocean to western Pacific region have a northwest to southeast elongation. The shift and shape of the OLR signal makes sense because northward propagation around Southeast Asia is a primary feature of the summertime MJO (Kiladis et al, 2014;Wang et al, 2018). After MJO phase-pair 8-1, the main convective signal still remains inside the purple box (the tropical eastern Pacific).…”