Distinct Propagation Characteristics of Intraseasonal Variability Over the Tropical West Pacific

Tropical intraseasonal variability, with the Madden-Julian oscillation (MJO) as its most prominent mode, exerts extensive influences on global weather extremes. It is found that strong interannual variability of intraseasonal convection exists in the west Pacific (WP), in the form of years with strong eastward propagation (i.e., associated with the MJO) and years with strong westward propagation. Years with strong westward propagation on intraseasonal timescales are dominated by a westward propagating intraseasonal mode (WPIM), which is the second leading intraseasonal mode after the MJO over the tropical WP. Initiated over the central Pacific, the WPIM exhibits slow equatorial westward propagation (5m/s) with a period of 25days and a spatial scale of zonal wave number 3-4. Unlike the MJO, the WPIM lacks a significant tilt with height in specific humidity and vertical velocity. A strong anticorrelation is found between MJO and WPIM activity on interannual timescales over the WP. Budget analyses of the moist static energy suggest that both modes are driven by horizontal moist static energy advection and that substantial differences in winter mean large-scale moisture and zonal winds largely define their distinct propagation behaviors. The WPIM is favored, while the MJO is suppressed when mean equatorial low-level easterlies between 150 degrees E and 160 degrees W are enhanced and equatorial mean low-level moisture is reduced near the Dateline and enhanced in the off-equatorial WP (110 degrees-150 degrees E). While the WPIM bears resemblance to low-frequency equatorial Rossby waves, a more detailed analysis must be conducted to affirm if they are the same phenomenon.