The MJO and Northern Hemisphere Winter Blocking
MJO teleconnections have a significant influence on blocking. A blocking pattern occurs when a large-scale high pressure anomaly becomes stagnant and persist for several days or weeks. It is called a block because it "blocks" the westerly flow, redirecting precipitation and air masses. Due to its persistence and quasi-stationary nature, blocks are associated with weather extremes such as sudden and persistent cold snaps, drought, heat waves, and flooding.
In Henderson et al. (2016), we investigated how blocking frequency in the North Pacific, North Atlantic, and European regions are impacted by MJO teleconnections during winter, when Northern Hemisphere blocking occurs more frequently and the MJO is the most active.
In Henderson et al. (2016), we investigated how blocking frequency in the North Pacific, North Atlantic, and European regions are impacted by MJO teleconnections during winter, when Northern Hemisphere blocking occurs more frequently and the MJO is the most active.
(click figure for high resolution image) Figure: Blocking frequency (%) averaged between 40 and 60N for the eight phases of the MJO. Blocking frequencies are shown during an MJO phase (blue), and 5 (green), 10 (orange), and 15 (red) days after each MJO phase. The mean Dec-Feb blocking frequency is overlaid in black in all panels for reference. Asterisks highlight those regions found to be 95% significantly different than the mean. Each region is color dotted as defined in the legend below for reference. Adapted from Henderson et al. (2016) |
MJO impacts on blocking during ENSO events
In Henderson et al. (2018), we examine this relationship further by specifically focusing on MJO teleconnections during certain phases of the El Niño-Southern Oscillation (ENSO). ENSO alters the large-scale background flow as well as MJO convection, thereby modifying the characteristics of MJO teleconnections. Furthermore, ENSO and MJO teleconnections can act constructively or destructively, dampening or amplifying the circulation response, respectively. As a result, the MJO impact on blocking is also altered.
For example, when all winter seasons are considered (e.g. Henderson et al. 2016, discussed above), there is a significant increase in Atlantic blocking following MJO phase 7 (e.g. bottom left panel in above figure), with up to a doubling in blocking frequency relative to the mean. However, the significant increase in blocking frequency is almost entirely during warm ENSO events (up to 3x the winter mean!), with little impact during neutral and cold ENSO events (see figure below).
For example, when all winter seasons are considered (e.g. Henderson et al. 2016, discussed above), there is a significant increase in Atlantic blocking following MJO phase 7 (e.g. bottom left panel in above figure), with up to a doubling in blocking frequency relative to the mean. However, the significant increase in blocking frequency is almost entirely during warm ENSO events (up to 3x the winter mean!), with little impact during neutral and cold ENSO events (see figure below).
(click figure for high resolution image) Figure: Atlantic blocking frequency anomalies relative to the Dec-Feb mean during (top) neutral, (middle) cold, and (bottom) warm ENSO conditions (left to right) 5, 10, and 15 days after MJO phase 7. Dotting indicates anomalies found to be 95% significantly different from zero. Adapted from Henderson and Maloney (2018) |