Mechanisms Relating the Madden-Julian Oscillation (MJO) to Intraseasonal Variability of the Surface Climate in the Americas

This study uses NCEP's Climate Forecast System Reanalysis (CFSR) to revisit the dynamical mechanisms associated with the Madden-Julian Oscillation (MJO) that affect the surface climate over the Americas. During the boreal winter North America sees a notable eastward propagation in temperature anomalies through the MJO evolution. With smaller land area, South America sees a weaker (but still well-defined) connection to MJO convection during the austral winter. These temperature structures are related to the MJO enhanced tropical convection that can induce upper-tropospheric cyclonic circulation anomalies on the leading edge and anti-cyclonic anomalous circulations on the trailing edge. During each hemisphere's winter season, MJO convection can modulate Rossby wave sources and emanating wave trains, as indicated in wave activity fluxes, propagating into the extratropics to induce near-surface temperature and precipitation anomalies. Wintertime precipitation anomalies can also be connected to shifts in the prevailing storm track of the given region. Storm track proxies indicate a poleward shift associated with the passing of suppressed convection and an equatorward shift or enhancement of the subtropical storm track when the enhanced convection is over the western hemisphere. During the corresponding summer seasons, the diagnostics show that precipitation patterns are more directly connected to the convection anomalies rather than the circulation anomalies. The Americas see enhanced precipitation in the monsoon regions and under the inter-tropical convergence zone when the large scale upper-tropospheric divergent anomalies associated with the MJO convection pass over the western hemisphere. Similarly, precipitation is suppressed in these regions when the large scale upper-tropospheric convergence passes over the continents.