Anticyclonic Wavebreaking and the MJO

Abstract:

The Madden Julian Oscillation (MJO) is the dominant atmospheric circulation in the tropics on intraseasonal timescales. The MJO often couples with convection as it moves over the Indian Ocean and decouples as it leaves the west Pacific Ocean. The association between convection coupled to the MJO and the midlatitude flow pattern has been the primary focus of numerous recent studies. In particular, it has been shown that there is a statistical relationship between the position and strength of the MJO and anticyclonic wavebreaking (AWB). This study expands on this statistical relationship and examines the physical mechanisms by which the MJO is able to induce AWB.

We have constructed a climatology of AWB events during the boreal cold season from 1979 through 2010 over the North Pacific. The subset of the AWB events that are associated with the MJO is used as the basis of this study. Composite analyses will be shown to highlight the large-scale, high amplitude features associated with the MJO and AWB.

It will be shown that convection associated with the MJO and convectively coupled Kelvin Waves over the Indian Ocean creates middle tropospheric heating anomalies near the equator, which reduce upper-tropospheric PV across the low latitudes. This process tightens the pre-existing climatologically located meridional PV gradient at the periphery of the tropics. The meridional PV gradient is proportional to the Rossby wave restoring force, and as Rossby waves propagate eastward from the region of stronger to the region of weaker meridional PV gradient, they encounter an insufficient restoring force, causing them to break. The location of the breaking point depends on the strength and zonal extent of the meridional PV gradient as well as the strength and location of the MJO.

Since the MJO is often used as the basis of intraseasonal forecasts, it is important to understand how AWB can affect the relationship between the MJO and sensible weather over North America. It will be shown that MJO events with AWB over the North Pacific affect the temperature and precipitation patterns over North America differently than MJO events without AWB.