3.2.2 The Influence of ENSO and the MJO on Cutoff Cyclone Frequency
In comparing the frequency of 500-hPa cutoff cyclones in the Northeast US by phase of the MJO for ENSO warming and cooling, there is a discernable difference. During ENSO cooling, the frequency peaks at 11 cutoff cyclones during phases 3 and 4 of the MJO (Fig. 3.9), when enhanced convection is located over the eastern Indian Ocean or the Maritime Continent (Figs. 3.7c,d). In addition, there is a distinct minimum in cutoff cyclones, with only one cutoff cyclone, during ENSO cooling and phase 7 of the MJO, when enhanced convection is entering the Western Hemisphere (Fig. 3.4g). The distribution of cutoff cyclone frequency by MJO phase during ENSO warming appears to be reversed from that of ENSO cooling. During ENSO warming, there is a maximum in cutoff cyclone occurrence during phase 8 of the MJO, with 14 cutoff cyclones, and a minimum in phase 4, with only 2 cutoff cyclones.
The cutoff cyclone frequency distribution by MJO phase for ENSO warming agrees reasonably well with the 500-hPa geopotential height anomaly composites. The peak in cutoff cyclones during ENSO warming and phase 8 of the MJO coincides with strong negative 500-hPa geopotential height anomalies over the eastern US (Fig. 3.8h). In addition, the minimum in cutoff cyclones during ENSO warming and phase 4 of the MJO agrees with the presence of strong positive 500-hPa geopotential height anomalies over the Northeast US (Fig. 3.8d); however, this agreement is likely related to the very small number of cutoff cyclone days (n=2) included in the composite for these conditions. In contrast, the cutoff cyclone frequency distribution by MJO phase for ENSO cooling does not agree well with the composites of 500-hPa geopotential height anomalies. While there is a minimum in cutoff cyclones for ENSO cooling and phase 7 of the MJO, the composite for this phase indicates a large negative geopotential height anomaly over eastern Canada extending into the Northeast US, suggesting favorable conditions for cutoff cyclone development (Fig. 3.7g). Similarly, the peak in cutoff cyclones during ENSO cooling and phases 3 and 4 of the MJO disagrees with the positive 500-hPa geopotential height anomalies over the Northeast US in the composite for these phases (Fig. 3.7c,d).
