Impact of the Boreal Summer Quasi-biweekly Oscillation on Eastern North Pacific Tropical Cyclone Activity

Several studies have focused on the impact of the Madden-Julian Oscillation (MJO, with a period between 30-60 days) on boreal summer tropical cyclone (TC) activity over the Eastern North Pacific (ENP) basin. The quasi-biweekly oscillation (QBWO), with a period of about 20 days, is another dominant mode of intra-seasonal variability during the boreal summer over the ENP basin. There have been fewer studies focused on the influence of the QBWO on tropical cyclogenesis over the ENP basin. The exploratory analyses performed in this study suggest that the QBWO has a strong impact on boreal summer ENP TC activity, with enhanced TC activity during the convectively active phase.

The significant increase (decrease) of tropical cyclogenesis events during the convectively active (inactive) QBWO phase is found to be closely associated with the strengthening (weakening) of the low-level cyclonic circulation and increasing (decreasing) mid-level relative humidity (vertical wind shear) over the ENP. Mid-level relative humidity and low-level vorticity are found to be the two most important players in modulating TC genesis location and frequency, based upon the analyses of the anomalous genesis potential index (GPI) pattern and its magnitude associated with the QBWO. Associated with changes in TC location and large-scale steering flows, distinct difference in TC tracks during different QBWO phases can be readily found and thus cause substantial differences in basin-wide TC intensity. This study enhances our understanding of the modulation of intra-seasonal oscillations on boreal summer ENP TC activity and has the potential to aid in the sub-seasonal prediction of ENP TC activity.