Impact of different ENSO regimes on southwest pacific tropical cyclones
- Authors: Chand, Savin , McBride, John , Tory, Kevin , Wheeler, Matthew , Walsh, Kevin
- Date: 2013
- Type: Text , Journal article
- Relation: Journal of Climate Vol. 26, no. 2 (2013), p. 600-608
- Full Text: false
- Reviewed:
- Description: The influence of different types of ENSO on tropical cyclone (TC) interannual variability in the central southwest Pacific region (58-258S, 1708E-1708W) is investigated. Using empirical orthogonal function analysis and an agglomerative hierarchical clustering of early tropical cyclone season Pacific sea surface temperature, years are classified into four separate regimes (i.e., canonical El Niño, canonical La Niña, positive-neutral, and negative-neutral) for the period between 1970 and 2009.These regimes are found to have a large impact on TC genesis over the central southwest Pacific region. Both the canonical El Niño and the positive-neutral years have increased numbers of cyclones, with an average of 4.3 yr-1 for positive-neutral and 4 yr-1 for canonical El Niño. In contrast, during a La Niña and negative-neutral events, substantially fewer TCs (averages of ;2.2 and 2.4 yr-1, respectively) are observed in the central southwest Pacific. The enhancement of TC numbers in both canonical El Niño and positive-neutral years is associated with the extension of favorable low-level cyclonic relative vorticity, and low vertical wind shear eastward across the date line. Relative humidity and SST are also very conducive forgenesis in this region during canonical El Niño and positiveneutral events. The patterns are quite different, however, with the favorable conditions concentrated in the date line region for the positive-neutral, as compared with conditions farther eastward for the canonical El Niño regime. A significant result of the study is the demonstration that ENSO-neutralevents can be objectively clustered into two separate regimes, each with very different impacts on TCgenesis. © 2013 American Meteorological Society.
The importance of low-deformation vorticity in tropical cyclone formation
- Authors: Tory, Kevin , Dare, Richard , Davidson, Noel , McBride, John , Chand, Savin
- Date: 2013
- Type: Text , Journal article
- Relation: Atmospheric Chemistry and Physics Vol. 13, no. 4 (2013), p. 2115-2132
- Full Text: false
- Reviewed:
- Description: Studies of tropical cyclone (TC) formation from tropical waves have shown that TC formation requires a wave-relative quasi-closed circulation: the "marsupial pouch" concept. This results in a layerwise nearly contained region of atmosphere in which the modification of moisture, temperature and vorticity profiles by convective and boundary layer processes occurs undisturbed. The pouch concept is further developed in this paper. TCs develop near the centre of the pouch where the flow is in near solid body rotation. A reference-frame independent parameter is introduced that effectively measures the level of solid-body rotation in the lower troposphere. The parameter is the product of a normalized Okubo-Weiss parameter and absolute vorticity (OWZ). Using 20 yr of ERA-interim reanalysis data and the IBTrACS global TC database, it is shown 95% of TCs including, but not limited to, those forming in tropical waves are associated with enhanced levels of OWZ on both the 850 and 500 hPa pressure levels at the time of TC declaration, while 90% show enhanced OWZ for at least 24 h prior to declaration. This result prompts the question of whether the pouch concept extends beyond wave-type formation to all TC formations world-wide. Combining the OWZ with a low vertical shear requirement and lower troposphere relative humidity thresholds, an imminent genesis parameter is defined. The parameter includes only relatively large-scale fluid properties that are resolved by coarse grid model data (>150 km), which means it can be used as a TC detector for climate model applications. It is also useful as a cyclogenesis diagnostic in higher resolution models such as real-time global forecast models. © 2013 Author(s).