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Kyle S. Griffin
and
Jonathan E. Martin

Abstract

Time-extended EOF (TE-EOF) analysis is employed to examine the synoptic-scale evolution of the two leading modes of the North Pacific jet stream variability, namely, its zonal extension–retraction (TE-EOF 1) and the north–south shift of its exit region (TE-EOF 2). Use of the TE-EOF analysis enables a temporally coherent examination of the synoptic-scale evolution preceding and following peaks in each of the two leading modes that provides insight into the preferred evolutions of the North Pacific jet.

Composite analyses are constructed based upon selecting peaks in the principal component time series of both phases of each TE-EOF whose magnitude exceeded 1.5 standard deviations. Jet extension events are associated with an anomalous cyclonic circulation over the Gulf of Alaska that induces a low-level warm anomaly over western North America. Jet retractions are associated with a nearly opposite configuration characterized by an anomalous anticyclonic circulation over the Aleutians and anomalous low-level cold anomaly over western North America. Similar but lower-amplitude upper-level patterns are noted in the composites of the corresponding poleward-/equatorward-shifted jet phases, with the poleward shift of the jet exit region tied to anomalously low geopotential heights over Alaska and anomalous low-level warmth over north-central North America. An equatorward shift of the exit region is tied to positive height anomalies over Alaska with downstream cold anomalies occurring in western North America. The more extreme downstream impacts that characterize TE-EOF 2 are also longer lasting (>5 days), suggesting potential utility in medium-range forecasts.

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Kyle S. Griffin
and
Lance F. Bosart

Abstract

Documentation of southwest Indian Ocean (SWIO) tropical cyclones (TCs) and extratropical transition (ET) events is sparse in the refereed literature. The authors present a climatology of SWIO TC and ET events for 1989–2013. The SWIO averages ~9 tropical cyclones (TCs) per year in this modern era. Of these TCs, ~44% undergo extratropical transition (ET), or ~four per year. A case study of TC Edisoana (1990), the most rapidly intensifying SWIO post-ET TC between 1989 and 2013, shows that extratropical interactions began when an approaching trough embedded in the subtropical jet stream (STJ) induced ET on 7 March. As Edisoana underwent ET, a subtropical ridge downstream amplified in response to poleward-directed positive potential vorticity (PV) advection associated with diabatically (convectively) driven upper-level outflow from TC Edisoana. This amplifying lower-latitude ridge phased with a lower-amplitude higher-latitude ridge embedded in the polar front jet (PFJ), resulting in the merger of the two jets. This ridge phasing and jet merger, combined with the approach of an upstream trough embedded in the PFJ, resulted in a decrease in the half-wavelength between the approaching trough and the downstream phased ridges and provided extratropical cyclone Edisoana with a prime environment for rapid reintensification (RI). Poleward-directed positive PV advection into the phased ridge strengthened the upper-level jet downstream of Edisoana, which provided the primary baroclinic forcing throughout the RI phase. A backward trajectory analysis suggests that strong diabatic heating enhanced favorable synoptic-scale forcing for ascent from the upstream and downstream jet streaks and played a crucial role in the deepening of Edisoana through the ET and RI periods.

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Sarah A. Monette
,
Christopher S. Velden
,
Kyle S. Griffin
, and
Christopher M. Rozoff

Abstract

A geostationary satellite–derived cloud product that is based on a tropical-overshooting-top (TOT) detection algorithm is described for applications over tropical oceans. TOTs are identified using a modified version of a midlatitude overshooting-top detection algorithm developed for severe-weather applications. The algorithm is applied to identify TOT activity associated with Atlantic Ocean tropical cyclones (TCs). The detected TOTs can serve as a proxy for “hot towers,” which represent intense convection with possible links to TC rapid intensification (RI). The purpose of this study is to describe the adaptation of the midlatitude overshooting-top detection algorithm to the tropics and to provide an initial exploration of possible correlations between TOT trends in developing TCs and subsequent RI. This is followed by a cursory examination of the TOT parameter’s potential as a predictor of RI both on its own and in multiparameter RI forecast schemes. RI forecast skill potential is investigated by examining empirical thresholds of TOT activity and trends within prescribed radii of a large sample of developing North Atlantic TC centers. An independent test on Atlantic TCs in 2006–07 reveals that an empirically based TOT scheme has potential as a predictor for RI occurring in the subsequent 24 h, especially for RI maximum wind thresholds of 25 and 30 kt (24 h)−1 (1 kt ≈ 0.5 m s−1). As expected, the stand-alone TOT-based RI scheme is comparatively less accurate than existing objective multiparameter RI prediction methods. A preliminary experiment that adds TOT-based predictors to an objective logistic regression-based scheme is shown to improve slightly the forecast skill of RI, however.

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