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Kevin C. Prince and Clark Evans

Abstract

Cold surges represent one of several phenomena by which midlatitude features can modulate the atmosphere, both dynamically and thermodynamically, deep into the tropics. This study involves the construction of a climatology of the strongest South American cold surges that follow along the Andes Mountains to quantify the extent to which these surges modulate the atmosphere from the midlatitudes to the tropics. Cold surges occurring during June–September (austral winter) from 1980 to 2017 are considered. In this study, cold-surge events are identified using standardized anomalies of 925-hPa meridional wind and 925-hPa temperature. As compared with previous cold-surge investigations, the use of standardized anomalies better enables spatial variation in cold-surge intensity and impacts to be quantified. A strong cold surge is defined as one in which the 925-hPa temperature is at least 3 standardized anomalies below 0 and the 925-hPa meridional wind is at least 3 standardized anomalies above 0 on the meso-α scale or larger. Using these criteria, 67 events are identified. The composite cold surge is characterized by highly anomalous cold, southerly flow that originates in northern Argentina and progresses northward, significantly modulating lower-tropospheric kinematic and thermodynamic fields across the entire Amazon basin over a period of 2 to as many as 8 days.

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Kevin C. Prince and Clark Evans

Abstract

While it is understood that a recurving tropical cyclone (TC) that interacts with the midlatitude flow can cause large changes to the midlatitude flow pattern, it is much less understood if, and how, such events could impact a downstream tropical cyclone. Here, an indirect TC interaction is defined as one in which a primary TC perturbs the downstream midlatitude waveguide within one synoptic-scale wavelength of a secondary TC. In this study, a climatology and composite analysis using ERA-Interim reanalysis data is completed for all indirect interactions occurring between two tropical and/or subtropical cyclones in the North Atlantic and western North Pacific basins between 1989 and 2018. In all, 26 cases are identified in the North Atlantic and 56 cases are identified in the western North Pacific. The composite-mean interaction between a primary TC and upstream trough amplifies the immediate downstream ridge, increasing the tropospheric-deep vertical wind shear on its poleward and, in the western North Pacific, eastern, and equatorward flanks. An amplified downstream trough is detectable farther downstream in the western North Pacific 1–2 days after interaction onset; however, the same is not true in the North Atlantic, in which some cases exhibit anticyclonic Rossby wave breaking of the immediate downstream ridge. Secondary TCs that weaken following the indirect-interaction events are primarily located along the gradient between the downstream ridge and trough (North Atlantic) or at high latitudes (western North Pacific); those that strengthen are primarily located equatorward of the downstream ridge, particularly in the western North Pacific.

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Kevin C. Prince and Clark Evans

Abstract

A tropical cyclone (TC) that recurves into the midlatitudes can lead to significant downstream flow amplification by way of a favorable interaction with the midlatitude waveguide. Current conceptualizations emphasize the role of the meso-α- to synoptic-scale diabatically enhanced vertical redistribution of potential vorticity in facilitating downstream flow amplification following the interaction of a TC with the midlatitude waveguide. Less understood, however, is the extent to which this downstream flow amplification may be facilitated by the convective-scale diabatically enhanced horizontal redistribution of potential vorticity. Consequently, this study aims to diagnose the role that deep, moist convection in an associated predecessor rain event north of the TC played in influencing the midlatitude waveguide and potentially the downstream evolution.

A convection-allowing numerical simulation is performed on a predecessor rain event which precedes the interaction of North Atlantic TC Irma in September 2017 with the midlatitude waveguide. Horizontal gradients in microphysical heating result in intense convective-scale potential vorticity dipoles aligned perpendicular to the vertical wind shear vector, with the negative anomaly poleward (and thus closer to the midlatitude waveguide) of the large-scale southwesterly vertical wind shear vector. Regions of intensely negative potential vorticity persist for multiple hours after their formation as they become deformed by the large-scale strain field that is aligned parallel to the background vertical wind shear vector. The deformation-driven thinning of the negative potential-vorticity is associated with a transfer of energy to the large-scale flow, suggesting a non-negligible impact to the TC-midlatitude waveguide interaction by the collection of convective cells embedded in the predecessor rain event.

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