Identification and characterization of an atmospheric blocking event over the South Pacific from August 31 to September 05, 2019

<p>Atmospheric circulation in mid-latitudes is characterized by a westerlies zonal flow. On blocking conditions, this flow is interrupted by a large almost-stationary anticyclone. This situation, there is a splitting of the jet stream, what modify zonal flow pattern and change the normal eastward displacement of transients. There are two blocking types frequently observed in South Hemisphere (SH): dipole type blocking – occurs when a cut-off low is located north of the anticyclone, which characterize a dipole; omega type blocking – occurs when there is an arrangement of two cut-off lows and the blocking high like Greek letter Ω (omega, inverted in SH). First, the subjective methods were created to identify these systems, later, aiming at numerical modeling, the objective methods, called zonal index, were created. Thus, the purpose of this study was to identify, through subjective and objective methods, a blocking system that occurred over South Pacific, on the west coast of South America, from August 31 to September 05, 2019. In this study, surface synoptic chart from Navy Hydrography Center (NHC) and images from Geostationary Operational Environmental Satellite (GOES-16) in channel 13 (infrared) were used. In addition, data from Era5 reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF), with a horizontal resolution of 0.25°, were used to elaborate meteorological fields and zonal index calculation. The identification criteria proposed by Casarin and Kousky (1982) were used for subjective analysis, and the Lejeñas (1984) for objective one. The analyzed fields indicate that the system had persisted for six days. In this period, the flow was split, the blocking high didn’t move more 25º of longitude and the zonal index remained negative, what satisfied all criteria used. Therefore, this event was characterized as atmospheric blocking of dipole kind.</p>

Modified, threshold-based circulation type classification for Central Europe, on the basis of Lityński’s classification

There are many classifications of atmospheric circulation types. In Poland, the most important one, used by Polish weather services, is the classification of circulation types developed by Lityński. This paper proposes four modifications to the currently used algorithm for determining atmospheric circulation types. The proposed algorithms have been compared and it has been shown which one produces a catalogue of circulation types in which the division of the distribution of the values of the three indices (the zonal index Ws, meridional index Wp and cyclonicity index Cp) into three classes is the closest to being equally likely. In 1986-2015, the classification scheme that deemed to be the best, differed in above 19% of cases from the currently used classification.

Plant Diversity of The Mid Silurian (Lower Wenlock, Sheinwoodian) Terrestrial Vegetation Preserved in Marine Sediments from The Barrandian Area, The Czech Republic

Plant mega- and microfossils are described from the middle Sheinwoodian of the Barrandian area. The material comes from the Loděnice locality and the same horizon as the earliest unequivocal land plant, Cooksonia barrandei LIBERTÍN, J.KVAČEK, BEK, ŽÁRSKÝ et ŠTORCH. Its age (432 Myr) is inferred from the associated graptolite fauna, including the zonal index graptolite Monograptus belophorus. Megafossils have clear similarity with Cooksonia, due to their dichotomised axes with slightly widened subtending axes bearing putative sporangia. They document some of the plant diversity that was in place when the first proven representative of the genus Cooksonia appeared, and together with dispersed spores they provide strong and important evidence that a diversified terrestrial ecosystem had developed on the Barrandian volcanic archipelago in the peri-Gondwanan realm by the end of the Sheinwoodian Stage of the Silurian Period.

First record of the Mediterranean zonal index Mesosimoceras cavouri (Gemmellaro, 1872) in the Upper Jurassic (Pseudomutabilis Zone, semicostatum γ horizon) of SW Germany and its stratigraphical significance

The ammonite species Mesosimoceras cavouri (Gemmellaro), the index of the Mediterranean Cavouri Zone, is recorded for the first time from the Submediterranean Upper Jurassic of SW Germany. It occurs in the Untere-Felsenkalke Formation, within a rich ammonite association of the late Pseudomutabilis Zone characterized by fine-ribbed variants of Aulacostephanus pseudomutabilis (de Loriol) and a still undescribed flexuous-ribbed variant of Aulacostephanus eudoxus (d’Orbigny). This ammonite association represents the here newly introduced Lingulaticeras semicostatum γ horizon. The record of Mesosimoceras cavouri (Gemmellaro) confirms at least a partial correlation of the Mediterranean Cavouri Zone with the Submediterranen Pseudomutabilis Zone and the Subboreal Eudoxus Zone.

Diabatic Damping of Zonal Index Variations

Zonal index variations, or north–south shifts of the midlatitude jet, are the dominant mode of zonal wind variability in the Southern Hemisphere. Previous studies have shown that synoptic-time-scale eddy momentum flux provides a positive feedback and acts to increase the persistence and low-frequency variance of the zonal index. However, the impact of diabatic heating due to the precipitation associated with these eddies has not been investigated. In this study, regression analyses have been conducted to demonstrate that a robust precipitation anomaly can be found to accompany the jet and eddy momentum flux anomalies associated with a poleward shift of the jet, with enhanced precipitation on the poleward flank of the jet and reduced precipitation on the equatorward flank. Diabatic heating associated with such a precipitation anomaly is expected to reduce the temperature gradient across the jet anomaly, thus decreasing eddy generation and damping the anomaly. This expectation is confirmed by three sets of mechanistic model experiments, using three different ways to mimic the impact of moist heating in a dry model. Results of this study suggest that diabatic heating provides a negative feedback to zonal index variations, partially offsetting the positive feedback provided by eddy momentum flux. These results could partially explain why zonal index variations have been found to be very persistent in dry mechanistic model experiments since this negative diabatic feedback is absent in dry models. These results suggest that these models may be overly sensitive to climate forcings that produce a jet shift response.