Planetary- and Synoptic-Scale Influences on Eastern Pacific Tropical Cyclogenesis
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The structure and evolution of lowpass-filtered background flow and synoptic-scale easterly waves were examined during the 1991 eastern Pacific hurricane season. Active and inactive cyclogenesis periods conformed well to the sign of the near-equatorial, lowpass-filtered, 850-mb zonal wind anomaly, consistent with the recent results of Maloney and Hartmann. This behavior emphasizes the importance of westerly wind bursts associated with the Madden–Julian oscillation (MJO) in creating an environment favorable for eastern Pacific tropical cyclogenesis. Synoptic-scale easterly waves reached the western Caribbean and eastern Pacific regularly from upstream, usually from Africa. The amplitude of waves leaving Africa had little correlation with the likelihood of a wave producing an eastern Pacific storm. Rather, easterly waves intensified, and tropical depressions formed, during the convectively active phase of the MJO in the western Caribbean and eastern Pacific. Wave growth, measured by strengthening of convection within the waves, occurred in the regions of sign reversal of the meridional potential vorticity gradient found previously. For the 1991 season cyclogenesis occurs when westward-moving synoptic-scale waves amplify within the superclusters that represent the favorable MJO envelope. Analogously, waves existed but failed to grow during the unfavorable part of the MJO. During each active period of the MJO, the region of active convection moved eastward and northward with time in the eastern Pacific, with strongest convection reaching as far as the southwestern Gulf of Mexico by the end of such periods. The locations of tropical depression formation followed a similar path, shifting eastward with time following the MJO, and northward following the eastern Pacific intertropical convergence zone. The latter was defined by the locations of low-pass-filtered background vorticity maxima at 1000 mb. It is argued based on previous work in the literature that the western Pacific might behave similarly, with upstream easterly waves growing and producing depressions within the convectively active envelope of the MJO.Keywords:
Madden–Julian oscillation
Tropical cyclogenesis
Cyclogenesis
African easterly jet
Equatorial waves
Thirty four cases of topical cyclogenesis that occurred in boreal winter(November to February of next year) during 12-yr(1995—2006) have been analyzed by using the composite technique for further understanding of the larger-scale circulation features and the mechanism of the formation of tropical cyclone in the western north Pacific(WNP)during wintertime.It has been found that the easterly waves play a major role in the tropical cyclogenesis.The existence of low-level cross-equatorial tropical cyclone pairs is a remarkable feature in the formation of tropical cyclogenesis.The equatorial mixed Rossby-gravity(MRG) waves in lower troposphere propagated northwestward from the central Pacific.These waves developed into the TD-type disturbances in place where the strong convection overlaps,which provided disturbances for the tropical cyclogenesis.Further analysis in terms of eddy kinetic energy budgets for the TC(tropical cyclone) related with circulations indicates that the available potential energy of disturbances and the energy that converts barotropically from the mean kinetic energy facilitates the TC formation.These two kinds of energy are associated with cumulus convection heating and the asymmetric horizontal flow respectively.The barotropic energy conversion mainly occurs in the low and mid-troposphere while the available energy conversion locates in the mid and upper troposphere.The tropic easterly waves get barotropic instability energy from the mean flow and coupled with the deep convection,inducing the formation of TCs under the interaction of dynamics and thermodynamics.
Tropical cyclogenesis
Cyclogenesis
African easterly jet
Barotropic fluid
Monsoon trough
Madden–Julian oscillation
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We examined the Cyclone Global Navigation Satellite System (CYGNSS) retrievals of surface winds and enthalpy fluxes in African easterly waves that led to the formation of 31 Atlantic tropical cyclones from 2018–2021. Lag composites show a cyclonic proto-vortex as early as 3 days prior to tropical cyclogenesis. The distribution of enthalpy fluxes within the proto-vortex does not vary substantially prior to cyclogenesis, but subsequently, there is an increase in the upper extreme values. A negative radial gradient of enthalpy fluxes becomes apparent as early as 2 days before cyclogenesis. These results—based on a novel data blending satellite retrievals and global reanalysis—are consistent with recent studies that have found that tropical cyclone spin-up is associated with a shift of peak convection towards the vortex-core and a radially inward increase of enthalpy fluxes. They provide additional evidence for the importance of surface enthalpy fluxes and their radial structure for tropical cyclogenesis.
Tropical cyclogenesis
Cyclogenesis
African easterly jet
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Abstract Eastern Africa is a common region of African easterly wave (AEW) onset and AEW early life. How the large-scale environment over East Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared with nondeveloping AEWs. Using a 21-yr August–September (1990–2010) climatology of AEWs, differences in the large-scale environment between developers and nondevelopers are identified and are proposed to be used as key predictors of subsequent tropical cyclone (TC) formation and could inform tropical cyclogenesis prediction. TC precursors when compared with nondeveloping AEWs experience an anomalously active West African monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet. These large-scale conditions are linked to near-trough attributes of developing AEWs that favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, a deeper wave pouch, and arguably more AEW and mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or West Africa. Developing AEWs are more likely than nondeveloping AEWs to be southern-track AEWs.
African easterly jet
Tropical cyclogenesis
Cyclogenesis
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Tropical cyclogenesis
African easterly jet
Cyclogenesis
Tropical Atlantic
Atlantic hurricane
Extratropical cyclone
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Abstract This paper provides new information on the low-level (850 hPa) structure and behavior of African easterly waves (AEWs) and relates this information to previous studies. Individual AEWs that occurred during June–September of 2001 are studied by a synoptic approach that employs Hovmöller diagrams, wave track maps, and case studies. The focus is on two AEW regimes in the lower troposphere over North Africa: a dry regime to the north of the African easterly jet (AEJ) coincident with the surface position of the monsoon trough near 20°N, and a wet regime to the south of the jet coincident with the near-equatorial rainbelt near 10°N. The following issues are addressed: the origin of the waves seen in the two wave regimes, relation of the wave activity to the mean positions of the surface monsoon trough and the 600–700-hPa AEJ, collocation of the tracks of the two wave regimes off the African coast, and diversity in low-level wave behavior that includes merging, splitting, and dissipation of the cyclonic vorticity centers associated with the wave troughs. The relationship between the waves following the two tracks is examined as well as the relationship between the low-level wave activity and Atlantic tropical cyclogenesis in 2001. It is shown that the two wave regimes can interact, and that both regimes were instrumental in Atlantic tropical cyclogenesis in 2001.
African easterly jet
Tropical cyclogenesis
Cyclogenesis
Monsoon trough
Trough (economics)
Tropical Atlantic
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Abstract In this study, a wave-following Lagrangian framework was used to examine the evolution of tropical easterly wave structure, circulation, and convection in the days leading up to and including tropical cyclogenesis in the Atlantic and east Pacific basins. After easterly waves were separated into northerly, southerly, trough, and ridge phases using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis 700-hPa meridional wind, waves that developed a tropical cyclone [developing waves (DWs)] and waves that never developed a cyclone [nondeveloping waves (NDWs)] were identified. Day zero (D0) was defined as the day on which a tropical depression was identified for DWs or the day the waves achieved maximum 850-hPa vorticity for NDWs. Both waves types were then traced from five days prior to D0 (D − 5) through one day after D0. Results suggest that as genesis is approached for DWs, the coverage by convection and cold cloudiness (e.g., fractional coverage by infrared brightness temperatures ≤240 K) increases, while convective intensity (e.g., lightning flash rate) decreases. Therefore, the coverage by convection appears to be more important than the intensity of convection for tropical cyclogenesis. In contrast, convective coverage and intensity both increase from D − 5 to D0 for NDWs. Compared to NDWs, DWs are associated with significantly greater coverage by cold cloudiness, large-scale moisture throughout a deep layer, and large-scale, upper-level (~200 hPa) divergence, especially within the trough and southerly phases, suggesting that these parameters are most important for cyclogenesis and for distinguishing DWs from NDWs.
Tropical cyclogenesis
Cyclogenesis
African easterly jet
Trough (economics)
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Abstract African easterly waves (AEWs) are objectively tracked between West Africa and the tropical Atlantic based on the CFSRv2 data for 1979 to 2012. The characteristics of the troughs of the AEWs at the West African coast are explored and related to whether they favor tropical cyclogenesis over the eastern Atlantic. A logistic regression model was used to determine the optimum combination of predictors that relate AEW characteristics to tropical cyclogenesis. The most skillful model for genesis over the eastern Atlantic consisted of four variables of the AEWs dynamics over the coastal region and the absolute number of days from the peak in the AEW season. Using this diagnostic an equal number of favorable developing and nondeveloping waves were compared through a composite difference analysis. Favorable developing waves had significantly higher moisture content in the lower troposphere to the northwest of the trough as they exited the West African coast compared to favorable nondeveloping waves. Trajectory analysis for all the waves revealed that as the AEWs transition over the West African coast the troughs are typically open to the environment ahead and to the northwest of the trough. For developing waves this means that moist air is ingested into the lower levels of the system, while for nondeveloping waves dry air is ingested. At this point in the AEW life cycle this difference may be fundamental in determining whether a favorable wave can develop or not.
African easterly jet
Tropical cyclogenesis
Cyclogenesis
Trough (economics)
Tropical Atlantic
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Abstract We examined the Cyclone Global Navigation Satellite System (CYGNSS) retrievals of surface wind speeds and enthalpy fluxes in African easterly waves that led to the formation of 30 Atlantic tropical cyclones during 2018–2021. Lag composites show a cyclonic proto‐vortex as early as 3 days prior to tropical cyclogenesis. The enthalpy flux distribution does not vary substantially before cyclogenesis, but subsequently, there is a marked increase in the extreme upper values. In the composites, a negative radial gradient of enthalpy fluxes becomes apparent 2–3 days before cyclogenesis. These results—based on novel data blending satellite retrievals and global reanalysis—support the findings from recent studies that the spin‐up of tropical cyclones is associated with a shift of peak convection toward the vortex core and an inward increase of enthalpy fluxes.
Tropical cyclogenesis
African easterly jet
Cyclogenesis
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Abstract Over one hundred years of vigorous progress in tropical cyclone (TC) research, the genesis of the cyclone (hereafter, tropical cyclogenesis) is remarkable as a doubtful subject. Furthermore, predicting tropical cyclogenesis, particularly in the lesser latitude, remains a significant challenge. Therefore, understanding the complex interactions in developing tropical cyclogenesis over the region is vital to improving tropical cyclogenesis forecasting. Hence, the Indonesia Maritime Continent is a tropical cyclone-free region due to decreasing the Coriolis effect. However, Seroja TC hits Flores (8.6 o S, 120 o E), east Nusa Tenggara, Indonesia, on 4 April 2021, and recorded as the first TC occurred over the mainland, which brought a catastrophic disaster in the region. This study investigated the tropical cyclogenesis of Seroja by using both observational and numerical studies. The results indicate that a marine heatwave and double vortices were favorable conditions produced preconditions to developing tropical cyclogenesis over the Maluku Sea. Thus, tropical cyclogenesis is formed by the breakdown of the intertropical convergence zone (ITCZ) associated with synoptic-scale wave train driven under the interaction of the Madden Julian oscillation (MJO) and equatorial Rossby waves. Moreover, our finding suggested that an extensive background cyclonic vorticity under the cold pool mechanisms is responsible for maintaining tropical cyclogenesis into a persistent Seroja TC.
Tropical cyclogenesis
Cyclogenesis
African easterly jet
Intertropical Convergence Zone
Madden–Julian oscillation
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