scholarly journals Potential Large-Scale Forcing Mechanisms Driving Enhanced North Atlantic Tropical Cyclone Activity since the Mid-1990s

2018 ◽  
Vol 31 (4) ◽  
pp. 1377-1397 ◽  
Author(s):  
Haikun Zhao ◽  
Xingyi Duan ◽  
G. B. Raga ◽  
Fengpeng Sun
Keyword(s):  
Tropical Cyclone ◽  
North Atlantic ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Low Level ◽  
The North ◽  
Recent Increase ◽  
Forcing Mechanisms ◽  
Interdecadal Changes

A significant increase of tropical cyclone (TC) frequency is observed over the North Atlantic (NATL) basin during the recent decades (1995–2014). In this study, the changes in large-scale controls of the NATL TC activity are compared between two periods, one before and one since 1995, when a regime change is observed. The results herein suggest that the significantly enhanced NATL TC frequency is related mainly to the combined effect of changes in the magnitudes of large-scale atmospheric and oceanic factors and their association with TC frequency. Interdecadal changes in the role of vertical wind shear and local sea surface temperatures (SSTs) over the NATL appear to be two important contributors to the recent increase of NATL TC frequency. Low-level vorticity plays a relatively weak role in the recent increase of TC frequency. These changes in the role of large-scale factors largely depend on interdecadal changes of tropical SST anomalies (SSTAs). Enhanced low-level westerlies to the east of the positive SSTAs have been observed over the tropical Atlantic since 1995, with a pattern nearly opposite to that seen before 1995. Moreover, the large-scale contributors to the NATL TC frequency increase since 1995 are likely related to both local and remote SSTAs. Quantification of the impacts of local and remote SSTAs on the increase of TC frequency over the NATL basin and the physical mechanisms require numerical simulations and further observational analyses.

scholarly journals Large-Scale Controls on Atlantic Tropical Cyclone Activity on Seasonal Time Scales

2016 ◽  
Vol 29 (18) ◽  
pp. 6727-6749 ◽  
Author(s):  
Young-Kwon Lim ◽  
Siegfried D. Schubert ◽  
Oreste Reale ◽  
Andrea M. Molod ◽  
Max J. Suarez ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
North Atlantic ◽  
General Circulation ◽  
Large Scale ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Circulation Model ◽  
The North ◽  
Predictive Skill ◽  
The North Atlantic

Abstract Interannual variations in seasonal tropical cyclone (TC) activity (e.g., genesis frequency and location, track pattern, and landfall) over the Atlantic are explored by employing observationally constrained simulations with the NASA Goddard Earth Observing System, version 5 (GEOS-5), atmospheric general circulation model. The climate modes investigated are El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Atlantic meridional mode (AMM). The results show that the NAO and AMM can strongly modify and even oppose the well-known ENSO impacts, like in 2005, when a strong positive AMM (associated with warm SSTs and a negative SLP anomaly over the western tropical Atlantic) led to a very active TC season with enhanced TC genesis over the Caribbean Sea and a number of landfalls over North America, under a neutral ENSO condition. On the other end, the weak TC activity during 2013 (characterized by weak negative Niño index) appears caused by a NAO-induced positive SLP anomaly with enhanced vertical wind shear over the tropical North Atlantic. During 2010, the combined impact of the three modes produced positive SST anomalies across the entire low-latitudinal Atlantic and a weaker subtropical high, leading to more early recurvers and thus fewer landfalls despite enhanced TC genesis. The study provides evidence that TC number and track are very sensitive to the relative phases and intensities of these three modes and not just to ENSO alone. Examination of seasonal predictability reveals that the predictive skill of the three modes is limited over tropics to subtropics, with the AMM having the highest predictability over the North Atlantic, followed by ENSO and NAO.

scholarly journals Observations of a Nondeveloping Tropical Disturbance in the Western North Pacific during TCS-08 (2008)

2015 ◽  
Vol 143 (7) ◽  
pp. 2459-2484 ◽  
Author(s):  
Andrew B. Penny ◽  
Patrick A. Harr ◽  
Michael M. Bell
Keyword(s):  
Tropical Cyclone ◽  
Large Scale ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Low Level ◽  
The North ◽  
Cloud Clusters ◽  
Potential Formation ◽  
Shearing Deformation ◽  
Upper Level

Abstract Large uncertainty still remains in determining whether a tropical cloud cluster will develop into a tropical cyclone. During The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment, over 50 tropical cloud clusters were monitored for development, but only 4 developed into a tropical cyclone. One nondeveloping tropical disturbance (TCS025) was closely observed for potential formation during five aircraft research missions, which provided an unprecedented set of observations pertaining to the large-scale and convective environments of a nondeveloping system. The TCS025 disturbance was comprised of episodic convection that occurred in relation to the diurnal cycle along the eastern extent of a broad low-level trough. The upper-level environment was dominated by two cyclonic cells in the tropical upper-tropospheric trough (TUTT) north of the low-level trough in which the TCS025 circulation was embedded. An in-depth examination of in situ observations revealed that the nondeveloping circulation was asymmetric and vertically misaligned, which led to larger system-relative flow on the mesoscale. Persistent environmental vertical wind shear and horizontal shearing deformation near the circulation kept the system from becoming better organized and appears to have allowed low equivalent potential temperature () air originating from one of the TUTT cells to the north (upshear) to impact the thermodynamic environment of TCS025. This in turn weakened subsequent convection that might otherwise have improved alignment and contributed to the transition of TCS025 to a tropical cyclone.

The Role of Equatorial Rossby Waves in Tropical Cyclogenesis. Part I: Idealized Numerical Simulations in an Initially Quiescent Background Environment

2010 ◽  
Vol 138 (4) ◽  
pp. 1368-1382 ◽  
Author(s):  
Jeffrey S. Gall ◽  
William M. Frank ◽  
Matthew C. Wheeler
Keyword(s):  
Tropical Cyclone ◽  
Large Scale ◽  
Phase Speed ◽  
Tropical Cyclogenesis ◽  
Initial Amplitude ◽  
Low Level ◽  
Wave Simulation ◽  
Horizontal Momentum ◽  
Good Agreement

Abstract This two-part series of papers examines the role of equatorial Rossby (ER) waves in tropical cyclone (TC) genesis. To do this, a unique initialization procedure is utilized to insert n = 1 ER waves into a numerical model that is able to faithfully produce TCs. In this first paper, experiments are carried out under the idealized condition of an initially quiescent background environment. Experiments are performed with varying initial wave amplitudes and with and without diabatic effects. This is done to both investigate how the properties of the simulated ER waves compare to the properties of observed ER waves and explore the role of the initial perturbation strength of the ER wave on genesis. In the dry, frictionless ER wave simulation the phase speed is slightly slower than the phase speed predicted from linear theory. Large-scale ascent develops in the region of low-level poleward flow, which is in good agreement with the theoretical structure of an n = 1 ER wave. The structures and phase speeds of the simulated full-physics ER waves are in good agreement with recent observational studies of ER waves that utilize wavenumber–frequency filtering techniques. Convection occurs primarily in the eastern half of the cyclonic gyre, as do the most favorable conditions for TC genesis. This region features sufficient midlevel moisture, anomalously strong low-level cyclonic vorticity, enhanced convection, and minimal vertical shear. Tropical cyclogenesis occurs only in the largest initial-amplitude ER wave simulation. The formation of the initial tropical disturbance that ultimately develops into a tropical cyclone is shown to be sensitive to the nonlinear horizontal momentum advection terms. When the largest initial-amplitude simulation is rerun with the nonlinear horizontal momentum advection terms turned off, tropical cyclogenesis does not occur, but the convectively coupled ER wave retains the properties of the ER wave observed in the smaller initial-amplitude simulations. It is shown that this isolated wave-only genesis process only occurs for strong ER waves in which the nonlinear advection is large. Part II will look at the more realistic case of ER wave–related genesis in which a sufficiently intense ER wave interacts with favorable large-scale flow features.

scholarly journals A Look at the Relationship between the Large-Scale Tropospheric Static Stability and the Tropical Cyclone Maximum Intensity

2020 ◽  
Vol 33 (3) ◽  
pp. 959-975
Author(s):  
Alexandria Downs ◽  
Chanh Kieu
Keyword(s):  
Tropical Cyclone ◽  
Negative Correlation ◽  
North Atlantic ◽  
Large Scale ◽  
Static Stability ◽  
Maximum Intensity ◽  
Northwestern Pacific ◽  
The North ◽  
The Future ◽  
The North Atlantic

AbstractVarious modeling and observational studies have suggested that tropical cyclone (TC) intensity tends to increase in the future due to projected warmer sea surface temperature (SST). This study examines the effects of the tropospheric stratification that could potentially offset the direct increase of TC intensity associated with the warmer SST. Using reanalysis datasets and TC records in the northwestern Pacific and the North Atlantic basins, it is shown that there exists a consistently negative correlation between the annually averaged TC intensity and the basinwide average of the tropospheric static stability. This negative correlation is more robust in the northwestern Pacific basin when using the TC lifetime maximum intensity but is somewhat less significant in the North Atlantic basin. Further separation of the troposphere into a lower (1000–500 hPa) and an upper layer (500–200 hPa) reveals that it is the upper-tropospheric static stability that plays a more dominant role in governing the TC intensity variability. The negating effects of a stable troposphere on TC intensity as found in this study suggest a partial offset of the projected increase in the TC potential intensity due to the future warmer SST. Thus, the tropospheric static stability is one of the key large-scale factors that need to be properly taken into account in studies of long-term TC intensity change.

scholarly journals Dynamic wavelet correlation analysis for multivariate climate time series

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Josué M. Polanco-Martínez ◽  
Javier Fernández-Macho ◽  
Martín Medina-Elizalde
Keyword(s):  
Time Series ◽  
Tropical Cyclone ◽  
North Atlantic ◽  
Large Scale ◽  
Southern Oscillation ◽  
Atlantic Multidecadal Oscillation ◽  
Last Millennium ◽  
Climate Data ◽  
The North ◽  
Climate Time Series

AbstractThe wavelet local multiple correlation (WLMC) is introduced for the first time in the study of climate dynamics inferred from multivariate climate time series. To exemplify the use of WLMC with real climate data, we analyse Last Millennium (LM) relationships among several large-scale reconstructed climate variables characterizing North Atlantic: i.e. sea surface temperatures (SST) from the tropical cyclone main developmental region (MDR), the El Niño-Southern Oscillation (ENSO), the North Atlantic Multidecadal Oscillation (AMO), and tropical cyclone counts (TC). We examine the former three large-scale variables because they are known to influence North Atlantic tropical cyclone activity and because their underlying drivers are still under investigation. WLMC results obtained for these multivariate climate time series suggest that: (1) MDRSST and AMO show the highest correlation with each other and with respect to the TC record over the last millennium, and: (2) MDRSST is the dominant climate variable that explains TC temporal variability. WLMC results confirm that this method is able to capture the most fundamental information contained in multivariate climate time series and is suitable to investigate correlation among climate time series in a multivariate context.

Modulation of North Pacific and North Atlantic tropical cyclones by tropical trans-basin variability and ENSO during May-October

2020 ◽  
pp. 1
Author(s):  
Shaohua Chen ◽  
Haikun Zhao ◽  
Graciela B. Raga ◽  
Philip J. Klotzbach
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
North Pacific ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Relative Vorticity ◽  
Low Level ◽  
Scale Factors ◽  
Frequency Changes ◽  
Dipole Modulation

AbstractThis study highlights the distinct modulation of May-October tropical cyclones (TCs) in the western North Pacific (WNP), eastern North Pacific (ENP) and North Atlantic (NATL) basins by tropical trans-basin variability (TBV) and ENSO. The pure TBV significantly modulates total TC counts in all three basins, with more TCs in the WNP and ENP and fewer TCs in the NATL during warm TBV years and fewer TCs in the WNP and ENP and more TCs in the NATL during cold TBV years. By contrast, the pure ENSO signal shows no impact on total TC count over any of the three basins. These results are consistent with changes in large scale factors associated with TBV and ENSO. Low-level relative vorticity (VOR) is an important driver of WNP TC genesis frequency, with broad agreement between the observed spatial distribution of TC genesis and TBV/ENSO-associated VOR anomalies. TBV significantly affects ENP TC frequency due to changes in basin wide vertical wind shear and sea surface temperatures, while the modulation in TC frequency by ENSO is primarily caused by a north-south dipole modulation of large-scale atmospheric and oceanic factors. The pure TBV-related low-level VOR changes appear to be the most important factor modulating NATL TC frequency. Changes in large-scale factors compare well with the budget of synoptic-scale eddy kinetic energy. Possible physical processes associated with pure TBV and pure ENSO that modulate TC frequency are further discussed. This study contributes to the understanding of TC inter-annual variability and could thus be helpful for seasonal TC forecasting.

scholarly journals The role of North Atlantic-European weather regimes in the surface impact of sudden stratospheric warming events

2020 ◽  
Author(s):  
Daniela I. V. Domeisen ◽  
Christian M. Grams ◽  
Lukas Papritz
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Western Europe ◽  
Storm Track ◽  
Stratospheric Warming ◽  
Sudden Stratospheric Warming ◽  
Weather Regimes ◽  
The North ◽  
The North Atlantic

Abstract. Sudden stratospheric warming (SSW) events can significantly impact tropospheric weather for a period of several weeks, in particular over the North Atlantic and Europe. However, not all SSW events exhibit the same tropospheric response, if any, and it remains an open question what determines the existence, location, timing, and strength of the downward impact. We here explore the role of the state of the tropospheric flow in the North Atlantic region at the onset of SSW events for determining the subsequent surface impact. A refined definition of seven North Atlantic tropospheric weather regimes indicates the Greenland blocking (GL) and Atlantic Trough (AT) regimes as the most frequent large-scale patterns following the weeks after an SSW. While GL is dominated by high pressure over Greenland, AT is dominated by a southeastward shifted storm track in the North Atlantic. We find that a blocking situation over western Europe and the North Sea (European Blocking) at the time of the SSW onset favours the GL response and the associated cold conditions over Europe. In contrast, an AT response and mild conditions are more likely if GL occurs already at SSW onset. For the remaining tropospheric flow regimes during SSW onset, we find no clear response. The results indicate that the tropospheric impact of SSW events critically depends on the tropospheric state during the onset of the SSW, which could provide crucial guidance for subseasonal prediction.

A Climatology of Indirect Tropical Cyclone Interactions in the North Atlantic and Western North Pacific Basins

2020 ◽  
Vol 148 (10) ◽  
pp. 4035-4059
Author(s):  
Kevin C. Prince ◽  
Clark Evans
Keyword(s):  
Tropical Cyclone ◽  
North Atlantic ◽  
North Pacific ◽  
Western North Pacific ◽  
Vertical Wind Shear ◽  
Reanalysis Data ◽  
Synoptic Scale ◽  
Rossby Wave Breaking ◽  
The North ◽  
The North Atlantic

AbstractWhile 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.

scholarly journals On the Seasonal and Synoptic Time-Scale Variability of the North Atlantic Trade Wind Region and Its Low-Level Clouds

2015 ◽  
Vol 72 (4) ◽  
pp. 1428-1446 ◽  
Author(s):  
Matthias Brueck ◽  
Louise Nuijens ◽  
Bjorn Stevens
Keyword(s):  
Wind Speed ◽  
Time Scales ◽  
North Atlantic ◽  
Large Scale ◽  
Sensible Heat Flux ◽  
Cloud Amount ◽  
Trade Wind ◽  
Low Level ◽  
The North ◽  
The North Atlantic

Abstract The seasonality in large-scale meteorology and low-level cloud amount (CClow) is explored for a 5° × 5° area in the North Atlantic trades, using 12 years of ERA-Interim and MODIS data, supported by 2 years of Barbados Cloud Observatory (BCO) measurements. From boreal winter to summer, large-scale subsiding motion changes to rising motion, along with an increase in sea surface temperature, a clockwise turning and weakening of low-level winds, and reduced cold-air advection, lower-tropospheric stability (LTS), and surface fluxes. However, CClow is relatively invariant around 30%, except for a minimum of 20% in fall. This minimum is only pronounced when MODIS scenes with large high-level cloud amount are excluded, and a winter maximum in CClow is more pronounced at the BCO. On monthly time scales, wind speed has the best correlation with CClow. Existing large-eddy simulations suggest that the wind speed–CClow correlation may be explained by a direct deepening response of the trade wind layer to stronger winds. Large correlations of wind direction and advection with CClow also suggest that large-scale flow patterns matter. Smaller correlations with CClow are observed for LTS and surface evaporation, as well as negligible correlations for relative humidity (RH) and vertical velocity. However, these correlations considerably increase when only summer is considered. On synoptic time scales, all correlations drop substantially, whereby wind speed, RH, and surface sensible heat flux remain the leading parameters. The lack of a single strong predictor emphasizes that the combined effect of parameters is necessary to explain variations in CClow in the trades.

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