Search Results

You are looking at 1 - 10 of 37 items for

  • Author or Editor: Chris Thorncroft x
  • Refine by Access: All Content x
Clear All Modify Search
Chris Thorncroft and Ioannis Pytharoulis

Abstract

Analysis of ECMWF reanalyses and operational analyses covering the period between 1979–98 has confirmed that seasonal Atlantic tropical cyclone activity is strongly and negatively correlated with the observed vertical wind shear present in the main development region (MDR) between July and September. In 1983 and 1995, the least active and most active tropical cyclone years, respectively, anomalous shear was shown to be present in spring and to persist throughout each of the tropical cyclone seasons. While monitoring of MDR shear is recommended for highlighting the risk of such extreme events, the springtime MDR shear is not generally a good indicator of shear in the summer months.

Seasonal forecasts of MDR shear made with the U.K. Met Office (UKMO) atmospheric GCM (AGCM) and observed SSTs for the years 1979–97 have been analyzed. The model possesses potential skill for predicting the MDR shear as determined by a consideration of the ensemble mean shear variability and an evaluation of the relative operating characteristics (ROC). The ROC analysis indicates high probabilistic skill, in particular for anomalously low shear events. Analysis of seasonal forecasts of MDR shear made with the UKMO AGCM with persisted SST anomalies for the years 1979–97 was also performed. Skill in predicting MDR shear is reduced but still significant. ROC analysis indicates probabilistic skill for the anomalously low shear events, which may be useful for some applications.

Based on this work, the authors conclude that a dynamical approach to the seasonal forecasting of Atlantic tropical cyclone activity, which combines predicted MDR shear with a statistical model should be developed.

Full access
Anantha Aiyyer and Chris Thorncroft

Abstract

Spatiotemporal patterns of tropics-wide vertical shear variability are extracted after separating a 58-yr data record into high-frequency (HF, periods of 1.5–8 yr) and low-frequency (LF, periods greater than 8 yr) components. The HF vertical shear variability is dominated by circulation anomalies associated with the El Niño–Southern Oscillation (ENSO). The LF variability is primarily contained in two multidecadal patterns and a near-decadal pattern.

The multidecadal modes are strongest within the tropical Atlantic and are correlated with Sahel precipitation and interhemispheric sea surface temperature (SST) anomalies. The results suggest that the multidecadal variability of vertical shear over the Atlantic is linked to atmospheric circulation anomalies forced by the variability in Sahel precipitation. The decadal mode is strongest within the central Pacific and is correlated with Pacific decadal oscillation (PDO)-like SST anomalies. The circulation associated with this anomalous shear pattern appears to be consistent with the atmospheric response to the PDO-related diabatic heating anomaly over the central Pacific.

The relationship between vertical shear and seasonal tropical cyclone activity, as defined by the accumulated cyclone energy (ACE), is examined for the Atlantic, eastern Pacific, and western Pacific Oceans. The results show that global modes of vertical shear and seasonal average ACE are not consistently related in all three regions. It is only in the Atlantic Ocean that seasonal ACE is most consistently limited by vertical shear. This calls into question the utility of vertical shear as an independent predictor of seasonal tropical cyclone activity, particularly over the western Pacific Ocean.

Full access
Chris Thorncroft and Kevin Hodges

Abstract

Automatic tracking of vorticity centers in European Centre for Medium-Range Weather Forecasts analyses has been used to develop a 20-yr climatology of African easterly wave activity. The tracking statistics at 600 and 850 mb confirm the complicated easterly wave structures present over the African continent. The rainy zone equatorward of 15°N is dominated by 600-mb activity, and the much drier Saharan region poleward of 15°N is more dominated by 850-mb activity. Over the Atlantic Ocean there is just one storm track with the 600- and 850-mb wave activity collocated. Based on growth/decay and genesis statistics, it appears that the 850-mb waves poleward of 15°N over land generally do not get involved with the equatorward storm track over the ocean. Instead, there appears to be significant development of 850-mb activity at the West African coast in the rainy zone around (10°N, 10°W), which, it is proposed, is associated with latent heat release.

Based on the tracking statistics, it has been shown that there is marked interannual variability in African easterly wave (AEW) activity. It is especially marked at the 850-mb level at the West African coast between about 10° and 15°N, where the coefficient of variation is 0.29. For the period between 1985 and 1998, a notable positive correlation is seen between this AEW activity and Atlantic tropical cyclone activity. This correlation is particularly strong for the postreanalysis period between 1994 and 1998. This result suggests that Atlantic tropical cyclone activity may be influenced by the number of AEWs leaving the West African coast, which have significant low-level amplitudes, and not simply by the total number of AEWs.

Full access
Ioannis Pytharoulis and Chris Thorncroft

Abstract

The existence of African easterly waves (AEWs) north of the African easterly jet (AEJ) core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the U.K. Meteorological Office global model analysis from the hurricane season of 1995. At Bamako (12.5°N, 8.0°W) the AEWs were characterized mainly by maximum amplitudes at the level of the AEJ (around 700 mb), whereas at Dakar (14.7°N, 17.5°W) the waves were characterized by maxima between 850 and 950 mb. The low-level waves to the north of the AEJ arise in association with baroclinic interactions between the negative meridional potential vorticity (PV) gradients in the jet core and the positive low-level gradient of potential temperature, θ, enhanced by the presence of low-static-stability air north of the AEJ. These waves follow the positive meridional θ gradients over northern Africa in contrast to the jet-level AEWs that follow the meridional PV gradients at the level of the AEJ. Cross-correlation analysis shows that there is strong coherence between the low-level AEWs and the well-known cold core AEWs that propagate south of the jet, confirming that AEWs are associated with a combined barotropic–baroclinic instability mechanism.

Full access
Chris Thorncroft and Sarah C. Jones

Abstract

The extratropical transitions of Hurricanes Felix and Iris in 1995 are examined and compared. Both systems affected northwest Europe but only Iris developed significantly as an extratropical system. In both cases the hurricane interacts with a preexisting extratropical system over the western Atlantic. The remnants of the exhurricanes can be identified and tracked across the Atlantic as separate low-level potential vorticity (PV) anomalies. The nature of the baroclinic wave involved in the extratropical transition is described from a PV perspective and shown to differ significantly between the two cases.

The role of vertical shear in modifying the hurricane structure during the early phase of the transition is investigated. Iris moved into a region of strong shear. The high PV tower of Iris developed a marked downshear tilt. Felix moved into a vertically sheared environment also but the shear was weaker than for Iris and the PV tower of Felix did not tilt much.

Iris maintained its warm-core structure as it tracked across relatively warm water. It moved into the center of a large-scale baroclinic cyclone. The superposition of the two systems gave rise to strong low-level winds. The resulting strong surface latent heat fluxes helped to keep the boundary layer equivalent potential temperature (θe) close to the saturated equivalent potential temperature of the underlying sea surface temperature. This high equivalent potential temperature air was redistributed in the vertical in association with deep convection, which helped maintain the warm core in a similar way to that in tropical cyclones.

Felix did not maintain its warm-core structure as it tracked across the Atlantic. This has been shown to be linked to its more poleward track across colder water. It is argued that negative surface fluxes of latent and sensible heat decrease the boundary layer θ e, resulting in low-cloud formation and a decoupling of the cyclone boundary layer from the the deep troposphere.

In order to forecast these events there is a need for skill in predicting both the nature of the large-scale baroclinic wave development and the structural evolution of the exhurricane remnants.

Full access
Alan Brammer and Chris D. Thorncroft

Abstract

This study presents a large-scale trajectory analysis of African easterly waves (AEWs) across West Africa and the eastern Atlantic. Back trajectories were initialized at multiple pressure levels from in and around the vortex center of the AEW troughs to reveal the source regions of environmental inflow. The trajectory analysis highlights a changing influence of environmental air on AEW troughs. Over West Africa, monsoonal flow dominates with source regions of air from the southwest and east to northeast influencing the trough. As the AEW troughs leave West Africa, flow from the northwest becomes increasingly important. Cluster analysis highlighted that the contribution of trajectories from the northwest increased as the AEW troughs move westward and that this cluster also had high variability in environmental characteristics.

Correlation analysis of outgoing longwave radiation around the troughs with environmental characteristics 72 h earlier was conducted on 443 AEWs. This analysis reveals that the impact of the various source regions on convective activity within the AEW troughs is consistent with the cluster trajectory analysis. While the AEW troughs were over West Africa, convection was sensitive to midlevel equivalent potential temperature around the trough and to the northeast of the trough axis. Over the West African coast and Cape Verde basin, the correlation analysis captures the changing flow regime with sensitivity to west of the trough axis at midlevels and northwest at low levels. These results highlight that the cool and dry low-level northerly trade winds over the Canary and Cape Verde basin can be a crucial influence on AEWs as they leave West Africa.

Full access
Alan Brammer and Chris D. Thorncroft

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.

Full access
Elinor R. Martin and Chris Thorncroft

Abstract

African easterly waves (AEWs) can act as seed disturbances for tropical cyclones (TCs) in the Atlantic, and changes in future AEW activity may have important consequences for development of TCs. The simulation of AEWs was investigated using output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) suite of experiments, including coupled historical and future simulations and atmosphere only (AMIP) simulations. Large biases exist in the simulation of low- (850 hPa) and midlevel (700 hPa) eddy kinetic energy (EKE, a proxy for AEW activity) in AMIP and historical simulations. CMIP5 models simulate excessive EKE and deficient rainfall south of 17°N. The same biases exist in historical and AMIP models and are not a consequence of errors in sea surface temperatures. The models also struggle to accurately couple AEWs and rainfall, with little improvement from CMIP3 models. CMIP5 models are unable to propagate AEWs across the coast and into the Atlantic, which is shown to be related to the resolution of the Guinea Highlands. Future projections of the annual cycle of AEW activity show a reduction in late spring and early summer and a large increase between July and October that is consistent with rainfall projections in the Sahel, but large differences exists in future projections between high- and low-resolution models. The simulation of AEWs is challenging for CMIP5 models and must be further diagnosed in order to accurately predict future TC activity and rainfall in the Sahel.

Full access
Anantha R. Aiyyer and Chris Thorncroft

Abstract

The spatiotemporal variability of the 200–850-hPa vertical wind shear over the tropical Atlantic is examined for a period of 46 yr. This work extends and updates past studies by considering a longer data record as well as a tropospheric-deep measure of vertical wind shear. Composite fields are constructed to illustrate the spatial pattern of the large-scale circulation associated with the mean and extreme cases of vertical shear within the tropical Atlantic. The contemporaneous relationship of vertical shear with El Niño–Southern Oscillation (ENSO) and Sahel precipitation are also examined. While the ENSO–shear correlation appears to have slightly strengthened during the past decade, the Sahel–shear correlation has become significantly degraded.

A combined empirical orthogonal function (EOF) analysis of the zonal and meridional components of the vertical shear reveals interannual and multidecadal modes. The leading EOF exhibits mainly interannual variability and is highly correlated with ENSO. The second EOF is associated with a multidecadal temporal evolution and is correlated with Sahel precipitation. Both EOFs correlate at the same level with tropical cyclones in the main development region of the tropical Atlantic.

Full access
Gareth J. Berry and Chris Thorncroft

Abstract

The life cycle of an intense African easterly wave (AEW) over the African continent is examined using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, Meteosat satellite images, and synoptic observations. This system, the strongest AEW of 2000, can be tracked from central North Africa into the eastern Atlantic Ocean, where it is associated with the genesis of Hurricane Alberto. Synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity (PV), allowing exploration at the role of multiple scales in the evolution of this AEW.

The authors’ analysis promotes the division of the AEW life cycle into three distinctive phases. (i) Initiation: The AEW development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in Sudan. This convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical North Africa. (ii) Baroclinic growth: A low-level warm anomaly, generated close to the initial convection, interacts with a midtropospheric strip of high PV that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. This interaction is reinforced by the generation of subsynoptic-scale PV anomalies by deep convection that is embedded within the baroclinic AEW structure. (iii) West coast development: Near the West African coast, the baroclinic structure weakens, but convection is maintained. The midtropospheric PV anomalies embedded within the AEW merge with one another and with PV anomalies that are generated by convection over topography ahead of the system. These mergers result in the production of a significant PV feature that leaves the West African coast and rapidly undergoes tropical cyclogenesis.

Full access