scholarly journals A Characterization of African Easterly Waves on 2.5–6-Day and 6–9-Day Time Scales

2013 ◽  
Vol 26 (18) ◽  
pp. 6750-6774 ◽  
Author(s):  
Man-Li C. Wu ◽  
Oreste Reale ◽  
Siegfried D. Schubert
Keyword(s):  
Time Scales ◽  
Ensemble Empirical Mode Decomposition ◽  
African Easterly Waves ◽  
Pressure System ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
The North ◽  
Easterly Wave ◽  
Mode Decomposition ◽  
The Tropics

Abstract This study shows that the African easterly wave (AEW) activity over the African monsoon region and the northern tropical Atlantic can be divided in two distinct temporal bands with time scales of 2.5–6 and 6–9 days. The results are based on a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The novel result of this investigation is that the 6–9-day waves appear to be located predominantly to the north of the African easterly jet (AEJ), originate at the jet level, and are different in scale and structure from the well-known low-level 2.5–6-day waves that develop baroclinically on the poleward flank of the AEJ. Moreover, they appear to interact with midlatitude eastward-propagating disturbances, with the strongest interaction taking place at the latitudes where the core of the Atlantic high pressure system is located. Composite analyses applied to the mode decomposition indicate that the interaction of the 6–9-day waves with midlatitude systems is characterized by enhanced southerly (northerly) flow from (toward) the tropics. This finding agrees with independent studies focused on European floods, which have noted enhanced moist transport from the ITCZ toward the Mediterranean region on time scales of about a week as important precursors of extreme precipitation.

scholarly journals Intermittent African Easterly Wave Activity in a Dry Atmospheric Model: Influence of the Extratropics

2011 ◽  
Vol 24 (20) ◽  
pp. 5378-5396 ◽  
Author(s):  
Stephanie Leroux ◽  
Nicholas M. J. Hall ◽  
George N. Kiladis
Keyword(s):  
Storm Track ◽  
Atmospheric Model ◽  
Convective Heating ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Global Circulation ◽  
African Easterly Jet ◽  
African Easterly Wave ◽  
The North ◽  
Easterly Wave

Abstract A dynamical model is constructed of the northern summertime global circulation, maintained by empirically derived forcing, based on the same dynamical code that has recently been used to study African easterly waves (AEWs) as convectively triggered perturbations (Thorncroft et al.; Leroux and Hall). In the configuration used here, the model faithfully simulates the observed mean distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to investigate the origin and intraseasonal intermittency of AEWs in an artificially dry (no convection) context. A long integration of the model produces a summertime climatology that includes a realistic African easterly jet and westward-propagating 3–5-day disturbances over West Africa. These simulated waves display intraseasonal intermittency as the observed AEWs also do. Further experiments designed to discern the source of this intermittency in the model show that the simulated waves are mainly triggered by dynamical precursors coming from the North Atlantic storm track. The model is at least as sensitive to this remote influence as it is to local triggering by convective heating.

scholarly journals An Evaluation of the Parallel Ensemble Empirical Mode Decomposition Method in Revealing the Role of Downscaling Processes Associated with African Easterly Waves in Tropical Cyclone Genesis

2016 ◽  
Vol 33 (8) ◽  
pp. 1611-1628 ◽  
Author(s):  
Yuling Wu ◽  
Bo-Wen Shen
Keyword(s):  
Tropical Cyclone ◽  
Empirical Mode Decomposition ◽  
Ensemble Empirical Mode Decomposition ◽  
Reanalysis Data ◽  
Tropical Cyclone Genesis ◽  
African Easterly Waves ◽  
Horizontal Shear ◽  
Easterly Waves ◽  
Mode Decomposition

AbstractIn this study the parallel ensemble empirical mode decomposition (PEEMD) is applied for an analysis of 10-yr (2004–13) ERA-Interim global reanalysis data in order to explore the role of downscaling processes associated with African easterly waves (AEWs) in tropical cyclone (TC) genesis. The focus of the study was aimed at understanding the downscaling process in multiscale flows during storm intensification. To represent the various length scales of atmospheric systems, intrinsic mode functions (IMFs) were extracted from the reanalysis data using the PEEMD. It was found that the nonoscillatory trend mode can be used to represent large-scale environmental flow and that the third oscillatory mode (IMF3) can be used to represent AEW/TC scale systems. The results 1) identified 42 developing cases from 272 AEWs, where 25 of them eventually developed into hurricanes; 2) indicated that the maximum for horizontal shear largely occurs over the ocean for the IMF3 and over land near the coast for the trend mode for developing cases, suggesting shear transfer between the trend mode and the IMF3; 3) displayed opposite wind shear tendencies for the trend mode and the IMF3 during storm intensification, signifying that the downscaling process was active in 13 hurricane cases along their tracks; and 4) showed that among the 42 developing cases, only 13 of the 25 hurricanes were found to have significant downscaling transfer features, so other processes such as upscaling processes may play an important role in the other developing cases, especially for the remaining 12 hurricane cases. In a future study, the authors intend to investigate the upscaling process between the convection scale and AEWs/TCs, which requires data at a finer grid resolution.

scholarly journals North Atlantic Hurricanes Contributed by African Easterly Waves North and South of the African Easterly Jet

2008 ◽  
Vol 21 (24) ◽  
pp. 6767-6776 ◽  
Author(s):  
Tsing-Chang Chen ◽  
Shih-Yu Wang ◽  
Adam J. Clark
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Monsoon Trough ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
The North ◽  
West African Coast ◽  
Atlantic Hurricanes ◽  
Population Ratio

Abstract A majority of tropical cyclones in the North Atlantic develop from African easterly waves (AEWs), which originate along both the southern and northern flanks of the midtropospheric African easterly jet (AEWS and AEWn, respectively). The purpose of this note is to identify the contribution of AEWSs and AEWns to North Atlantic tropical cyclones that develop from AEWs. Applying a manual backtracking approach to identify the genesis locations of AEWS, it was found that the population ratio of tropical cyclones formed from AEWSs to those formed from AEWns is 1:1.2. Because the population ratio of AEWSs to AEWns is 1:2.5, the conversion rate of the former AEWS to tropical cyclones is twice as effective as the latter waves. In addition, it was found that AEWns travel farther and take longer to transform into tropical cyclones than AEWSs, which is likely because the AEWns are drier and shallower than AEWSs. An analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough over West Africa provides moisture and enhances low-level vorticity for both AEWns and AEWSs as they move off the West African coast. The monsoon trough appears to be of particular importance in supplying AEWns with enough moisture so that they have similar properties to AEWSs after they have traveled a considerable westward distance across the tropical Atlantic.

scholarly journals Observed Change in Sahel Rainfall, Circulations, African Easterly Waves, and Atlantic Hurricanes Since 1979

2011 ◽  
Vol 2011 ◽  
pp. 1-14 ◽  
Author(s):  
Shih-Yu Wang ◽  
Robert R. Gillies
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Dynamic Properties ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
The North ◽  
Sahel Rainfall ◽  
The North Atlantic ◽  
Atlantic Hurricanes

Here, we examine the dynamic properties associated with the recent increase in the Sahel rainfall using an ensemble of five global reanalysis datasets (1979–2010). The rainfall that has been observed to be increasing over the Sahel is accounted for by enhancements in both the tropical easterly jet and the African easterly jet, both of which are known to induce wet anomalies. Moreover, positional shifts in the African easterly jet and African easterly waves (AEWs) accompanied the northward migration of the Sahel rainband. Change in the African easterly jet and AEWs are coupled to a northward shift and amplification of convective activity; this signals an increased potential for the occurrence of flash floods along the northern Sahel. In addition, the result from a wave tracking analysis suggests that the change in AEWs is closely linked to increased activity of intense hurricanes in the North Atlantic. The synoptic concurrence of AEWs in driving the dynamics of the Sahel greening and the increase in tropical cyclogeneses over the North Atlantic is an important aspect in the evaluation of climate model projections.

scholarly journals On the Relationship between African Easterly Waves and the African Easterly Jet

2009 ◽  
Vol 66 (8) ◽  
pp. 2303-2316 ◽  
Author(s):  
Stephanie Leroux ◽  
Nicholas M. J. Hall
Keyword(s):  
Intraseasonal Variability ◽  
Equation Model ◽  
Dominant Mode ◽  
Vertical Shear ◽  
Convective Heating ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
Easterly Wave ◽  
Basic States

Abstract This idealized modeling study investigates how convectively triggered African easterly waves (AEWs) are influenced by the intraseasonal variability of the African easterly jet (AEJ). A set of 10-day averaged zonally varying basic states is constructed with the NCEP-2 reanalysis (1979–2006). A primitive equation model is used to simulate linear AEWs on each of these basic states using the same idealized convective heating localized over the Darfur mountains as an initial trigger. It is shown that the transient response depends strongly on the basic state. With the same trigger, many configurations of the AEJ fail to produce a wave disturbance, while others produce strong easterly wave structures. Necessary conditions for the development of strong waves can be characterized by a strong jet, a strong vertical shear, or a strong and extended potential vorticity reversal. In strong-wave cases the jet is extended to the south and west, and the jet core is aligned with the maximum of surface westerlies, maximizing the vertical shear. The pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the AEJ revealed by an empirical orthogonal function (EOF) analysis of the set of basic states.

scholarly journals Decadal-to-Multidecadal Variability of Seasonal Land Precipitation in Northern Hemisphere in Observation and CMIP6 Historical Simulations

Atmosphere ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 195
Author(s):  
Hua Chen ◽  
Zhenchen Xu
Keyword(s):  
Northern Hemisphere ◽  
Atlantic Multidecadal Oscillation ◽  
Ensemble Empirical Mode Decomposition ◽  
Dominant Mode ◽  
Coupled Models ◽  
Multidecadal Variability ◽  
The North ◽  
Mode Decomposition ◽  
Four Seasons ◽  
Oceanic Forcing

Based on the centennial-scale observations and CMIP6 historical simulations, this paper employs the ensemble empirical mode decomposition to extract the decadal-to-multidecadal variability of land precipitation (DMVLP) in the northern hemisphere. The spatial distributions of the dominant mode from the empirical orthogonal function are different in four seasons. Regions with the same sign of precipitation anomalies are likely to be teleconnected through oceanic forcing. The temporal evolutions of the leading modes are similar in winter and spring, with an amplitude increasing after the late 1970s, probably related to the overlap of oceanic multidecadal signals. In winter and spring, the Interdecadal Pacific Oscillation (IPO) and the Atlantic Multidecadal Oscillation (AMO) play a joint role. They were in phase before late 1970s and out of phase after then, weakening/strengthening the impacts of the North Pacific and North Atlantic on the DMVLP before/after late 1970s. In summer and autumn, AMO alone plays a part and the amplitude of time series does not vary as in winter and spring. The ability of the coupled models from CMIP6 historical simulations is also evaluated. The good-models average largely captures the spatial structure in four seasons and the associated oceanic signals. The poor-models average is hardly or weakly correlated with observation.

Predictability of North Atlantic Tropical Cyclone Activity on Intraseasonal Time Scales

2010 ◽  
Vol 138 (12) ◽  
pp. 4362-4374 ◽  
Author(s):  
James I. Belanger ◽  
Judith A. Curry ◽  
Peter J. Webster
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Large Scale ◽  
North Atlantic Ocean ◽  
Relative Vorticity ◽  
Vertical Shear ◽  
Forecast System ◽  
Ensemble Forecasts ◽  
Easterly Waves ◽  
The North

Abstract Recent work suggests that there may exist skill in forecasting tropical cyclones (TC) using dynamically based ensemble products, such as those obtained from the ECMWF Monthly Forecast System (ECMFS). The ECMFS features an ensemble of 51 coupled ocean–atmosphere simulations integrated to 32 days once per week. Predicted levels of TC activity in the North Atlantic Ocean with these monthly ensemble forecasts is compared with the observed variability during the months of June–October during 2008 and 2009. Results indicate that the forecast system can capture large-scale regions that have a higher or lower risk of TC activity and that it has skill above climatology for the Gulf of Mexico and the “Main Development Region” on intraseasonal time scales. Regional forecast skill is traced to the model’s ability to capture the large-scale evolution of deep-layer vertical shear, the frequency of easterly waves, and the variance in 850-hPa relative vorticity. The predictability of TC activity, along with the forecast utility of the ECMFS, is shown to be sensitive to the phase and intensity of the Madden–Julian oscillation at the time of model initialization.

Are Tropical Cyclones Less Effectively Formed by Easterly Waves in the Western North Pacific than in the North Atlantic?

2008 ◽  
Vol 136 (11) ◽  
pp. 4527-4540 ◽  
Author(s):  
Tsing-Chang Chen ◽  
Shih-Yu Wang ◽  
Ming-Cheng Yen ◽  
Adam J. Clark
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
North Pacific ◽  
Western North Pacific ◽  
Monsoon Trough ◽  
Easterly Waves ◽  
The North ◽  
Easterly Wave ◽  
Monsoon Gyre ◽  
The North Atlantic

Abstract It has been observed that the percentage of tropical cyclones originating from easterly waves is much higher in the North Atlantic (∼60%) than in the western North Pacific (10%–20%). This disparity between the two ocean basins exists because the majority (71%) of tropical cyclogeneses in the western North Pacific occur in the favorable synoptic environments evolved from monsoon gyres. Because the North Atlantic does not have a monsoon trough similar to the western North Pacific that stimulates monsoon gyre formation, a much larger portion of tropical cyclogeneses than in the western North Pacific are caused directly by easterly waves. This study also analyzed the percentage of easterly waves that form tropical cyclones in the western North Pacific. By carefully separating easterly waves from the lower-tropospheric disturbances generated by upper-level vortices that originate from the tropical upper-tropospheric trough (TUTT), it is observed that 25% of easterly waves form tropical cyclones in this region. Because TUTT-induced lower-tropospheric disturbances often become embedded in the trade easterlies and resemble easterly waves, they have likely been mistakenly identified as easterly waves. Inclusion of these “false” easterly waves in the “true” easterly wave population would result in an underestimation of the percentage of easterly waves that form tropical cyclones, because the TUTT-induced disturbances rarely stimulate tropical cyclogenesis. However, an analysis of monsoon gyre formation mechanisms over the western North Pacific reveals that 82% of monsoon gyres develop through a monsoon trough–easterly wave interaction. Thus, it can be inferred that 58% (i.e., 82% × 71%) of tropical cyclones in this region are an indirect result of easterly waves. Including the percentage of tropical cyclones that form directly from easterly waves (∼25%), it is found that tropical cyclones formed directly and indirectly from easterly waves account for over 80% of tropical cyclogeneses in the western North Pacific. This is more than the percentage that has been documented by previous studies in the North Atlantic.

scholarly journals The Internal Multidecadal Variability of SST in the Pacific and Its Impact on Air Temperature and Rainfall over Land in the Northern Hemisphere

Atmosphere ◽  
2019 ◽  
Vol 10 (3) ◽  
pp. 153
Author(s):  
Hua Chen ◽  
Donglin He ◽  
Zhiwei Zhu
Keyword(s):  
Northern Hemisphere ◽  
Air Temperature ◽  
Critical Role ◽  
Surface Air Temperature ◽  
Ensemble Empirical Mode Decomposition ◽  
Near Surface ◽  
Multidecadal Variability ◽  
The North ◽  
Mode Decomposition ◽  
The Pacific

Based on the centennial-scale observations and re-analyses, this paper employs the ensemble empirical mode decomposition to separate the internal multidecadal variability (IMV) from the externally-forced variability of sea surface temperature (SST), and then defines new indices that represent the IMV of SST in the North Pacific (NPIMV) and South Pacific (SPIMV), respectively. The spatial structure of NPIMV/SPIMV shows remarkably positive SST anomaly only in the index-defined region; meanwhile, the temporal evolutions of NPIMV and SPIMV are uncorrelated, indicating their independence of each other. Both NPIMV and SPIMV play a critical role in the near-surface air temperature and rainfall over land in the Northern hemisphere, especially in the season when their intensity is the strongest. It is through teleconnection wave trains that NPIMV and SPIMV exert influences on remote regions. Results from another two rainfall datasets are found to be consistent in the majority of the Northern hemisphere in response to NPIMV/SPIMV, yet disagreement exists in certain regions due to large uncertainties of rainfall datasets.

scholarly journals Hybrid improved empirical mode decomposition and BP neural network model for the prediction of sea surface temperature

Ocean Science ◽  
2019 ◽  
Vol 15 (2) ◽  
pp. 349-360 ◽  
Author(s):  
Zhiyuan Wu ◽  
Changbo Jiang ◽  
Mack Conde ◽  
Bin Deng ◽  
Jie Chen
Keyword(s):  
Neural Network ◽  
Empirical Mode Decomposition ◽  
Bp Neural Network ◽  
Back Propagation ◽  
Back Propagation Neural Network ◽  
Ensemble Empirical Mode Decomposition ◽  
The North ◽  
Mode Decomposition ◽  
Bpnn Model

Abstract. Sea surface temperature (SST) is the major factor that affects the ocean–atmosphere interaction, and in turn the accurate prediction of SST is the key to ocean dynamic prediction. In this paper, an SST-predicting method based on empirical mode decomposition (EMD) algorithms and back-propagation neural network (BPNN) is proposed. Two different EMD algorithms have been applied extensively for analyzing time-series SST data and some nonlinear stochastic signals. The ensemble empirical mode decomposition (EEMD) algorithm and complementary ensemble empirical mode decomposition (CEEMD) algorithm are two improved algorithms of EMD, which can effectively handle the mode-mixing problem and decompose the original data into more stationary signals with different frequencies. Each intrinsic mode function (IMF) has been taken as input data to the back-propagation neural network model. The final predicted SST data are obtained by aggregating the predicted data of individual series of IMFs (IMFi). A case study of the monthly mean SST anomaly (SSTA) in the northeastern region of the North Pacific shows that the proposed hybrid CEEMD-BPNN model is much more accurate than the hybrid EEMD-BPNN model, and the prediction accuracy based on a BP neural network is improved by the CEEMD method. Statistical analysis of the case study demonstrates that applying the proposed hybrid CEEMD-BPNN model is effective for the SST prediction. Highlights include the following: Highlights. An SST-predicting method based on the hybrid EMD algorithms and BP neural network method is proposed in this paper. SST prediction results based on the hybrid EEMD-BPNN and CEEMD-BPNN models are compared and discussed. A case study of SST in the North Pacific shows that the proposed hybrid CEEMD-BPNN model can effectively predict the time-series SST.

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