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Anantha Aiyyer and John Molinari

Abstract

The role of the Madden–Julian oscillation (MJO) in modulating the frequency and location of tropical cyclogenesis over the eastern Pacific and the Gulf of Mexico during August–September 1998 is examined. During the nonconvective phase of the MJO, convection and low-level cyclonic vorticity occurred primarily in conjunction with the intertropical convergence zone (ITCZ). During the convective phase, convection, low-level cyclonic vorticity, and convergence expanded into the northeastern Pacific and the Gulf of Mexico. This was accompanied by enhanced eddy kinetic energy and barotropic energy conversions as compared to the nonconvective phase, consistent with previous research. During the nonconvective phase of the MJO, vertical shear was relatively weaker but tropical cyclones tended to form mainly within the ITCZ. On the contrary, during the convective phase, vertical wind shear exceeded 10 m s−1 over much of this region and tropical cyclone development occurred north of the ITCZ, near the Mexican Pacific coast and the Gulf of Mexico.

Idealized numerical experiments are conducted using a barotropic model with time-invariant basic states representative of the nonconvective and convective phases of the MJO. The simulations indicate that the propagation paths as well as the amplification of the eddies differ substantially between the two phases. During the nonconvective phase, the waves tend to propagate westward into the eastern Pacific. During the convective phase, stronger southerlies steer the waves into the Gulf of Mexico. The MJO-related modulation of tropical cyclogenesis in the eastern Pacific and Gulf of Mexico thus appears to involve anomalous convergence, cyclonic vorticity, vertical wind shear, and differing tracks of easterly waves.

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Michael Diaz and Anantha Aiyyer

Abstract

The existence of an upstream (eastward) group velocity for African easterly waves (AEWs) is shown based on single-point lag regressions using gridded reanalysis data from 1990 to 2010. The eastward energy dispersion is consistent with the direction of ageostrophic geopotential flux vectors. A local eddy kinetic energy (EKE) budget reveals that, early in the life cycle of AEWs, growth rate due to geopotential flux convergence exceeds baroclinic and barotropic growth rates. Later in the life cycle, EKE decay due to geopotential flux divergence cancels or exceeds baroclinic and barotropic growth. A potential vorticity (PV) budget is used to diagnose tendencies related to group propagation. Although both upstream and downstream group speeds are possible because of the reversal in the mean meridional PV gradient, upstream propagation associated with the positive poleward PV gradient dominates wave packet evolution. Analogous to the concept of downstream development of midlatitude baroclinic waves, new AEWs develop preferentially upstream of the older ones, thus providing a mechanism for seeding new waves. It is suggested that these results are also relevant to AEW intermittency and storm-track structure.

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Bryce Tyner and Anantha Aiyyer

Abstract

The evolution of African easterly waves (AEWs) leading to tropical cyclones (TCs) in the Atlantic during 2000–08 is examined from isentropic potential vorticity (PV) and Lagrangian streamline perspectives. Tropical cyclone formation is commonly preceded by axisymmetrization of PV, scale contraction of the wave, and formation of a closed circulation within the wave. In these cases, PV associated with the synoptic-scale wave is irreversibly deformed and subsumed within the developing vortex. Less commonly, filamentation of the PV leads to separation and independent propagation of the wave and the TC vortex. In an example presented here, the remnant wave with a closed circulation persisted for several days after separation from the TC. A second TC did not result, consistent with several past studies that show that a midtropospheric closed gyre is not sufficient for TC genesis. Sometimes, an AEW and a weak TC remain coupled for a few days, followed by the dissipation of the TC and the continued propagation of the wave. Merger of tropical and extratropical PV anomalies is also often observed and likely helps maintain some waves. The results of this study are broadly consistent with recent Lagrangian analyses of AEW evolution during TC genesis.

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Michael Diaz and Anantha Aiyyer

Abstract

The stability of the African easterly jet (AEJ) is examined using idealized numerical simulations. It is found that a zonally homogeneous representation of the AEJ can support absolute instability in the form of African easterly waves (AEWs). This finding is verified through a local energy budget, which demonstrates the presence of both upstream and downstream energy fluxes. These energy fluxes allow unstable wave packets to spread upstream and downstream relative to their initial point of excitation. This finding is further verified by showing that the ground-relative group velocity of these wave packets has both eastward and westward components. In contrast with normal-mode instability theory, which emphasizes wave growth through energy extraction from the basic state, the life cycle of the simulated AEWs is strongly governed by energy fluxes. Convergent fluxes at the beginning of the AEW storm track generate new AEWs, whereas divergent fluxes at the end of the storm track lead to their decay. It is argued that, even with small normal-mode growth rates and a short region of instability, the presence of absolute instability allows AEWs to develop through the mixed baroclinic–barotropic instability mechanism, because upstream energy fluxes allow energy extracted through baroclinic and barotropic conversion to be recycled between successive AEWs.

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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.

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Michael Diaz and Anantha Aiyyer

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A genesis mechanism for African easterly waves (AEWs) is proposed. In the same manner that new troughs and ridges in the midlatitudes form downstream of existing ones through a mechanism known as downstream development, it is proposed that new AEWs can be generated upstream of existing AEWs. A local eddy kinetic energy budget of the AEW that ultimately became Hurricane Alberto (2000) demonstrates that upstream development explains its genesis more convincingly than previous theories of AEW genesis. The energetics and ageostrophic secondary circulation of a composite AEW are consistent with a new AEW forming as a result of this mechanism. Some strengths and weaknesses of upstream development as a paradigm for AEW genesis are discussed with respect to other potential mechanisms.

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Anantha R. Aiyyer and John Molinari

Abstract

A linear shallow water model is used to simulate the evolution of mixed Rossby–gravity (MRG) waves in background states representative of the convective phase of the Madden–Julian oscillation (MJO). Initial MRG wave structures are obtained analytically. The MJO basic state is defined by the steady response of the tropical atmosphere to localized heating. Results from the simulations reveal that variations in the background flow play a significant role in the evolution of the MRG waves. When the basic state is symmetric about the equator, the MRG wave amplifies within the convergent region of the background flow and the ensuing development remains symmetric. When the heating is asymmetric, both the basic state and the MRG wave evolution exhibit significant asymmetries. Prominent features of this simulation are the development and growth of a series of small-scale, off-equatorial eddies that resemble tropical-depression-type disturbances.

The results suggest that a persistent large-scale heating that is asymmetric with respect to the equator may lead to the growth of off-equatorial disturbances from an equatorial mode. These disturbances, approximately 1000–2000 km in scale, are considerably smaller than the initial wavelength of the MRG wave. It is suggested that the cyclonic elements among them could serve as seedlings for tropical cyclones. This process may be particularly relevant to cyclogenesis in the tropical western Pacific, a region where the MJO and MRG waves are frequently observed.

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Walter M. Hannah and Anantha Aiyyer

Abstract

African easterly wave (AEW) activity is examined in quadrupled CO2 experiments with the superparameterized CESM (SP-CESM). The variance of 2–10-day filtered precipitation increases with warming over the West African monsoon region, suggesting increased AEW activity. The perturbation enstrophy budget is used to investigate the dynamic signature of AEW activity. The northern wave track becomes more active associated with enhanced baroclinicity, consistent with previous studies. The southern track exhibits a surprising reduction of wave activity associated with less frequent occurrence of weak waves and a slight increase in the occurrence of strong waves. These changes are connected to changes in the profile of vortex stretching and tilting that can be understood as interconnected consequences of increased static stability from the lapse rate response, weak temperature gradient balance, and the fixed anvil temperature hypothesis.

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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.

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Badrinath Nagarajan and Anantha R. Aiyyer

Abstract

The quality of the ECMWF operational analyses is evaluated against independent upper-air sounding data collected during the Joint Air–Sea Monsoon Interaction Experiment (JASMINE; April–May 1999) and the Indian Ocean Experiment (INDOEX; February–March 1999).

Statistics of the difference between observations and analyses are compiled for temperature, humidity, and wind speed. The results show that the analyzed temperature has a cold bias between 1000 and 750 hPa. However, in the upper troposphere, a warm bias occurs between 350 and 150 hPa, while a cold bias is seen above 150 hPa. Compared to the observations, the analyzed humidity is lower between 1000 and 950 hPa and higher between 950 and 750 hPa. The analyzed wind speeds are close to observations over much of the troposphere, except near the tropopause, where they are overestimated by 1–2 m s−1 in the analyses. The low-level (1000– 750 hPa) biases in moisture and temperature in the ECMWF analyses over the Indian Ocean are similar to those reported for the tropical Pacific Ocean in past studies.

The occurrence of a cold and dry bias in the lowest 50 hPa indicates reduced convective available potential energy, which will render difficult the initiation and development of convection in numerical models initialized with these analyses. The moisture biases arise most likely because of the poor fit to humidity observations by the four-dimensional variational data assimilation scheme. This suggests that a better fit to humidity observations will yield an improved water vapor climatology over the Arabian Sea and Indian Ocean.

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