An analytical study of easterly waves over southern peninsular India during the northeast monsoon 2010

Easterly wave characteristics over southern peninsular India during the northeast monsoon season of 2010 are examined by means of synergetic analysis involving synoptic, statistical and numerical methods. NCEP 6-hourly reanalysis datasets of zonal and meridional winds, vertical velocity, temperature and net long wave radiation at 2.5° × 2.5° grid resolution for the period 20th October to 31st December, 2010 form the main database for the analysis. During this period, 3 easterly waves could be identified to have passed over this region and the time period of these waves are determined to be 4.2 days (4.5 days) by statistical methods (synoptic methods). The speed of movement, wavelength and amplitude of the waves are determined to be 7.28 ms-1, 2800 km and 6.7 ms-1 respectively. While rising motion is observed at most of the tropospheric levels over and behind the trough, subsidence occurs ahead of the trough. Divergence is noted over and behind the trough at upper troposphere while convergence occurs in the lower to mid-troposphere. Concomitantly warming is noted ahead of the trough while colder anomalies are noted in the lower levels over and behind the trough.

The Interaction between African Easterly Waves and Different Types of Deep Convection and Its Influence on Atlantic Tropical Cyclones

This study revisited the association of African easterly waves (AEWs) to Atlantic tropical cyclone (TC) development using weather states (WSs) from the International Satellite Cloud Climatology Project, National Hurricane Center best track hurricane data (HURDAT2), and reanalysis products. The WS data are used as a proxy for two different types of deep convection. This study covers July–October 1984–2009. Statistical analysis based on HURDAT2 and objectively tracked AEWs has shown that a small fraction (~20%) of the AEWs that propagate from Africa serve as TC precursors. About 80% of the AEWs from the continent were non-developing. As in the past work, our study showed an important difference between developing and non-developing AEWs. Composites based on developing AEWs revealed well-organized large scale deep convection (one type, composed of mesoscale systems and thick anvil clouds) is tightly coupled to the AEW trough, while scattered, less well-organized deep convection (second type, isolated cumulonimbus and cumulus congestus clouds) dominated a large area downstream of the developing AEW trough. Developing AEWs propagate westwards while strengthening. In contrast, non-developing AEWs showed that the peak well-organized deep convection is located either behind (to the east of) or far ahead (to the west) of the AEW trough (peaks values are not in close proximity). Moreover, well-organized deep convections associated with non-developing AEWs were weaker than those associated with developing AEWs. The results indicated that convective activity ahead of the non-developing AEWs is weak. Positive relative humidity (RH) anomalies dominate the area around AEWs and downstream over the main TC development region. In contrast, negative RH dominated the main TC development region ahead of non-developing AEWs, suggesting an unfavorable environment downstream of the AEWs. The results also showed that developing AEWs maintained stronger features in the lower and middle troposphere, while non-developing AEWs exhibited weaker structures, in agreement with past work. (Supplemental information related to this paper is available at the journal’s website of this edition).

Passive versus Active Transport of Saharan Dust Aerosols by African Easterly Waves

Theory and modeling are combined to reveal the physical and dynamical processes that control Saharan dust transport by amplifying African easterly waves (AEWs). Two cases are examined: active transport, in which the dust is radiatively coupled to the circulation; passive transport, in which the dust is radiatively decoupled from the circulation. The theory is built around a dust conservation equation for dust-coupled AEWs in zonal-mean African easterly jets. The theory predicts that, for both the passive and active cases, the dust transports will be largest where the zonal-mean dust gradients are maximized on an AEW critical surface. Whether the dust transports are largest for the radiatively passive or radiatively active case depends on the growth rate of the AEWs, which is modulated by the dust heating. The theoretical predictions are confirmed via experiments carried out with the Weather Research and Forecasting model, which is coupled to a dust conservation equation. The experiments show that the meridional dust transports dominate in the passive case, while the vertical dust transports dominate in the active case.

Energetics of Interactions between African Easterly Waves and Convectively Coupled Kelvin Waves

Perturbation kinetic and available energy budgets are used to explore how convectively coupled equatorial Kelvin waves (KWs) impact African easterly wave (AEW) activity. The convective phase of the Kelvin wave increases the African easterly jet’s meridional shear, thus enhancing the barotropic energy conversions, leading to intensification of southern track AEWs perturbation kinetic energy. In contrast, the barotropic energy conversion is reduced in the suppressed phase of KW. Baroclinic energy conversion of the southern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. AEWs in the convective phase of a Kelvin wave have stronger perturbation available potential energy generation by diabatic heating and stronger baroclinic overturning circulations than in the suppressed phase of a Kelvin wave. These differences suggest that southern track AEWs within the convective phase of Kelvin waves have more vigorous convection than in the suppressed phase of Kelvin waves. Barotropic energy conversion of the northern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. The convective phase of the Kelvin wave increases the lower-tropospheric meridional temperature gradient north of the African easterly jet, thus enhancing the baroclinic energy conversion, leading to intensification of northern track AEWs perturbation kinetic energy. In contrast, the baroclinic energy conversion is reduced in the suppressed phase of KW. These results provide a physical basis for the modulation of AEWs by Kelvin waves arriving from upstream.

Easterly waves in the East Pacific during the OTREC 2019 field campaign

Easterly waves (EWs) are off-equatorial tropical synoptic disturbances with a westward phase speed between 11-14 m s−1. Over the East Pacific in boreal summer, the combination of EWs and other synoptic disturbances, plus local mechanisms associated with sea surface temperature (SST) gradients, define the climatological structure of the Intertropical Convergence Zone (ITCZ). The East Pacific ITCZ has both deep and shallow convection that is linked to deep and shallow meridional circulations, respectively. The deep convection is located around 9°N over warm SSTs. The shallow convection is located around 6°N and is driven by the meridional SST gradient south of the ITCZ. This study aims to document the interaction between East Pacific EWs and the deep and shallow meridional circulations during the Organization of Tropical East Pacific Convection (OTREC) field campaign in 2019 using field campaign observations, ERA5 reanalysis, and satellite precipitation. We identified three EWs during the OTREC period using precipitation and dynamical fields. Composite analysis shows that the convectively active part of the EW enhances ITCZ deep convection and is associated with an export of column-integrated moist static energy (MSE) by vertical advection. The subsequent convectively suppressed, anticyclonic part of the EW produces an increase of moisture and column-integrated MSE by horizontal advection that likely enhances shallow convection and the shallow overturning flow at 850 hPa over the southern part of the ITCZ. Therefore, EWs appear to strongly modulate shallow and deep circulations in the East Pacific ITCZ.

Model Sensitivity Study of the Direct Radiative Impact of Saharan Dust on the Early Stage of Hurricane Earl

Current studies report inconsistent results about the impacts of Saharan dust on the development of African Easterly Waves (AEWs), the African Easterly Jet (AEJ), and tropical cyclones (TCs). We present a modeling case study to further elucidate the direct radiative impacts of dust on the early development stage of a TC. We conducted experiments using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem-V3.9.1) to simulate Hurricane Earl (2010) which was influenced by the dusty Saharan Air Layer (SAL). We used the aerosol product from ECMWF MACC-II as the initial and boundary conditions to represent aerosol distribution, along with typical model treatment of its radiative and microphysical effects in WRF. Our simulations at 36-km resolution show that, within the first 36 h, the presence of dust weakens the low-pressure system over North Africa by less than 1 hPa and reduces its mean temperature by 0.03 K. Dust weakens and intensifies the AEJ at its core and periphery, respectively, with magnitudes less than 0.2 m/s. Dust slightly shifts the position of 600 hPa AEW to the south and reduces its intensity prior to impacting the TC. Finally, TC with dust remains weaker.

Genesis of Easterly Waves over the Tropical Eastern Pacific and the Intra-Americas Sea

This paper explores a new mechanism for in situ genesis of Easterly Waves (EWs) over the tropical Eastern Pacific (EPAC). Using an idealized primitive equation model, it is shown that EWs can be triggered by finite-amplitude transient heating close to the mid-level jet at about 15°N over the EPAC and Intra Americas Seas region. The atmospheric response to heating initiates EWs downstream showing an EW structure within 4 days, with a wavelength and propagation speed of about 2000 km and 4.6 ms- 1, respectively; resembling EWs described in the literature. The most sensitive location for EW initiation from finite amplitude transient heating is located over the northern part of South America and extending to the EPAC. The closer the heating is to the jet, the bigger the response. A stratiform heating profile is the most efficient at triggering EPAC EWs. Comparisons of simulated EWs over the EPAC and West Africa reveal similar structures but with a shorter wavelength and much weaker amplitudes over the EPAC. EPAC EWs are dominated by horizontal tilts against the shear on the equatorial side of the jet consistent with barotropic growth with weaker low-level amplitudes compared to those seen over West Africa. These differences arise due to differences in the mean state EPAC having a shorter and weaker mid-level jet with less baroclinicity.

Effects of Saharan Dust on African Easterly Waves: The Impact of Aerosol-Affected Satellite Radiances on Data Assimilation

This study incorporates time-varying aerosols into satellite radiance calculations within the Global Data Assimilation System (GDAS) to investigate its impact on African easterly waves (AEWs) and their environment. Comparison of analysis fields from the aerosol-aware experiment and an aerosol-blind control during August 2017 showed that the aerosol-affected radiances accelerated the African easterly jet and West African monsoon flow; warmed the Saharan boundary layer; and modified the AEW vorticity structure, with increases in the northern circulation and decreases in the southern circulation. Analysis fields from each experiment were used in the Global Forecast System (GFS) to examine differences in forecasting two AEW cases that developed hurricanes over the Atlantic, but were structurally different over North Africa. The aerosol-aware experiment reduced errors in forecasting the AEW case whose northern circulation interacted with a large-scale Saharan dust plume; neutral improvement was found for the other AEW, which did not contain a northern circulation nor interacted with a dust plume. The changes to the analysis fields by the aerosol-aware assimilation are reminiscent of dust radiative effects that operate on AEWs and their environment. That is, the aerosol-affected radiances produce corrections to the brightness temperatures that modify the analysis fields like dust aerosols that are radiatively coupled to the atmospheric variables in the forecast model. We show qualitatively that dust radiative effects are captured by the aerosol-affected radiances for the AEW case that interacted with a dust plume, which served to improve forecasts of the wave downstream.

Favorable monsoon environment over eastern Africa for subsequent tropical cyclogenesis of African easterly waves

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 to non-developing AEWs. Using a 21-year August-to-September (1990-2010) climatology of AEWs, differences in the large-scale environment between developers and non-developers are identified and are propose to be used as key predictors of subsequent tropical cyclone formation and could informtropical cyclogenesis prediction. TC precursors when compared to non-developing 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 (AEJ). These large-scale conditions are linked to near-trough attributes of developing AEWs which favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, 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 to be southern-track AEWs than non-developing AEWs.