Westward Propagation Research Articles

NEOTECTONIC BASIN EVOLUTION IN CENTRAL-EASTERN MAINLAND GREECE: AN OVERVIEW

The neotectonic evolution of central-eastern mainland Greece (Sterea Hellas) is documented in the result of local extensional tectonics within a regional transtensional field, which is related to the westward propagation of the North Anatolian Fault. The observed tectonic structures within the neotectonic basins and their margins (range-bounding faults and fault zones, rotation of tectonic blocL·) suggest a close relation to the Parnassos Detachment Fault (PDF), which is a reused alpine thrust surface. Lokris basin (LB) occupied a central position in this neotectonic configuration, having received its first sediments in the Uppermost Miocene and subsequently been greatly affected by tectonic episodes, which continue until nowadays. LB is considered to have been separated from the present-day North Gulf of Evia not earlier than the Lower Pleistocene. Voiotihos Kifissos Basin, on the other hand, is tightly related to the activation of PDF, occupying the position of a frontal basin and having developed along the main detachment front.

Semiannual variation of zonal current along the equatorial Indian Ocean

Semiannual current variation in the equatorial Indian Ocean is examined using currents velocity records from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) Program. Spectral analysis shows that semiannual variation is the strongest variation in equatorial Indian Ocean. Further analysis exhibits that this variation is associated with Wyrtki Jet phenomenon and forced by wind along the equator. Upward phase propagation (downward energy propagation) is clearly shown as indication of the propagation of equatorial Kelvin wave. Cross correlation of two mooring data captures westward propagation of reflected Rossby wave that interferes the propagation of Kelvin wave. During IOD positive event, stronger reflected Rossby wave occurs and causes weakening of upward phase propagation. This result reveals that interferences of equatorial wave have important role on forming vertical structure of currents along equatorial Indian Ocean.

Sensitivity of Tropical Cyclone Track to the Vertical Structure of a Nearby Monsoon Gyre

Abstract The impact of different vertical structures of a nearby monsoon gyre (MG) on a tropical cyclone (TC) track is investigated using idealized numerical simulations. In the experiment with a relatively deeper MG, the TC experiences a sharp northward turn at a critical point when its zonal westward-moving speed slows down to zero. At the same time, the total vorticity tendency for the TC wavenumber-1 component nearly vanishes as the vorticity advection by the MG cancels the vorticity advection by the TC. At this point, the TC motion is dominated by the beta effect, as in a no-mean-flow environment, and takes a sharp northward turn. In contrast, the TC does not exhibit a sharp northward turn with a shallower MG nearby. In the case with a deeper MG, a greater relative vorticity gradient of the MG promotes a quicker attraction between the TC and MG through the vorticity segregation process. In addition, a larger outer size of the TC also favors a faster westward propagation from its initial position, thus having more potential to collocate with the MG. Once the coalescence is in place, the Rossby wave energy dispersion associated with the TC and MG together is enhanced and rapidly strengthens the southwesterly flow on the eastern flank of both systems. The steering flow from both the beta gyre and the Rossby wave dispersion leads the TC to take a sharp northward track when the total vorticity tendency is at its minimum. This study indicates the importance of good representations of the TC structure and its nearby environmental flows in order to accurately predict TC motions.

Mesozoic plate subduction in West Pacific and tectono-magmatic response in the East Asian ocean-continent connection zone

The Mesozoic impact to the North China and the South China blocks in the East Asian Continental Margin by the West Pacific subduction processes and deep processes, the tectonic, magmatic mineralization and geomorphic evolutionary trends show some similarities, but there are also significant differences beween them, the dynamic mechanism is generally believed to be caused by the Paleo-Pacific Plate subduction. But how they are related is a long-term unresolved question as a conundrum and a focus of geological researches, and a huge controversy is still remained. This paper systematically summarizes the comprehensive achievements of deformation analysis, igneous geochronology, tomography and other field observations in the East Asian Continental Margin in recent decade, briefly discussing the pre-Yanshanian final assembly of blocks and the formation of the unified continental margin in China, and mainly focusing on the review of Jurassic and Cretaceous geological processes in the East Asian Ocean-Continent Connection Zone in the Yanshanian period. The temporal and spatial evolution of structural propagation, age zonation and migration of igneous rocks and huge tectono-morphological inversion in the Earth surface system, combined with deep processes are jointly to put forward: the destruction of the North China Craton was mainly affected by the westard early-stage layerred rollback, delamination and thinning of the deep lithosphere, resulting in early Yanshanian westward migration of tectonism and magmatism following the middle-stage multiple-directional delamination and the late-stage eastward withdrawal of delamination; coevally, the combined effect of the closure of the Okhotsk Ocean to the north and the subduction of the Bangong-Co-Nujiang Ocean to the south made the East Asian Ocean-Continent Connection Zone under the tectonic setting of overall compression, the asthenosphere upwelling at depth continued an eastward migration to drive lithosphere continuously eastward and periodical, alternative extension and compression. The South China Block experienced the westward flat subduction at early stage, resulting in the westward propagation of deformation and magmatism, following the late two-stage delamination to induce the eastward tectono-magmatism. In summary, the tectono-magmatic differences between the North China and South China blocks are caused by their difference of the deep mantle processes and the East Asian plate superconvergence.

A Statistical Study of Inertia Gravity Waves in the Lower Stratosphere Over the Arctic Region Based on Radiosonde Observations

AbstractRelative to many investigations of inertial gravity waves (IGWs) in the Antarctic, IGW activity in the Arctic region was paid less attention to. We use radiosonde observations at the Ny‐Alesund station (78.9°N, 11.9°E) from April 2012 to June 2016 to study the IGW characteristics in the lower stratosphere over the Arctic. The observation reveals a prevailing eastward zonal background wind below 20 km and an obvious annual cycle of the background temperature from the troposphere to the lower stratosphere, which is different from the results in the middle and low latitudes. By combining Lomb‐Scargle spectrum and hodograph technique, case study demonstrates that the lower stratospheric IGWs exhibit a feature of freely propagating waves. Statistical analysis indicates that the IGWs have dominant horizontal (vertical) wavelength of 50–1,050 km (1–4 km) and ratio (1–2.5) of the intrinsic to inertia frequencies. Wave energy exhibits an annual oscillation with the maximum in winter and the minimum in summer. In winter, the downward propagating waves increase to about 20% due to polar stratospheric vortex. Because of the lower atmospheric filtering, the IGWs display a dominant direction of westward propagation, thus have a mean vertical flux of −0.647 mPa for the zonal momentum, which indicates that the IGWs can put a westward drag on the atmospheric wind field over the Arctic as they break and dissipate. All the vertical wavenumber spectra have spectral slopes from −2.23 to −2.99 close to the universal spectrum index of −3.

Opening of Hess Deep Rift at the Galapagos Triple Junction

AbstractAt the Galapagos triple junction, the westward propagating Cocos‐Nazca (C‐N) Rift breaks into ~0.5 Ma crust accreted at the East Pacific Rise. Rifting transitions to full magmatic seafloor spreading in the wake of the propagating tip. The 25‐km‐long Hess Deep rift is the transitional segment from rifting to spreading. Intrarift ridge (IRR), located within Hess Deep rift, is interpreted as a detachment fault, which exhumes deep‐seated rocks to the seafloor. Although transitional segments must have occurred throughout the westward propagation of C‐N Rift, IRR is the only obvious detachment fault along the base of the Rift scarps in the last ~5 Ma of its propagation. IRR formation may be in response to a decrease in spreading rate (~40 to <20 mm/yr) and presumed lower melt supply, resulting from the formation of the Galapagos microplate ~1.4 Ma, which now controls the opening at the C‐N Rift tip.

Interannual Variability of Eddy Kinetic Energy in the Subtropical Southeast Indian Ocean Associated With the El Niño‐Southern Oscillation

AbstractInterannual variability of eddy kinetic energy (EKE) in the subtropical southeast Indian Ocean (SEIO) is investigated using satellite observations in three regions in the 20°S–35°S latitude band: R1 (108°E–115°E), R2 (100°E–108°E), and R3 (60°E–100°E). The El Niño‐Southern Oscillation (ENSO) plays an important role in modulating the interannual variability of EKE in the SEIO. EKE in the three regions shows negative correlations with the Nino3.4, lagging Nino3.4 by 2, 14, and 22 months, respectively. In R1, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of meridional velocity shear between the upper poleward Leeuwin Current (LC) and the underlying equatorward Leeuwin Undercurrent (LUC). During La Niña (El Niño) year, both the poleward LC and equatorward LUC strengthened (weakened) due to the high (low) sea level anomaly (SLA) propagating from the Pacific Ocean as Rossby wave under geostrophic equilibrium, and baroclinic instability of vertical shear enhanced (slackened), further induced the strong (weak) EKE in the eastern boundary. In R2, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the central and southern branches of the upper eastward South Indian Countercurrent (SICC) and the lower extending westward South Equatorial Current (SEC). In R3, both the ENSO and the Southern Annual Mode modulate the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the SICC and SEC. The interannual variability of EKE in the interior SEIO could be influenced by westward propagation of EKE originated from eastern boundary.

Abundant aftershock sequence of the 2015 Mw7.5 Hindu Kush intermediate-depth earthquake

The 2015 M_w7.5 Hindu Kush earthquake occurred at a depth of 213 km beneath the Hindu Kush region of Afghanistan. While many early aftershocks were missing from the global earthquake catalogues, this sequence was recorded continuously by eight broad-band stations within 500 km. Here we use a waveform matching technique to systematically detect earthquakes around the main shock. More than 3000 events are detected within 35 d after the main shock, as compared with 42 listed in the Advanced National Seismic System catalogue (or 196 in the International Seismological Centre catalogue). The aftershock sequence generally follows the Omori's law with a decay constant p = 0.92. We also apply the recently developed double-pair double-difference technique to relocate all detected aftershocks. Most of them are located to the west of the hypocentre of the main shock, consistent with the westward propagation of the main-shock rupture. The aftershocks outline a nearly vertical southward dipping plane, which matches well with one of the nodal planes of the main shock. We conclude that the aftershock sequence of this intermediate-depth earthquake shares many similarities with those for shallow earthquakes and infer that there are some common mechanisms responsible for shallow and intermediate-depth earthquakes.

Does the MJO Have a Westward Group Velocity?

AbstractCurrent theoretical studies have a debate on whether the Madden–Julian oscillation (MJO) has a zero or westward group velocity. A recent analysis of the observed Hovmöller diagram of MJO signals suggested that the MJO has a significant westward group velocity. Here it is shown that the observed MJO has a negligibly small group velocity, which is manifested in two aspects. First, on the wavenumber–frequency spectra diagram the precipitation spectra indicate quasi independence of the MJO frequency on wavenumber, suggesting a nearly vanishing group velocity. Second, on the Hovmöller diagram of the regressed intraseasonal daily precipitation, the MJO group velocity is defined by the propagation of the wave envelopes of the precipitation and is shown to be negligibly small for the eastward propagating signals. The causes of the discrepancy between this study and the recent study mentioned above are the calculating method and the data filtering process. The group velocity in the recent study is calculated by the propagation of local convection extrema, which does not necessarily indicate the propagation of the wave envelopes. More importantly, the westward propagation of the local convection extrema is an artifact of the data filtering. The Hovmöller diagram in the recent study was constructed by using only the eastward propagating wavenumber-1–5 signals. This truncation of data onto the planetary scales of the eastward wavenumber domain fails to resolve the Maritime Continent “barrier effect,” causing significant artificial westward propagation of local convection extrema.

A Positive Feedback Process Between Tropical Cyclone Intensity and the Moisture Conveyor Belt Assessed With Lagrangian Diagnostics

AbstractUsing a cloud‐resolving regional model and Lagrangian diagnostics, we assess a positive feedback process between tropical cyclone (TC) intensity and the moisture conveyor belt (MCB), which connects a TC and the Indian Ocean (IO), the South China Sea (SCS), and the Philippine Sea (PS) vapors, from a macroscopic view. We performed sensitivity experiments that modified the observed sea surface temperature field over the IO and the SCS to regulate the MCB behavior, and we examined the remote response of a prototypical TC. The results show that the connection between MCB formation and TC development is very robust, which was also observed in another TC's case. The MCB plays a vital role in transporting lots of moist air parcels toward the TC from the IO, SCS, and PS regions. The transported parcels, which further gained the underlying ocean vapor along the MCB, are easily trapped in the inner core by radial inflow in the atmospheric boundary layer and, subsequently, release latent heat around the eyewall, resulting in the TC's intensifying. This acts to further penetrate the moist parcels of remote ocean origin into the inner core through the enhanced and expanded inflow. An additional experiment suggested that the MCB is not formed unless the westward propagation of equatorial waves induced by TC heating overlaps with the background monsoon westerlies. These findings support the reliability and validity of TC–MCB feedback.

On the Seasonal Variability of Sea Surface Temperature and Air–Sea Fluxes in the Lakshadweep Sea

Using satellite-sensed sea surface temperature (SST), winds, and air–sea fluxes, we studied seasonal variability of SST and air–sea fluxes in the Lakshadweep Sea (LS) and quantified their relations. Seasonal variability of SST exhibits a bimodal nature, for that air–sea fluxes and oceanic processes operate differently. Analysis demonstrates that seasonal SST variability due to air–sea fluxes in the LS arises with a time lag of 1 or 2 months. For winter cooling, low solar insolation plays a cardinal role and the effects due to net longwave radiation (NLWR) and latent heat flux (LHF) are surpassed. Although net shortwave radiation (NSWR) is minimum in December, low winter SST with a lag of 1 month occurs in January. All air–sea fluxes are favorable for summer monsoon cooling though cooling is also set by several oceanic processes such as advection, vertical overturning, and turbulent mixing. During pre-monsoon warming months March–April, all air–sea fluxes are favorable. But during intense warming in May, though SST is high (> 29.8 °C) in the LS, air–sea fluxes are not supportive as incoming solar insolation is low and evaporative heat loss is high. This indicates that warming during May is the resultant of the expansion of Indian Ocean warm pool into the LS and westward propagation of the Lakshadweep high (LH) as downwelling Rossby wave. For the post-monsoonal warming during October–November, low NSWR is unfavorable but distributions of LHF and NLWR demonstrate suitable condition. For warming during November, vertical ocean heat convergence and less evaporation act favorably.

Saharan dust and the African easterly jet–African easterly wave system: Structure, location and energetics

The radiative effects of Saharan mineral dust (SMD) aerosols on the structure, location and energetics of the African easterly jet–African easterly wave (AEJ‐AEW) system are examined for July–September 2006. Experiments are conducted with and without SMD using the Weather Research and Forecasting (WRF) model, which is radiatively coupled to an interactive dust model. The SMD‐modified heating field shifts the AEJ northward, upward and westward, and enhances its zonal asymmetry. These SMD‐induced changes to the AEJ are manifest in the AEWs: the northern and southern tracks of the AEWs shift northward (like the AEJ); and the zonal‐scale of the AEWs expands and their westward propagation increases.The SMD also strengthens the energetics of the AEJ‐AEW system. The domain and temporally averaged baroclinic energy conversion, which is an order of magnitude larger than the barotropic conversion, increases by a factor of 2.5. The eddy kinetic energy and generation of available potential energy increase by factors of 1.5 and 2.7, respectively.The implications of an SMD‐modified AEJ‐AEW system for West African precipitation and tropical cyclogenesis in the eastern Atlantic Ocean are discussed.

Syn‐orogenic fluid flow in the Jaca basin (south Pyrenean fold and thrust belt) from fracture and vein analyses

AbstractThis study aims at understanding the origin and nature of syn‐orogenic fluid flow in the Jaca basin from the South Pyrenean fold and thrust‐belt, as recorded in calcite and quartz veins of the Sierras Interiores (Spain) and the turbiditic basin, which cover upper Cretaceous to Late Eocene syntectonic deposits. The fracture network consists of a classical pattern of transverse and longitudinal fractures related to Layer Parallel Shortening (LPS) and folding respectively. Veins filled equally about the third of fractures in the carbonate shelf and turbidites. Carbon and oxygen isotopes of calcite veins mostly indicate precipitation from isotopically buffered water, consistent with high water‐rock interaction. In the Sierras Interiores, petrographical observations and fluid inclusion microthermometry are consistent with two distinct stages of precipitation. The first stage is characterized by relatively low Th and low salinities (155–205 °C and 0.5–3.2 wt% eq. NaCl). The second stage, which was characterized both by the formation of mode‐I joints and by mode‐I reactivation of pre‐existing veins, shows higher Th and salinities (215–270 °C and 2.2–5.7 wt% eq. NaCl). Waters recorded in the second stage are interpreted to have interacted with underlying Triassic evaporites and flowed along major thrusts before vein precipitation, which are locally in thermal disequilibrium with host‐rocks. We suggest the transition from a rather closed hydrological system during the first stage of vein formation, interpreted to have occurred during Eaux‐chaudes thrusting (upper Lutetian‐Bartonian), to a more open hydrological system during the second stage, which likely occurred during Gavarnie thrusting (Priabonian‐early Rupelian). Finally, we also document the migration in space and time of hydrothermal pulses along the South Pyrenean Foreland Basin, related to the westward propagation of major thrusts during the Pyrenean orogeny.

Trapped planetary (Rossby) waves observed in the Indian Ocean by satellite borne altimeters

Abstract. Using 20 years of accurately calibrated, high-resolution observations of sea surface height anomalies (SSHAs) by satellite borne altimeters, we show that in the Indian Ocean south of the Australian coast the low-frequency variations of SSHAs are dominated by westward propagating, trapped, i.e., non-harmonic, Rossby (Planetary) waves. Our results demonstrate that the meridional-dependent amplitudes of the SSHAs are large only within a few degrees of latitude next to the southern Australian coast while farther in the ocean they are uniformly small. This meridional variation of the SSHA signal is typical of the amplitude structure in the trapped wave theory. The westward propagation speed of the SSHA signal is analyzed by employing three different methods of estimation. Each one of these methods yields speed estimates that can vary widely between adjacent latitudes but the combination of at least two of the three methods yields much smoother variation. The estimates obtained in this manner show that the observed phase speeds at different latitudes exceed the phase speeds of harmonic Rossby (planetary) waves by 140 to 200 % (which was also reported in previous studies). In contrast, the theory of trapped Rossby (planetary) waves in a domain bounded by a wall on its equatorward side yields phase speeds that approximate more closely the observed phase speeds in the study area.

Equatorial Indian Ocean subsurface current variability in an Ocean General Circulation Model

The variability of subsurface currents in the equatorial Indian Ocean is studied using high resolution Ocean General Circulation Model (OGCM) simulations during 1958–2009. February–March eastward equatorial subsurface current (ESC) shows weak variability whereas strong variability is observed in northern summer and fall ESC. An eastward subsurface current with maximum amplitude in the pycnocline is prominent right from summer to winter during strong Indian Ocean Dipole (IOD) years when air-sea coupling is significant. On the other hand during weak IOD years, both the air-sea coupling and the ESC are weak. This strongly suggests the role of ESC on the strength of IOD. The extension of the ESC to the summer months during the strong IOD years strengthens the oceanic response and supports intensification and maintenance of IODs through modulation of air sea coupling. Although the ESC is triggered by equatorial winds, the coupled air-sea interaction associated with IODs strengthens the ESC to persist for several seasons thereby establishing a positive feedback cycle with the surface. This suggests that the ESC plays a significant role in the coupled processes associated with the evolution and intensification of IOD events by cooling the eastern basin and strengthening thermocline-SST (sea surface temperature) interaction. As the impact of IOD events on Indian summer monsoon is significant only during strong IOD years, understanding and monitoring the evolution of ESC during these years is important for summer monsoon forecasting purposes. There is a westward phase propagation of anomalous subsurface currents which persists for a year during strong IOD years, whereas such persistence or phase propagation is not seen during weak IOD years, supporting the close association between ESC and strength of air sea coupling during strong IOD years. In this study we report the processes which strengthen the IOD events and the air sea coupling associated with IOD. It also unravels the connection between equatorial Indian Ocean circulation and evolution and strengthening of IOD.

Equatorward propagating auroral arcs driven by ULF wave activity: Multipoint ground‐ and space‐based observations in the dusk sector auroral oval

AbstractObservations of multiple equatorward propagating arcs driven by a resonant Alfvén wave on closed field lines are presented. Data sets from the European Incoherent Scatter Svalbard Radar (ESR) and Meridian Scanning Photometer in Longyearbyen, All‐Sky Camera in Ny Ålesund, ground magnetometer data in Svalbard, and Defense Meteorological Satellite Program (DMSP) F16 satellite were utilized to study the arc structures. The arcs had an equatorward phase propagation of ~0.46 km s−1 and were observed in the dusk ionosphere from 1800 to 2030 magnetic local time. Analysis of the optical data indicates that the Alfvén wave had a frequency of 1.63 mHz and an azimuthal wave number, m ~ −20 (the negative sign indicating a westward propagation). Inverted‐V electron populations associated with field‐aligned currents of between 0.5 and 0.8 μA m−2 are observed by DMSP F16 inside the arc structures. In addition to electron density enhancements associated with the arcs, the ESR data show elevated ion temperatures in between the arcs consistent with electric field enhancements and ionospheric heating effects. The combination of ESR and DMSP F16 data indicates that the wave energy was dissipated through ionospheric Joule and/or ion frictional heating and acceleration of particles into the ionosphere, generating the auroral displays. The fine‐scale structuring, in addition to the propagation direction and scale size, would suggest that the auroral features are the signatures of a field line resonance driven by an interaction with a compressional fast mode wave propagating earthward from the magnetotail.

The role of intraseasonal variability at mid‐high latitudes in regulating Pacific blockings during boreal winter

ABSTRACTBy using daily National Centers for Environmental Prediction‐Department of Energy reanalysis data from 1979 to 2015, we investigated the connection between Pacific blockings and mid‐high latitude atmospheric intraseasonal oscillation (ISO) at 500 hPa in boreal winter. The ISO perturbation over a key area of 150°E–155°W, 60°–75°N is most co‐variated with Pacific blockings. The averaged geopotential height over this key area exhibits a statistically significant 10–35‐day periodicity. The ISO perturbation associated with the Pacific blocking activity shows pronounced westward propagation. The perturbation initiates at about 120°W, triggered by intraseasonal wave energy accumulation. A geopotential tendency diagnosis reveals that the time change rate of the geopotential height over the key area is primarily attributed to the advection of planetary vorticity by the ISO meridional flow. A mechanism is put forward in which the westward‐propagating ISO perturbations induce a blocking over the Pacific sector. When a strong northward ISO flow arrives in the key area, it acts against the gradient of planetary vorticity, leading to a negative vorticity advection. This negative advection tends to strengthen local anticyclonic vorticity and geopotential height anomalies. Thus, the westward‐propagating ISO meridional flow plays an active role in inducing the low‐frequency blocking activity over the Pacific sector.

Upper-ocean dynamical features and prediction of the super El Niño in 2015/16: A comparison with the cases in 1982/83 and 1997/98

The 2015/16 super El Nino event has been widely recognized as comparable to the 1982/83 and 1997/98 El Nino events. This study examines the main features of upper-ocean dynamics in this new super event, contrasts them to those in the two historical super events, and quantitatively compares the major oceanic dynamical feedbacks based on a mixed-layer heat budget analysis of the tropical Pacific. During the early stage, this new event is characterized by an eastward propagation of SST anomalies and a weak warm-pool El Nino; whereas during its mature phase, it is characterized by a weak westward propagation and a westward-shifted SST anomaly center, mainly due to the strong easterly wind and cold upwelling anomalies in the far eastern Pacific, as well as the westward anomalies of equatorial zonal current and subsurface ocean temperature. The heat budget analysis shows that the thermocline feedback is the most crucial process inducing the SST anomaly growth and phase transition of all the super events, and particularly for this new event, the zonal advective feedback also exerts an important impact on the formation of the strong warming and westward-shifted pattern of SST anomalies. During this event, several westerly wind burst events occur, and oceanic Kelvin waves propagate eastwards before being maintained over eastern Pacific in the mature stage. Meanwhile, there is no evidence for westward propagation of the off-equatorial oceanic Rossby waves though the discharg-ing process of equatorial heat during the development and mature stages. The second generation El Nino prediction system of the Beijing Climate Center produced reasonable event real-time operational prediction during 2014–16, wherein the statistical prediction model that considers the preceding oceanic precursors plays an important role in the multi-method ensemble prediction of this super.

Effects of the South Asian Monsoon Intraseasonal Modes on Genesis of Low Pressure Systems over Bangladesh

The quasi-biweekly oscillation (QBW) is a dominant intraseasonal mode in summer rainfall over Bangladesh. Active phases of the QBW are often accompanied by low pressure systems (LPSs) such as vortex-type lows. This study investigated the effects of two intraseasonal modes, the QBW and the boreal summer intraseasonal oscillation (BSISO), on the genesis of LPSs over Bangladesh during 29 summer monsoon seasons. Daily lag composites of convection and low-level atmospheric circulation were constructed for active-phase cases with LPSs (LPS case) and without LPSs (non-LPS case) based on rainfall in the QBW over Bangladesh. In the QBW mode, a westward propagation of an anticyclonic anomaly from the western Pacific to the Bay of Bengal (BoB) is common in both cases. However, the anticyclonic center in the LPS case is located slightly to the east of that in the non-LPS case, which results in stronger cyclonic vorticity over and around Bangladesh. In contrast, the BSISO mode shows an opposite phase between the two cases: a cyclonic (anticyclonic) anomaly propagating northward from the equator to the BoB in the LPS case (non-LPS case). In the LPS case, the cyclonic anomaly in the BSISO mode enhances the westerly (easterly) flow over the BoB (Bangladesh) in the active phase, resulting in the enhancement of cyclonic vorticity over the northern BoB and Bangladesh, in cooperation with the QBW mode. These results suggest that both the QBW and BSISO modes have significant influence on the environmental conditions for LPS genesis over Bangladesh.

Subcritical Destabilization of African Easterly Waves by Saharan Mineral Dust

Abstract A theoretical framework is presented that exposes the radiative–dynamical relationships that govern the subcritical destabilization of African easterly waves (AEWs) by Saharan mineral dust (SMD) aerosols. The framework is built on coupled equations for quasigeostrophic potential vorticity (PV), temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of the AEWs. The expressions are functions of the domain-averaged wave activity, which is generated by the direct radiative effects of the SMD. The wave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in PV and SMD. Using an idealized version of the Weather Research and Forecasting (WRF) Model coupled to an interactive dust model, a linear analysis shows that, for a subcritical African easterly jet (AEJ) and a background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their westward propagation, in agreement with the theoretical prediction. The SMD-induced growth rates are commensurate with, and can sometimes exceed, those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic–baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs.