Convective Destabilization Research Articles

A high-resolution seismic velocity model for East Asia using full-waveform tomography: Constraints on India-Asia collisional tectonics

East Asia's complex tectonic history makes it a unique place for plate-tectonic studies. While the crust and mantle structures beneath this region have been extensively investigated by receiver function and many different kinds of tomography studies, discrepancies among seismic velocity models have complicated our understanding of East Asia's tectonic evolution. Here, we use full-waveform adjoint tomography to study the seismic P- and S-wave velocity structures of the crust and mantle beneath East Asia. We use a large waveform dataset from 141 earthquakes recorded by 4640 broadband seismic stations. Our inversion optimizes the normalized correlation coefficient between synthetic and observed seismograms within individual time windows of different seismic phases, from P arrivals to the slowest surface waves. This approach allows us to fit regional multipathed waveforms and provides very well-resolved seismic P- and S-wave velocity structures from the crust to depths of about 800 km. Furthermore, we are able to obtain this resolution over most of East Asia allowing us to compare the amplitude and shapes of anomalies across the entire region. The observed structures in our new full-waveform tomography model (FWEA23) correlate well with tectonic units, revealing sharp contrasts across tectonic boundaries. Our model reveals narrow and strong linear fast anomalies associated with subduction zones in the western Pacific and Southeast Asia. Compared to these oceanic slabs, we observe different and more complicated mantle structures beneath the India-Asia continental collision zone. While our model spans a wide region in East Asia, we focus our interpretations on the seismic velocity anomalies beneath Tibet and surrounding regions, as our model provides significant implications for India-Asia continental collision tectonics. Our analysis suggests that cratonic Indian lithosphere collided with Eurasian plate around 25–20 Ma, subsequently underthrusting beneath Tibet horizontally with little deformation. This resulted in thickening, destabilization, and delamination of the rheologically weaker and less buoyant mantle lithosphere beneath north-central Tibet. Moreover, our model reveals isolated lithospheric fragments within the mantle transition zone and lower mantle beneath southern Tibet and northern India, suggesting that non-cratonic Greater India continental lithosphere has broken off and sunk into the deeper mantle. The lack of continuity among these lithospheric fragments suggests a convective destabilization process, distinct from typical oceanic plate subduction. Our results emphasize the important role of rheological properties of continental lithospheres in shaping the dynamics of continental collision.

Fast Physics and Slow Physics in the Nonlinear Dansgaard–Oeschger Relaxation Oscillation

AbstractThe Dansgaard–Oeschger (D-O) relaxation oscillation that governed glacial climate variability during marine isotope stage 3 has been accurately simulated using a high-resolution coupled climate model. Here the authors present additional detailed analyses of both the slow physics transition between warm and cold states and the fast physics transition between cold and warm states of the D-O cycle. First, the authors demonstrate that the mechanisms active during the slow transition from interstadial to stadial conditions involves the continuous flux of thick and old sea ice from the Arctic basin into the North Atlantic subpolar gyre region along the East Greenland Current. During this slow physical process, the freshwater input from sea ice melting as it moves over the surface of the warm ocean restratifies the high-latitude North Atlantic and leads to a significant reduction in the rate of North Atlantic Deep Water formation. A detailed freshwater budget and hydrography analysis is also presented to demonstrate that the D-O cycle is a low-latitude–high-latitude salt oscillator as the authors have previously argued. Second, the authors provide a more detailed analysis than previously of the fast-time-scale processes that govern the extremely rapid transition from cold stadial conditions back to the warm interstadial state. These are associated with the onset of a sub-sea ice thermohaline convective instability, which opens a massive polynya to the north of the southern boundary of the extensive North Atlantic sea ice lid that is characteristic of stadial conditions. This instability is enabled by the continuous increase of salinity above the sub-sea ice pycnocline, which eliminates the vertical salinity gradient that prevents convective destabilization of the water column under full stadial conditions. This reduction in the vertical salinity gradient beneath the sea ice lid results from the continuing northward salt transport by the North Atlantic gyre circulation once the expansion of the stadial sea ice lid has ceased. The onset of instability occurs in the Irminger basin to the south of Denmark Strait, and the authors discuss the reason for this localization of instability onset.

Evolution of the upper‐level thermal structure in tropical cyclones

AbstractTropical cyclones (TCs) are associated with tropopause‐level cooling above tropospheric warming. We collect temperature retrievals from 2007 to 2014 near worldwide hurricane‐strength TCs using three remote sensing platforms: the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), the Advanced Microwave Sounding Unit‐A (AMSU‐A), and geostationary infrared (IR) imagery. These retrievals are composited about the lifetime maximum intensity (LMI) to examine the evolution of the fine‐scale temperature structure within TCs. The convective structure evolves highly asymmetrically about LMI, while intensity evolution shows a much weaker degree of asymmetry. Relative to the far‐field structure, tropopause‐level cooling occurs before a tropospheric warm core is established. We speculate that the associated convective destabilization exerts a positive feedback on TC development by increasing the depth of existing convection. Tropopause‐level cold anomalies move away from the storm after LMI, potentially increasing the near‐surface horizontal pressure gradient toward the storm center and increasing the maximum winds.

Cross-diffusion-driven hydrodynamic instabilities in a double-layer system: General classification and nonlinear simulations.

Cross diffusion, whereby a flux of a given species entrains the diffusive transport of another species, can trigger buoyancy-driven hydrodynamic instabilities at the interface of initially stable stratifications. Starting from a simple three-component case, we introduce a theoretical framework to classify cross-diffusion-induced hydrodynamic phenomena in two-layer stratifications under the action of the gravitational field. A cross-diffusion-convection (CDC) model is derived by coupling the fickian diffusion formalism to Stokes equations. In order to isolate the effect of cross-diffusion in the convective destabilization of a double-layer system, we impose a starting concentration jump of one species in the bottom layer while the other one is homogeneously distributed over the spatial domain. This initial configuration avoids the concurrence of classic Rayleigh-Taylor or differential-diffusion convective instabilities, and it also allows us to activate selectively the cross-diffusion feedback by which the heterogeneously distributed species influences the diffusive transport of the other species. We identify two types of hydrodynamic modes [the negative cross-diffusion-driven convection (NCC) and the positive cross-diffusion-driven convection (PCC)], corresponding to the sign of this operational cross-diffusion term. By studying the space-time density profiles along the gravitational axis we obtain analytical conditions for the onset of convection in terms of two important parameters only: the operational cross-diffusivity and the buoyancy ratio, giving the relative contribution of the two species to the global density. The general classification of the NCC and PCC scenarios in such parameter space is supported by numerical simulations of the fully nonlinear CDC problem. The resulting convective patterns compare favorably with recent experimental results found in microemulsion systems.

The Influence of Time-Dependent Melting on the Dynamics and Precipitation Production in Maritime and Continental Storm Clouds

Abstract Simulations of one maritime and four continental observed cases of deep convection are performed with the Hebrew University Cloud Model that has spectral bin microphysics. The maritime case is from observations made on 18 September 1974 during the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE). The continental storm cases are those of summertime Texas clouds observed on 13 August 1999, and green-ocean, smoky, and pyro-clouds observed during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia–Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA–SMOCC) campaign on 1–4 October 2002. Simulations have been performed for these cases with a detailed melting scheme. This scheme allows calculation of liquid water fraction within each mass bin for the melting of graupel, hail, snowflakes, and crystals, as well as alteration of the sedimentation velocity of ice particles in the course of their melting. The results obtained with the detailed melting scheme are compared with corresponding results from simulations involving instantaneous melting at the freezing (0°C) level. The detailed melting scheme allows penetration of ice from the freezing level down into the boundary layer by distances ranging from a few hundred meters for the numerous, smaller particles to ∼1.5 km for the largest particles, which are much scarcer. In these simulations, most of the mass of ice falling out melts over this short distance of a few hundred meters. The deepening and intensification of the layer of latent cooling enhances the convective destabilization of the troposphere. This effect is especially pronounced under continental conditions, causing significant changes in the accumulated rain amount.

Palaeo-altimetry of Tibet (reply)

Molnar et al.1 question our conclusion on the role of convective destabilization of thickened mantle lithosphere in determining the surface elevation history of the Tibetan plateau. The primary argument depends on our interpretation2,3 of oxygen-isotope-based estimates of palaeo-elevation of the Lunpola basin, a locality in the centre of the plateau.

Contributions from California Coastal-Zone Surface Fluxes to Heavy Coastal Precipitation: A CALJET Case Study during the Strong El Niño of 1998

Abstract Analysis of the case of 3 February 1998, using an extensive observational system in the California Bight during an El Niño winter, has revealed that surface sensible and latent heat fluxes within 150 km of the shore contributed substantially to the destabilization of air that subsequently produced strong convection and flooding along the coast. Aircraft, dropsonde, and satellite observations gathered offshore documented the sea surface temperatures (SSTs), surface fluxes, stratification, and frontal structures. These were used to extrapolate the effects of the fluxes on the warm-sector, boundary layer air ahead of a secondary cold front as this air moved toward the coast. The extrapolated structure was then validated in detail with nearshore aircraft, wind profiler, sounding, and buoy observations of the frontal convection along the coast, and the trajectory transformations were confirmed with a model simulation. The results show that the surface fluxes increased CAPE by about 26% such that the nearshore boundary layer values of 491 J kg−1 were near the upper end of those observed for cool-season California thunderstorms. The increased CAPE due to upward sensible and latent heat fluxes was a result of the anomalously warm coastal SSTs (+1°–3°C) typical of strong El Niño events. Applications of the extrapolation method using a surface flux parameterization scheme and different SSTs suggested that convective destabilization due to nearshore surface fluxes may only occur during El Niño years when positive coastal SST anomalies are present. The fluxes may have no effect or a stabilizing effect during non–El Niño years, characterized by zero or negative coastal SST anomalies. In short, during strong El Niños, it appears that the associated coastal SST anomalies serve to further intensify the already anomalously strong storms in southern California, thus contributing to the increased flooding. This modulating effect by El Niño–Southern Oscillation (ENSO) of a mesoscale process has not been considered before in attempts at assessing the impacts of ENSO on U.S. west coast precipitation.

Numerical simulations of subduction zones: Effect of slab dehydration on the mantle wedge dynamics

In oceanic subduction zones, dehydration of slab’s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength. This should enhance the upper plate base reheating and markedly alter its thermal structure. To quantify this phenomenon, we model slab subduction within a viscous mantle, dehydration–hydration processes, and the rock strength dependence on water content. We use accurate phase diagrams for a H 2O-satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock. Transition phases are supposed to be not metastable. Water is released from the oceanic crust and from the altered peridotite portion of the slab. Dehydration of the subducting lithosphere occurs first at 60–75 km depth, when the crust is eclogitized, and second, deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable. For high convergence rates, because of cold P – T – t paths in the slab, serpentine can be transformed into the hydrated phase A and water is recycled by the slab until great depth. However, in all investigated cases, the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 ± 55 km away from the trench and, as it goes up, to form hydrated minerals in the overlying lithosphere over a significant volume. The convergence rate slightly shifts dehydration fronts location, and consequently widens or reduces the hydrated mantle wedge. Note that for the dynamic corner flow modelled here, with a non-Newtonian rheology, the slab surface is significantly warmer than for an isoviscous analytical corner flow model, yielding plausible crust melting. We assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water. The rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge. This results in cooler temperatures along the slab top, but in an enhanced corner flow effect on the upper plate thermal structure. For large hydration strength reductions, a strong thermal erosion of the overlapping lithosphere develops in less than 15 Myr due to convective destabilization. This weakens drastically the upper plate at a distance from 110 to 220 km away from the trench. The geometry of the eroded region could correspond to the low-velocity zone observed below the arc region.

Convective destabilization of a thickened continental lithosphere

Removal or delamination of the lithospheric mantle in a late stage of mountain building is a process often invoked to explain syn orogenic extension, high temperature metamorphism, magmatism and uplift. One mechanism that could explain the lithospheric root detachment is the development of convective instabilities within the peridotitic lithosphere due to its high density. This mechanism is studied by two-dimensional convective numerical simulations in the simple case of a strongly temperature dependent viscous rheology appropriate for upper mantle rocks. We neglect here the weakening effect of a brittle rheology and of a crustal layer, and therefore we did not model tectonic deformations. Depending on the upper mantle viscosity and activation energy, a 300 km thick root can be inferred to be either indefinitely stable or to thicken with time or to thin with time. When the lithosphere is initially thicker than its equilibrium thickness, the convective flow at the base and on the sides of the lithospheric root is strong enough to cancel downwards heat conduction and to progressively remove the root. This flow is due to the finite density perturbations induced by the topography of the isotherms on the base and at the sides of the root. We derive two general parameterizations of the convective removal duration as a function of the equilibrium thickness, the thickening factor, the root width, and the rheological temperature scale. Using these relationships, and assuming that the lithospheric equilibrium thickness is about 100 km, the removal duration of a 250 km thick root ranges from 55 to 750 Myr depending on the root width. It is too small to explain the long term stability of cratonic lithospheric root, but too long to explain any sudden change in the stress and strain states in mountain belts development.

Monitoring High-Temporal-Resolution Convective Stability Indices Using the Ground-Based Atmospheric Emitted Radiance Interferometer (AERI) during the 3 May 1999 Oklahoma–Kansas Tornado Outbreak

Abstract The Department of Energy Atmospheric Radiation Measurement Program has funded the development and installation of five atmospheric emitted radiance interferometer (AERI) systems around the Southern Great Plains Cloud and Radiation Test Bed located in Oklahoma and Kansas. The AERI instruments measure atmospheric emitted radiance to within 1% ambient radiance at 1 cm–1 spectral resolution from 520 to 3000 cm–1 (3–20 μm) at 10-min temporal resolution. This high-spectral-resolution radiance information is inverted through a form of the infrared radiative transfer equation to produce temperature and water vapor profiles within the planetary boundary layer (to 3 km), effectively mapping the thermodynamic state of the lower troposphere. Taking advantage of the 10-min resolution of the AERI thermodynamic profiles, the convective destabilization during the 3 May 1999 Oklahoma–Kansas tornado outbreak is analyzed. Tropospheric changes involving the rapid (on the order of 1–2 h) dissipation of a capping temp...

Moist Baroclinic Instability in the Presence of Surface–Atmosphere Coupling

The influence of convective heating on baroclinic instability in the presence of surface sensible heat and moisture fluxes is investigated. Following previous numerical work, a two-dimensional continuous model on an f plane incorporates diabatic heating effects due to cumulus convection and surface sensible heat flux using parameterizations based on a wave-induced unstable boundary layer and associated moist convective destabilization. The temperature-damping effect of surface sensible heat flux is assumed to decrease exponentially with height, and the vertical distribution of convective heating uses a prescribed profile. The atmosphere is assumed to overlie an oceanic surface. In this configuration, convective heating occurs in the wave’s cold sector. General forms of the dispersion relation and eigenfunction are derived analytically. Results show that the most unstable wave is modified by the effect of convective latent heating. With weak convection, the wave’s structure does not change much, while the wave’s energy generation is hampered by the negative contribution of convection. In the presence of moderate convective heating, although the wave’s energy generation is decreased by convection, the wave adjusts its structure to minimize the negative effect of convection and retain growth. In the region with strong convective heating, convective heating significantly changes the wave’s temperature structure. Above and below the strong heating region, the wave structure still retains some features of the Eady mode. The results have bearing on how the structure of oceanic storms may be altered by convection.

The anatomy of the mixing transition in homogeneous and stratified free shear layers

We investigate the detailed nature of the ‘mixing transition’ through which turbulence may develop in both homogeneous and stratified free shear layers. Our focus is upon the fundamental role in transition, and in particular the associated ‘mixing’ (i.e. small-scale motions which lead to an irreversible increase in the total potential energy of the flow) that is played by streamwise vortex streaks, which develop once the primary and typically two-dimensional Kelvin–Helmholtz (KH) billow saturates at finite amplitude.Saturated KH billows are susceptible to a family of three-dimensional secondary instabilities. In homogeneous fluid, secondary stability analyses predict that the stream-wise vortex streaks originate through a ‘hyperbolic’ instability that is localized in the vorticity braids that develop between billow cores. In sufficiently strongly stratified fluid, the secondary instability mechanism is fundamentally different, and is associated with convective destabilization of the statically unstable sublayers that are created as the KH billows roll up.We test the validity of these theoretical predictions by performing a sequence of three-dimensional direct numerical simulations of shear layer evolution, with the flow Reynolds number (defined on the basis of shear layer half-depth and half the velocity difference) Re = 750, the Prandtl number of the fluid Pr = 1, and the minimum gradient Richardson number Ri(0) varying between 0 and 0.1. These simulations quantitatively verify the predictions of our stability analysis, both as to the spanwise wavelength and the spatial localization of the streamwise vortex streaks. We track the nonlinear amplification of these secondary coherent structures, and investigate the nature of the process which actually triggers mixing. Both in stratified and unstratified shear layers, the subsequent nonlinear amplification of the initially localized streamwise vortex streaks is driven by the vertical shear in the evolving mean flow. The two-dimensional flow associated with the primary KH billow plays an essentially catalytic role. Vortex stretching causes the streamwise vortices to extend beyond their initially localized regions, and leads eventually to a streamwise-aligned collision between the streamwise vortices that are initially associated with adjacent cores.It is through this collision of neighbouring streamwise vortex streaks that a final and violent finite-amplitude subcritical transition occurs in both stratified and unstratified shear layers, which drives the mixing process. In a stratified flow with appropriate initial characteristics, the irreversible small-scale mixing of the density which is triggered by this transition leads to the development of a third layer within the flow of relatively well-mixed fluid that is of an intermediate density, bounded by narrow regions of strong density gradient.

Atmospheric and surface variations during westerly wind bursts in the tropical western pacific

AbstractAn analysis is made of variations in both the surface energy balance and the regional atmospheric dynamic and thermal structure during 44 westerly wind bursts (WWBs) in the western equatorial Pacific Ocean from 1979 to 1995. The study assesses winds, convective available potential energy, cloud properties, precipitation, surface temperature, and surface heat flux while distinguishing between brief (5–25 day periodicity) and sustained (30–90 day) WWBs. Datasets used in the study include fields from the NCEP/NCAR and ECMWF re‐analyses, and satellite retrievals of clouds (ISCPP), precipitation (MSU), moisture (TOVS), and surface solar flux.Both brief and sustained WWBs, by definition, experience strong low‐level westerly winds that typically induce an increased surface latent‐heat flux of approximately 30 W m−2. Enhanced cloud thickness, precipitation, and upper tropospheric easterly wind anomalies accompany surface westerly winds, though maxima in winds lag those in clouds and precipitation by about one day for brief WWBs and four days for sustained events. WWBs of both types experience strong seasonality, occurring frequently in all seasons except boreal summer.Important distinctions between brief and sustained WWBs can also be made. Westerly anomalies typically extend above 200 hPa during brief WWBs but are generally confined to the lower troposphere (below 400 hPa) for sustained events. Sustained WWBs are also preceded by a quiescent period of reduced cloud thickness and surface winds that is accompanied by strong incident solar flux. Convective instability, as judged by a variety of techniques, increases by approximately 30% during this quiescent period. Brief WWBs do not include the precursory surface warming or convective destabilization of sustained WWBs. Notwithstanding the warming episodes before the events, sustained WWBs are associated with a net surface cooling approximately 40% larger than brief WWBs.The relationship between brief and sustained WWBs and the phase of the Madden‐Julian Oscillation (MJO) (as judged from outgoing long‐wave radiation) is also examined. Results support the classification of events as ‘brief’ and ‘sustained’ as used in this study, with brief WWBs occurring frequently during both wet and dry phases of the MJO, while sustained WWBs occur uniquely during the MJO wet phase. The association of brief and sustained WWBs with the MJO is shown to be independent of the El Niiio Southern Oscillation phase. It is therefore proposed that some, but not all, WWBs may be viewed as the surface signature of the MJO and that the mechanisms responsible for the MJO play an integral role in the formation and sustenance of sustained WWBs.

Convective destabilization by a tropopause fold diagnosed using potential‐vorticity inversion

AbstractThere is observational evidence to suggest that at the time tropopause folds in extra‐tropical cyclones are descending into the mid troposphere there is frequently a destablization of the lower troposphere to moist convection. Here we consider the dynamical reasons for such a linkage using so‐called potential vorticity (PV) attribution concepts. In particular PV inversion is used to find the wind shear attributable to the tropopause fold itself. Then this shear is used to quantify the tendency to generate regions of potential instability. For an observed case of a modest tropopause fold it is found that the PV anomaly contained in the tropopause fold contributed substantially to the instantaneous convective destabilization in the sense of increasing the potential instability. Indeed, at some locations, it overcame a tendency to stabilize by the atmosphere without the fold. On the other hand, only a fraction of the region with increased potential instability experienced sufficient large‐scale ascent to lead to convection and it remains to be seen whether folds in general make a significant contribution to the generation of potential instability that can actually be realized. The importance of tropopause folds, or indeed of tropopause depressions in general in the context of convection, is that their small horizontal scale has the potential to focus and amplify any convective destabilization locally. This may account for the localization of outbreaks of severe convective weather which it is important to forecast accurately. Currently numerical weather‐prediction models often do not handle these small scales very accurately.

Convective destabilization by upper-level troughs

Convective destabilization by upper-level troughs

Convective destabilization by upper‐level troughs

AbstractThe approach of an upper‐level trough is accompanied by a local ascent of the isentropic surfaces in the troposphere. The associated cooling and moistening destabilizes the atmosphere to convection. In this paper we attempt to quantify the maximum amount of destabilization as a function of the amplitude and horizontal scale of the trough. The trough is represented by an elongated potential‐temperature perturbation on the tropopause. Its structure, including the isentrope displacement it produces, is calculated from the balanced shear‐line solutions of Juckes. These solutions are used to estimate the amount by which mid‐ and lower‐tropospheric air masses are lifted when an upper‐tropospheric trough passes overhead on the assumption that isentropic potential‐vorticity anomalies are localized in the upper troposphere. The amount of destabilization is characterized by the maximum change in convective available potential energy (CAPE) and convective inhibition (CIN) brought about by the passage of the trough. The calculations seek to isolate one aspect of the interaction between an upper‐level trough and a tropical cyclone. We show that the changes in CAPE are significant in the Tropics (up to 83% for the soundings examined) and even more so in the middle latitudes (up to 130%) and that the CIN may be completely removed.

Convective destabilization by a tropopause fold diagnosed using potential-vorticity inversion

Convective destabilization by a tropopause fold diagnosed using potential-vorticity inversion

Surface-Flux Regulation of the Coupling between Cumulus Convection and Baroclinic Waves

Abstract The authors examine the role of convection in the dynamics of eddy life cycles through numerical experiments using initial states that are baroclinically and conditionally unstable in midlatitudes. The location of wave-induced convection and its influence on the growing wave depends on how strongly the wave is coupled to the lower boundary through surface fluxes. For all three convective schemes used here (Emanuel, modified Grell, modified Kuo), convective destabilization is favored in the wave’s warm sector when there are no surface fluxes included in the simulation and in the cold sector when there are. Convection is also shallower when it occurs in the cold sector, though still precipitating. For simulations using Emanuel convection, the relatively shallow convection plays a central role in a water cycle wherein 1) evaporation gives moisture to the cold sector’s boundary layer, 2) convection pumps some of the moisture into the lower troposphere above the boundary layer, 3) the large-scale circ...

Finite-amplitude baroclinic instability of a mesoscale gravity current in a channel

Abstract A finite amplitude theory is developed for the evolution of marginally unstable modes for a mesoscale gravity current on a sloping bottom. The theory is based on a nonquasigeostrophic, baroclinic model of the convective destabilization of gravity currents which allows for large amplitude isopycnal deflections while filtering out barotropic instabilities. Two calculations are presented. First, a purely temporal amplitude equation is derived for marginally unstable modes not located at the minimum of the marginal stability curve. These modes eventually equilibrate with a new finite amplitude periodic solution formed. Second, the evolution of a packet of marginally unstable modes located at the minimum of the marginal stability curve is presented. These two models are dramatically different due to fundamental physical differences. For marginally unstable modes not located at the minimum of the marginal stability curve, it is possible to determine the evolution of a single normal mode amplitude. For ...

Thinning of the lithosphere by small-scale convective destabilization

Gravitational, topographical and seismological studies of uplift regions around hot spots on oceanic,1,2 and continental plates3–7 have suggested that considerable lithospheric thinning has taken place beneath these areas. Previous models used to study this phenomenon have been based on thermal conduction8–11 and on the dynamic interaction between large-scale plume structures12 and the lithosphere, for a purely temperature-dependent viscosity. However, their predictions of the time scale for direct erosion12 of the lithosphere by these upwellings are too long. We propose here a new dynamical mechanism which works most efficiently for a temperature and pressure-dependent rheology. Our idea is that thinning of the lithosphere is accomplished by strong secondary convection enhanced by the presence of a low-viscosity zone which grows from a plume associated with a hot spot. This process is capable of producing the rapid time scale of the swell uplift13, of the order of 10 Myr, and the rates of thinning, of order 10 km Myr−1, required by both geophysical13,14 and geochemical15 observations.