ALBERT

Description: Interbasin and intrabasin gradients play an important role as a part of a regional system of Caribbean climate drivers, which include the Atlantic warm pool (AWP) and the Caribbean low-level jet (CLLJ). When the Caribbean is conditioned to be wet between May and November, near-surface geopotentials in the Caribbean are lower than in the nearby eastern tropical Pacific and east tropical Atlantic. As a result, there is vertical ascent in the Caribbean through to the middle troposphere which connects to zonal circulations with both the eastern tropical Pacific and the eastern tropical Atlantic. The Caribbean Sea is also warm, and there is a moderate easterly flow regime, indicating a weakening of the trade winds. Deviations from this state caused by changes in one or both sides of the Pacific-Caribbean and Caribbean-Atlantic circulations (and diagnosed by changes in their geopotential gradients) reasonably track the transition of the Caribbean from wet to dry and vice versa on intraseasonal and interannual time scales. The study also uses changes to the gradients to offer insight into why the Caribbean region is projected to be drier during its traditional rainy season in the face of warmer surface temperatures under global warming. The Caribbean seemingly enters into a “July” mode, which persists for the duration of the boreal summer. The mode is characterized by higher (lower) geopotentials in the Caribbean (Pacific and Atlantic), a stronger CLLJ, and anomalous descent in the Caribbean in spite of the warmer surface temperatures.

Description: A step in the depth of the lithosphere base, associated with lateral variations in the upper mantle temperature structure, can trigger mantle flow that is referred to as edge-driven convection. This paper aims at outlining the implications of such edge-driven flow at a lateral temperature transition from a hot and thin to a cold and thick lithosphere of a continental back-arc. This configuration finds application in the southern Canadian Cordillera, where a hot and thin back-arc is adjacent to the cold and thick North American Craton. A series of geodynamical models tested the thermodynamical behavior of the lithosphere and upper mantle induced by a step in lithosphere thickness. The mantle flow patterns, thickness and heat flow evolution of the lithosphere, and surface topography are examined. We find that the lateral temperature transition shifts cratonward due to the vigorous edge-driven mantle flow that erodes the craton edge, unless the craton has a distinct high viscosity mantle lithosphere. The mantle lithosphere viscosity structure determines the impact of edge-driven flow on crustal deformation and surface heat flow; a dry olivine rheology for the craton prevents the edge from migrating and supports a persistent surface heat flow contrast. These phenomena are well illustrated at the transition from the hot Canadian Cordillera to craton that is supported by a rheological change and that coincides with a lateral change in surface heat flow. Fast seismic wave velocities observed in the upper mantle cratonward of the step can be explained as downwellings induced by the edge-driven flow.

Description: Observations made near the Celtic Sea shelf edge are used to investigate the interaction between wind generated near-inertial oscillations and the semi-diurnal internal tide. Linear, baroclinic energy fluxes within the near-inertial ( f ) and semi-diurnal (M 2 ) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the two week deployment period the semi-diurnal tide drove 28-48 W m -1 of energy directly on-shelf. Little spring-neap variability could be detected. Horizontal near-inertial energy fluxes were an order of magnitude weaker, but non-linear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semi-diurnal internal tide led to a 25-43% increase in positive on-shelf energy flux. The phase relationship between f and M 2 determines whether this non-linear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2-day counter clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations.

Description: Geodynamic models of mantle convection predict that Mexico and western North America share a history of dynamic support. We calculate admittance between gravity and topography, which indicates that the elastic thickness of the plate in Mexico is 11 km and in western North America it is 12 km. Admittance at wavelengths 〉 500 km in these regions suggests that topography is partly supported by sub-crustal processes. These results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of North American topography supported by the mantle may exceed 1 km. The Cenozoic history of magmatism, sedimentary flux, thermochronometric denudation estimates and uplifted marine terraces imply that North American lithosphere was uplifted and eroded during the last 30 Ma. We jointly invert 533 Mexican and North American longitudinal river profiles to reconstruct a continent-scale rock uplift rate history. Uplift rate is permitted to vary in space and time. Erosional parameters are calibrated using incision rate data in southwest Mexico and the Colorado Plateau. Calculated rock uplift rates were 0.15–0.2 mm/yr between 25–10 Ma. Central Mexico experienced the highest uplift rates. Central and southern Mexico continued to uplift at 0.1 mm/yr until recent times. This uplift history is corroborated by independent constraints. We predict clastic flux to the Gulf of Mexico and compare it to independent estimates. We tentatively suggest that the loop between uplift, erosion and deposition can be closed here. Mexico's staged uplift history suggests that its dynamic support has changed during the last 30 Ma. This article is protected by copyright. All rights reserved.

Description: The microtopography of two sandstone blocks with and without colonization of biofilms were measured with a traversing micro‐erosoin meter (TMEM) under different simulated environmental conditions. Two‐hourly microtopographic fluctuations of supratidal sandstone were mainly induced by the colonized biofilms and influenced by environmental factors. By increasing the magnitude and number of cycles of expansion and contraction, lithobiontic biofilms were proposed to play an erosive role in rock decay at hourly scale. Abstract In this study laboratory experiments were used to explore the role of biofilms, formed by lithobiontic microorganism communities, in causing hourly surface changes of supratidal sandstone and the potential linkage to long‐term rock decay. To isolate the influence of individual environmental factors (temperature and humidity) on rock surface changes (expansion and contraction), a colonized (biofilm‐covered) and a non‐colonized sandstone block (biofilm‐free) underwent the same univariate microclimatic simulations closely controlled by an environmental chamber. Simulations were run under three different light conditions, with a natural light lamp on, on and off at 20‐min intervals and off, to investigate the impact of light on rock surface dynamics. Measured with a traversing micro‐erosion meter (TMEM), two‐hourly microtopographic fluctuations of these two sandstone blocks were compared in the same environment. Induced by microclimatic variations, surface movements of significantly higher magnitude (12–120% under varying tempeature and 121–154% under varying humidity) and different change patterns were observed on the colonized block, indicating the primary role of biofilm in driving microtopographic fluctuations of supratidal sandstone. However, thermally driven changes of similar magnitude and pattern were observed on both surfaces, suggesting other mechanisms also operating on the non‐colonized rock surface in this process. Due to the sensitivity of biofilm microorganism communities to light, the magnitude and pattern of surface changes was impacted by light condition. Because biofilms increased the magnitude and number of cycles of expansion and contraction of the experimental rock surface, we propose that lithobiontic biofilms facilitate the detachment of grains and granular disintegration on the rock surface, consequently contributing to rock decay and accelerating the rate of breakdown of supratidal rock. This short‐term episode therefore needs to be superimposed on longer term studies to fully understand the role of biofilms in rock surface change. © 2019 John Wiley & Sons, Ltd.

Description: This study assesses the skill of four statistical models in hindcasting North Atlantic annual tropical cyclone (TC) frequency over 1950–2008 with the aim of projecting future activity. Three of the models are motivated by operational statistical forecast schemes and are premised on standard hurricane predictors including sea surface temperatures (SSTs) and near-surface zonal winds. The fourth model uses an SST-gradient index previously proposed for Caribbean seasonal rainfall prediction. The statistical models, created from backward regression, explain 24-48% of the observed variability in 1950-2008 annual TC frequency. The future state of the predictors are extracted from the ECHAM5, HadCM3, MRI CGCM2.3.2a and MIROC3.2 GCM simulations under the Coupled Model Intercomparison Project Phase 3 (CMIP3). Models utilizing SST and near-surface wind predictors suggest significant increases in mean annual frequency by 2-8 TCs by 2070-2090, compared to a single surface wind predictor model, indicating that positive trends in SSTs under global warming have a larger relative influence on projections than changes in the variability of the surface winds. Wind-only models exhibit declines in TC frequency while the SST-gradient model yields little change relative to the present-day mean. Backward regression reapplied against the 1990-2008 period, analogous to future warmer oceanic and atmospheric state relative to the earlier years in the record, retains only the CLLJ-type predictors, explaining up to 82% of TC frequency variability and suggesting a more dominant role for the CLLJ in a warmer climate. Projections using the new models show either a more conservative increase or a stronger decrease in frequency, consistent with a stronger CLLJ.

Description: A 16-year record of expendable bathythermograph (XBT) transects across Drake Passage is used to examine variability in upper ocean heat content that is not associated with the annual cycle. Links between upper ocean heat content and anomalous heat fluxes, winds, two large-scale climate indices, and mesoscale eddies and meanders are examined. Results suggest that interannual variations in surface heat fluxes explain ~5-10% of the variance in upper-ocean heat content. Anomalous surface heat fluxes are linked to meridional wind anomalies upstream of Drake Passage, which in turn are linked to forcing by El Niño / Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). ENSO and SAM are correlated with upper ocean heat content at near-zero lags, and statistically significant correlations occur at longer time lags as well. The impact of mesoscale eddies and meanders on upper ocean heat content is explored using a tracked eddy database. An empirical relationship is constructed relating upper ocean heat content anomalies to eddy length scales and amplitudes. Eddies and meanders are estimated to account for more than one third of the non-annual-cycle variance in Drake Passage upper-ocean heat content.

Description: [1]  The Grizzly Valley fault system (GVFS) strikes northwestward across Sierra Valley, a low-relief basin situated within a network of active dextral-slip faults in the northern Walker Lane, California. Quaternary motion along the Grizzly Valley fault system has not been previously documented. We used high-resolution (0.25 m) airborne LiDAR data in combination with high-resolution, P-wave, seismic-reflection imaging to evaluate Quaternary deformation associated with the GVFS. We identified suspected tectonic lineaments using the LiDAR data and collected seismic-reflection data along six profiles across the lineaments. The seismic-reflection images reveal a deformed basal marker that we interpret to be the top of Tertiary volcanic rocks overlain by a 120- to 450-m-thick suite of subhorizontal reflectors that we interpret to be Plio-Pleistocene lacustrine deposits. Three profiles image features that we interpret to be the principle active trace of the GVFS, which is a steeply dipping fault zone that vertically offsets the volcanic rocks and the lacustrine basin fill. These data suggest that the GVFS may have been active in latest Quaternary time because: 1) the LiDAR data show subtle surficial geomorphic features that are typical of youthful faulting, including a topographic lineament marked by a ~1-m-high ridge composed of discontinuous, left-stepping lobes; and 2) the seismic profiles demonstrate shallow faulting of the lacustrine strata that coincides with the left-stepping ridge. This investigation illustrates the potential for unidentified, low rate, strike-slip faults in transtensional basins and emphasizes the value of high-resolution topographic data and subsurface imaging as a means of identifying these structures.

Description: ABSTRACT The Statistical Downscaling Model (SDSM) is used to investigate future projections of daily minimum and maximum temperature extremes for 45 stations and rainfall extremes for 39 stations across the Caribbean and neighbouring regions. Models show good skill in reproducing the monthly climatology of the mean daily temperatures and the frequencies of warm days, warm nights, cool days and cool nights between 1961 and 2001. Models for rainfall exhibit lower skill but generally capture the monthly climatology of mean daily rainfall and the spatial distribution of the mean annual maximum number of consecutive dry days (CDD) and mean annual count of days with daily rainfall above 10 mm (R10). Future projections suggest an increase (decrease) in warm (cool) days and nights by 2071–2099 under the A2 and B2 scenarios relative to 1961–1990. An increase in CDD is suggested for most stations except some eastern Caribbean stations and Bahamas. Decreases in RX1 (monthly maximum 1-day precipitation), R10 and R95p (annual total rainfall above the 95th percentile) are also suggested for some northern Caribbean locations and Belize under the A2 scenario, compared to a mixture of increases and decreases for the eastern Caribbean. Atmospheric predictors used in SDSM correlate well with known oceanic and atmospheric drivers of Caribbean climate, e.g. the Atlantic Multidecadal Oscillation (AMO) on a seasonal timescale. Atlantic sea surface temperatures and the Caribbean low level jet appear to have significant influence on Caribbean temperature and rainfall extremes. Initiatives have explored future Caribbean rainfall and temperature extremes. These relied on outputs from global and regional climate models and a weather generator. Here, the Statistical Downscaling Model (SDSM), a hybrid of regression and stochastic weather generator approaches is used. Results suggest more frequent warm events and variable responses in extreme rainfall by 2071–2099 under A2 and B2 scenarios. Atmospheric predictors in SDSM correlate well with known oceanic and atmospheric drivers of Caribbean climate, e.g. Atlantic Multidecadal Oscillation (AMO) on seasonal timescales.

Description: We investigate how waves are transformed across a shore platform as this is a central question in rock coast geomorphology. We present results from deployment of three pressure transducers over four days, across a sloping, wide (~200 m) cliff-backed shore platform in a macrotidal setting, in South Wales, United Kingdom. Cross shore variations in wave heights were evident under the predominantly low to moderate (significant wave height 〈 1.4 m) energy conditions measured. At the outer transducer 50 m from the seaward edge of the platform (163 m from the cliff) high tide water depths were 8+ m meaning that waves crossed the shore platform without breaking. At the mid platform position water depth was 5 m. Water depth at the inner transducer (6 m from the cliff platform junction) at high tide was 1.4 m. This shallow water depth forced wave breaking, thereby limiting wave heights on the inner platform. Maximum wave height at the middle and inner transducers were 2.41 and 2.39 m respectively and significant wave height 1.35 m and 1.34 m respectively. Inner platform high tide wave heights were generally larger where energy was up to 335% greater than near the seaward edge where waves were smaller. Infragravity energy was less than 13% of the total energy spectra with energy in the swell, wind and capillary frequencies accounting for 87% of the total energy. Wave transformation is thus spatially variable and is strongly modulated by platform elevation and the tidal range. While shore platforms in microtidal environments have been shown to be highly dissipative, in this macro-tidal setting up to 90% of the offshore wave energy reached the landward cliff at high tide, so that the shore platform cliff is much more reflective.