ALBERT

Description: Abstract We construct a novel framework to investigate the uncertainties and biases associated with estimates of deep ocean temperature change from hydrographic sections and demonstrate this framework in an eddy‐permitting ocean model. Biases in estimates from observations arise due to sparse spatial coverage (few sections in a basin), low frequency of occupations (typically 5–10 years apart), mismatches between the time period of interest and span of occupations, and from seasonal biases relating to the practicalities of sampling during certain times of year. Between the years 1990 and 2010, the modeled global abyssal ocean biases are small, although regionally some biases (expressed as a heat flux into the 4,000‐ to 6,000‐m layer) can be up to 0.05 W/m2. In this model, biases in the heat flux into the deep 2,000‐ to 4,000‐m layer, due to either temporal or spatial sampling uncertainties, are typically much larger and can be over 0.1 W/m2 across an ocean. Overall, 82% of the warming trend deeper than 2,000 m is captured by hydrographic section‐style sampling in the model. At 2,000 m, only half the model global warming trend is obtained from observational‐style sampling, with large biases in the Atlantic, Southern, and Indian Oceans. Biases due to different sources of uncertainty can have opposing signs and differ in relative importance both regionally and with depth, revealing the importance of reducing temporal and spatial uncertainties in future deep ocean observing design.

Description: Observational analyses of running 5-year ocean heat content trends (H t ) and net downward top of atmosphere radiation (N) are significantly correlated (r~0.6) from 1960 to 1999, but a spike in H t in the early 2000s is likely spurious since it is inconsistent with estimates of N from both satellite observations and climate model simulations. Variations in N between 1960 and 2000 were dominated by volcanic eruptions, and are well simulated by the ensemble mean of coupled models from the Fifth Coupled Model Intercomparison Project (CMIP5). We find an observation-based reduction in N of -0.31±0.21 Wm -2 between 1999 and 2005 that potentially contributed to the recent warming slowdown, but the relative roles of external forcing and internal variability remain unclear. While present-day anomalies of N in the CMIP5 ensemble mean and observations agree, this may be due to a cancellation of errors in outgoing longwave and absorbed solar radiation.

Description: [1]  A new generation of aeromagnetic data documents the post-Caledonide rift evolution of the southwestern Barents Sea (SWBS) from the Norwegian mainland up to the continent-ocean transition. We propose a geological and tectonic scenario of the SWBS in which the Caledonian nappes and thrust sheets, well constrained onshore, swing from a NE-SW trend onshore Norway to NW-SE/NNW-SSE across the SWBS platform area. On the Finnmark and Bjarmeland platforms, the dominant inherited magnetic basement pattern may also reflect the regional and post-Caledonian development of the Late Palaeozoic basins. Farther west, the pre-breakup rift system is characterised by the Loppa and Stappen highs, which are interpreted as a series of rigid continental blocks (ribbons) poorly thinned as compared to the adjacent grabens and sag basins. As part of the complex western rift system, the Bjørnøya Basin is interpreted as a propagating system of highly thinned crust, which aborted in Late Mesozoic time. This thick Cretaceous sag basin is underlain by a deep-seated high-density body, interpreted as exhumed high-grade metamorphic lower crust. The abortion of this propagating basin coincides with a migration and complete reorganisation of the crustal extension towards a second necking zone defined at the level of the western volcanic sheared margin and proto-breakup axis. The abortion of the Bjørnøya Basin may be partly explained by its trend oblique to the regional, inherited, structural grain, revealed by the new aeromagnetic compilation, and by the onset of further weakening later sustained by the onset of magmatism to the west.

Description: An isotope-enabled ocean-atmosphere general circulation model (GISS ModelE-R) is used to estimate the spatial gradients of the oxygen isotopic composition of seawater (δ18Osw, where δ is the deviation from a known reference material in per mil) during the early Paleogene (45–65 Ma). Understanding the response of δ18Osw to changes in climatic and tectonic boundary conditions is important because records of carbonate δ18O document changes in hydrology, as well as changes in temperature and global ice-volume. We present results from an early Paleogene configuration of ModelE-R which indicate that spatial gradients of surface ocean δ18Osw during this period could have been significantly different to those in the modern ocean. The differences inferred from ModelE-R are sufficient to change early Paleogene sea surface temperature estimates derived from primary carbonate δ18O signatures by more than ±2°C in large areas of the ocean. In the North Atlantic, Indian, and Southern Oceans, the differences in δ18Osw inferred from our simulation with ModelE-R are in direct contrast with those from another δ18O-tracing model study which used different, but equally plausible, early Paleogene boundary conditions. The large differences in δ18Osw between preindustrial and early Paleogene simulations, and between models, emphasizes the sensitivity of δ18Osw to climatic and tectonic boundary conditions. For this reason, absolute estimates of Eocene/Paleocene temperature derived from carbonate δ18O alone are likely to have larger uncertainties than are usually assumed.

Description: [1]  Observations show that the upper 2 km of the subtropical North Atlantic Ocean cooled throughout 2010, and remained cold until at least December 2011. We show that these cold anomalies are partly driven by anomalous air-sea exchange during the cold winters of 2009/10 and 2010/11 and more surprisingly, by extreme interannual variability in the ocean's northward heat transport at 26.5°N. This cooling driven by the ocean's meridional heat transport affects deeper layers isolated from the atmosphere on annual timescales and water that is entrained into the winter mixed layer thus lowering winter sea-surface temperatures. Here we connect, for the first time, variability in the northward heat transport carried by the Atlantic Meridional Overturning Circulation (AMOC) to widespread sustained cooling of the subtropical North Atlantic, challenging the prevailing view that the ocean plays a passive role in the coupled ocean-atmosphere system on monthly-to-seasonal time scales.

Description: The net surface energy flux is central to the climate system yet observational limitations lead to substantial uncertainty. A combination of satellite-derived radiative fluxes at the top of atmosphere (TOA) adjusted using the latest estimation of the net heat uptake of the Earth system, and the atmospheric energy tendencies and transports from the ERA-Interim reanalysis are used to estimate surface energy flux globally. To consider snowmelt and improve regional realism, land surface fluxes are adjusted through a simple energy balance approach at each grid point. This energy adjustment is redistributed over the oceans to ensure energy conservation and maintain realistic global ocean heat uptake, using a weighting function to avoid meridional discontinuities. Calculated surface energy fluxes are evaluated through comparison to ocean reanalyses. Derived turbulent energy flux variability is compared with the OAFLUX product and inferred meridional energy transports in the global ocean and the Atlantic are also evaluated using observations. Uncertainties in surface fluxes are investigated using a variety of approaches including comparison with a range of atmospheric reanalysis products. Decadal changes in the global mean and the inter-hemispheric energy imbalances are quantified and present day cross-equator heat transports are reevaluated at 0.22 ± 0.15 PW southward by the atmosphere and 0.32 ± 0.16 PW northward by the ocean considering the observed ocean heat sinks.

Description: We present an observation-based heat budget analysis for seasonal and interannual variations of ocean heat content ( H ) in the mixed layer ( H mld ) and full depth ocean ( H tot ). Surface heat flux and ocean heat content estimates are combined using a novel Kalman smoother-based method. Regional contributions from ocean heat transport convergences are inferred as a residual and the dominant drivers of H mld and H tot are quantified for seasonal and interannual time scales. We find that non-Ekman ocean heat transport processes dominate H mld variations in the equatorial oceans and regions of strong ocean currents and substantial eddy activity. In these locations, surface temperature anomalies generated by ocean dynamics result in turbulent flux anomalies that drive the overlying atmosphere. In addition, we find large regions of the Atlantic and Pacific oceans where heat transports combine with local air-sea fluxes to generate mixed layer temperature anomalies. In all locations except regions of deep convection and water mass transformation, interannual variations in H tot are dominated by the internal rearrangement of heat by ocean dynamics rather than the loss or addition of heat at the surface. Our analysis suggests that, even in extra-tropical latitudes, initialization of ocean dynamical processes could be an important source of skill for interannual predictability of H mld and H tot . Furthermore, we expect variations in H tot (and thus thermosteric sea level) to be more predictable than near surface temperature anomalies due to the increased importance of ocean heat transport processes for full-depth heat budgets. This article is protected by copyright. All rights reserved.

Description: [1]  Mechanisms of sustained multidecadal changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are investigated in a set of simulations with a new state-of-the-art earth system model. Anthropogenic aerosols have previously been highlighted as a potential mitigator of AMOC weakening. In this study we explain the oceanic mechanisms behind how anthropogenic aerosols force a strengthening of the AMOC by up to 20% in our state-of-the-art earth-system model. This strengthening is driven via atmospheric circulation changes which subsequently modulate the salinity budget of the North Atlantic subpolar gyre. Gradual salinification occurs via increased evaporation and decreased fluxes of ice through the Fram Straits. A component of the salinification is a positive feedback from the AMOC bringing more saline water northwards from the subtropical Atlantic. Salinification of the subpolar gyre results in increased deep convection and a strengthening of the AMOC. Following a reduction in aerosol concentrations the AMOC rapidly weakens, approximately three times faster than in the case where anthropogenic aerosol concentrations had never been increased. Similarities and differences with available observational records and long term reanalysis products are also discussed.

Description: Abstract Aim Phylogenies are increasingly used in community ecology, biogeography and macroecology. However, sourcing a phylogeny comprising the entire species pool for a focal region can be difficult. Typically, a bespoke phylogeny must be created requiring considerable data manipulation and the use of many standalone software packages. Here we present a suite of methodological tools within the popular R environment that help to build molecular phylogenies appropriate for ecological studies with a regional focus. Innovation Our R package regPhylo provides a pipeline to construct a Bayesian posterior distribution of time‐calibrated trees suitable to address ecological questions. The novel contributions of regPhylo include options to: use prior phylogenetic knowledge through flexible topological constraints; include spatial metadata in sourcing DNA sequences; and include taxa without DNA sequences and then infer consequent phylogenetic uncertainty. Specifically, regPhylo helps researchers: retrieve DNA sequences; enhance available metadata; select DNA sequences based on their length or spatial proximity to the region of study; align sequences; and perform quality control. Output from the pipeline is a file ready to run in the Bayesian tree reconstruction software beast2, appropriate for estimating time‐calibrated trees and including phylogenetic uncertainty for downstream analyses. Main conclusions Overall, regPhylo improves the integration of popular standalone phylogenetic software into the flexible R environment. It provides a novel approach to include topological constraints based on prior knowledge, include taxa without DNA sequences, and select spatially appropriate DNA sequences. When coupled with a Bayesian tree‐building process, our approach provides estimates of uncertainty in both topology and branch lengths. We demonstrate the utility of the package by constructing a posterior distribution of time‐calibrated phylogenies for the New Zealand marine ray‐finned fishes (Actinopterygii) providing the unprecedented opportunity to include phylogenetic information in downstream ecological analyses for marine fishes in this region.