Letters in 2013

In the tropics, substantially more rain falls just north of the Equator. An analysis of satellite observations, reanalysis data and model simulations suggests that the meridional ocean overturning circulation contributes significantly to the tropical rainfall peak north of the Equator.

The position of the intertropical convergence zone is thought to be linked to changes in the Earth’s climate state. Analyses of marine sediment show that over the past 25,000 years, the intertropical convergence zone over the Atlantic Ocean migrated in response to climate changes.

The recent rise in surface air temperatures over southern Africa is thought to largely result from the increase in atmospheric greenhouse gas concentrations. An analysis of climate data from the past four decades suggests that the warming may be linked to changes in Southern Hemisphere circulation induced by Antarctic ozone loss.

Subglacial meltwater channels beneath the Antarctic Ice Sheet have been reported, but the nature and distribution of these meltwater pathways are unclear. Remote sensing observations reveal persistent channelized features beneath the Filchner–Ronne Ice Shelf in West Antarctica, suggesting widespread channelized flow driven by melting.

The differentiation of the Earth into mantle and core implies that there is a mechanism to separate iron from silicates. Three-dimensional imaging of samples experimentally subjected to high pressures reveals that liquid iron forms interconnected melt networks at lower mantle conditions, suggesting pathways through which iron can percolate towards the core.

The end of the African Humid Period about 6,000 years ago was associated with vegetation change and decreased precipitation. Conceptual modelling suggests that the nature of the feedback between climate and vegetation is dependent on vegetation type and diversity.

Field measurements have revealed much higher concentrations of hydroxyl radicals than expected in regions with high loads of the biogenic volatile organic compound isoprene. Results from isoprene oxidation experiments suggest that the additional recycling of radicals in the presence of isoprene contributes to hydroxyl radical enhancement in these regions.

Spectral observations from the Mars Express spacecraft have revealed an ozone layer that forms at night in south polar Mars. Data analysis and climate models suggest that poleward transport of oxygen and seasonal changes in hydrogen radicals explain the ozone layer’s presence in the southern hemisphere, and its absence in the north.

Little is known about the structure of possible mantle materials of extra-solar super-Earths with interior pressures of up to 1,000 GPa. Dynamic X-ray diffraction measurements of ramp-compressed magnesium oxide, an important component of Earth’s mantle, show a solid–solid state transition at about 600 GPa, with a high-pressure structure that is stable up to 900 GPa.

The surface expressions of mantle plumes—known as hotspot tracks—are rarely observed on continents because the lithosphere is so thick. Analysis of seismic data from the eastern United States, combined with geodynamical modelling, reveals a linear, east–west-trending seismic anomaly that may represent a hidden hotspot track extending from Missouri to Virginia.

The long-term stability of the continents has been attributed to a trade-off between thermal and compositional effects. Numerical simulations of the evolution of continents over 3 billion years, however, show that this state is ephemeral, and continents that are neutrally buoyant today were more (or less) buoyant in the geologic past.

Mid-ocean ridges are composed of segmented faults and magma reservoirs. Seismic images from the East Pacific Rise show that the magma reservoirs are segmented on the same fine scale as the surface faults, and that distinct lava eruptions are sourced from largely isolated magma lenses.

Volcanic eruptions can be cyclical, alternating between intense activity and repose over periods of hours to days. Numerical simulations of a viscous, gas-rich magma show that ascent through the volcanic conduit naturally induces a periodic pulse of pressurized gas to travel through the magma, which, on reaching the surface, can trigger the cyclical eruptions.

Palaeoclimate records indicate lower El Niño/Southern Oscillation (ENSO) variance during the middle Holocene compared with today, but the mechanisms leading to this muted variability are not clear. A 175-year oxygen isotope record from a Porites coral microatoll in the NINO3.4 region records persistently reduced ENSO variance about 4,300 years ago, and season-specific analyses of the record suggest that insolation played an important role in this change.

The buoyancy of magma should cause it to rise into the crust, preventing it from ponding in the uppermost mantle. Magnetotelluric data from the Dabbahu rift segment, Ethiopia, identify a magma reservoir that extends well into the mantle beneath the rift, and is so large that it should persist for thousands of years.

Between 80,000 and 70,000 years ago, climate cooled and ice sheets in the high northern latitudes expanded. Pollen and microfossil data from marine sediments indicate that an increasing thermal gradient between cold air and warmer oceans could have supported continental ice growth.

Phosphate is thought to be a chemical nutrient essential for life, but the low solubility of phosphate minerals means that abiogenesis on Earth had to overcome the hurdle of phosphate-limited environments. Dissolution experiments of phosphate minerals commonly found on Mars suggest that phosphate may have been more readily available in early martian environments.

The subduction zone beneath Costa Rica experiences infrequent large earthquakes in its northwestern part, whereas slow slip dominates in the southeast. Seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour, implying that spatial gradients in fluid content may control subduction-zone seismicity.

Anthropogenic aerosols are highly spatially variable, whereas greenhouse gases are largely well-mixed at the global scale, but both affect climate. Nevertheless, climate simulations suggest that regional changes in sea surface temperature and precipitation to changes in greenhouse gas and aerosol forcings are similar.

The remote detection of surface water indigenous to the Moon has proved difficult because of alternative sources, such as the solar wind. Spectroscopic observations of hydroxyl-bearing materials in Bullialdus Crater by the Chandrayaan-1 spacecraft are consistent with indigenous magmatic water that was excavated by impact from the lunar interior.