Decadal predictions of the cooling and freshening of the North Atlantic in the 1960s and the role of ocean circulation

In the 1960s North Atlantic sea surface temperatures (SST) cooled rapidly. The magnitude of the cooling was largest in the North Atlantic subpolar gyre (SPG), and was coincident with a rapid freshening of the SPG. Here we analyze hindcasts of the 1960s North Atlantic cooling made with the UK Met Office’s decadal prediction system (DePreSys), which is initialised using observations. It is shown that DePreSys captures—with a lead time of several years—the observed cooling and freshening of the North Atlantic SPG. DePreSys also captures changes in SST over the wider North Atlantic and surface climate impacts over the wider region, such as changes in atmospheric circulation in winter and sea ice extent. We show that initialisation of an anomalously weak Atlantic Meridional Overturning Circulation (AMOC), and hence weak northward heat transport, is crucial for DePreSys to predict the magnitude of the observed cooling. Such an anomalously weak AMOC is not captured when ocean observations are not assimilated (i.e. it is not a forced response in this model). The freshening of the SPG is also dominated by ocean salt transport changes in DePreSys; in particular, the simulation of advective freshwater anomalies analogous to the Great Salinity Anomaly were key. Therefore, DePreSys suggests that ocean dynamics played an important role in the cooling of the North Atlantic in the 1960s, and that this event was predictable.     

BUILDING A DIGITAL CLOSE RANGE THREE DIMENSIONAL MEASURING SYSTEM FOR LESS THAN £5000

The use of CCD cameras for digital photogrammetry has only slowly been taken up by the photogrammetric community. However, the pace of change is quickening with better understanding and cheaper equipment. Recently a decision was made to start some research into close range three dimensional digital measurement. The overall objective was to develop a low cost system for the automated measurement of small industrial objects. This shorter contribution describes the initial stages of this work.     

Reliability of experimental partition coefficients in carbonate systems: Evidence for inhomogeneous distribution of impurity cations

Electron spin resonance (ESR) spectroscopy, thermal analysis and X-ray diffraction (XRD) analysis are used to study the mode of incorporation of a wide range of impurity ions (up to 1% Ni²⁺, Sr²⁺, Cd²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Zn²⁺, Pb²⁺, Mn²⁺ and Ba²⁺) in calcites synthesised in aqueous solutions. It is shown that when certain impurity ions are incorporated in these calcites they become concentrated, together with other trace elements such as Mn²⁺, into hitherto unsuspected trace phases. The important conclusion is drawn that unless the scavenging of trace elements by these phases can be avoided or corrected for, any calcite—liquid partition coefficients measured may have limited significance. ESR spectroscopy has revealed the intense strain common in calcites grown by different methods, whilst thermal analysis suggested that such strain may significantly alter the thermodynamic properties of these calcites. ESR spectroscopic analysis of aragonites synthesised by methods previously reported in the literature has revealed the common occurrence of trace calcite at concentrations below the XRD detection limit and proves the tendency of this calcite to scavenge trace Mn. Using ESR spectroscopy to monitor trace calcite levels and structural imperfections in synthetic aragonites, the urea hydrolysis technique is modified to provide a method for the synthesis of high-purity strain-free aragonite available for experimental studies.     

On the effects of wind and tides on the hydrodynamics of a shallow mediterranean estuary

A study is made of the effect of wind and tides on the hydrodynamics of the shallow inner basins of mediterranean estuaries. The paper includes a case study of Harvey Estuary in southwestern Australia where salinity and temperature data exist for 11 years during the 1980s and 1990s when that estuary experienced massive annual blue-green algal blooms. An analysis is made of salt exchange through the channels that join estuarine basins of this class to either the ocean or, as in the case of Harvey Estuary, to another shallow estuarine basin. A detailed three-dimensional numerical model is also implemented for the basin of Harvey Estuary. It is concluded that exchange through the channel is dominated by the (mainly diurnal) tides, despite the general micro-tidal nature of this class of estuary, although the efficiency of this process is found to be controlled by the length of the channel. Wind set-up in the basin also produces channel exchange and for Harvey Estuary this is about 20% of the exchange due to tides. Baroclinic flow through the channel is also capable of producing significant exchange but this is suppressed by the tidal currents in the channel except immediately after riverflow. Salt transport along the basins of this class of estuary is mainly driven by the longitudinal density gradient and the strength of this process is controlled by vertical mixing from the wind. However, there is also significant salt transport from wind-induced advection, the effect of which changes seasonally with the direction of the salt gradient.