Neil C. Wells

Similar meltwater contributions to glacial sea level changes from Antarctic and northern ice sheets

The period between 75,000 and 20,000 years ago was characterized by high variability in climate and sea level. Southern Ocean records of ice-rafted debris suggest a significant contribution to the sea level changes from melt water of Antarctic origin, in addition to likely contributions from northern ice sheets, but the relative volumes of melt water from northern and southern sources have yet to be established. Here we simulate the first-order impact of a range of relative meltwater releases from the two polar regions on the distribution of marine oxygen isotopes, using an intermediate complexity model. By comparing our simulations with oxygen isotope data from sediment cores, we infer that the contributions from Antarctica and the northern ice sheets to the documented sea level rises between 65,000 and 35,000 years ago were approximately equal, each accounting for a rise of about 15 m. The reductions in Antarctic ice volume implied by our analysis are comparable to that inferred previously for the Antarctic contribution to meltwater pulse 1A (refs 16, 17), which occurred about 14,200 years ago, during the last deglaciation.

Re‐emerging ocean temperature anomalies in late‐2010 associated with a repeat negative NAO

Northern Europe was influenced by consecutive episodes of extreme winter weather at the start and end of the 2010 calendar year. A tripole pattern in North Atlantic sea surface temperature anomalies (SSTAs), associated with an exceptionally negative phase of the North Atlantic Oscillation (NAO), characterized both winter periods. This pattern was largely absent at the surface during the 2010 summer season; however equivalent sub-surface temperature anomalies were preserved within the seasonal thermocline throughout the year. Here, we present evidence for the re-emergence of late-winter 2009/10 SSTAs during the following early winter season of 2010/11. The observed re-emergence contributes toward the winter-to-winter persistence of the anomalous tripole pattern. Considering the active influence of the oceans upon leading modes of atmospheric circulation over seasonal timescales, associated with the memory of large-scale sea surface temperature anomaly patterns, the re-emergence of remnant temperature anomalies may have also contributed toward the persistence of a negative winter NAO, and the recurrence of extreme wintry conditions over the initial 2010/11 winter season.

Global secular changes in different tidal high water, low water and range levels

Tides exert a major control on the coastal zone by influencing high sea levels and coastal flooding, navigation, sediment dynamics, and ecology. Therefore, any changes to tides have wide ranging and important implications. In this paper, we uniquely assess secular changes in 15 regularly used tidal levels (five high water, five low water and five tidal ranges), which have direct practical applications. Using sea level data from 220 tide gauge sites, we found changes have occured in all analyzed tidal levels in many parts of the world. For the tidal levels assessed, between 36% and 63% of sites had trends significantly different (at 95% confidence level) from zero. At certain locations, the magnitude of the trends in tidal levels were similar to trends in mean sea level over the last century, with observed changes in tidal range and high water levels of over 5 mm yr−1 and 2 mm yr−1, respectively. More positive than negative trends were observed in tidal ranges and high water levels, and vice versa for low water levels. However we found no significant correlation between trends in mean sea level (MSL) and any tidal levels. Spatially coherent trends were observed in some regions, including the north-east Pacific, German Bight and Australasia, and we also found that differences in trends occur between different tidal levels. This implies that analyzing different tidal levels is important. Because changes in the tide are widespread and of similar magnitude to MSL rise at a number sites, changes in tides should be considered in coastal risk assessments.

Depth-integrated vorticity budget of the Southern Ocean from a general circulation model

An analysis of the depth-integrated vorticity budget of the U.K. Fine Resolution Antarctic Model is used to investigate the mechanisms that maintain and dissipate vorticity in the Antarctic Circumpolar Current (ACC) and adjacent circulations of the Southern Ocean. The individual contributions to the vorticity budget are evaluated over the last six years when the model had reached a quasi-steady state. It is shown that the primary balance in the ACC is between the curl of the wind stress and the bottom pressure torque (BPT). Regional analysis reveals that the predominant contribution to BPT is in the Drake Passage, Scotia Sea, and the Argentine Basin. The region to the south of Tasmania and New Zealand also contributes significantly to BPT in the ACC. In addition it is shown that Drake Passage and the Falkland Current are responsible for the major dissipation of the ACC. Within the ACC between 320° and 290°E, there is good correspondence between the integrated southward flow and the curl of the wind stress.

On the accuracy of North Atlantic temperature and heat storage fields from Argo

The accuracy with which the Argo profiling float dataset can estimate the upper ocean temperature and heat storage in the North Atlantic is investigated. A hydrographic section across 36°N is used to assess uncertainty in Argo-based estimates of the temperature field. The root-mean-square (RMS) difference in the Argo-based temperature field relative to the section measurements is about 0.6°C. The RMS difference is smaller, less than 0.4°C, in the eastern basin and larger, up to 2.0°C, toward the western boundary. In comparison, the difference of the section with respect to the World Ocean Atlas (WOA) is 0.8°C. For the upper 100 m, the improvement with Argo is more dramatic, the RMS difference being 0.56°C, compared to 1.13°C with WOA. The Ocean Circulation and Climate Advanced Model (OCCAM) is used to determine the Argo sampling error in mixed layer heat storage estimates. Using OCCAM subsampled to typical Argo sampling density, it is found that outside of the western boundary, the mixed layer monthly heat storage in the subtropical North Atlantic has a sampling error of 10–20 Wm-2 when averaged over a 10° A~ 10° area. This error reduces to less than 10 Wm-2 when seasonal heat storage is considered. Errors of this magnitude suggest that the Argo dataset is of use for investigating variability in mixed layer heat storage on interannual timescales. However, the expected sampling error increases to more than 50 Wm-2 in the Gulf Stream region and north of 40°N, limiting the use of Argo in these areas.

The diurnal mixed layer and upper ocean heat budget in the western equatorial Pacific

This paper presents the results of an experiment in the western equatorial Pacific centered on the equator at 165°E which was designed to study the changes to the structure of the upper ocean on timescales of a few days and spatial scales of tens of kilometers. The results show that the response of the upper ocean to atmospheric forcing is very sensitive to the vertical structure of both the temperature and salinity. The diurnal response of the near-surface temperature to daytime heating and nighttime cooling was found to have an amplitude of a few tenths of a degree Celsius, This compares with a horizontal variation of temperature on scales of a few tens of kilometers of a similar magnitude. Even away from the very fresh surface layers typical of the area, salinity is found to play an important role in limiting the depth of nighttime mixing. In this case a subsurface salinity maximum restricts the depth to around 40 m. The nighttime convection is severely limited by either a small change in the surface forcing or the horizontal advection of slightly cooler waters from the east; we are unable to determine which is the dominant mechanism in the present case. The reduced mixing leads to an increase of the diurnal variation of sea surface temperature to over 1°C. The estimated net surface heat flux from the atmosphere to the ocean was found to be not significantly different from zero at 10 W m?2, in agreement with recent measurements. The net surface heat flux during the period of the heat budget experiment, which took place on the equator, was substantially higher at 65 W m?2. Changes of in situ temperature are found to be dominated by advection. The vertical velocity is estimated to be of order 10 m d?1 and to be caused by advection along east-west sloping density surfaces. Changes to the temperature structure of the upper ocean induced by motions with a timescale of a few days (possibly planetary waves) are found to be significantly greater than longer-term wind-induced upwelling or advection

Modelling the trajectories of migrating Atlantic salmon (Salmo salar)

This paper describes a model for simulating the trajectories of migrating Atlantic salmon (Salmo salar) in the ocean. Surface current and temperature representations were used as boundary conditions for simulation of migration trajectories. Representations of surface currents were derived from a general circulation model forced by realistic winds and then tested through comparisons with observed trajectories of drifting buoys. Observed climatology data were used to represent sea surface temperature patterns. The model was used to simulate the trajectories of 15 individual salmon that were tagged in their home rivers and subsequently recaptured at sea. In contrast to a random swimming direction model, trajectories simulated using both rheotaxis and thermotaxis as direction-finding mechanisms passed close to the recapture locations of the salmon. The timings and positions of the trajectories simulated using rheotaxis corresponded more closely with the observed data than those simulated using thermotaxis. This work indicates that either rheotaxis or thermotaxis, or a combination of the two, are possible direction-finding mechanisms for migrating Atlantic salmon.

Identifying the roles of the ocean and atmosphere in creating a rapid equatorial response to a Southern Ocean anomaly

Recent research has identified a rapid ocean response mechanism to salinity anomalies in the Southern Ocean using an idealised ocean model. Here we examine the relative importance of the ocean and atmosphere in creating an equatorial response to a Southern Ocean anomaly. Using a coupled climate model with realistic bottom topography and land relief, two rapid teleconnections are produced from a high latitude anomaly. An equatorial ocean response can be seen after 30 days. The mechanism producing this response is shown to rely on barotropic and baroclinic oceanic wave propagation. A second, atmospheric, response is seen in the Northern Hemisphere (NH) high latitudes, driven by atmospheric Rossby waves. The ocean quickly responds to the atmospheric signal above it, resulting in sea surface temperature anomalies at NH high latitudes.

Bioenergetic modelling of the marine phase of Atlantic salmon (Salmo salar L.)

A bioenergetic model of marine phase, wild Atlantic salmon was constructed to investigate the potential effects on post-smolt growth of predicted changes in oceanic conditions. Short-term estimates of growth in weight were similar to measurements in captivity and simulated growth varied with water temperature and swimming speed as expected. Longer-term estimates of growth in length were less than that achieved by wild salmon, particularly with constant swimming assumed. The model was sensitive to parameters relating to maximum daily food consumption, respiration and the relationships between body energy content, length and weight. Some of the sensitive parameters were based on substantive information on Atlantic salmon and their realistic ranges are likely to be much narrower than those tested. However, other parameter values were based on scant data, farmed Atlantic salmon or other salmonid species, and are therefore less certain and indicate where future empirical research should be focussed.

Modeling the Antarctic Circumpolar Current: A comparison of FRAM and equivalent barotropic model results

Analyzing the FRAM simulations, Killworth (1992) noticed a strong tendency for self-similarity in the vertical structure of the velocity field of the Antarctic Circumpolar Current (ACC). By assuming the self-similarity as a hypothesis, Krupitsky et al. (1996) developed an equivalent barotropic (EB) model of the ACC capable of describing the horizontal structure of the ACC. Compared to the multi-level-primitive-equation GCM, the EB model appeared substantially simpler and therefore useful in pilot process-oriented and sensitivity studies. In the present study dynamical and kinematical comparisons of the EB and FRAM outputs are given.