John R. Hunter

Tidal dissipation and stratification in the Gulf of California

The effect of tidal mixing upon the distribution of stratification and sea surface temperature (SST) in the Gulf of California is studied by direct observation and by numerical modeling. Data from conductivity-temperature-depth surveys made in September and December of 1986 are used to describe the distribution of stratification and SST. A vertically integrated finite differences numerical model was used to obtain the geographical distribution of available turbulent kinetic energy (TKE) from the M2 tide. The following three areas were found to be strongly affected by tidal mixing: the shallow coastal area at the head of the gulf, the shelf south of Tiburon island, and the area of the midgulf archipelago. The first two are vertically well mixed and are therefore similar to other shelf sea frontal areas. Dissipation occurs all around the islands of the archipelago, and the sills are foci of the most intense tidal dissipation. Stratification in this area was reduced with respect to the background, but complete vertical mixing does not occur. Therefore the strong SST fronts that occur in this area do not mark the boundary between well-mixed and stratified water. The Simpson-Hujiter criterion (Simpson and Hunter, 1974) for stratification and frontal position produced 2.75 ≤ λcrit ≤ 3.0 for the frontal position in the shallow northern gulf. This value of λcrit is larger than in other shelf sea fronts, which may be due to horizontal advection, which is not included in the criterion. We found that the effect of advection may be of the same order of magnitude as that of buoyancy flux. In the area over the sills, λ > λcriti the available TKE is not enough to produce complete mixing over the ∼ 400 m of water over the sills.

A simple technique for estimating an allowance for uncertain sea-level rise

Projections of climate change are inherently uncertain, leading to considerable debate over suitable allowances for future changes such as sea-level rise (an ‘allowance’ is, in this context, the amount by which something, such as the height of coastal infrastructure, needs to be altered to cope with climate change). Words such as ‘plausible’ and ‘high-end’ abound, with little objective or statistically valid support. It is firstly shown that, in cases in which extreme events are modified by an uncertain change in the average (e.g. flooding caused by a rise in mean sea level), it is preferable to base future allowances on estimates of the expected frequency of exceedances rather than on the probability of at least one exceedance. A simple method of determining a future sea-level rise allowance is then derived, based on the projected rise in mean sea level and its uncertainty, and on the variability of present tides and storm surges (‘storm tides’). The method preserves the expected frequency of flooding events under a given projection of sea-level rise. It is assumed that the statistics of storm tides relative to mean sea level are unchanged. The method is demonstrated using the GESLA (Global Extreme Sea-Level Analysis) data set of roughly hourly sea levels, covering 198 sites over much of the globe. Two possible projections of sea-level rise are assumed for the 21st century: one based on the Third and Fourth Assessment Reports of the Intergovernmental Panel on Climate Change and a larger one based on research since the Fourth Assessment Report.

Past and Future Changes in Extreme Sea Levels and Waves

(1) Proudman Oceanographic Laboratory, Liverpool, UK (plw@pol.ac.uk) (2) The Hadley Centre, Met Office, UK (jason.lowe@metoffice.gov.uk) (3) Geophysical Fluid Dynamics Laboratory, Princeton, USA (tom.knutson@noaa.gov) (4) The Hadley Centre, Met Office, UK (ruth.mcdonald@metoffice.gov.uk) (5) CSIRO, Aspendale, Australia (kathleen.mcinnes@csiro.au) (6) GKSS, Geesthacht, Germany (woth@gkss.de) (7) GKSS, Geesthacht, Germany (storch@gkss.de) (8) Proudman Oceanographic Laboratory, Liverpool, UK (jaw@pol.ac.uk) (9) Environment Canada, Downsview, Canada (val.swail@ec.gc.ca) (10) Dalhousie University, Halifax, Canada (natacha.bernier@phys.ocean.dal.ca) (11) P.P. Shirshov Institute of Oceanology, Moscow, Russia (gul@sail.msk.ru) (12) Proudman Oceanographic Laboratory, Liverpool, UK (kevinh@pol.ac.uk) (13) National Institute of Oceanography, Goa, India (unni@darya.nio.org) (14) University of Hobart, Tasmania, Australia (john.hunter@utas.edu.au)

Modeling the basal melting and marine ice accretion of the Amery Ice Shelf

The basal mass balance of the Amery Ice Shelf (AIS) in East Antarctica is investigated using a numerical ocean model. The main improvements of this model over previous studies are the inclusion of frazil formation and dynamics, tides and the use of the latest estimate of the sub-ice-shelf cavity geometry. The model produces a net basal melt rate of 45.6 Gt year�1 (0.74 m ice year�1) which is in good agreement with reviewed observations. The melting at the base of the ice shelf is primarily due to interaction with High Salinity Shelf Water created from the surface sea-ice formation in winter. The temperature difference between the coldest waters created in the open ocean and the in situ freezing point of ocean water in contact with the deepest part of the AIS drives a melt rate that can exceed 30 m of ice year�1. The inclusion of frazil dynamics is shown to be important for both melting and marine ice accretion (refreezing). Frazil initially forms in the supercooled water layer adjacent to the base of the ice shelf. The net accretion of marine ice is 5.3 Gt year�1, comprised of 3.7 Gt year�1 of frazil accretion and 1.6 Gt year�1 of direct basal refreezing.

Betting strategies on fluctuations in the transient response of greenhouse warming.

We examine a series of betting strategies on the transient response of greenhouse warming, expressed by changes in 15-year mean global surface temperature from one 15-year period to the next. Over the last century, these bets are increasingly dominated by positive changes (warming), reflecting increasing greenhouse forcing and its rising contribution to temperature changes on this time scale. The greenhouse contribution to 15-year trends is now of a similar magnitude to typical naturally occurring 15-year trends. Negative 15-year changes (decreases) have not occurred since about 1970, and are still possible, but now rely on large, and therefore infrequent, natural variations. Model projections for even intermediate warming scenarios show very low likelihoods of obtaining negative 15-year changes over the coming century. Betting against greenhouse warming, even on these short time scales, is no longer a rational proposition.

The sea level at Port Arthur, Tasmania, from 1841 to the Present

Observations of sea level at Port Arthur, Tasmania, southeastern Australia, based on a two-year record made in 1841–1842, a three-year record made in 1999–2002, and intermediate observations made in 1875–1905, 1888 and 1972, indicate an average rate of sea level rise, relative to the land, of 0.8 ± 0.2 mm/year over the period 1841 to 2002. When combined with estimates of land uplift, this yields an estimate of average sea level rise due to an increase in the volume of the oceans of 1.0 ± 0.3 mm/year, over the same period. These results are at the lower end of the recent estimate by the Intergovernmental Panel on Climate Change of global average rise for the 20th century. They provide an important contribution to our knowledge of past sea level rise in a region (the Southern Hemisphere) where there is a dearth of other such data.

Initial borehole results from the Amery Ice Shelf hot-water drilling project

Abstract The Amery Ice Shelf Ocean Research (AMISOR) project aims to examine and quantify processes involved in the interaction between the ice shelf, the interior grounded ice and the oceanic water masses that circulate beneath it. Two boreholes were melted through the shelf, within 100 km of the calving front, to access the ocean cavity. One (AM02) was at a site where it was believed that basal melt was occurring, and the other (AM01) was in a region with accreted marine ice. At both sites the summertime ocean structure revealed meltwater-modified boundary layers up to 100 m thick immediately beneath the shelf. Salinity and temperature data in the upper cavity at AM02 showed a strong seasonal cycle as a result of a combination of ice-shelf basal melt, and the intrusion of ocean water masses modified by sea-ice processes in Prydz Bay. At AM01, a 200m thick layer of marine ice underlay the meteoric ice, and showed an increase in salinity and decrease in stable-isotope fractionation with depth. The lowest 100m of marine ice was highly permeable, with a rectangular banded textural facies. Other preliminary results from this study are also reported.

The cavity under the Amery Ice Shelf, East Antarctica

Ocean circulation under ice shelves and associated rates of melting and freezing are strongly influenced by the shape of the sub-ice-shelf cavity. We have refined an existing method and used additional in situ measurements to estimate the cavity shape under the Amery Ice Shelf, East Antarctica. A finite-element hydrodynamic ocean-tide model was used to simulate the major tidal constituents for a range of different sub-Amery Ice Shelf cavity water-column thicknesses. The data are adjusted in the largely unsurveyed southern region of the ice-shelf cavity by comparing the complex error between simulated tides and in situ tides, derived from GPS observations. We show a significant improvement in the simulated tides, with a combined complex error of 1.8 cm, in comparison with past studies which show a complex error of ∼5.3 cm. Our bathymetry incorporates ice-draft data at the grounding line and seismic surveys, which have provided a considerable amount of new data. This technique has particular application when the water column beneath ice shelves is inaccessible and in situ GPS data are available.

Scientific uncertainty and climate change: Part I. Uncertainty and unabated emissions

Uncertainty forms an integral part of climate science, and it is often used to argue against mitigative action. This article presents an analysis of uncertainty in climate sensitivity that is robust to a range of assumptions. We show that increasing uncertainty is necessarily associated with greater expected damages from warming, provided the function relating warming to damages is convex. This constraint is unaffected by subjective or cultural risk-perception factors, it is unlikely to be overcome by the discount rate, and it is independent of the presumed magnitude of climate sensitivity. The analysis also extends to “second-order” uncertainty; that is, situations in which experts disagree. Greater disagreement among experts increases the likelihood that the risk of exceeding a global temperature threshold is greater. Likewise, increasing uncertainty requires increasingly greater protective measures against sea level rise. This constraint derives directly from the statistical properties of extreme values. We conclude that any appeal to uncertainty compels a stronger, rather than weaker, concern about unabated warming than in the absence of uncertainty.

On the Temperature Correction of the Aquatrak Acoustic Tide Gauge

Temperature corrections for the Aquatrak acoustic tide gauge are derived. These account for variable temperature (and hence sound velocity) along the sounding tube, thermal expansion of the calibration tube, and the thermal response of the transducer/controller combination. When averaged over long periods (e.g., seasonal), the corrections are typically several millimeters in magnitude, with short-period (e.g., diurnal) variations as large as several centimeters. The correction associated with thermal expansion of the calibration tube is of the same order as the correction for variable sound velocity, which is already applied at some Aquatrak installations. The corrections derived here relate to the situation in which an initial adjustment has been made by the manufacturer (making the calibration tube a fixed known length under factory conditions), and a subsequent calibration is performed to define a single zero offset for the instrument.