Angela Hibbert

Sustained monitoring of the southern ocean at Drake Passage: Past achievements and future priorities

Drake Passage is the narrowest constriction of the Antarctic Circumpolar Current (ACC) in the Southern Ocean, with implications for global ocean circulation and climate. We review the long-term sustained monitoring programs that have been conducted at Drake Passage, dating back to the early part of the twentieth century. Attention is drawn to numerous breakthroughs that have been made from these programs, including (1) the first determinations of the complex ACC structure and early quantifications of its transport; (2) realization that the ACC transport is remarkably steady over interannual and longer periods, and a growing understanding of the processes responsible for this; (3) recognition of the role of coupled climate modes in dictating the horizontal transport and the role of anthropogenic processes in this; and (4) understanding of mechanisms driving changes in both the upper and lower limbs of the Southern Ocean overturning circulation and their impacts. It is argued that monitoring of this passage remains a high priority for oceanographic and climate research but that strategic improvements could be made concerning how this is conducted. In particular, long-term programs should concentrate on delivering quantifications of key variables of direct relevance to large-scale environmental issues: In this context, the time-varying overturning circulation is, if anything, even more compelling a target than the ACC flow. Further, there is a need for better international resource sharing and improved spatiotemporal coordination of the measurements. If achieved, the improvements in understanding of important climatic issues deriving from Drake Passage monitoring can be sustained into the future.

Mixing in cyclonic eddies in the Antarctic Circumpolar Current

Observations over a period of 39 days of the increasing minimum core temperature of Winter Water trapped within a mature cyclonic eddy in the Antarctic Circumpolar Current encouraged us to assess the mixing processes which might lead to this temperature rise. Using diffusive heat budgets for alternatively diapycnic (∼vertical) and then isopycnic (∼horizontal) mixing combined with the observed rate of warming allowed upper limits for diapycnic and isopycnic diffusivities to be inferred. This gave values of (3.3 ± 0.8) × 10−4m2s−1 and 87 ± 20m2s−1 for the diapycnic and isopycnic diffusivities respectively. These were then in turn applied to the isopycnic temperature distributions of a juvenile eddy observed earlier in the cruise and integrated forward in time. While both forms of diffusion undoubtedly play a role in modifying the temperature, it was the horizontal diffusivity which was better able to reproduce the tighter θS relationship and the horizontal spread of the temperature minimum observed in the mature eddy

An empirical approach to improving tidal predictions using recent real-time tide gauge data

Harmonic tidal prediction methods are often problematic in estuaries owing to the distortion of tidal fluctuations in shallow water, causing disparity between predicted and observed sea levels. The UK National Tidal and Sea Level Facility attempted to reduce prediction errors for the short-term forecasting of High Water (HW) extremes using three alternative techniques to the Harmonic Method in the Bristol Channel, where prediction errors are relatively large. A simple procedure for correcting Harmonic Method HW predictions using recent observations (referred to as the Empirical Correction Method) proved most effective and was also successfully applied to sea-level records from 42 of the 44 UK Tide Gauge Network locations. It is to be incorporated into the operational systems of the UK Coastal Monitoring and Forecasting Partnership to improve UK short-term sea level predictions.

A Century of Sea Level Measurements at Newlyn, SW England

ABSTRACT The Newlyn Tidal Observatory is the most important sea level station in the United Kingdom. It commenced operations in 1915 as part of the Second Geodetic Levelling of England and Wales, and the mean sea level determined from the tide gauge during the first six years (May 1915–April 1921) defined Ordnance Datum Newlyn (ODN) which became the national height datum for the whole of Great Britain. The 100 years of sea level data now available have contributed significantly to many studies in oceanography, geology, and climate change. This paper marks the centenary of this important station by reviewing the sea level (and, more recently, detailed land level) measurements and Newlyns contributions to UK cartography, geodesy and sea-level science in general. Recommendations are made on how sea and land level measurements at Newlyn might be enhanced.

A Rossby whistle: A resonant basin mode observed in the Caribbean Sea

We show that an important source of coastal sea level variability around the Caribbean Sea is a resonant basin mode. The mode consists of a baroclinic Rossby wave which propagates westward across the basin and is rapidly returned to the east along the southern boundary as coastal shelf waves. Almost two wavelengths of the Rossby wave fit across the basin, and it has a period of 120 days. The porous boundary of the Caribbean Sea results in this mode exciting a mass exchange with the wider ocean, leading to a dominant mode of bottom pressure variability which is almost uniform over the Grenada, Venezuela, and Colombia basins and has a sharp spectral peak at 120 day period. As the Rossby waves have been shown to be excited by instability of the Caribbean Current, this resonant mode is dynamically equivalent to the operation of a whistle.

Sea-level monitoring in the British Overseas Territories

A review is conducted on the status of sea-level monitoring in the British Overseas Territories (BOTs), showing where measurements have been made in the last two decades by various groups and thereby indicating where investments in future recording are needed. The sea level has risen in every territory since the early 1990s, and is predicted to increase by approximately 0.25–1.0 m by the end of the twenty-first century, depending on the emissions scenario. As a result, it is maintained that sea-level monitoring is needed in all BOTs, both for local coastal applications and as British contributions to the worldwide sea-level monitoring programme.

Sea level changes at Ascension Island in the last half century

An exercise in ‘data archaeology’ at Ascension Island has provided an estimate of sea level change between 1955 and 2001.5 (the mid-point of a recent dataset spanning 1993–2009). That average trend of 0.93 mm y−1 (SE 0.69) compares to a larger rate during 1993–2009 itself of 2.55 (SE 0.13) and 2.07 (SE 0.30) mm y−1 from tide gauge and altimeter data respectively, suggesting a recent acceleration in sea level rise. An ocean model and steric height datasets have been used for comparison to the measurements, with the conclusion that the acceleration was probably at least partly due to a steric height increase. This exercise is based on only one month of historical tide gauge data and is admittedly on the useful limit for long-term sea level studies. In addition, it is unfortunate that the tide gauge benchmark installed in 1955 has disappeared, even if one can estimate its height relative to modern marks. Nevertheless, the study does provide information of interest to climate studies, enables limits to be inferred on the real changes, and provides background information for other coastal studies. Most importantly, it is intended as a demonstration of the value of similar exercises where short historical records exist.

The nodal dependence of long-period ocean tides in the Drake Passage

Almost three decades of bottom pressure recorder (BPR) measurements at the Drake Passage, and 31 years of hourly tide gauge data from the Vernadsky Research Base on the Antarctic Peninsula, have been used to investigate the temporal and spatial variations in this region of the three main long-period tides Mf, Mm and Mt (in order of decreasing amplitude, with periods of a fortnight, a month and one-third of a month, respectively). The amplitudes of Mf and Mt, and the phase lags for all three constituents, vary over the nodal cycle (18.61 years) in essentially the same way as in the equilibrium tide, so confirming the validity of Doodson’s “nodal factors” for these constituents. The amplitude of Mm is found to be essentially constant, and so inconsistent at the 3σ level from the ± 13 % (or ∼±0.15 mbar) anticipated variation over the nodal cycle, which can probably be explained by energetic non-tidal variability in the records at monthly timescales and longer. The north–south differences in amplitude for all three constituents are consistent with those in a modern ocean tide model (FES2014), as are those in phase lag for Mf and Mt, while the phase difference for Mm is smaller than in the model. BPR measurements are shown to be considerably superior to coastal tide gauge data in such studies, due to the larger proportion of non-tidal variability in the latter. However, correction of the tide gauge records for non-tidal variability results in the uncertainties in nodal parameters being reduced by a factor of 2 (for Mf at least) to a magnitude comparable (approximately twice) to those obtained from the BPR data.

Comment on "Use of local tidal records to identify relative sea level change: accuracy and error for decision makers" by Powell VA, McGlashan DJ, Duck RW (2012) J Coast Conserv

In this Comment we refer to our strong reservations concerning the paper by Powell et al. J Coast Conserv, (2012) recently published on the Online First web site of the Journal of Coastal Conservation. The paper makes a number of comments on data obtained from the Permanent Service for Mean Sea Level (PSMSL) and British Oceanographic Data Centre (BODC) which are incorrect or misleading. In addition, some of their comments on sea-level science in general need to be challenged and corrected.