Philip L. Woodworth

Evidence for enhanced coastal sea level rise during the 1990s

into 13 regions with near global coverage and using a Glacial Isostatic Adjustment (GIA) model to correct for land movements. We present evidence from altimeter data that the rate of sea level rise around the global coastline was significantly in excess of the global average over the period 1993–2002. We also show that the globally-averaged rate of coastal sea level rise for the decade centered on 1955 was significantly larger than any other decade during the past 55 years. In some models of sea level rise, enhanced coastal riseisapre-cursorofglobalaveragerise.Itremainstobe seen whether the models are correct and whether global-average rates in the future reflect the high rates of coastal rise observed during the 1990s. INDEX TERMS: 1635 Global Change: Oceans (4203); 1223 Geodesy and Gravity: Ocean/ Earth/atmosphere interactions (3339); 4215 Oceanography: General: Climate and interannual variability (3309); 4556 Oceanography: Physical: Sea level variations. Citation: Holgate, S. J., and P. L. Woodworth (2004), Evidence for enhanced coastal sea level rise during the 1990s, Geophys. Res. Lett., 31, L07305,

Accuracy assessment of recent ocean tide models

Over 20 global ocean tide models have been developed since 1994, primarily as a consequence of analysis of the precise altimetric measurements from TOPEX/POSEIDON and as a result of parallel developments in numerical tidal modeling and data assimilation. This paper provides an accuracy assessment of 10 such tide models and discusses their benefits in many fields including geodesy, oceanography, and geophysics. A variety of tests indicate that all these tide models agree within 2-3 cm in the deep ocean, and they represent a significant improvement over the classical Schwiderski 1980 model by approximately 5 cm rms. As a result, two tide models were selected for the reprocessing of TOPEX/POSEIDON Geophysical Data Records in late 1995. Current ocean tide models allow an improved observation of deep ocean surface dynamic topography using satellite altimetry. Other significant contributions include theft applications in an improved orbit computation for TOPEX/POSEIDON and other geodetic satellites, to yield accurate predictions of Earth rotation excitations and improved estimates of ocean loading corrections for geodetic observatories, and to allow better separation of astronomical tides from phenomena with meteorological and geophysical origins. The largest differences between these tide models occur in shallow waters, indicating that the current models are still problematic in these areas. Future improvement of global tide models is anticipated with additional high-quality altimeter data and with advances in numerical techniques to assimilate data into high-resolution hydrodynamic models.

New Data Systems and Products at the Permanent Service for Mean Sea Level

ABSTRACT Holgate, S.J.; Matthews, A.; Woodworth, P.L.; Rickards, L.J.; Tamisiea, M.E.; Bradshaw, E.; Foden, P.R.; Gordon, K.M.; Jevrejeva, S., and Pugh, J., 2013. New data systems and products at the Permanent Service for Mean Sea Level. Sea-level rise remains one of the most pressing societal concerns relating to climate change. A significant proportion of the global population, including many of the worlds large cities, are located close to the coast in potentially vulnerable regions such as river deltas. The Permanent Service for Mean Sea Level (PSMSL) continues to evolve and provide global coastal sea-level information and products that help to develop our understanding of sea-level and land motion processes. Its work aids a range of scientific research, not only in long-term change, but also in the measurement and understanding of higher frequency variability such as storm surges and tsunamis. The PSMSL has changed considerably over the past 10 years, and the aim of this paper is to update the community about these changes as well as provide an overview of our continuing work.

Evidence for Systematic Changes in Extreme High Waters since the Mid-1970s

Sea level data from a set of 141 tide gauges with a reasonable global distribution have been used to seek evidence for significant changes during recent decades in the occurrence of extreme high-water levels, and for systematic differences between changes in high waters and mean sea levels. The relationships between the occurrence of extreme high waters and of indices of regional climate have also been investigated. It is found that there is indeed evidence for a general worldwide increase in extreme high-water levels since 1975, and that the variations in extremes in this period are closely related to changes in regional climate. However, for most of the stations in the dataset used here, the secular changes and the interannual variability in extremes are similar to those in mean sea level. Consequently, changes in both sea level parameters are consistent with being produced by the same type of atmospheric and/or oceanic forcing.

Understanding sea level rise and variability

The coastal zone changed considerably during the twentieth century due to growing populations and increasing urbanization. A recent study indicated that in 1990, 23% of the worlds population (1.2 billion people) were living within both a 100-kilometer distance and a 100-meter elevation of the coast at densities 3 times higher than the global average. Society is becoming increasingly vulnerable to sea level extremes, as Hurricane Katrina demonstrated. Rising levels will result in more flooding, even if storm intensities do not increase. Improved understanding of the reasons for sea level rise and variability is required to reduce the uncertainties in sea level rise projections, and this improved understanding could contribute to more effective coastal planning and management.

Dedicated gravity field missions—principles and aims

Abstract Current knowledge of the Earths gravity field and its geoid, as derived from various observing techniques and sources, is incomplete. Within a reasonable time, substantial improvement can only come by exploiting new approaches based on satellite gravity observation methods. For this purpose three satellite missions will be realised, starting with CHAMP in 2000, followed by GRACE in 2002 and GOCE in 2004. Typical for all three missions is their extremely low and (almost) polar orbit, continuous and three-dimensional tracking by GPS and their ability to separate non-gravitational from gravitational signal parts. A further amplification of the gravity signal is achieved by inter-satellite tracking between two low orbiters in the case of GRACE and by gravity gradiometry in the case of GOCE. The rationale of GOCE will be discussed in more detail. The missions have a wide range of applications in solid Earth physics, oceanography, ice research, climatology, geodesy and sea level research.

Intercomparison of recent ocean tide models

One of the tremendous achievements of the TOPEX/POSEIDON (T/P) satellite mission is the release of 12 new global ocean tide models during 1994. Virtually all of these models are based on the high-precision altimetry obtainable from the T/P satellite, and all of the models are superior to both the Cartwright and Ray (1990, 1991) model as well as to the Schwiderski (1980a, b) model when compared to tide gauges. In the present intercomparison, only global ocean tide models released during 1994 have been tested. As T/P continues to operate, new ocean tide models will continue to be released and existing ocean models will be updated. Hence this intercomparison can only present a snapshot of the “state of the art” within ocean tide modeling. However, the intercomparison demonstrates the high accuracy and consistency of all these recent ocean tide models. From a comparison with a common 104 tide gauge data set compiled by Le Provost and others (unpublished data), the general result is that, six of the new ocean tide models have RMS agreement better than 3 cm. The obtainable accuracy with different tide gauge types raises the question about the accuracy of “ground truth.” As ocean tide models become increasingly more accurate, the accuracy of the harmonic constants derived from the tide gauge recordings becomes significant. Generally, the RMS agreement between models based on 2 years of T/P altimetry is significantly better than the agreement between models based on 1 year of T/P altimetry. For models based on two years of T/P altimetry the RMS agreement has values around 2.0, 1.4, 1.0, and 0.7 cm for the M2, S2, K1 and O1 constituents, respectively. As a preliminary evaluation, these values might be taken as a measure of the consistency of ocean tides estimated from the first part of the T/P mission, which is extremely promising for future ocean tide models derived from T/P altimetry using 3 or more years of data. This also suggests that further improvement using 3 or more years of data will only be marginal. The differences arise when studying the ocean tides on continental shelves surrounding the ocean. Major differences are seen, especially on the Patagonian shelf. As the continental shelves are critical for tidal dissipation, it will be very important to improve tide models on the shelves in future global ocean tide models.

Changes in the ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode

[1] We present the first direct evidence that interannual changes in ocean transport through Drake Passage are forced by variability in the Southern Annular Mode (SAM). This evidence is derived from two decades (1980s and 1990s) of subsurface pressure measurements from the tide gauge at Faraday station (western Antarctic Peninsula), combined with the output of an ocean general circulation model. In recent decades, the SAM has moved toward a higher-index state (stronger circumpolar winds); this trend is not simply monotonic, but is the product of a long-term change in the seasonality of the SAM. Whilst we cannot address directly the effect of the long-term trend on circumpolar transport, bottom pressure data from Drake Passage during the 1990s demonstrate that ocean transport showed the same changes in seasonality as did the SAM. This offers a mechanism for atmospheric climate change to influence directly the largescale ocean circulation. INDEXTERMS: 4207 Oceanography: General: Arctic and Antarctic oceanography; 4215 Oceanography: General: Climate and interannual variability (3309); 4532 Oceanography: Physical: General circulation; 1635 Global Change: Oceans (4203); 4556 Oceanography: Physical: Sea level variations. Citation: Meredith, M. P., P. L. Woodworth, C. W. Hughes, and V. Stepanov (2004), Changes in the ocean transport through Drake Passage during the 1980s and 1990s, forced by changes in the Southern Annular Mode, Geophys. Res. Lett., 31, L21305, doi:10.1029/2004GL021169.

High waters at Liverpool since 1768: the UK's longest sea level record

An exercise of ‘data archaeology’ of high water tidal information from Liverpool, NW England has resulted in the construction of a time series of ‘Adjusted Mean High Water’ spanning 1768 to the present which can be employed as a record of proxy Mean Sea Level (MSL). The time series, although gappy, is arguably the second oldest sea level-related record in the world, after Amsterdams (1682, although the data we hold are from 1700 only) and of comparable age to Stockholms (1774). It describes a secular trend for the period up to 1880 of 0.39+/−0.17 mm/year, a trend for the twentieth century of 1.22plus;/−0.25 mm/year, and an overall low frequency acceleration of 0.33+/−0.10 mm/year/century. When considered alongside geological sea level information from the area, the evidence suggests that the greater secular trend of sea level in the twentieth century, compared to long term projections derived from geological information, is primarily the result of an acceleration towards the second half of the last century, consistent with conclusions inferred from previous analyses of the very small number of other long European MSL records.

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.