Frits B Koek

Advances in the Use of Historical Marine Climate Data

SUMMARY OF DISCUSSION AND RECOM-MENDATIONS. Discussions were held in breakout groups, each focusing on a range of marine variables, and the following key conclusions were drawn:• An overarching recommendation was for con-tinuing augmentation of ICOADS with in situ marine meteorological data and enhanced links to ocean data repositories, such as the World Ocean Database (e.g., Levitus et al. 1998). At least 25 million undigitized ship logbook reports exist, for instance, in U.K. national archives. In view of scarce resources, the need for data inventories and assessments to help identify priorities for digitiza-tion and datasets for incorporation into ICOADS was also highlighted.• Concern was expressed that the marine obser-vation system is in decline. Observations from VOS have decreased by more than a half since 1990, and there are now fewer than a third of the number of VOS participating in the program. As a result, the uncertainty of in situ surface products is increasing. All of the discussion groups were concerned about the diminishing data quantities, which represent a huge challenge for the future. It is essential that the marine climate community makes assessments both of its future data require-ments and the adequacy of the surface marine climate observing system, and feeds this infor-mation through to the appropriate operational bodies.562 |

Ship logbooks help analyze pre-instrumental climate

The Climatological Database for the Worlds Oceans: 1750–1854 (CLIWOC) project, which concluded in 2004, abstracted more than 280,000 daily weather observations from ships logbooks from British, Dutch, French, and Spanish naval vessels engaged in imperial business in the eighteenth and nineteenth centuries. These data, now compiled into a database, provide valuable information for the reconstruction of oceanic wind field patterns for this key period that precedes the time in which anthropogenic influences on climate became evident. These reconstructions, in turn, provide evidence for such phenomena as the El Nino-Southern Oscillation and the North Atlantic Oscillation. Of equal importance is the finding that the CLIWOC database—the first coordinated attempt to harness the scientific potential of this resource [Garcia-Herrera et al., 2005]—represents less than 10 percent of the volume of data currently known to reside in this important but hitherto neglected source.

Searching for Factors that Limit Observed Extreme Maximum Wave Height Distributions in the North Sea

The probability that individual waves are much larger than the significant wave height is studied in a large set of observations. It is investigated whether steepness and shallow water effects are limiting factors for extreme wave heights. The relation between observations and a model freak wave index is examined. Measurements from two locations in the North Sea are used, one with a depth of 80 m and one with a depth of 20 m. The data consist of significant wave height, wave period and maximum wave height of 20 min records. The total amount of the records covers several years. The freak wave model index from the European Centre for Medium-Range Weather Forecasts (ECMWF) wave model is collocated with the observations. The instrumental data show Rayleigh-like distributions for the ratio of maximum wave height to significant wave height. Our analysis is limited by uncertainties in the instrumental response in measuring maximum wave height. The data indicate that steepness is a limiting factor for extreme wave height. At the shallow water location, extreme waves are not more frequently observed than at the deep water location. The relation between the freak wave index of the ECMWF wave model and enhanced extreme wave probability is studied.