Review of Area Climatology

Monthly frequency tables of wave height against direction, and wind force against direction, together with information on ice conditions and the occurrence of tropical cyclones, were used to decide upon the seasonal subdivisions. For each sea area the months were grouped together to form four seasons in the manner which appeared to give the best possible fit to the available climatological data. It was, however, necessary to take some account of the total number of observations available for the months concerned. It is counter-productive to choose a one-month season in an area with a relatively small amount of data, as the number of observations is likely to be too low to give a representative picture of the true climatology. Thus, when choosing the seasons for some areas, it was necessary to balance the requirement for climatological homogeneity with the conflicting requirement for an adequate number of observations.

In some areas with relatively low total observation counts, or where there was a marked difference in the numbers of observations at different times of the year, there were seasons where the observation-count was considered to be too low to carry out the analysis.

Months with similar predominant wave and wind directions, or similar severity of wind and wave conditions, were generally grouped together but, in areas where there was a marked difference between the severity or directionality of winds and waves, greater weight was given to the wave climatology when deciding upon the groupings. In areas where sea ice occurs, consideration was given to grouping ice-free months together, and months with the greatest ice cover together, but in practice there are few ships observations from ice-affected areas in the coldest months. Where relevant, months in which tropical cyclones are likely to occur were also grouped together, according to the expected frequency of occurrence, but only when there was no significant conflict with the wind and wave climatology.

For many sea areas, the year can be divided into two distinct seasons (for example, the North-East monsoon and the South-West monsoon), separated by two so-called 'transition' periods. In other areas, however, there may be no obvious seasonality in the wind and wave conditions, or there may be several alternative seasonal breakdowns, but a decision was made as to the best one to use, and the seasonally-analysed data were checked for any obvious inconsistencies before being used for the NMIMET analysis. The monthly groupings of December to February, March to May, June to August and September to November were taken to be the 'standard seasons'. For areas where it was necessary to choose between two alternative monthly groupings of approximately equal merit, the standard breakdown was generally chosen. This was mainly because it was thought advantageous to have approximately similar numbers of observations available for analysis in each season.

Summary of Database Organisation

To summarise, the Global Wave Statistics Online database contains a distinct record for each directional sector of each season of each sea area. Thus, as there are 104 sea areas, 5 seasons and 9 directional sectors, the database contains a total of 4680 records.

Each record contains the wind speed probability distribution for that area / season / direction, plus sufficient NMIMET coefficients, quality control flags and other information to enable Global Wave Statistics Online to reconstitute the wave scatter tables, and perform the extreme wave and persistence analysis.

Some records have insufficient wave data or have been rejected by the NMIMET analysis for other reasons, and in these cases quality control flags prevent the wave data from being accessed by the Global Wave Statistics Online system.

Wind probability data is available for all 4680 records.