The global coverage and continuity made possible by satellite altimetry enables scientists to provide ocean wave climatology and to study large-scale patterns of wave variability. Below is a short example based on one year’s satellite altimetry datasets using the Broadview Radar Altimetry Toolbox (BRAT).

Data used

We have focused on 2004 and have used gridded Jason-1 Near-Real Time (NRT) Significant Wave Height (SWH) datasets. Merged gridded SWHs (with improved quality) are not included as they have only been available since the end of 2005. It is also possible to use historical along-track SWH datasets and then to grid them.

 

printemps04_sm ete04_sm
automne04_sm hiver05_sm
echelle_sm

fig 1.Seasonal map of global significant wave height derived from Jason-1 data acquired from March 2004 to March 2005. From top to bottom: spring (March-May), summer (June-August), autumn (September-November) and winter (December-February).

 

Methodology

Name the dedicated BRAT workspace you are using for this job. Within this workspace, name your dataset; as the study is based on four seasons, you can for example name your dataset ‘winter_swh’, ‘summer_swh’ etc., which will allow you to easily identify the kind of file in your dataset.

Temporal extraction

Download Jason-1 NRT MSWH. The time period ranges from early March 2004 (nrt_mswh_j1_19783.nc.gz) to the end of February 2005 (nrt_mswh_j1_20089.nc.gz).

Geographic extraction

No specific focus here.

Mapping the distribution of SWH

In the ‘Operations’ tab, name your computation (for example, ‘seasonal_mean’), then select your dataset and data. In ‘Data Computation’ keep ‘MEAN’ selected
Enter your data expression: specify to BRAT that the data field is Grid_0001 and rename it (‘MeanSWH’). For the X field the variable is ‘Longitude’, and for Y it is ‘Latitude’. In both X and Y field options, enter 1 as the step value to obtain a 1°x1° map (which is the original resolution of this NetCDF file). Now your output file is ready to be executed.
In the ‘Views’ menu, you now only have to name your plot file, give the plot a title, select your NetCDF computed file and click on ‘execute’ to view it.

Results and comments

The global distribution of significant wave height displays a zonal structure, with a large band of high waves in the Southern Oceans that reaches its maximum around 50°S.
In the Northern Hemisphere’s winter, the highest waves are located in the mid-latitudes, both in the central North Atlantic and North Pacific Oceans.
In the Northern Hemisphere’s summer, high waves disappear in the North while they become stronger and larger in the Southern band.
Spring and autumn maps appear as transitional periods between the two previous ocean states.
Note also the signature of the monsoon in the Arabian Sea, where the increasing wave height reaches up to 3 metres in Summer.

Wave climatology suggests possible links between the state of the oceans on inter-annual and seasonal scales (for example, are the wave climates of the North Atlantic and North Pacific connected?). It has also now been proved that North Atlantic wave climatology is linked to North Atlantic Oscillation.

Using more than 25 years of altimeter data, this investigation is still ongoing.