Global ocean winds and waves are changing

Predicting ocean weather can be difficult at the best of times, but monitoring ocean waves during extreme storms is particularly important, as they can be more than 20m high – as tall as a 5-storey building. These waves can pose a critical threat to ocean shipping, the stability of beaches, coastal inundation and the design of coastal and offshore structures.

New research which I have undertaken with my colleague Agustinus Ribal at the University of Melbourne, recently published in the prestigious journal Science has shown that these waves, and the winds that generate them, are increasing in magnitude and have been doing so for the last 30 years.

The research was undertaken using extensive satellite measurements.  These measurements show that mean conditions are increasing but, more importantly, the extreme conditions are increasing even more rapidly. These increases are global but the largest increases are occurring in the Southern Ocean.

Global trends in extreme (90th percentile) wind speed (top) and wave height (bottom) over the period 1985-2018. Areas which a red indicate increasing values, wheras blue indicates decreases.

Extreme winds in the Southern Ocean have increased by approximately 1.5 m/s or 8% over the last 30 years. Similarly, extreme waves in this same region have increased by 30cm or 5%. Generally, winds are increasing at a faster rate than the waves.

In addition to the increases in the Southern Ocean, extreme winds have also increased in the equatorial Pacific and Atlantic and the north Atlantic by approximately 0.6m/s over the 30-year period. The increases in extreme wave height are less uniform than the winds. In addition to increases in the Southern Ocean, extreme waves are also increasing in the North Atlantic. The rate of increase in wind speed and wave height is shown in the figures below.

The determination of these changes in ocean wind and wave climate were made possible by the development of long-term databases of measurements of wind speed and wave height from orbiting satellites.

Data from a total of 31 satellites which were in orbit during the period 1985-2018 were used in the study. Over this period of time, these satellites made approximately 4 billion measurements of wind speed and wave height. Although the data set is huge, to be useful all the satellites needed to be very precisely calibrated. This was done by comparing the satellite measurements with more than 80 ocean buoys deployed around the world. This is the largest and most detailed database of its type ever compiled.

Importantly, within the combined database, there are three different forms of satellites – altimeters, radiometers and scatterometers. They all work differently but all give essentially the same result. This adds confidence that the results are robust.

Although increases of 5% for waves and 8% for winds may not seem like much, if sustained into the future such changes to our climate will have major impacts. The potential impacts of climate induced sea level rise are well known. What most people don’t understand is that the actual flooding events are caused by storm surge and breaking waves associated with storms.

The increased sea level just makes these events more serious and more frequent. Increases in wave height and other properties such as wave direction will further increase the probability of coastal flooding. Such changes will also cause enhanced coastal erosion, putting at risk coastal settlements and infrastructure.

We still don’t know if the historical increases will be sustained into the future. One of the important uses of the extensive satellite database will be to calibrate and validate the next generation of global climate models which are now including ocean wave predictions. Early results from such models yield similar results to the historical record and particularly point to changes in the Southern Ocean.

These changes in the Southern Ocean are important, as this is the origin for swell that dominates the wave climate of the south Pacific, south Atlantic and Indian Oceans. Changes in the Southern Ocean thus have impacts that are felt around the world.