Wind speeds over the world’s oceans have increased over the past two decades, as have wave heights. Studies of climate change typically consider measurements or predictions of temperature over extended periods of time. Climate, however, is much more than temperature. Over the oceans, changes in wind speed and the surface gravity waves generated by such winds play an important role. We used a 23-year database of calibrated and validated satellite altimeter measurements to investigate global changes in oceanic wind speed and wave height over this period. We find a general global trend of increasing values of wind speed and, to a lesser degree, wave height, over this period. The rate of increase is greater for extreme events as compared to the mean condition.

https://researchbank.swinburne.edu.au/file/bca43746-6f16-4ad2-a4b7-27f8d8d60fed/1/PDF (Published version).pdf

Global trends in wind speed and wave height over the past 25 years

Dominant climatic factors controlling the lifetime peak intensity of typhoons are determined from six decades of Pacific typhoon data We find that upper ocean temperatures in the low-latitude northwestern Pacific (LLNWP) and sea surface temperatures in the central equatorial Pacific control the seasonal average lifetime peak intensity by setting the rate and duration of typhoon intensification, respectively An anomalously strong LLNWP upper ocean warming has favored increased intensification rates and led to unprecedentedly high average typhoon intensity during the recent global warming hiatus period, despite a reduction in intensification duration tied to the central equatorial Pacific surface cooling Continued LLNWP upper ocean warming as predicted under a moderate [that is, Representative Concentration Pathway (RCP) 45] climate change scenario is expected to further increase the average typhoon intensity by an additional 14% by 2100

Northwestern Pacific Typhoon Intensity Controlled by Changes in Ocean Temperatures

Sea state information is needed for many applications, ranging from safety at sea and on the coast, for which real time data are essential, to planning and design needs for infrastructure that require long time series. The definition of the wave climate and its possible evolution requires high resolution data, and knowledge on possible drift in the observing system. Sea state is also an important climate variable that enters in air-sea fluxes parameterizations. Finally, sea state patterns can reveal the intensity of storms and associated climate patterns at large scales, and the intensity of currents at small scales. A synthesis of user requirements leads to requests for spatial resolution at kilometer scales, and estimations of trends of a few centimeters per decade. Such requirements cannot be met by observations alone in the foreseeable future, and numerical wave models can be combined with in situ and remote sensing data to achieve the required resolution. As today's models are far from perfect, observations are critical in providing forcing data, namely winds, currents and ice, and validation data, in particular for frequency and direction information, and extreme wave heights. In situ and satellite observations are particularly critical for the correction and calibration of significant wave heights to ensure the stability of model time series. A number of developments are underway for extending the capabilities of satellites and in situ observing systems. These include the generalization of directional measurements, an easier exchange of moored buoy data, the measurement of waves on drifting buoys, the evolution of satellite altimeter technology, and the measurement of directional wave spectra from satellite radar instruments. For each of these observing systems, the stability of the data is a very important issue. The combination of the different data sources, including numerical models, can help better fulfill the needs of users.

Observing Sea States

The climate over the north Indian Ocean (NIO) is one of the most dynamic in the world because of seasonally reversing monsoon winds. In this study, the climate of the NIO and the variability of its surface waves using the European Centre for Medium-Range Weather Forecasts (ECMWF) global atmospheric reanalysis product (ERA-Interim) for the period 1979–2012 are analyzed. Annual average significant wave height (SWH) of the NIO ranges from 1.5 to 2.5 m and the seasonal average is highest (3–3.5 m) during the monsoon period [June–September (JJAS)]. Swells propagating from the Southern Hemisphere are present in the NIO during the premonsoon [February–May (FMAM)] and postmonsoon [October–January (ONDJ)] periods. The waves are separated into wind seas and swells based on the wave energy statistical method. The results show that the NIO is swell dominated and that wind sea heights are lower compared to the swell heights. Higher wind sea and swell heights are observed during the monsoon in the western NIO b...

/pdf/surface-wave-climatology-and-its-variability-in-the-north-g2s2k02v2c.pdf

Surface Wave Climatology and Its Variability in the North Indian Ocean Based on ERA-Interim Reanalysis*

Two empirical algorithms are developed for wave mode images measured from the synthetic aperture radar aboard Sentinel-1 A. The first method, called CWAVE_S1A, is an extension of previous efforts developed for ERS2 and the second method, called Fnn, uses the azimuth cutoff amongst other parameters to estimate significant wave heights and average wave periods without using a modulation transfer function. Neural networks are trained using co-located data generated from WAVEWATCH III and independently verified with data from altimeters and in-situ buoys. We use neural networks to relate the nonlinear relationships between the input SAR image parameters and output geophysical wave parameters. CWAVE_S1A performs well and has reduced precision compared to Fnn with Hs root mean square errors within 0.5 and 0.6 m respectively. The developed neural networks extend the SAR's ability to retrieve useful wave information under a large range of environmental conditions including extra-tropical and tropical cyclones.

/pdf/significant-wave-heights-from-sentinel-1-sar-validation-and-49vkzem0z2.pdf

Significant wave heights from Sentinel-1 SAR: Validation and applications

In this study, a global climatology of swells and wind seas was investigated using near-10-yr collocated wind speed and significant wave height (SWH) measurements from the basic Geophysical Data Record (GDR) of the Jason-1 mission. A statistical method to estimate the wind sea and swell SWHs, respectively, on the basis of wave energy and wind sea/swell probability was proposed. The global distributions of swell/wind sea probability displayed the swell's dominance in the World Ocean. Their seasonal variation showed not only the regions called “swell pools” with high swell probability throughout the year at low latitudes, which have been found in previous studies, but also the regions with high swell probability only in hemispheric summer, termed “seasonal swell pools,” located at the midlatitudes of open oceans. The seasonal geographical patterns of the swell SWH were similar to those of the SWH due to the swell's dominance, and the patterns of the wind SWH were similar to those of the wind speed b...

A Global View on the Swell and Wind Sea Climate by theJason-1Mission: A Revisit

Swells are found in all oceans and strongly influence the wave climate and air-sea processes The poorly known swell dissipation is the largest source of error in wave forecasts and hindcasts We use synthetic aperture radar data to identify swell sources and trajectories, allowing a statistically significant estimation of swell dissipation We mined the entire Envisat mission 2003–2012 to find suitable storms with swells (13   T   18 s) that are observed several times along their propagation This database of swell events provides a comprehensive view of swell extending previous efforts The analysis reveals that swell dissipation weakly correlates with the wave steepness, wind speed, orbital wave velocity, and the relative direction of wind and waves Although several negative dissipation rates are found, there are uncertainties in the synthetic aperture radar-derived swell heights and dissipation rates An acceptable range of the swell dissipation rate is −01 to 6 × 10−7 m−1 with a median of 1 × 10−7 m−1

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015GL067566

Swell dissipation from 10 years of Envisat advanced synthetic aperture radar in wave mode

The arrival time of ocean swells is an important factor for offshore and coastal engineering and naval and recreational activities, which can also be used in evaluating the numerical wave model Using the continuity and pattern of wave heights during the same swell event, a methodology is developed for identifying swell events and verifying swell arrival time in models from buoy data The swell arrival time in a WAVEWATCH III hindcast database is validated with in situ measurements The results indicate that the model has a good agreement with the observations but usually predicts an early arrival of swell, about 4 h on average A histogram shows that about one-quarter of swell events arrive early and three-quarters late by comparison with the model Many processes that may be responsible for the arrival time errors are discussed, but at this stage it is not possible to distinguish between them from the available data

https://researchbank.swinburne.edu.au/file/eef4ba3e-3057-4c91-afbd-5a7ffc786726/1/2016_jiang_event based validation of.pdf

Event-Based Validation of Swell Arrival Time

Numerical investigation of tsunami wave impacts on different coastal bridge decks using immersed boundary method

A method for systematically tracking swells across oceanic basins is developed by taking advantage of high-quality data from space-borne altimeters and wave model output. The evolution of swells is observed over large distances based on 202 swell events with periods ranging from 12 to 18 s. An empirical attenuation rate of swell energy of about 4 × 10−7 m−1 is estimated using these observations, and the nonbreaking energy dissipation rates of swells far away from their generating areas are also estimated using a point source model. The resulting acceptance range of nonbreaking dissipation rates is −2.5 to 5.0 × 10−7 m−1, which corresponds to a dissipation e-folding scales of at least 2000 km for steep swells, to almost infinite for small-amplitude swells. These resulting rates are consistent with previous studies using in-situ and synthetic aperture radar (SAR) observations. The frequency dispersion and angular spreading effects during swell propagation are discussed by comparing the results with other studies, demonstrating that they are the two dominant processes for swell height attenuation, especially in the near field. The resulting dissipation rates from these observations can be used as a reference for ocean engineering and wave modeling, and for related studies such as air-sea and wind-wave-turbulence interactions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JC011536

Haoyu Jiang

Papers

A Global View on the Swell and Wind Sea Climate by theJason-1Mission: A Revisit

Swell dissipation from 10 years of Envisat advanced synthetic aperture radar in wave mode

Event-Based Validation of Swell Arrival Time

Numerical investigation of tsunami wave impacts on different coastal bridge decks using immersed boundary method

Tracking the attenuation and nonbreaking dissipation of swells using altimeters