Tomoya Shimura

Ocean Waves and Teleconnection Patterns in the Northern Hemisphere

AbstractUnderstanding long-term, ocean wave climate variability is important to assess climate change impacts on coastal and ocean physics and engineering. Teleconnection patterns can represent wave climate variability in the context of climate change. The objective of this study is to identify how large-scale spatial distributions of wave heights vary on a monthly basis and how they are influenced by various teleconnection patterns using reanalysis datasets. The wave height climate responses to teleconnection patterns in the eastern part of the North Pacific and North Atlantic are more sensible than in the corresponding western parts. The dominant spatial patterns of monthly averaged wave height variability in winter were obtained by empirical orthogonal function analysis. The three dominant patterns in the North Pacific and North Atlantic are similar. It is remarkable that one of the three dominant patterns, a band-shaped pattern, exhibits a strong relation to the teleconnection pattern in each ocean. T...

Future Projection of Ocean Wave Climate: Analysis of SST Impacts on Wave Climate Changes in the Western North Pacific

AbstractChanges in ocean surface waves elicit a variety of impacts on coastal environments. To assess the future changes in the ocean surface wave climate, several future projections of global wave climate have been simulated in previous studies. However, previously there has been little discussion about the causes behind changes in the future wave climate and the differences between projections. The objective of this study is to estimate the future changes in mean wave climate and the sensitivity of the wave climate to sea surface temperature (SST) conditions in an effort to understand the mechanism behind the wave climate changes by specifically looking at spatial SST variation. A series of wave climate projections forced by surface winds from the MRI-AGCM3.2 were conducted based on SST ensemble experiments. The results yield future changes in annual mean wave height that are within about ±0.3 m. The future changes in summertime wave height in the western North Pacific (WNP), which are influenced by tro...

Over 5,000 Years of Ensemble Future Climate Simulations by 60-km Global and 20-km Regional Atmospheric Models

AbstractAn unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the a...

Future Projections of Extreme Ocean Wave Climates and the Relation to Tropical Cyclones: Ensemble Experiments of MRI-AGCM3.2H*

AbstractFuture projections of extreme ocean surface wave climates were carried out with single-model ensemble experiments of the atmospheric global climate model MRI-AGCM3.2H. The ensemble experiments of MRI-AGCM3.2H consist of four future sea surface temperature (SST) ensembles and three perturbed physics (PP) ensembles. This study showed that future changes in extreme wave heights strongly depend on the global climate model (GCM) performance to simulate tropical cyclones (TCs), indicating a need to acknowledge that results in a study that employs a low-performance model are not able to account for extreme waves associated with TCs (TC waves). The spatial distribution of future changes in non-TC extreme wave heights on the global scale was similar to that for mean wave heights; namely, wave heights increase over the middle-to-high latitudes in the Southern Ocean and central North Pacific and decrease over midlatitudes and the North Atlantic, although the magnitude of future changes for extreme wave heigh...

Variability and future decreases in winter wave heights in the Western North Pacific

Ocean surface wave climate is a key consideration for a number of industrial and environmental systems, both offshore and at the coast. A dynamical spectral wave model forced with global climate models (GCMs) was used to produce an ensemble of simulations of both historical and projected future wave climate. To estimate the uncertainty of the projected wave climate, we combined a multimember ensemble experiment using MRI-AGCM3.2H with a multimodel ensemble using eight CMIP5 GCMs. Future changes in wintertime wave heights from the end of the 20th to the 21st century were analyzed. Projected decreases in wave heights over the Western North Pacific are highly consistent among the ensemble. The future decreases in wave heights are significantly related to changes in the West Pacific pattern. Both locally generated waves and remotely generated swells are important to estimate future changes in the wave climate on a regional scale.

Long‐term impacts of ocean wave‐dependent roughness on global climate systems

Ocean surface waves can play an active role in climate systems, but they are often ignored in Global Climate Models (GCMs). Wave-dependent surface roughness was implemented within the Atmospheric GCM (MRI-AGCM) using the spectral wave model WAVEWATCH III. Two types of wave-dependent roughness, due to wave steepness and to wave age, were considered. Climate simulations with wave-dependent roughness were compared to simulations with just wind speed-dependent roughness. In climate simulation with wave steepness-dependent roughness, the spatial distribution of roughness is correlated to that of swell dominance. In simulation with wave age-dependent roughness, the spatial distribution of roughness is correlated to that of wind direction stationarity. Both simulations show reduced roughness in the tropics, which leads to an enhancement of surface wind speeds by up to 15%; these enhanced wind speeds are closer to observations compared with the baseline simulation with just wind speed-dependent roughness. We find that the reduced roughness and the enhanced wind speeds in the tropics lead to significant changes in atmospheric circulation, as in Hadley circulation and precipitation. The characteristic responses of the Hadley circulation and precipitation to changing sea surface roughness are presented.

Estimation of Wind Vector Profile Using a Hexarotor Unmanned Aerial Vehicle and Its Application to Meteorological Observation up to 1000 m above Surface

AbstractSmall unmanned aerial vehicles (UAVs), also known as drones, have recently become promising tools in various fields. We investigated the feasibility of wind vector profile measurement using...