Stokes Drift and Meshless Wave Modeling

This dissertation is loosely organized around efforts to improve vertical ocean mixing in global climate models and includes an in-depth analysis of Stokes drift, optimization of a new global climate model wave component, and development of a meshless spectral wave model. Stokes drift (hereafter SD) is an important vector component that appears often in wave-averaged dynamics. Mathematically, SD is the mean difference between Eulerian and Lagrangian velocities and intuitively can be thought of as the near-surface ocean current induced from wave motion. Increasingly, spectral wave models are being used to calculate SD globally. These models solve a 5D wave action balance equation and typically require large computational resources to make short to medium-range forecasts of the sea state. In the first part, a hierarchy of SD approximations are investigated and new approximations that remove systematic biases are derived. A new 1D spectral approximation is used to study the effects of multidirectional waves and directional wave spreading on SD. It is shown that these effects are largely uncorrelated and affect both the magnitude and direction of SD in a nonlinear fashion that is sensitive with depth. In the second part, efforts to add a wave model component to the NCAR Community Earth System Model are discussed. This coupled component will serve as the backbone to a new Langmuir mixing parameterization and uses a modified version of NOAA WAVEWATCH III (a third-generation spectral wave model). In addition, the governing wave action balance equation is reviewed and several variations are derived and formulated. In the third part, construction of a monochromatic spectral wave model using RBF-generated finite differences is described. Several numerical test cases are conducted to measure performance and guide further development. In kinematic comparisons with WAVEWATCH III, the meshless prototype is approximately 70–210 times more accurate and uses a factor of 12 to 17 less unknowns

Future changes in extreme storm surge based on a maximum potential storm surge model for East Asia

We analyzed tropical cyclones (TC) based on the theory of Maximum Potential Intensity (MPI) and Maximum Potential Surge (MPS) for a long-term assessment of extreme TC intensity and storm surge heights. We investigated future changes in the MPI fields and MPS for different global warming levels based on 150-year continuous scenario projections (HighResMIP) and large ensemble climate projections (d4PDF/d2PDF). Focusing on the Western North Pacific Ocean (WNP), we analyzed future changes in the MPI and found that it reached a maximum in the latitudinal range of 30–40°N in September. We also analyzed future changes in the MPS in major bays of East Asia and along the Pacific coast of Japan. Future changes in the MPS were projected, and it was confirmed that changes in the MPS are larger in bays where large storm surge events have occurred in the past

The Assessment of the Impact of Climate Change on Typhoons Using a Slab-Ocean Coupled Atmospheric Global Circulation Model with Month Fixed Event Attribution Experiments

New climate experiment was proposed using a model that combines the Meteorological Research Institute's Atmospheric Global Circulation Model (MRI-AGCM) with a slab ocean model to evaluate the impact of warming on tropical cyclones. A month-fixed EA experiment was conducted under the present and future climate under ssp585 scenarios in the climate projection experiment. The statistical characteristics of typhoons passing near Japan were evaluated. The increases in the number of strong typhoons were obtained as a change in typhoon intensity characteristics caused by climate change. Comparison with the results of a typical scenario run, confirms the validity of fixed-month EA climate experiment

Development of Coupled Atmospheric-SLAB Ocean Model Global Climate Model and Climate Change Impacts on Tropical Cyclones

In this study, the effects of global warming on tropical cyclones (TCs) were evaluated by using a slab-ocean model coupled with the Atmospheric Global Climate Model (MRI-AGCM). The warming conditions for the MRI-AGCM climate simulations were obtained from the latest climate models (CMIP6). We have conducted two types of global climate simulations, typical and newly proposed experiments. The former is time slice experiments and latter is the climate experiment fixed single month condition. Both experiments are conducted on present and future periods, to estimate the effects of climate change impacts on TC characteristics. The slab ocean model which considers sea surface cooling due to strong winds, significantly contributes to a reduction in TC intensity. Moreover, we have confirmed that climate change can reduce the TC frequency and enhance the TC intensity

Robustness and uncertainties in global multivariate wind-wave climate projections

Understanding climate-driven impacts on the multivariate global wind-wave climate is paramount to effective offshore/coastal climate adaptation planning. However, the use of single-method ensembles and variations arising from different methodologies has resulted in unquantified uncertainty amongst existing global wave climate projections. Here, assessing the first coherent, community-driven, multi-method ensemble of global wave climate projections, we demonstrate widespread ocean regions with robust changes in annual mean significant wave height and mean wave period of 5–15% and shifts in mean wave direction of 5–15°, under a high-emission scenario. Approximately 50% of the world’s coastline is at risk from wave climate change, with ~40% revealing robust changes in at least two variables. Furthermore, we find that uncertainty in current projections is dominated by climate model-driven uncertainty, and that single-method modelling studies are unable to capture up to ~50% of the total associated uncertainty

Recent nationwide climate change impact assessments of natural hazards in Japan and East Asia

Climate change due to global warming is expected to have major impacts on phenomena such as tropical cyclones (TCs), Baiu, precipitation, and seasonal storms. Many natural disasters in East Asia are driven by TC (typhoon) activity in particular and their associated hazards are sensitive to local-scale characteristics. As such, it is critically important to numerically simulate TC activity (and other phenomenon) on local scales in order to properly assess climate change impacts on natural hazards in the region. In addition, projecting future changes of many TC-related hazards and/or their potential economic impacts can be challenging due to their low occurrence frequencies in any one particular area. With these views in mind, a collaborative research program was formed in Japan to project long-term changes in natural hazards in Japan and East Asia based on local-scale and large-ensemble numerical experiments. This paper reviews recent climate change impact assessments (written in both English and Japanese) from the program and summarizes the projected future changes in precipitation, river flooding, and coastal hazards, and their associated economic impacts