Akiyoshi Wada

The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report indicated that the global proportion of Saffir-Simpson category 3–5 tropical cyclones (TCs) has increased over the past four decades. Warming in the upper ocean is considered one of the factors. However, since the number, lifetime maximum intensity, phase transition, and track of TCs vary depending on atmospheric internal factors, it is uncertain whether the increase in upper ocean heat content (UOHC) will have a similar effect on worldwide TCs. This chapter aims to present the representative modes of atmospheric geopotential heights and upper ocean heat contents mainly in the tropical and subtropical oceans. In addition, the representative modes in the inner core of TCs were obtained from the empirical orthogonal analysis with infrared brightness temperatures. The similarities and differences in the relationship among the representative modes, TC central pressure, the lowest temporally average brightness temperature in the inner core from the genesis of a TC to first reaching minimum central pressure, and the spatiotemporally average upper-ocean heat content were examined. Each representative mode for geopotential heights and UOHC was related to climate indices based on sea surface temperature variability, but the relationship varied depending on the region. They were unrelated to the representative modes of infrared brightness temperature in the inner core of TCs. However, the axisymmetric pattern in the inner core was correlated with UOHC beneath a TC and infrared brightness temperature in the inner core of TCs, although their contribution rates varied depending on the region. Therefore, the TC intensity is not determined by the combination of climate indices but is directly influenced by the upper ocean warming beneath a TC.

Over two consecutive days in September 2025, a Japanese aircraft that reached the center of Typhoon Neoguri conducted observations during the typhoon’s mature and weakening phases. Dropsondes were launched at intervals of 5 min or less from approximately 14 km altitude to capture vertical profiles of the inner core and the environmental fields. In the mature phase, we observed a warm-core structure with warm-anomaly maxima in both the upper and lower troposphere in the eye, and a vertical shear of the tangential wind near the lower part of the warm-anomaly maxima. A combined analysis of the dropsondes and satellite-based atmospheric motion vectors in the low-level eye revealed a potential-vorticity bridge structure; this is the first time that such a structure is detected in tropical-cyclone observations over the western North Pacific. In the weakening phase, the lower part of the warm-anomaly maxima disappeared, the central pressure increased, and the profile of equivalent potential temperature in the eye was vertically uniform. The temporal changes of the inner-core structure during the aircraft observations are discussed. We highlight the importance of the synergetic utilization of data from aircraft and satellites for a better understanding of inner-core dynamics.

This study investigates the forecast bust of Typhoon SHANSHAN (2024) characterized by large track errors using the four major interactive grand global operational ensemble data and the atmospheric reanalysis data. Ensemble space empirical orthogonal function (EOF) analysis is applied to 850, 500, and 300 hPa geopotential heights at three target times to diagnose how synoptic-scale uncertainty contributed to the erroneous motions of SHANSHAN. We align the multi-level EOF bases to a reference-time basis via a weighted Procrustes rotation and evaluate similarity to the atmospheric reanalysis data in the aligned principal component (PC) space, enabling robust, distance-based conditioning of ensemble members. Results show that ensemble spread is consistently larger in the mid-latitudes, with relatively large uncertainty concentrated around the upper-tropospheric trough and lower-tropospheric structure near SHANSHAN. The dominant EOF modes differ by phase of SHANSHAN: lower-tropospheric modes govern the westward-moving stage, whereas mid- and upper-tropospheric modes dominate after recurvature. Selecting members whose EOF-based PC structures most closely match the atmospheric reanalysis effectively suppresses large-error outliers and yields improved conditional track predictions. These findings highlight phase-dependent synoptic controls and demonstrate that adaptive, reference-consistent conditioning can enhance the track guidance of tropical cyclones during difficult forecast situations.

The near‐surface specific humidity is critical for accurately estimating the enthalpy flux from the ocean, which plays an important role in tropical cyclone intensification. However, under the severe oceanic and atmospheric conditions of these storms, even spaceborne microwave radiometers struggle to retrieve reliable humidity data. This limitation stems primarily from heavy precipitation, which masks the water‐vapor signal in the observed brightness temperatures. To overcome these challenges, this study introduces a two‐pronged approach: (a) a neural network‐based screening of brightness temperatures to filter out precipitation‐contaminated measurements, and (b) the first use of low‐frequency microwave channel for humidity estimation in tropical cyclones. Comparisons with independent in situ observations demonstrate that our method provides more accurate near‐surface humidity estimates than previous techniques that do not use low‐frequency channels. These improvements will enhance the estimation of ocean‐to‐atmosphere enthalpy fluxes during tropical cyclones and may support better understanding of air‐sea interaction processes under extreme weather conditions.

Typhoons AMPIL and SHANSHAN (2024) slowed down as they approached and moved parallel to the Kuroshio Current. Both typhoons reached peak intensity at relatively high latitudes while they were moving northward. Various atmospheric and oceanic datasets were used to investigate atmospheric and oceanic thermodynamic similarities and differences between AMPIL and SHANSHAN. In addition, numerical simulations and sensitivity experiments for high sea surface temperature in the Kuroshio Current region were performed with a nonhydrostatic atmosphere model and an atmosphere-wave-ocean coupled model to understand related intensification mechanism. The 26°C isotherm was relatively deep and the upper-ocean heat content was relatively

The IPCC Sixth Assessment Report (AR6) indicates that the global proportion of Category 3–5 tropical cyclones (TCs) has increased over the past four decades. One of the possible reasons is an increase in the upper-ocean heat content (UOHC). However, it is questionable to explain the long-term trend only based on the upper-ocean warming because Category 3–5 TCs enhance the ocean response in the form of sea surface cooling. Here we calculated a new heat content metric by using 2-m air and dew-point temperatures from 2000 to 2021 with oceanic reanalysis data under the consideration of atmospheric baroclinicity. Since 2000, the new heat content from July to December has not significantly increased in the tropical and subtropical Northwest Pacific (NWP). Although the median for conventional UOHC calculated based on the 26 °C-isotherm depth has increased significantly, the trends in the median and extreme values of TC central pressures obtained from multiple best track datasets were consistent with the trends of the new heat content metric. The results suggest that upper ocean warming alone cannot explain an increase in extremely intense TCs, while new heat content has a potentially closer relationship with TC activity than conventional UOHC.

This study investigated the relationship between the predicted track of Typhoon Mawar (2023) and the quasi-stationary front along the southern coast of Japan where heavy rainfall occurred. Also, the role of ocean coupling was explored by using global model predictions and numerical simulations conducted by a regional atmosphere–wave–ocean coupled model. The track predictions by four major global models showed that the prediction errors became significantly larger after the recurvature. One of the global models could reasonably predict both the track and the location of the front, even after five days. The results of numerical simulations of which the initial and boundary conditions were based on the successful predictions suggest that ocean coupling contributes to the improvement of central pressure simulations compared with fixed oceanic conditions. More northward translation of Mawar after the recurvature simulated by the coupled model could be explained by the separation of the inner-core vortex into two parts in the upper and lower troposphere. However, the predictability of the Subtropical High was more important in determining not only the track but also environmental southerly flow over the moisture road formed between Mawar and the Subtropical High and in accurately predicting the location of the front.

Typhoon Faxai hit Japan in 2019 and severely damaged the Tokyo metropolitan area. To mitigate such damages, a good track forecast is necessary even before the typhoon formation. To investigate the predictability of the genesis and movement of a precursor vortex and its relationship with the synoptic‐scale flow, 100‐member ensemble simulations of Typhoon Faxai were performed using a 14‐km mesh global nonhydrostatic atmospheric model, which started from 16 different initial days (i.e., 1,600 members in total). The results show that the model could predict an enhanced risk of a Faxai‐like vortex heading toward Japan 2 weeks before landfall, which was up to 70%. The reason for the enhancement was a rapid increase in the members reproducing a precursor vortex from 15 to 12 days before landfall in Japan. In addition, the upper‐tropospheric vortex played an essential role in the track simulation of Faxai.

An empirical orthogonal function (EOF) analysis was performed for the inner core of Typhoon Hagibis (2019) in the intensification phase. The Himawari–8 geostationary infrared (IR) brightness temperature (BT) collocated at the Hagibis’s center was combined with the IR BT simulated by a radiative transfer model, with 1 km ensemble simulations conducted by an atmosphere model and the coupled atmosphere–wave–ocean model. The ensemble simulations were conducted under one control atmospheric initial condition and the 26 perturbed ones with two different oceanic initial conditions. The first four EOF modes showed symmetric and asymmetric patterns such as a curved band, cloud dense overcast, and eye pattern used in the classification of the Dvorak technique. The influence of ocean coupling on the modes appeared only in the early intensification phase but was relatively small compared to the difference from the Himawari–8 observations. While ocean coupling and different oceanic initial condition quantitatively affected the IR BT, the normalized amplitude for the first EOF mode did not become close to that of the Himawari–8 observation in the late intensification phase. The intensification rate in the late intensification phase was inconsistent between the simulation results and the estimate from the Himawari–8 normalized amplitude by multiple linear regression analysis.

Insufficient in situ observations in high winds make it difficult to verify climatological data sets and the results of tropical cyclone (TC) simulations. Reliable data sets are necessary for developing numerical models that predict TCs more accurately. This study attempted to compare the third-generation Japanese Ocean Flux Data Sets with Use of Remote-Sensing Observations (J-OFURO3) data, with TC simulations conducted by a 2 km mesh coupled atmosphere-wave-ocean model. This is a case study of Typhoon Dujuan (2015) and the area of approximately 20̊N, 130̊E, south of Okinawa, was selected. The comparison reveals that J-OFURO3 data are reliable for verifying the atmospheric and oceanic components of TC simulations with two different initial sea surface temperature (SST) conditions, although the blank area remains within the inner core area for air temperature, specific humidity, and latent heat flux owing to issues with the construction method. Simulated maximum surface wind speeds (MSWs) are significantly correlated with J-OFURO3 MSWs. The asymmetrical distribution of simulated surface wind speeds within the inner core area can be reproduced well in the J-OFURO3 data set. In terms of the oceanic response to the TC, TC-induced sea surface cooling was reproduced well in the J-OFURO3 data set and is consistent with the simulation results. Unlike simulated SST, simulated surface wind speeds, surface air temperature, and surface specific humidity are still inconsistent with the J-OFURO3 data, even when the J-OFURO3 SST is used as the initial condition. New algorithms, more satellite data used, and model improvement are expected in the future.