Abstract
This study identifies seasonally-reversed trends in Kuroshio strength and sea surface temperatures (SSTs) within the western North Pacific (WNP) since the 1990s, specifically in the 22° ...N–28° N region. These trends are characterized by increases during summer and decreases during winter. The seasonally-reversed trends are a result of the asymmetric responses of the WNP to a shift towards the positive phase of the Atlantic multidecadal oscillation (AMO) around the same period. The positive AMO induces an anomalous descent over the North Pacific during summer, leading to the direct strengthening of the gyre. However, during winter, it triggers an anomalous descent over the tropical Pacific, which excites a poleward wavetrain impacting the WNP and causing gyre weakening. The associated responses of the East Asian monsoon and China Coastal Current contribute to the observed seasonally-reversed SST trends. It is noteworthy that the seasonally-reversed trends in gyre strength and SSTs are predominantly observed north of 20° N in the WNP. This limitation arises because the anomalous cyclone within the winter poleward wavetrain is located north of this latitude boundary. Specifically, the clearest trends in gyre strength are observed in the northern segment of the Kuroshio, while the manifestation of SST trends in the Taiwan Strait could potentially be attributed to the influence and enhancement of the East Asian monsoon and the China Coastal Current. Due to the limited length of observational data, statistical significance of some of the signals discussed is rather limited. A CESM1 pacemaker experiments is further conducted to confirm the asymmetric responses of the North Pacific to the AMO between the summer and winter seasons.
Intense eruptions of the Tonga volcano activated prominent traveling atmospheric disturbances (TADs) at 04:05UT on 15 January 2022. Himawari‐8 satellite images depict that TADs of the tropospheric ...Lamb wavefront propagate with a speed of 315 m/s and arrive in Taiwan at 11:30UT. Networks of 98 barometers, 28 tide gauges, an ionosonde, and 10 magnetometers are used to study the responses of magnetic fields to TADs in Taiwan. The horizontal components in magnetic field changes of the Taiwan magnetometers all point toward and away from the Tonga volcano at 11:00–12:00UT upon the tropospheric Lamb wavefront arrival and at 22:00–23:00UT when the thermospheric Lamb wavefront with speeds of 487 m/s coming, respectively. Analyses of the raytracing and beamforming techniques on the horizontal components in magnetic field changes of 69 INTERMAGNET magnetometers show that both tropospheric and thermospheric Lamb waves efficiently activate traveling ionospheric disturbances and modify ionospheric currents of the globe.
Plain Language Summary
At 04:05UT on 15 January 2022, intense Tonga volcanic eruptions induce prominent atmospheric disturbances and tsunami waves. Himawari‐8 meteorological satellite images depict the induced upper‐level tropospheric disturbances with horizontal speeds of about 315 m/s at 8.2 km altitude in the Lamb wave mode travel worldwide. Upon the traveling atmospheric disturbances (TADs) of the tropospheric Lamb wavefront arriving in Taiwan at 11:30UT, 98 ground‐based barometers register increases and reach peaks at about 11:50UT in the atmospheric pressure; 28 tide gauges record enhancements and maximums of sea level fluctuations at about 14:30–17:30UT; and a local ionosonde observes that the ionosphere reaches the highest altitude at 14:30UT. The changes of the horizontal component of the Earth's magnetic fields measured by 10 Taiwan magnetometers almost all point exactly toward the Tonga volcano upon the tropospheric Lamb wavefront arrival at 11:00–12:00UT, and away from the volcano at 22:00–23:00UT, which suggests a 487 m/s TAD (or thermospheric Lamb wavefront) at about 130 km altitude also being activated. The horizontal components in magnetic field changes of 69 INTERMAGNET magnetometers show that both tropospheric and thermospheric Lamb waves triggered by Tonga volcanic eruptions are very powerful, and can induce intense dynamo currents and electric fields on the globe.
Key Points
Tropospheric and thermospheric Lamb waves of the Tonga volcanic eruption activate dynamo currents and electric fields
Traveling atmospheric disturbances of the Tonga volcanic eruption significantly uplift the ionosphere
Tropospheric Lamb waves of the Tonga volcanic eruption modulate ground‐based air pressures and sea levels
Tropical cyclones (TCs) cause severe natural hazards and drive intense upper ocean cooling through a series of oceanic and atmospheric physical processes, including vertical mixing and upwelling. ...Among these processes, TC-induced warming of near-surface waters in the open ocean has rarely been noted. This study provides a detailed analysis of upper ocean responses to 30 TC events observed by two buoys in the western North Pacific between 2016 and 2021. Supplemented with numerical experiments, we suggest that downwelling frequently occurs at the periphery of upwelling regions (around the radius of the 34 knot wind speed) following the passage of a TC. Downwelling is identified via pronounced warm anomalies under a shallow mixed layer depth, and its dynamics are attributed to negative wind stress curl and current-induced convergence. These findings highlight the important role played by TC-induced downwelling and offer insights for reconsidering the influence of TCs on biogeochemical processes.
Global warming has enduring consequences in the ocean, leading to increased sea surface temperatures (SSTs) and subsequent environmental impacts, including coral bleaching and intensified tropical ...storms. It is imperative to monitor these trends to enable informed decision-making and adaptation. In this study, we comprehensively examine the methods for extracting long-term temperature trends, including STL, seasonal-trend decomposition procedure based on LOESS (locally estimated scatterplot smoothing), and the linear regression family, which comprises the ordinary least-squares regression (OLSR), orthogonal regression (OR), and geometric-mean regression (GMR). The applicability and limitations of these methods are assessed based on experimental and simulated data. STL may stand out as the most accurate method for extracting long-term trends. However, it is associated with notably sizable computational time. In contrast, linear regression methods are far more efficient. Among these methods, GMR is not suitable due to its inherent assumption of a random temporal component. OLSR and OR are preferable for general tasks but require correction to accurately account for seasonal signal-induced bias resulting from the phase–distance imbalance. We observe that this bias can be effectively addressed by trimming the SST data to ensure that the time series becomes an even function before applying linear regression, which is named “evenization”. We compare our methods with two commonly used methods in the climate community. Our proposed method is unbiased and better than the conventional SST anomaly method. While our method may have a larger degree of uncertainty than combined linear and sinusoidal fitting, this uncertainty remains within an acceptable range. Furthermore, linear and sinusoidal fitting can be unstable when applied to natural data containing significant noise.
The forecast of typhoon tracks remains uncertain and is positively related to the accuracy of the storm surge forecast. The storm surge prediction error increases dramatically when the forecast track ...error is larger than 100 km. This study aims to develop an ensemble storm surge prediction system using parametric weather models to account for the uncertainty in typhoon track prediction. The storm surge model adopted in this study is COMCOT-SS storm surge forecast system. Two methods are introduced and analyzed to generate the ensemble members in this study. One is from the weather ensemble prediction system (WEPS), and the other is from the error distribution of the deterministic forecasts (EDF). The ensemble prediction results show that the ensemble mean of WEPS performs similarly to the deterministic forecast. However, the maximum surge height of WEPS is often lower than one from EDF. The verification results suggest that, for disaster prevention, EDF provides stronger warnings to the coastal region than WEPS. However, it may provide overestimated forecasts in some cases.
Abstract
This paper introduces a relocation scheme for tropical cyclone (TC) initialization in the Advanced Research Weather Research and Forecasting (ARW-WRF) model and demonstrates its application ...to 70 forecasts of Typhoons Sinlaku (2008), Jangmi (2008), and Linfa (2009) for which Taiwan’s Central Weather Bureau (CWB) issued typhoon warnings. An efficient and dynamically consistent TC vortex relocation scheme for the WRF terrain-following mass coordinate has been developed to improve the first guess of the TC analysis, and hence improves the tropical cyclone initialization. The vortex relocation scheme separates the first-guess atmospheric flow into a TC circulation and environmental flow, relocates the TC circulation to its observed location, and adds the relocated TC circulation back to the environmental flow to obtain the updated first guess with a correct TC position. Analysis of these typhoon cases indicates that the relocation procedure moves the typhoon circulation to the observed typhoon position without generating discontinuities or sharp gradients in the first guess.
Numerical experiments with and without the vortex relocation procedure for Typhoons Sinlaku, Jangmi, and Linfa forecasts show that about 67% of the first-guess fields need a vortex relocation to correct typhoon position errors while eliminates the topographical effect. As the vortex relocation effectively removes the typhoon position errors in the analysis, the simulated typhoon tracks are considerably improved for all forecast times, especially in the early periods as large adjustments appeared without the vortex relocation. Comparison of the horizontal and vertical vortex structures shows that large errors in the first-guess fields due to an incorrect typhoon position are eliminated by the vortex relocation scheme and that the analyzed typhoon circulation is stronger and more symmetric without distortions, and better agrees with observations. The result suggests that the main difficulty of objective analysis methods e.g., three-dimensional variational data assimilation (3DVAR), in TC analysis comes from poor first-guess fields with incorrect TC positions rather than not enough model resolution or observations. In addition, by computing the eccentricity and correlation of the axes of the initial typhoon circulation, the distorted typhoon circulation caused by the position error without the vortex relocation scheme is demonstrated to be responsible for larger track errors. Therefore, by eliminating the typhoon position error in the first guess that avoids a distorted initial typhoon circulation, the vortex relocation scheme is able to improve the ARW-WRF typhoon initialization and forecasts particularly when using data assimilation update cycling.
Storm surge is an important issue in Taiwan because our country is located at the typhoon prone area and surrounded by the Pacific Ocean and the Taiwan Strait. People live at coastal areas need to ...pay attention to the damages of storm surge, especially the inundation during typhoon’s duration. Therefore, the accurate and highly efficient operational storm surge system is required to provide not only the information of water elevations but also potential inundation areas. The large computational domain should be adopted to cover the complete typhoon life-cycle, and the atmospheric model and tidal model are expected to be coupled inside. COMCOT-SS (COrnell Multi-grid Coupled Tsunami Model – Storm Surge) fulfils above-mentioned requirements and solves nonlinear shallow water equations with multi-scale wave propagations and inundation calculation. In this study, 2015 category-5 Typhoon Soulik is chosen for model validation, and the results are in a good agreement with observed data. Coastal inundation induced by Typhoon Soulik is also simulated to study the variation of storm tides and tides. By a series of strict validations and operational experiments, COMCOT storm surge operational system has been the official forecasting model at Central Weather Bureau, Taiwan since July, 2016, and has successfully predicted the storm surges induced by 2016 category-5 Typhoon Nepartak.
Taiwan is subjected to significant storm surges, waves, and coastal inundation during frequent tropical cyclones. Along the west coast, with gentler bathymetric slopes, storm surges often cause ...significant coastal inundation. Along the east coast with steep bathymetric slopes, waves can contribute significantly to the storm surge in the form of wave setup. To examine the importance of waves in storm surges and quantify the significance of coastal inundation, this paper presents numerical simulations of storm surge and coastal inundation during two major typhoons, Fanapi in 2010 and Soulik in 2013, which impacted the southwest and northeast coasts of Taiwan, respectively. The simulations were conducted with an integrated surge-wave modeling system using a large coastal model domain wrapped around the island of Taiwan, with a grid resolution of 50 - 300 m. During Fanapi, the simulated storm surge and coastal inundation near Kaohsiung are not as accurate as those obtained using a smaller coastal domain with finer resolution (40 - 150 m). During Soulik, the model simulations show that wave setup contributed significantly (up to 20%) to the peak storm surge along the northeast coast of Taiwan. Three-D model simulations yield more accurate water level results and significant vertical variations in horizontal velocity at stations where wave-surge interaction is more pronounced. The simulated storm surge generally agreed well with the observed data at nearly 40 stations.
The coastal regions in Eastern Asia are under the potential threat of storm surges, especially the surge inundation caused by typhoons. In this study, the COMCOT (COrnell Multi-grid Coupled Tsunami ...Model) storm surge model has been developed for the approach of operational process and successfully coupled with dynamic atmospheric WRF model and global tidal TPXO 7.1 model. The large-scale and near-shore storm surge propagation could be simulated simultaneously with nested-grid scheme, and high-resolution inundated areas could also be calculated from the wet-dry-cell method. The destructive storm surge event induced by 2013 Typhoon Haiyan is chosen as the case study in this research, and the non-symmetric WRF fields and TPXO tidal effect are both considered in this case. From simulated results, the storm surges are concentric before Typhoon Haiyan landing off the Philippines because the pressure gradient was significant at that time, and then storm surges are concentrated at near-shore regions when wind shear stress turn to be the main force after landing off. The relationship between maximum storm surges and typhoon crack is also found in this study. More details would be presented at 2016 IUTAM Symposium.
This study investigates the characteristics of large-scale circulation and heating during the first transition of the Asian summer monsoon by a compositing technique. The first transition is ...characterized by a sudden change in large-scale atmospheric circulation and convective activity in South and Southeast Asia. The most notable features include 1) the development of the low-level cyclonic circulation and the upper-level anticyclone in South Asia, 2) the strong convection in the Bay of Bengal, the Indochina peninsula, and the South China Sea, and 3) the warming and the subsequent cooling of the SST in the Bay of Bengal.
Results show the close relationship between the fluctuations of atmospheric circulation, heating, and surface condition. It is suggested that the atmospheric circulation abruptly changes during the transition owing to the interaction between convection, large-scale circulation, and lower-boundary forcing that includes topographically lifting ocean and land surface heating.
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