Abstract The present study reveals climate features of low‐level jets (LLJs) over the Bohai Sea and Yellow Sea (BYS) based on a 35‐year (1979–2013) high‐resolution (7 km) atmospheric hindcast. The regional climate model COSMO‐CLM driven by the ERA‐Interim reanalysis data set was used to obtain the hindcast. Through comparison with observations, the hindcast was proved to robustly reproduce the climatology, the diurnal cycle, the variability of wind profiles, and specific LLJ cases. LLJs over the BYS feature a strong diurnal cycle, intra‐annual, and interannual variability but weak decadal variability. LLJs are more frequent in April, May, and June (LLJ season) and less frequent in winter over the Bohai Sea and western coastal areas of the Yellow Sea, which is due to the intra‐annual variations of large‐scale circulation and local land‐sea thermal contrast. In the LLJ season, the heights of jet cores are generally lower than 500 m above sea level. The maximum wind speed of LLJs is mostly in the range of 10–16 m/s, and prevailing wind directions are southerly and southwesterly. The LLJs are of the nocturnal type, with the highest occurrence frequency at approximately 2300 local time. Furthermore, a low‐frequency link between anomalies of LLJ occurrence and regional large‐scale barotropic circulation was identified using canonical correlation analysis and associated correlation patterns. Pressure systems over the East Asia‐northwest Pacific region are significantly correlated with the variations of LLJ occurrence over the BYS in terms of the intra‐annual and interannual variability.
Abstract Tropical cyclones (TCs) induce heat pump and cold suction in the upper ocean layer. However, limited research investigated whether this heat can be effectively transported into the deep ocean. The present study shows that seawater at Station S2 is anomalously warmer in the deep ocean layer than at S1 and S3 during Typhoon Kalmaegi (2014) in the South China Sea. The turbulence‐induced vertical heat flux is estimated based on fine‐scale mixing parameterization but it cannot explain the warming rate below the thermocline. Therefore, a new method is proposed to estimate the vertical velocity, allowing us to calculate the heat flux more accurately. To elucidate the underlying causes of the observed differences, we analyze horizontal velocity to examine the role of mesoscale eddies in modulating near‐inertial waves (NIWs)‐induced vertical heat flux. Station S1 is located inside a cyclonic eddy while S2 and S3 are within two distinct anticyclonic eddies. According to the “Chimney Effect” theory, cyclonic (anticyclonic) eddies tend to limit (enhance) the vertical propagation of the NIWs. While this theory explains the confined heat flux at S1, it fails to explain the differences observed at S2 and S3. Further examination of the vertical structures and intensities of the two anticyclonic eddies reveals that the eddy at S2 extends much deeper and is stronger than that at S3, allowing the NIWs to propagate and transport heat deeper at S2 than at S3. The study demonstrates the role of TC‐induced NIWs in deep ocean heat transport under the influence of mesoscale eddies.
Abstract Long‐range radiation and interference of M 2 internal tides from multiple sources in the Philippine Sea are examined by driving a high‐resolution numerical model. The M 2 internal tides are effectively generated around the boundary area, which includes the Luzon Strait, Ryukyu Island chain, Bonin Ridge, Mariana Arc, and Izu Ridge, favoring the occurrence of complex interference patterns. The local sources (mainly Daito Islands and Palau Ridge) inside the basin contribute to a small portion (~5%) of total energy but enhance the geographical inhomogeneity of the baroclinic field. The mode‐1 and mode‐2 M 2 tidal beams from boundary sources radiate a long distance into the basin but exhibit different interference‐modulated geography variations. A 2‐D line source model characterizing interference can reproduce the general baroclinic field. Two notable interference cases are investigated: (1) the superposition of internal tides from Luzon Strait and Miyako Strait bifurcates into several southeastward beams, consistent with previous numerical simulations and altimeter measurements, and (2) the interference between Tokara Strait and Bonin Ridge exhibits a multiscale spatial pattern, which is modulated by the local generated energy and bathymetry features. Energetic dissipation occurs both near the boundary sources and in the basin. A locally dissipated fraction q of ~0.4 is estimated at the Luzon Strait and Bonin Ridge with continuous bathymetry features, while q of ~0.6 is estimated at the Ryukyu Island chain and Mariana Arc with discrete topographic variability. A lower locally dissipated fraction indicates a stronger energy flux radiating into the basin, where enhanced dissipation coincides closely with the interference‐modulated flux field.
The generation processes and potential energy sources of internal solitary waves (ISWs) in the southern Taiwan Strait are investigated by driving a high resolution non-hydrostatic numerical model with realistic background conditions. Two main types of ISWs are clarified according to their different energy sources. One is generated by the nonlinear disintegration of remote internal tides emanating from Luzon Strait, and the other type is generated by local tide-topography interaction at the continental slope. The basic properties and evolution processes differ between these two kinds of ISWs. The waves originated from the remote internal tides at Luzon Strait have amplitudes comparable to previous field observations. In contrast, the ISWs generated locally are much weaker than observed waves, even in the presence of a steady offshore background current, which intensifies the generation of onshore ISWs. The ISWs induced by remotely generated M2 internal tides are stronger than those induced by K1 internal tides, and the fraction of internal wave energy transmitted onto the shelf is not significantly influenced by the intensity of remotely generated internal tides.
Abstract Interannual variations in eddy kinetic energy (EKE) in the Sulawesi Sea and their driving mechanisms are investigated based on the outputs of Ocean Forecasting Australia Model version 3 from 1979 to 2014. The interannual EKE variability is found to be primarily modulated by the Mindanao Current intrusion transport (MCIT). By regulating the intensity of barotropic instability of the cyclonic loop current in the Sulawesi Sea, the MCIT fluctuation leads to the downstream interannual EKE variations. Further analysis suggests that the paths of Mindanao Current (MC) and New Guinea Coastal Current and Undercurrent (NGCC/NGCUC) influence the interannual MCIT variability. During high‐EKE periods, the NGCC/NGCUC is weakened, and the MC retroflection extends to south of 5°N, which causes MCIT to increase by 0.60 Sv and strengthens the barotropic energy conversion from mean kinetic energy to EKE in the Sulawesi Sea. During low‐EKE periods, the NGCC/NGCUC is intensified whereas the MC retroflection retreats to a northernmost path, resulting in a decrease of 0.58 Sv in MCIT and thus a low EKE level. In addition, mesoscale eddies to the east of the Sulawesi Sea in the western Pacific also have an impact on the MC intrusion. This study highlights the significance of the nonlinear dynamics of western boundary currents in modulating eddy activities in the formation region of the Indonesian Throughflow.
Based on the observation data of a thermistor chain, highly nonlinear internal solitary waves (ISWs) on the continental shelf of the northwestern South China Sea (ISWs) have been observed. The characteristics of the highly nonlinear ISWs with amplitude as large as 45 m are investigated and comparisons are also made between the observation results and existing internal wave theories. Discussion is presented describing how the high-order extended Korteweg-de Vries (EKdV) theory may show better agreement with the observation than the KdV equation. Based on the EKdV theory, Internal wave forces on a cylindrical Pile on the northwestern shelf of SCS are also estimated with the Morison Formula. These results show that ISWs in our study area can cause serious threat to ocean engineering structures and should not be ignored in the design of oil platforms.
Abstract. Based on in-situ time series data from an array of temperature sensors and an acoustic Doppler current profiler on the continental shelf of the northwestern South China Sea, a sequence of internal solitary waves (ISWs) were observed during the passage of tropical storm Washi in the summer of 2005, which provided a unique opportunity to investigate the ISW response to the tropical cyclone. The passing tropical storm is found to play an important role in affecting the stratification structure of the water column, and consequently leading to significant variability in the propagating features of the ISWs, such as the polarity reversal and amplitude variations of the waves. The response of the ISWs to Washi can be divided into two stages, direct forcing by the strong wind (during the arrival of Washi) and remote forcing via the near-inertial internal waves induced by the tropical storm (after the passage of Washi). The field observations as well as a theoretical analysis suggest that the variations of the ISWs closely coincide with the changing stratification structure and shear currents in accompanied by the typhoon wind and near-inertial waves. This study presents the first observations and analysis of the ISW response to the tropical cyclone in the South China Sea.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)