Recent evidence shows that wind‐driven ocean currents, like the western boundary currents, are strongly affected by global warming. However, due to insufficient observations both on temporal and spatial scales, the impact of climate change on large‐scale ocean gyres is still not clear. Here, based on satellite observations of sea surface height and sea surface temperature, we find a consistent poleward shift of the major ocean gyres. Due to strong natural variability, most of the observed ocean gyre shifts are not statistically significant, implying that natural variations may contribute to the observed trends. However, climate model simulations forced with increasing greenhouse gases suggest that the observed shift is most likely to be a response of global warming. The displacement of ocean gyres, which is coupled with the poleward shift of extratropical atmospheric circulation, has broad impacts on ocean heat transport, regional sea level rise, and coastal ocean circulation.

In austral winter, biological productivity at the Angolan shelf reaches its maximum. The alongshore winds, however, reach their seasonal minimum suggesting that processes other than local wind‐driven upwelling contribute to near‐coastal cooling and upward nutrient supply, one possibility being mixing induced by internal tides (ITs). Here, we apply a three‐dimensional ocean model to simulate the generation, propagation, and dissipation of ITs at the Angolan continental slope and shelf. Model results are validated against moored acoustic Doppler current profiler and other observations. Simulated ITs are mainly generated in regions with a critical/supercritical slope typically between the 200‐ and 500‐m isobaths. Mixing induced by ITs is found to be strongest close to the coast and gradually decreases offshore thereby contributing to the establishment of cross‐shore temperature gradients. The available seasonal coverage of hydrographic data is used to design simulations to investigate the influence of seasonally varying stratification characterized by low stratification in austral winter and high stratification in austral summer. The results show that IT characteristics, such as their wavelengths, sea surface convergence patterns, and baroclinic structure, have substantial seasonal variations and additionally strong spatial inhomogeneities. However, seasonal variations in the spatially averaged generation, onshore flux, and dissipation of IT energy are weak. By evaluating the change of potential energy, it is shown, nevertheless, that mixing due to ITs is more effective during austral winter. We argue that this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing.

A global ocean circulation and tide model with nonuniform resolution is used in this work to resolve the ocean circulation globally as well as mesoscale eddies and internal tides regionally. Focusing on the northwest Pacific Ocean (NWP, 0°–35°N, 105°–150°E), a realistic experiment is conducted to simulate internal tides considering the background circulation and stratification. To investigate the influence of a background field on the generation and propagation of internal tides, idealized cases with horizontally homogeneous stratification and zero surface fluxes are also implemented for comparison. By comparing the realistic cases with idealized ones, the astronomical tidal forcing is found to be the dominant factor influencing the internal tide conversion rate magnitude, whereas the stratification acts as a secondary factor. However, stratification deviations in different areas can lead to an error exceeding 30% in the local internal tide energy conversion rate, indicating the necessity of a realistic stratification setting for simulating the entire NWP. The background shear is found to refract propagating diurnal internal tides by changing the effective Coriolis frequencies and phase speeds, while the Doppler-shifting effect is remarkable for introducing biases to semidiurnal results. In addition, nonlinear baroclinic tide energy equations considering the background circulation and stratification are derived and diagnosed in this work. The mean flow–baroclinic tide interaction and nonlinear energy flux are the most significant nonlinear terms in the derived equations, and nonlinearity is estimated to contribute approximately 5% of the total internal tide energy in the greater Luzon Strait area.

Internal solitary waves (ISWs) in the South China Sea (SCS) have received considerable attention. This paper reports on a strong ISW captured northeast of Dong-Sha Atoll on 22 May 2011 by shipboard Acoustic Doppler Current Profiler (ADCP), which had the largest velocity among the ISWs so far reported in the global ocean. The peak westward velocity (u) was 2.94 m/s, and the peak downward velocity (w) was 0.63 m/s, indicating a first baroclinic mode depression wave. The amplitude of ISW inferred from ADCP backscatter was about 97 m. 2.2 h later, a trailing wave was captured with a peak westward velocity and downward velocity of 2.24 m/s and 0.42 m/s, respectively, surprisingly large for a trailing wave, suggesting that the ISW is type-A wave. The estimated baroclinic current induced by the leading ISW was much larger than the barotropic current. The Korteweg-De Vries (KdV) theoretical phase speed and the phase speed inferred from the satellite images were 1.76 m/s and 1.59 m/s, respectively. The peak horizontal velocity exceeded the phase speed, suggesting the ISW was close to or already in the process of breaking and may have formed a trapped core.