Variability of the sea surface temperature (SST) in the equatorial Pacific plays an important role in the global climate. The El Niño-Southern Oscillation (ENSO) is a typical abnormal climate phenomenon. The ocean experiences both El Niño and La Niña climate patterns; however, the mechanism of El Niño is still not fully understood (L'Heureux et al., 2017;Li et al., 2019). Growing and breaking ocean waves play an important role in air-sea momentum and heat transfer and affect the mixing of the upper ocean (Janssen et al., 2013). Multiple large-scale effects of waves have been studied, including the driving effect of waves on ocean currents (

Wind-generated ocean surface waves give rise to a near-surface velocity known as Stokes drift (SD) (Stokes, 1847), which can contribute substantially to near-surface mass transport. SD is widely applied in representation of nearshore circulation in coastal zones and in modeling of tracer transport as the difference between Lagrangian and Eulerian averages, and it is responsible for the Coriolis-Stokes force in the upper-ocean layer in Eulerian models (van den Bremer & Breivik, 2018). It has been shown that wave-induced water transport due to SD can affect a wide range of ocean states in dramatic ways (Hasselmann, 1971). The magnitude of SD can reach the same as that of wind transport in high-and mid-latitude oceans (McWilliams & Restrepo, 1999;Shi et al., 2016). Tamura et al. (2012) proposed that SD plays an important role in the momentum balance of the upper ocean through introduction into the average Eulerian flow. Moreover, SD also influences the mixed layer of the ocean by causing Langmuir turbulence generation (McWilliams et al., 2014).

Surface waves perform a crucial role in modulating tropical cyclone (TC) systems and have proved to be key for numerical TC predictions. In this study, we investigate the effects of wave-induced mixing and wave-affected surface exchange coefficients using a coupled ocean–atmosphere–wave model for two real TC cases: Shanshan (2018) and Megi (2010). The results demonstrate that wave-affected surface exchange coefficients enhance air–sea heat fluxes and have a significantly positive effect on simulated TC intensity, size, and strengthening process. In contrast, the wave-induced mixing has a negative impact on TC intensity and size and is not conducive to TC intensification and maintenance. The net effect of these two factors is a balance between the wave-affected positive contribution and the negative contribution from wave-induced sea surface temperature cooling. We find that the effect of wave-induced mixing on a TC system depends on the local thermal structure of the ocean. When the thermocline is weak and there is warm water at the wave-induced mixing penetration depth, the negative effect of the wave-induced mixing is weak. However, when there is cold subsurface water and a strong thermocline, wave-induced mixing has a significant impact, which exceeds the wave-driven positive feedback.

In the Northwest Pacific (NWP), where a unique monsoon climate exists and where both typhoons and extratropical storms occur frequently, hazardous waves pose a significant risk to maritime safety. To analyze the 20-year variability of hazardous waves in this region, this study utilized hourly reanalysis data from the ECMWF ERA5 dataset covering the period from 2001–2020, alongside the wave risk assessment method. The ERA5 data exhibits better consistency, in both the temporal and spatial dimensions, than satellite data. Although hazardous wind seas occur more frequently than hazardous swells, swells make hazardous waves travel further. Notably, the extreme wave height (EWH) shows an increasing trend in high- and low-latitude areas of the NWP. The change in meridional wind speeds is the primary reason for the change in the total wind speed in the NWP. Notably, the maximum annual increase rate of 0.013 m/year for EWH exists in the region of the Japanese Archipelago. This study elucidated the distributions of wave height intensity and wave risk levels, noting that the EWHs of the 50-year and 100-year return periods can reach 20.92 m and 23.07 m, respectively.

Surface waves play an essential role in regulating the mixing processes in the upper ocean boundary, and then directly affect the air–sea exchange of mass and energy, which is important for the intensity prediction of tropical cyclones (TCs). The relative and integrated impacts of the wave breaking (WB) and the wave orbital motion (WOM) on the mixing and ocean response to TC forcing are investigated under typhoon Megi (2010), using the modeled data from a fully coupled air–sea–wave model. It is shown that the WOM can effectively increase the turbulence mixing in the upper ocean, thus significantly deepening the mixing layer depth and cooling the sea surface temperature. The WB can modulate the mixing layer depth and sea surface temperature to some extent in the cold tail zone with a shallow mixing layer (owing to typhoon forcing), whereas the WOM plays a predominant role. On the aspect of ocean currents driven by typhoon winds, the WOM-induced mixing significantly weakens the current velocity and shear strength in the upper ocean mixing layer, while the relative contribution for turbulence production between the WOM and the current shear differs at different vertical regions. Moreover, the effect of the WOM on the upper ocean turbulent mixing are dependent on the location with respect to the typhoon center, the local vertical thermal structure, and surface wave states.