The diffuse attenuation coefficient (K_d) is critical to understand the vertical distribution of underwater downwelling irradiance (E_d). Theoretically E_d is composed of the direct solar beam and the diffuse sky irradiance. Applying the statistical results from Hydrolight radiative transfer simulations, K_d is expressed into a mathematical equation (named as PZ06) integrated from the contribution of direct solar beam and diffuse sky irradiance with the knowledge of sky and water conditions. The percent root mean square errors (RMSE) for the vertical distribution of E_d(z) under various sky and water conditions between PZ06 and Hydrolight results are typically less than 4%. Field observations from the southern Middle Atlantic Bight (SMAB) and global in situ data set (NOMAD) also confirmed the validity of PZ06 in reproducing K_d. PZ06 provides an alternative and improvement to the simpler models (e.g., Gordon, 1989; and Kirk, 1991) and an operational ocean color algorithm, while the latter two kinds of models are valid to limited sky and water conditions. PZ06 can be applied to study K_d from satellite remotely sensed images and seems to improve K_d derivation over current operational ocean color algorithm.

Diffuse attenuation coefficient of photosynthetically available radiation (PAR), K_PAR, is a key product of ocean color remote sensing. Current ocean color algorithms generally detect only the average K_PAR within one optical depth, $K_{PAR}^{RS}$. Due to the marked vertical variations of K_PAR, knowledge of $K_{PAR}^{RS}$ is insufficient to accurately evaluate the submarine light field. By using field in situ observations, a two-step approach, based on the development of an ocean color algorithm for $K_{PAR}^{RS}$ and the relationships between $K_{PAR}^{RS}$ and the average K_PAR from the surface down to depth Z (${\overline{K}}_{PAR}^Z$), was developed to remotely estimate the vertical variations in ${\overline{K}}_{PAR}^Z$ in the North Pacific from the MODerate-resolution Imaging Spectrometer (MODIS) imagery. The root mean square difference of log(${\overline{K}}_{PAR}^Z$) in depths within the euphotic zone was around ±0.059 (in unit of m⁻¹ for ${\overline{K}}_{PAR}^Z$), which corresponded to a deviation of ±15% for the estimated ${\overline{K}}_{PAR}^Z$ and the penetration depths of PAR. Our study may provide a promising approach to detect the vertical variations of ${\overline{K}}_{PAR}^Z$ and underwater PAR distributions in the North Pacific Ocean.

The effects of the diel (involving a 24 hour period) variations in the surface concentrations of chlorophyll a (C) on the use of once-daily remotely sensed C as the diel average were assessed from the diel records in the derived depth-weighted C (C_d) that should be detected by remote sensing and the in situ surface C at two time-series stations in the North Pacific: the SEATS (SouthEast Asian Time-series Study) station in the northern South China Sea and the ALOHA (A Long-Term Oligotrophic Habitat Assessment) station in the North Pacific subtropical gyre. In situ surface C varied by a factor of about 2.0 and 1.3 over a diel cycle, and by ±20% and ±9% over the diel average at the SEATS and ALOHA stations, respectively. As the overpass-times of the different satellites were not identical, C_d was satellite-dependent. While the C_d corresponding to MODerate resolution Imaging Spectroradiometer on Aqua (MODIS-Aqua) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) overpass-times agreed to ±10%, the C_d corresponding to MEdium Resolution Imaging Spectrometer (MERIS) overpass-time could differ from the other two by −22% to +28% at the SEATS station and −1% to +12% at the ALOHA station. In addition, C_d corresponding to the overpass-times of the three satellites deviated from the observed diel average in situ surface C by −19% to +32% at the SEATS station and by −6% to +13% at the ALOHA station. These results indicate that, as a result of diel variations, neither a one-time remotely-sensed nor a one-time observed in situ surface C can represent the diel average in situ surface C accurately. Furthermore, diel variations are an inherent source of uncertainty when data from multiple satellites are pooled for use. The magnitudes of these discrepancies can be comparable to the commonly claimed uncertainties in remotely sensed C and thus should be taken into consideration in its interpretation and use.

The evaporation duct, formed above the ocean surface by sharp vertical gradients of humidity, would significantly influence electromagnetic wave propagation. It is a quasi-permanent feature over the sea, and its strength is quantified by the evaporation duct height (EDH). While previous studies have focused on how local factors influence evaporation ducts, the impact of El Niño–Southern Oscillation (ENSO) on EDH in the South China Sea (SCS) remains undocumented. Using correlation analysis, empirical orthogonal function (EOF) decomposition, and wavelet transform, this study shows that evaporation is the dominant environmental factor controlling EDH variability across seasonal and inter-annual timescales in the SCS, while wind speed and relative humidity play secondary roles with contrasting effects between the northern and southern regions. ENSO drives the inter-annual variability of EDH by modulating evaporation. During El Niño events, anomalous anticyclonic circulations near the Philippine Sea, which weaken (strengthen) the evaporation in the northern (southern) SCS, alter EDH and contribute to the formation of the meridional dipole structure, particularly within the 2-to-6-year ENSO band. These results provide new insights into the mechanisms controlling EDH in the SCS and highlight the critical role of ENSO in shaping its spatial distribution.

Field observations in the northern South China Sea (NSCS) indicate that elevated concentrations of chlorophyll a (Chl_a) associated with non-linear internal waves (NIWs) were more pronounced in the subsurface than at the sea-surface. At a given surface Chl_a, the vertically integrated Chl_a in an NIW-active area was 45% higher than that in an area where the NIW activities were minimal. As a result, net primary production (PP_eu) estimated from surface Chl_a from remote sensing by using the current Chl_a-based PP_eu model (e.g. VGPM) would have underestimated the true value by the same amount. If this correction is applied, the activities of NIWs would have elevated PP_eu in the NSCS by 89%, rather than by 15–37% as previously reported. The elevated PP_eu in the areas with high NIW activities would be at least 3.57 Tg-C yr^− 1. It may account for 2.0% of new production in the South China Sea.

Surface concentrations of particulate organic carbon (POC) and phytoplankton carbon (C_phy) in the northern South China Sea (NSCS) have been assessed successfully by remote sensing with appropriate regional tuning. Match-up comparisons between satellite derivations and field observations showed uncertainties of ± 28.5% for POC and ± 37.0% for C_phy. The satellite-derived climatological distributions are consistent with previous reports. In the open NSCS where phytoplankton growth is controlled primarily by the convective overturn, both POC and C_phy are higher in winter, associating with the effect of stronger northeast monsoons and surface cooling, than in summer, when surface heating and weaker southwest monsoons strengthen vertical stratification. Toward the coasts, because of terrestrial inputs, POC and C_phy increase, varying by one order of magnitude. Particularly, they are prominently enhanced along the Pearl River plume, which may be extensive to the middle shelf in the summer according to high river discharge and northeastward coastal currents. Elevated POC and C_phy are also found in the waters around the Taiwan Bank where upwelling occurs year round. Similar approaches on the remotely sensed POC and C_phy may be applicable to other shelf-seas in order to improve our understanding in ocean carbon cycle.