Surface Chlorophyll-A Fronts in the Yellow and Bohai Seas Based on Satellite Data

Chlorophyll fronts are important to monitor and map the oceanic front, especially in the season when sea surface temperature (SST) fronts weaken. In this study, surface chlorophyll-a (chl-a) fronts in the Yellow and Bohai seas were characterized for the first time using satellite data. Five distinct chl-a fronts (i.e., the Bohai Strait, Shandong Peninsula, Jiangsu, Liaodong Peninsula, and Korean Peninsula fronts) were observed in summer along the 40 m isobaths and faded in other seasons. Notably, these fronts coincided with SST fronts. Strong chl-a fronts emerged during summer due to chl-a blooms in eutrophic coastal waters paired with surface chl-a fading in strongly stratified offshore waters and coastal physical fronts. Although SST fronts were strong during winter, light limitation and strong vertical mixing in offshore waters led to low chl-a in both coastal and offshore waters, suppressing chl-a front formation. Both chl-a and SST fronts coincided with steep seabed slopes (slope ratio > 1), suggesting that seabed slope may be an indicator of oceanic front location.

Characteristics of Three-Dimensional Sound Propagation in Western North Pacific Fronts

Oceanic fronts involved by ocean currents led to strong gradients of temperature, density and salinity, which have significant effects on underwater sound propagation. This paper focuses on the impact of the oceanic front on three-dimensional underwater sound propagation. A joint experiment of ocean acoustic and physical oceanography at the western North Pacific fronts is introduced. The measurement data for sound waves passed through the oceanic front is processed. The results are analysed and compared with the numerical simulation. It was found that transmission loss presented some difference when the source was located in the front centre and sound waves propagated towards water mass on opposite sides of the front centre. And when the sound field is excited by the underwater explosion at a depth of 200 m, the effects of the horizontal refraction cannot be neglected. On the other hand, the transmission loss for sound pressure fell sharply and rose rapidly at the side of cold water masses.

A Nearshore Oceanic Front Induced By Wave Streaming

Coastal fronts impact cross-shelf exchange of materials, such as plankton and nutrients, which are important to the ecosystems in continental shelves. Here using numerical simulation we demonstrate a nearshore front induced by wave streaming. Wave streaming is a bottom Eulerian current along the surface wave direction, and it is caused by the wave bottom dissipation. Wave streaming drives a Lagrangian overturning circulation in the inner shelf and pumps up deep and cold water into the overturning circulation. The water inside the overturning circulation is quickly mixed and cooled because of the wave streaming-enhanced viscosity. However, the offshore water outside the overturning circulation remains stratified and warmer. Hence, a front develops between the water inside and outside the overturning circulation. The front is unstable and generates submesoscale shelf eddies, which lead the offshore transport across the front. This study presents a new mechanism for coastal frontogenesis.

What Kinds of Atmospheric Anomalies Drive Wintertime North Pacific Basin-Scale Subtropical Oceanic Front Intensity Variation?

ABSTRACTThe basin-scale subtropical oceanic front zone (STFZ) is a key region for midlatitude air–sea interaction in the North Pacific. However, previous studies considered midlatitude sea surface temperature (SST) variabilities as a response to atmospheric stochastic forcing. With reanalysis and observational data, this study investigates what kinds of atmospheric anomalies drive the wintertime North Pacific STFZ intensity variation. Lead correlations show that prior to the STFZ’s enhancement, there exist persistent atmospheric anomalies characterized by a negative-phase Arctic Oscillation (AO) and a positive-phase Pacific–North American (PNA) pattern, lasting for up to 80 and 50 days and peaking at 20- and 8-day leads, respectively. It is further found that the long-lasting negative-phase AO is conducive to stronger low-tropospheric baroclinicity at around 40°N over North Pacific where there is a climatological baroclinic region. The stronger baroclinicity leads to more synoptic transient eddy activities, promoting an equivalent barotropic low geopotential height anomaly north of STFZ via transient eddy vorticity forcing. The geopotential height anomaly propagates downstream, triggering a PNA-like pattern. With such an AO-promoted atmospheric internal wave–flow feedback, the regional PNA pattern is intensified and embedded in the annular AO mode, accompanied with an intensified Aleutian low and surface westerly wind that peak at an 8-day lead, preconditioning a persistent (nonstochastic) atmospheric forcing on the STFZ. The intensified surface westerly predominantly tends to drive a southward Ekman transport and increase upward surface turbulent heat fluxes into the atmosphere through increasing surface wind speed and sea–air temperature difference, amplifying the underlying negative SST anomaly and cross-frontal meridional SST gradient, ultimately intensifying the STFZ.