Journal of Geophysical Research-Oceans最新文献

Antarctic Bottom Water (AABW) is the densest water mass in the world and drives the lower limb of the global thermohaline circulation. AABW is formed in only four regions around Antarctica and Cape Darnley, East Antarctica, is the most recently discovered formation region. Here, we compile 40 years of oceanographic data for this region to provide the climatological oceanographic conditions, and review the water mass properties and their role in AABW formation. We split the region into three sectors (East, Central and West) and identify the main water masses, current regimes and their influence on the formation of Cape Darnley Bottom Water (CDBW). In the eastern sector, Prydz Bay, the formation of Ice Shelf Water preconditions the water (cold and fresh) that flows into the central sector to $��\sim �68.5�°�$E, enhancing sea ice production in Cape Darnley Polynya. This produces a high salinity variant of Dense Shelf Water (DSW) (up to 35.15 g/kg) that we coin Burton Basin DSW. In contrast, the western sector of the Cape Darnley Polynya produces a low salinity variant (up to 34.85 g/kg) we coin Nielsen Basin DSW. The resultant combined CDBW is the warmest (upper temperature bound of 0.05$��°�$C) AABW formed around Antarctica with an upper bound salinity of $��\sim �$34.845 g/kg. Our findings will contribute to planning future observing systems at Cape Darnley, determining the role that CDBW plays in our global oceanic and climate systems, and modeling past and future climate scenarios.

As a key dynamic element of the Bjerknes feedback mechanism, the thermocline effect (TE) is critically important to El Niño modeling. In this study, the potential influence of TE-related parametric uncertainties on El Niño is investigated using the conditional nonlinear optimal parametric perturbation (CNOP) method based on an intermediate coupled model (ICM). The optimal perturbation of the TE-related parameter (OTEP), which substantially affects El Niño simulations, is estimated through the CNOP approach. Results reveal that the El Niño simulation is highly sensitive to the TE uncertainty in the eastern equatorial Pacific, with OTEP-induced simulation errors demonstrating an El Niño-like growth trend. On one hand, as indicated by the simulated El Niño intensity, the uncertainty in the TE in the eastern region can easily affect the strength of the Bjerknes feedback-related thermocline effect and atmospheric circulation. On the other hand, the enhanced TE is highly favored to accelerate the growth of the SST error due to the air–sea interaction, thus severely affecting the El Niño simulations. Therefore, adequately representing the TE in the equatorial eastern Pacific is emphasized for effectively improving El Niño simulations.

Three dominant characteristics and underlying dynamics of the seasonal cycle in Baffin Bay are discussed. The study is based on a regional, high-resolution coupled sea ice-ocean numerical model that complements our understanding drawn from observations. Subject to forcing from the atmosphere, sea ice, Greenland, and other ocean basins, the ocean circulation exhibits complex seasonal variations that influence Arctic freshwater storage and export. The basin-scale barotropic circulation is generally stronger (weaker) in summer (winter). The interior recirculation (∼2 Sv) is primarily driven by oscillating along-topography surface stress. The volume transport along the Baffin Island coast is also influenced by Arctic inflows (∼0.6 Sv) via Smith Sound and Lancaster Sound with maximum (minimum) in June-August (October-December). In addition to the barotropic variation, the Baffin Island Current also has changing vertical structure with the upper-ocean baroclinicity weakened in winter-spring. It is due to a cross-shelf circulation associated with spatially variable ice-ocean stresses that flattens isopycnals. Greenland runoff and sea ice processes dominate buoyancy forcing to Baffin Bay. Opposite to the runoff that freshens the west Greenland shelf, stronger salinification by ice formation compared to freshening by ice melt enables a net densification in the interior of Baffin Bay. Net sea ice formation in the past 30 years contributes to ∼25% of sea ice export via Davis Strait. The seasonal variability in baroclinicity and water mass transformation changes in recent decades based on the simulation.

A single-column coupled physical and biological model based on the North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO) with nitrogen and silicon cycles is adapted for the Bay of Bengal (BoB) environment. The model simulated plankton biomass and nutrients along the track of Bio-Argos over East and West BoB (from 2016 to 2017) are validated with the observations. The model reasonably simulates the perennial structure of subsurface chlorophyll maximum (SCM). Further, three experiments are carried out to know the limitations in primary and secondary production in terms of nitrogen (NO₃ + NH₄) and silicate (Si(OH)₄) in the open ocean BoB. In a “no-NO₃”experiment, the nitrate limiting term [that is, NO₃/(NO₃ + KNO₃)] is set to zero so that the difference from the control case gives the role of “regenerated production” in the total primary and secondary production. Similarly, a no-NH₄ experiment was conducted to infer the role of “new production.” The new (regenerated) production fuels 85 ± 1% (28 ± 6%) of the living biomass in the East part of open ocean BoB (East BoB). The corresponding values for the west part (West BoB) are 86 ± 1% (42 ± 2%). Among the primary producers, the new (regenerated) production contributed 72 ± 1% (24 ± 6%) in the East BoB and 74 ± 1% (37 ± 2%) in the West BoB. The silicate limits the diatom production by 46% ± 22% (45% ± 27%) of the actual amount of diatom in the East BoB (West BoB) diatom. Surface to 60 m depth, we observed severe nitrate limitation over East BoB and equal limitation of nitrate and silicate over West BoB from June to September; the remaining months suffered from moderate nitrate limitation in both regions. This study shows that nitrate appears to be a limiting nutrient compared to silicate at the surface of both East and West BoB. Since silicicline is deeper than nitracline, silicate is the potential limiting nutrient in BoB from 65 to 105 m depth in all seasons.

Mooring observations have revealed that the daily reappeared time (DRt) of internal solitary waves (ISWs) in the northern South China Sea (SCS) manifests systematic shifts. This study aims to reveal the dynamic mechanisms behind this phenomenon using the theory of modulation of shear flow to the internal tide wave (IT) propagation. Approximate solutions for the frequency modulation function (FMF) and the amplitude modulation function (AMF) of ITs are derived and validated by the mooring array measurements on the northern SCS continental shelf in 2019 and 2020. Namely, the mechanisms of the ISW reappeared period shift (RPS derived from DRt) and amplitude variation are attributed to the modulation of low-frequency flow with a period of about 7 days to ITs. The FMF is induced by the vorticity of the low-frequency flow (FS1) and the Doppler shift (FS2). Thus, the sign of the FMF value may be positive (blue shift) and negative (red shift), depending on the algebraic summation of the two terms. The FS2 derived from mooring observed velocities confirms the modulation effects of mean current on the frequency shift and the amplitude variation of ITs and ISWs.

The Solomon Sea is a major contributor to (a) the volume transport into the Equatorial Undercurrent and (b) the associated heat transport, which has an order one effect on interannual temperature variability on the equator according to previous work. The narrow western boundary current (New Guinea Coastal Undercurrent, NGCU) merges with the broad, shallow North Vanuatu Jet within 100 km of the southern entry to the sea, which implies mixing. Existing estimates from observations suggest mixing is larger than in models with different mixing parameterizations, which produce disparate properties of these exiting waters. Here, we use sustained underwater glider measurements across the Solomon Sea from 2007 to 2020 to examine the spatial variability of mixing estimates and internal waves. We estimate diffusivity via a finescale parameterization using the vertical strain of isopycnal displacements from internal waves. A typical accuracy of this parameterization when compared to turbulence measurements is within a factor of 2–3. Our results and previous observations in this area agree within this factor. Our main results are: (a) vertical diffusivity estimates are about $��{10}^{�-�4�}�$ $��m^2�$ $��s^{�-�1�}�$ on the inshore, anticyclonic side of the NGCU, which are 10–100 times higher than offshore and (b) elevated near-inertial internal wave (NIW) amplitudes are also found inshore. Taken together, these results suggest trapping of NIW by the anticyclonic vorticity of the NGCU leads to the elevated mixing within 100 km of the entry to the Solomon Sea.