Biogeochemical impacts of flooding discharge with high suspended sediment on coastal seas: a modeling study for a microtidal open bay.

Freshwater, suspended sediment matter (SSM), and nutrients discharged from rivers into the ocean have large impacts on biological production. In particular, during floods, coastal areas are greatly stirred up and large amounts of nutrients are supplied to the sea surface. We investigate the biogeochemical impact of flooding river discharges containing a large amount of SSM by conducting numerical simulations for a specific flooding event of the Yura River, Japan. Parameters are varied over wide ranges of SSM properties and nutrient content in riverine water. Two qualitatively different regimes of the riverine plume, hypopycnal and hyperpycnal, appear within realistic parameter ranges. Compared with the reference case without SSM, the surface salinity (nutrients) within the riverine plume becomes lower (higher) in hypopycnal cases and higher (lower) in hyperpycnal cases within a few days after the flooding discharge. These results suggest the necessity of properly taking into account the effect of SSM in assessing the influence of high river discharges on coastal biogeochemistry. It is the case not only for the specific river and event we are dealing with but also for other flooding events and other rivers and connecting coastal seas.

Tidal control of the flow through long, narrow straits: a modeling study for the Seto Inland Sea.

Even in coastal oceans where tidal currents are predominant, long-term mean currents are of great interest since they are responsible for the transport of materials over long timescales. Tides could significantly affect mean currents in long, narrow straits due to tide-topography interaction, but it is yet unclear how and to what extent tides control throughflows. Here, we focus on the throughflow in the Seto Inland Sea, Japan, which has enormous impacts on the marine environment while its long-term mean characteristics, even the flow direction, are not well described by observations. By using a state-of-the-art ocean model, we show that the simulated throughflow is eastward on annual average and its volume transport is considerably suppressed by tides. It is found that tides enhance mixing and induce time-mean eddies, and both work to reduce the throughflow. A westward throughflow was previously estimated based on an acoustic measurement. The discrepancy between this estimate and our result would be due to whether or not such eddies are taken into account. These findings imply that tides may also suppress the throughflow of the other straits around the world. Revealing such tidal effects may contribute to a better performance of oceanic and climate simulations.

Impact of deep ocean mixing on the climatic mean state in the Southern Ocean.

The Southern Ocean is of great importance for the global stratification and biological carbon storage because it is connected to the global ocean conveyor by which atmospheric information absorbed in the Southern Ocean is redistributed globally and buffered over centuries. Therefore, understanding what controls the Southern Ocean climate, the global ocean conveyor, and links between them is a key to quantifying uncertainties in future climate projections. Based on a set of climate model experiments, here we show that the tide-induced micro-scale mixing in the Pacific deep ocean has significant impacts on the wintertime Southern Ocean climate through basin-scale reorganization of ocean stratification and resultant response of the global ocean conveyor. Specifically, Pacific deep water, which is modified by the deep ocean mixing while travelling south, reinforces the subsurface stratification and suppresses deep convection in the Southern Ocean. Resultant increase of the Ross Sea sea-ice leads to decrease of incoming shortwave radiation and strengthening of the westerly and storms. Because the Southern Ocean could regulate the global warming progress through its role as heat and carbon sink, our study implies that better representation of deep ocean mixing in climate models contributes to reliability improvement in regional-to-global climate projections.

Widespread collapse of the Ross Ice Shelf during the late Holocene.

The stability of modern ice shelves is threatened by atmospheric and oceanic warming. The geologic record of formerly glaciated continental shelves provides a window into the past of how ice shelves responded to a warming climate. Fields of deep (-560 m), linear iceberg furrows on the outer, western Ross Sea continental shelf record an early post-Last Glacial Maximum episode of ice-shelf collapse that was followed by continuous retreat of the grounding line for ∼200 km. Runaway grounding line conditions culminated once the ice became pinned on shallow banks in the western Ross Sea. This early episode of ice-shelf collapse is not observed in the eastern Ross Sea, where more episodic grounding line retreat took place. More widespread (∼280,000 km(2)) retreat of the ancestral Ross Ice Shelf occurred during the late Holocene. This event is recorded in sediment cores by a shift from terrigenous glacimarine mud to diatomaceous open-marine sediment as well as an increase in radiogenic beryllium ((10)Be) concentrations. The timing of ice-shelf breakup is constrained by compound specific radiocarbon ages, the first application of this technique systematically applied to Antarctic marine sediments. Breakup initiated around 5 ka, with the ice shelf reaching its current configuration ∼1.5 ka. In the eastern Ross Sea, the ice shelf retreated up to 100 km in about a thousand years. Three-dimensional thermodynamic ice-shelf/ocean modeling results and comparison with ice-core records indicate that ice-shelf breakup resulted from combined atmospheric warming and warm ocean currents impinging onto the continental shelf.

Impact of the Mertz Glacier Tongue calving on dense water formation and export.

Antarctic Bottom Water (AABW) is a critical component of the global climate system, occupying the abyssal layer of the World Ocean and driving the lower limb of the global meridional overturning circulation. Around East Antarctica, the dense shelf water (DSW) precursor to AABW is predominantly formed by enhanced sea ice formation in coastal polynyas. The dominant source region of AABW supply to the Australian-Antarctic Basin is the Adélie and George V Land coast, in particular, polynyas formed in the western lee of the Mertz Glacier Tongue (MGT) and the grounded iceberg B9b over the Adélie and the Mertz Depressions, respectively. The calving of the MGT, which occurred on 12-13 February 2010, dramatically changed the environment for producing DSW. Here, we assess its impact using a state-of-the-art ice-ocean model. The model shows that oceanic circulation and sea ice production in the region changes immediately after the calving event, and that the DSW export is reduced by up to 23%.