Search

Sinking of the dense water plays a significant role in the global climate system by driving thermohaline (overturning) circulation and biogeochemical cycles. Deep water convection occurs also in the Japan Sea, and the convection has been considered to be mainly caused by intense cooling of the sea surface. Another possible cause of the convection is brine rejection associated with high sea-ice production in a coastal polynya (thin sea-ice) area in the northern Japan Sea. We have developed an algorithm which detects the thin ice area and estimates the thickness using passive microwave satellite data. Based on a heat flux calculation with the satellite-derived ice thickness, the total sea-ice production in winter (December–March) averaged over 2002/03–2010/11 is estimated to be 4.27 × 10 10 m 3 . This indicates that the ice production was underestimated by about half in a previous study in which the polynya was unrealistically treated as a low ice concentration area. The main determinant factor for the total ice production is the surface air temperature in early winter (December–January), which shows a rapid warming trend of 0.7 °C/decade for this 40-years. Based on a linear regression approach, the total ice production is estimated to have decreased by ~5%/decade due to air temperature warming. If brine rejection due to the ice production contributes to the deep water formation in the Japan Sea, this is consistent with the fact that the deep water formation has been decreasing for the last 50–100 years. [ABSTRACT FROM AUTHOR]

At several locations around Antarctica, dense water is formed as a result of intense sea ice formation. When this dense water becomes sufficiently denser than the surrounding water, it descends the continental slope and forms Antarctic Bottom Water (AABW). This study presents the AABW formation off the coast of Cape Darnley [Cape Darnley Bottom Water (CDBW)] in East Antarctica, using a nonhydrostatic model. The model is forced for 8 months by a temporally uniform surface salt flux (because of sea ice formation) estimated from Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) data and a heat budget calculation. The authors reproduce AABW formation and associated periodic downslope flows of dense water. Descending pathways of dense water are largely determined by the topography; most dense water flows into depressions on the continental shelf, advects onto the continental slope, and is steered downslope to greater depths by the canyons. Intense sea ice formation is the most important factor in the formation of AABW off Cape Darnley, and the existence of depressions is of only minor importance for the flux of CDBW. The mechanism responsible for the periodic downslope flow of dense water is further analyzed using an idealized model setup. The period of dense water outflow is regulated primarily by the topographic beta effect. [ABSTRACT FROM AUTHOR]

Abstract: Strong vertical mixing in the Kuril Straits is believed to be an important contribution to the ventilation of the intermediate layer and water mass transformation in the North Pacific, especially for density greater than . Furthermore, a recent modeling study has suggested that the vertical profile of vertical diffusivity is a key factor in the determination of thermohaline circulation in the North Pacific. Here we report the distribution of vertical diffusivity in the Bussol' Strait, the main conduit of water exchange and a possible central site of strong mixing in the Kuril Straits. Our analysis is based on a set of highly densed CTD observations, with a total of 127 casts across the strait in 2001. Vigorous density inversions occurred in the strait with the largest vertical displacement being over 250m. We estimated the vertical diffusivity coefficient from the Thorpe scale for all the CTD data. The vertical average of estimated from all the casts is 60×10−4 m2 s−1. Overall, is relatively small in the upper 300m (density range approximately ), whereas it is relatively large below a depth of 500m (density range of ), with a maximum at the depths of 1100–1700m. The distributions of and the amplitude of the diurnal tidal current are similar, suggesting that the mixing is caused by the strong diurnal tidal current. The amplification of the diurnal (tidal) current over slopes near the bottom causes the maximum at depths of . We also introduce an empirical relationship between and the amplitude of the diurnal tidal current. The vertical diffusivity is one order of magnitude larger at the spring tide than at the neap tide, suggesting that there is extremely large variability of tidal mixing with the fortnightly modulation. In the intermediate layer at densities of , large values ( ) corresponds well to the colder and less-saline water mass characterized in the Bussol' Strait, confirming that water mass transformation occurs locally in the strait through strong diapycnal mixing. [Copyright &y& Elsevier]

Causes of the multidecadal-scale warming of the intermediate water in the Okhotsk Sea and the western subarctic North Pacific during 1980-2008 are investigated using an ice-ocean coupled model with interannually varying atmospheric forcing. A hindcast experiment qualitatively reproduces the warming and decadal fluctuations of the intermediate water that are similar to those of observations: the warming is significant along the western part of the Okhotsk Sea and subarctic frontal region. The effects of the thermohaline- and wind-driven ocean circulation on the warming are evaluated from perturbation experiments on thermohaline (turbulent heat and freshwater fluxes) and wind causes, respectively. The thermohaline causes are shown to contribute positively to warming in the Okhotsk Sea Intermediate Water (OSIW). The heat budget analysis for the OSIW indicates that the warming is related to a decrease in cold and dense shelf water (DSW) flux, which is caused by a decrease in sea ice and surface water freshening. In contrast, the wind cause has a cooling effect in the OSIW through an increase in DSW. In the subarctic frontal region, the warming is mainly caused by the wind stress change. The heat budget analysis indicates that the warming is related to an increase in the northward advection of the subtropical warm water. These results imply that both thermohaline- and wind-driven ocean circulation changes are essential components of the warming in the intermediate water. The atmospheric conditions responsible for the warming are related to a weakened Aleutian low and Siberian high in early and late winter. [ABSTRACT FROM AUTHOR]

The formation of Dense Shelf Water (DSW) and Antarctic Bottom Water (AABW) in the Southern Ocean is an essential part of the thermohaline circulation, and understanding this phenomenon is crucial for studying the global climate. AABW is formed as DSW flows down the continental slope and mixes with the surrounding waters. However, DSW formation and its descent remains a poorly resolved issue in many ocean models. We, therefore, simulated the formation and descent of DSW and investigated the model sensitivities to horizontal and vertical grid spacings. The Massachusetts Institute of Technology general circulation model (MITgcm) was used for the region off Cape Darnley in East Antarctica, one of the main AABW production areas, where historical and mooring data are available for comparison. Simulations with coarse horizontal grid resolutions of order (10 km) yielded high volumes of DSW on the shelf. However, the largest part of this DSW was transformed into intermediate water and advected westward. Horizontal model resolutions equal to or higher than 2 km were required to simulate the descent of DSW and a realistic AABW production. Simulated time series at a mooring located at a depth of 2,600 m showed periodic fluctuations in velocity and temperature of 0.3 m ⋅ s−1 and 0.5 °C, respectively, consistent with observations. We also found that high-resolution bathymetry datasets are crucial because the newly formed AABW volume was reduced by 20% when a smoother bathymetry was used on a 2-km resolution grid. Vertical resolution had little influence on model performance because the plume was much thicker (> 170 m) than the grids width. Therefore, reproducing the downslope flow of DSW and AABW formation in the Cape Darnley region can be achieved with a high horizontal resolution (≤ 2 km) and a relatively coarse vertical resolution (∼ 100 m on the continental slope). • We model AABW formation off Cape Darnley and test its sensitivity to grid spacings. • We simulate AABW formation in good agreement with observations in the region. • Fine horizontal resolutions ≥ 2 km yield realistic downslope flow, AABW properties. • Vertical resolution did not affect the simulation of AABW formation in the region. • Bathymetric data of O (100 km) is required to produce realistic amounts of AABW. [ABSTRACT FROM AUTHOR]