Frontogenesis and frontolysis of the subpolar front in the surface mixed layer of the Japan Sea

The frontogenesis and frontolysis processes of the subpolar front (SPF) in the surface mixed layer of the Japan Sea are investigated using state-of-the-art oceanic reanalysis data. The SPF experiences a 9 month weakening period from January to September, which shifts to a strengthening period in October. Our analysis shows that horizontal advection consistently contributes to the intensification of the SPF. After September, as the weakening effect of surface heat flux diminishes, horizontal advection becomes the dominant factor that contributes to changes in the SPF strength. Thus, the SPF enters a 3 month strengthening period. The geostrophic component of horizontal advection provides the most important contribution to strengthening the SPF, acting to intensify the SPF year-round. Ekman advection also promotes SPF strengthening with a smaller but still important contribution. During the weakening period, SPF strength is largely controlled by heat flux. The heat flux, especially the shortwave radiation component, is the primary cause of the surface front disappearance in the summer

A numerical experiment of a polar low observed over the Barents Sea in January 2011

第3回極域科学シンポジウム/特別セッション「これからの北極研究」11月28日（水） 国立極地研究所 2階大会議

Transient and local weakening of surface winds observed above the Kuroshio front in the winter East China Sea

To confirm whether surface winds strengthen above warm waters around oceanic fronts using in situ data, a field measurement was conducted using both expendable bathythermographs and Global Positioning System sondes released concurrently across the Kuroshio front in the East China Sea in December 2010. In contrast to previous studies mainly based on satellite observations, the finding of the present field survey is the local weakening of surface winds at the northern flank of the Kuroshio front. From the above field observation in conjunction with a regional numerical model experiment, it is suggested that the northwesterly winds crossing the Kuroshio front from the cooler side first weaken at the northern flank of the front because of the onset of upward transfer of the "nonslip" condition at the sea surface. Thereafter, as the atmospheric mixed layer with warm and humid air mass develops gradually downwind over the Kuroshio region, the surface winds are gradually accelerated by the momentum mixing with strong winds aloft. The surface winds remain strong over the cool East China Sea shelf, and it is thus considered that the surface winds only weaken at the northern flank of the Kuroshio front. However, numerical modeling indicates that this local weakening of the surface winds occurs as a transient state with a short duration and such a structure has thus rarely been detected in the long-term averaged wind fields observed by satellites. Key Points An XBT/GPS sonde observation was conducted across the Kuroshio front Local weakening of surface winds was found just above the Kuroshio front The weakening occurs at the onset of cross-frontal winds transientl

東シナ海の流れ藻に蝟集する稚魚の摂餌生態と海洋環境

Many commercially important fishes associate with drifting seaweeds in their juvenile stage, however, the ecological significance of drifting seaweeds for juvenile fishes is still unclear. We postulated that the following two hypotheses may be applicable for juvenile fishes associate with drifting seaweeds, the “concentration of food supply” hypothesis: juvenile fishes are attracted by phytal animals on the drifting seaweeds and the “indicator-log” hypothesis: fish use accumulations of drifting seaweed as an indicator of productive areas (e.g. frontal areas) for food. We investigated the frontal areas, zooplankton abundance around the drifting seaweed, and the food availability of fish juveniles associated with drifting seaweed accumulations in the East China Sea in 2012 and 2013. A total of 14 drifting seaweed mass and 22 species (n = 408) of fish juveniles were collected. We found that 49.7 - 99.7 % of the individual fed on planktonic food and the feeding incidence on phytal animals was less than 50 %. Although drifting seaweeds were aggregated around the frontal areas of surface currents, the zooplankton abundance was not significantly different between these frontal areas and other areas. Our findings indicate that ecological significance of drifting seaweeds as feeding habit is relatively low for juvenile fishes associated with drifting seaweeds.流れ藻には多くの水産上重要種の稚魚が付随するが，流れ藻の稚魚にとっての生態学的意義は明らかにされていない。筆者らは次の2仮説のいずれかが流れ藻付随稚魚に当てはまると考え，“concentration of food supply hypothesis”（流れ藻葉上生物を摂餌するため）と“indicator log hypothesis”（流れ藻をフロント域のような餌豊度の高い海域の目印とするため）を検証するため，2012年と2013年に東シナ海の流れ藻周辺の海洋環境，フロント域，動物プランクトン豊度，流れ藻付随稚魚の摂餌個体率を調べた。流れ藻は表層流の収束帯に集積されていたが，収束帯のプランクトン豊度は高くなかった。合計14個の流れ藻を採集し，合計22種（408尾）の稚魚の胃内容物を調査した結果，稚魚の49.7 - 99.7 %の個体はプランクトンを選択的に摂餌していたが，葉上生物の摂餌個体率は高くなかった（50 %未満）。以上の結果から流れ藻の稚魚にとっての生態学的意義は摂餌場でないことが示唆された

Impacts of a warming marginal sea on torrential rainfall organized under the Asian summer monsoon

Monsoonal airflow from the tropics triggers torrential rainfall over coastal regions of East Asia in summer, bringing flooding situations into areas of growing population and industries. However, impacts of rapid seasonal warming of the shallow East China Sea ECS and its pronounced future warming upon extreme summertime rainfall have not been explored. Here we show through cloudresolving atmospheric model simulations that observational tendency for torrential rainfall events over western Japan to occur most frequently in July cannot be reproduced without the rapid seasonal warming of ECS. The simulations also suggest that the future ECS warming will increase precipitation substantially in such an extreme event as observed in midJuly 2012 and also the likelihood of such an event occurring in June. A need is thus urged for reducing uncertainties in future temperature projections over ECS and other marginal seas for better projections of extreme summertime rainfall in the surrounding areas

Seasonal migration of the Baiu frontal zone over the East China Sea: Sea surface temperature effect

We propose a new scenario for the seasonal migration of the Baiu frontal zone over the East China Sea in which this migration is affected by variations in the sea surface temperature (SST). Using atmospheric and oceanic objective analysis datasets, a relationship was determined between the seasonal migration of the Baiu frontal zone and the decaying process of the cold high over the East China Sea. Before the middle of June, the cold high, cooled by the low SST, is present over the continental shelf, and the position of the Baiu frontal zone corresponds to that of the Kuroshio Front. After the middle of June, the cold high decays and is shifted northward in association with the warming SST over the shelf. As a result, the Baiu frontal zone migrates northward and ends in the middle of July due to the dissipation of the cold high

Numerical Experiment on the Fortnight Variation of the Residual Current in the Ariake Sea

The fortnight variations of the residual current in the Ariake Sea, a partly mixed estuary located in the western part of Japan, are investigated with a three-dimensional numerical ocean model. Regardless of season, lateral shear dominates at spring tide and vertical shear is intensified at neap tide. The densitydriven current dominates the overall structure of the residual current. The Ekman number is the dominant parameter that controls the overall structure of the densitydriven current, indicating that the magnitude of the tidal mixing is the most important factor that controls the structure of the density-driven current. The current structure is not sensitive to the Kelvin number, which suggests the bay width is not a constraint on the density-driven current.Nagasaki University Major Research Project: Restoration of Marine Environment and Resources in East Asi

Frontogenesis and frontolysis of the subpolar front in the surface mixed layer of the Japan Sea

The frontogenesis and frontolysis processes of the subpolar front (SPF) in the surface mixed layer of the Japan Sea are investigated using state-of-the-art oceanic reanalysis data. The SPF experiences a 9 month weakening period from January to September, which shifts to a strengthening period in October. Our analysis shows that horizontal advection consistently contributes to the intensification of the SPF. After September, as the weakening effect of surface heat flux diminishes, horizontal advection becomes the dominant factor that contributes to changes in the SPF strength. Thus, the SPF enters a 3 month strengthening period. The geostrophic component of horizontal advection provides the most important contribution to strengthening the SPF, acting to intensify the SPF year-round. Ekman advection also promotes SPF strengthening with a smaller but still important contribution. During the weakening period, SPF strength is largely controlled by heat flux. The heat flux, especially the shortwave radiation component, is the primary cause of the surface front disappearance in the summer