Nobuyuki Shikama

Seasonal temperature variation below the thermocline detected by Argo floats

[1] Strong seasonal temperature variation below the thermocline is detected by Argo profiling floats. The variation is associated with a long baroclinic Rossby wave. The results of this study show that seasonal signals can be captured at 1000-2000 dbar not only in basin boundary regions or deep convection regions but also in the interior ocean. A clear, westward propagation signal is observed. The phase speed is of the same order as a theoretically derived speed. Below the thermocline, the first mode of empirical orthogonal function analysis of potential density variations in the strong signal region shows a clear seasonal variation. Even in the middle layer, the amplitude of the temperature anomaly is 0.05-0.12° C at 1200 dbar, which is equivalent to the annual standard deviation from the WOA01. These results are obtained using the dense spatial and temporal distributions provided by the Argo observations.

Intermediate Circulation in the Northwestern North Pacific Derived from Subsurface Floats

In order to examine the formation, distribution and synoptic scale circulation structure of North Pacific Intermediate Water (NPIW), 21 subsurface floats were deployed in the sea east of Japan. A Eulerian image of the intermediate layer (density range: 26.6–27.0σθ) circulation in the northwestern North Pacific was obtained by the combined analysis of the movements of the subsurface floats in the period from May 1998 to November 2002 and historical hydrographic observations. The intermediate flow field derived from the floats showed stronger flow speeds in general than that of geostrophic flow field calculated from historical hydrographic observations. In the intermediate layer, 8 Sv (1 Sv ≡ 106 m3s−1) Oyashio and Kuroshio waters are found flowing into the sea east of Japan. Three strong eastward flows are seen in the region from 150°E to 170°E, the first two flows are considered as the Subarctic Current and the Kuroshio Extension or the North Pacific Current. Both volume transports are estimated as 5.5 Sv. The third one flows along the Subarctic Boundary with a volume transport of 5 Sv. Water mass analysis indicates that the intermediate flow of the Subarctic Current consists of 4 Sv Oyashio water and 1.5 Sv Kuroshio water. The intermediate North Pacific Current consists of 2 Sv Oyashio water and 3.5 Sv Kuroshio water. The intermediate flow along the Subarctic Boundary contains 2 Sv Oyashio water and 3 Sv Kuroshio water.

The Transition Region Mode Water of the North Pacific and Its Rapid Modification

AbstractUsing Argo float data, this study examined the formation region, spatial distribution, and modification of transition region mode water (TRMW), which is a recently identified pycnostad in the subtropical–subarctic transition region of the North Pacific, the basin-scale boundary region between subtropical and subarctic water masses. Analyses of the formation fields of water masses within and around the transition region reveal that TRMW forms in a wide area from the western to central transition region and is separated from the denser variety of central mode water (D-CMW) to the south by a temperature and salinity front. TRMW has temperatures of 4°–9°C and salinities of 33.3–34.0, making it colder and fresher than D-CMW. TRMW has a density range of 26.3–26.6 σθ, and thick TRMW is widely distributed in the transition region. However, the range of the T–S properties at TRMW cores is substantially reduced downstream within 10°–20° longitude from the formation region by gradually losing its fresh and c...

Pre-Japan-ARGO: Experimental Observation of Upper and Middle Layers South of the Kuroshio Extension Region Using Profiling Floats

We deployed two profiling floats in the region south of the Kuroshio Extension in March 2000. Temperature and salinity profiles from a depth of 1500 × 104 Pa to the surface are reported every two and four weeks, respectively. The floats performed very well for first four months after deployment. Later they failed in surfacing for a few months when the sea surface temperature in the region was high. The salinity sensors seemed to suffer from some damage during their failure-in-surfacing period. Despite this trouble, the results clearly demonstrate that the profiling float is a very useful and cost-effective tool for physical oceanographic observation in the open sea.

How to Set Surface Drifting Time of Profiling Floats to Obtain Profile Data with Less Observational Deficiency

プロファイリングフロートで得られる観測データの鉛直分解能は, Argos通信の処理能力に制約される。欠損のない観測プロファイルを得るためには, 多数に分割されたデータの全てを受信する必要がある。そのためにはフロートの通信時間(海面漂流時間)を長くしなければならないが, これは同時にフロート観測にとって深刻な問題を引き起こしうる。本稿では, 観測プロファイルに欠損が出現する割合(プロファイル欠損率)を統計的に求め, 調節すべき海面漂流時間とデータ量の基準を示した。プロファイル欠損率の理論値を正確に推定するためには, フロート展開域におけるデータ受信確率の正確な値を用いることが肝要である。データ受信確率は高緯度ほど高く, 同緯度では大陸から離れるほど高くなる傾向がある。データ量を固定して海面漂流時間だけを短くすると, データ受信確率の値によらずプロファイル欠損率は約3～5%を境として急速に増大した。欠損プロファイルの予期せぬ頻発を避けるために, プロファイル欠損率が3%以下になるようにフロートの海面漂流時間を設定すべきである。具体的には, 2005年4月現在と同様に5機のArgos衛星が稼働している場合, メッセージブロック数が24の場合は9時間以上, 同16の場合は6時間以上の海面漂流時間が必要である。