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海洋観測研究センター

大気海洋セミナー

第152回横須賀大気海洋セミナー

日時
2月25日(火)14:00~15:00
場所
横須賀本部 海洋研究棟3階セミナー室
発表者
名倉 元樹(GOORC)
タイトル
Interannual to decadal variability in subtropical circulation transport in the south Indian Ocean
要旨
This study estimates variability in meridional velocity and transport of the subtropical circulation in the south Indian Ocean using in-situ hydrographic observations, satellite altimetry and two reanalysis products for the period from 2006 to 2017. Previous studies used the zonal difference of satellite sea surface height (SSH) between the western and eastern parts of the basin as an index to variability in basinwide meridional geostrophic transport. This study estimates meridional geostrophic velocity in the upper 1800 m from in-situ observations and compares results with SSH variability. Results show that zonal SSH difference represents a surface trapped variability in meridional velocity, the amplitude of which is large in the upper 250 m and decreases to zero at about 1000 m depth. Zonal SSH difference is significantly correlated with zonally integrated meridional transport relative to 1000 m depth. It is likely that wind variability both in the south Indian Ocean and the tropical Pacific Ocean is responsible for this surface trapped variability, as is suggested by past studies. Results of this study also show meridional velocity variability at subsurface, which peaks in magnitude at about 400 to 800 m depth and is not correlated with zonal SSH difference. Waves radiated from the eastern boundary are possibly responsible for the generation of this subsurface flow, but detailed forcing mechanisms are not known in this study. This subsurface flow can contribute to interannual variability in mode water transport and warrants further study.

第151回横須賀大気海洋セミナー

日時
12月10日(火)14:00~15:00
場所
横須賀本部 海洋研究棟3階セミナー室
発表者
増田 周平(GOORC)
タイトル
Determining subsurface oceanic changes in the Indian sector of the Southern Ocean using Argo float data
要旨
The Southern Annular Mode (SAM) is one of the most prominent climate modes in the southern hemisphere. Although the influence of SAM on the surface ocean state has been revealed by remote-sensing data, its influence on the subsurface ocean state is not well-defined. Starting in the mid-2000s, the Argo float network was extended into higher-latitude regions in front of the sea ice zone, in particular in the Indian sector of the Southern Ocean. Integrated data, including several shipboard observations, enables us to investigate the relationship between interannual changes in the SAM and the surface-to-2000-m oceanic state. In this paper, we show that recent interannual changes in the wintertime subsurface oceanic state in the Indian sector are consistent with the fast response to the SAM proposed by previous model studies. In particular, the change in salinity is highly correlated with SAM variation, which has not been previously reported.

第150回横須賀大気海洋セミナー

日時
10月17日(木)14:00〜15:00
場所
横須賀本部 海洋研究棟3階セミナー室
発表者
深澤 理郎(JAMSTEC)
タイトル
Long Hydrographyの今昔物語
要旨
Before WOCE
The history of LHO may be Meteor’s Expedition in the Atlantic Ocean in 1932. An echo sounding was introduced firstly and seven zonal sections were occupied, though interval of stations was so coarse. The main purpose of the expedition was to know the stratification of seawater in the ocean. It is well known that this expedition located Mid-Atlantic Ridge, North Atlantic Deep Water, and Antarctic Bottom Water. Of these findings, Antarctic Bottom Water had attracted attentions of many chemical oceanographers to carry out new program of Geochemical Ocean Sections Study (GEOSECS). GEOSECS expeditions were carried out from 1972 to 1978 over the world ocean. The purpose of GEOSECS was to investigate the thermohaline circulation by measuring chemical tracers and radiotracers. So, the stations were lined up along assumed pathway of NADW and AABW in each ocean. GEOSECS succeeded to show schematic picture of global thermohaline circulation as the first step of global circulation study.

WOCE
After International Geophysical Year (IGY) from 1957 to 1958, corresponding to the increasing sense of crisis for the climate change or Global Warming, World Meteorological Organization (WMO) started World Climate Program (WCP) in 1979. World Climate Research Program (WCRP) was started in 1980 as a aub-program of WCP sponsored by WMO and International Council for Science (ICSU). IOC participated so late in 1993. Objectives of WCRP were to investigate the predictability of climate and to promote scientific understandings of climate syste. World Ocean Circulation Experiment (WOCE) was the stream III of WCRP from 1990 to 2002 with proper objectives to know ocean fluxes in the world ocean (1990-1998) and to develop a model for ocean data assimilation (1998 -2002 AIMS phase). Former objective was the reason why hydrographic lines were set to form a “mesh” by land-to-land, surface to bottom sections. There are some special characteristics of WOCE, e.g. 1. Essential Climate Variable (ECV)、which includes chemical and radio traces, was introduced as requirement to measure in hydrographic program of WOCE (WHP). 2. International office was established for each observational method (Data offices and Data Accumulation Center for WHP, XBT, ADCP etc) to adjust observation plans of participating countries, to prepare technical manuals of ECVs measurements in UNESCO technical papers, to synthesis compiled data, and to archive and open data within much shorter period than before.

After WOCE
Data from WHP were quite “FAIR” supported by DOs and DACs. Based on the “FAIR” characteristic of WHP, World Ocean Atlas (WOA) of NOAA (USA) and eWOCE of Scripps Institution of Oceanography could archive “epoch-making” progress. After WOCE era, the framework of WHP was taken over by CRIVAR and Carbon Hydrographic Program (CCHDP) incorporate with Joint Global Flux Study (JGOFS) of IGBP. In 2007, CCHDP was renewed as Global Ocean Ship-Based Hydrographic Investigation Program (GO-SHIP) under Global Ocean Observing System (GOOS) including biological study so far with a unique objective to develop a strategy for a sustained hydrography program.

Remark
There have been quite a few innovations in ocean observation. Argo float is one of such innovations. Now, Argo may be the most appropriate measuring platform of ocean monitoring. However, it should be noted here that ship based hydrography continues to give all oceanographers unrestricted possibility to understand ocean and seawater state INTERDISCIPLINARY with HIGHEST ACCURATE data which will be a historical record of our ocean and the Earth.

第149回横須賀大気海洋セミナー

日時
4月23日(火)14:00〜15:00
場所
横須賀本部 海洋研究棟3階セミナー室
発表者
高橋杏(東京大学)
タイトル
南極周極流域の乱流混合を想定した「波追跡シミュレーション」
要旨
乱流混合強度を比較的容易に推定する方法として考案されたファインスケール・パラメタリゼーションは, その有効性が多くの海域で確認されており, 南大洋を対象とした研究でも多用されている. しかしながら, これらのパラメタリゼーションは, 海洋内部領域に存在する平衡内部波スペクトルにおけるエネルギーカスケードから生じる乱流混合を対象としており, 南極周極流域に特有の平均流シアーや中規模渦が乱流混合過程に与える影響については全く考慮していない.TBA
Takahashi and Hibiya (2019) では , 2016 年に実施した南大洋におけるマイクロスケールの乱流混合強度とファインスケールの流速・密度の同時観測結果の解析を行い,南極周極流ジェット上に位置する観測点においてはファインスケール・パラメタリゼーションが乱流散逸率を 3 倍程度以上過大評価してしまう傾向を確認した . これらの過大評価傾向は内部波エネルギーと相関が大きく , 内部波スペクトルに着目した解析を通じて (i) 内部波場の非等方性 , (ii) 鉛直波数スペクトルの歪み , のいずれか又は両方がパラメタリゼーションによる過大評価の原因であるという可能性が示唆された .
本研究では, 背景内部波場を構成する内部波束の1 つ1 つの伝播・反射・屈折を追跡する ray-tracing という手法を用いて, 内部波の砕波に伴う乱流混合過程のシミュレーションを行い, (i)内部波場の非等方性, (ii) 鉛直波数スペクトルの歪みがファインスケール・パラメタリゼーションの推定精度にどのように影響するのか調べた. その結果, (i) 内部波場の非等方性は乱流混合強度およびファインスケール・パラメタリゼーションの推定精度に殆ど影響を与えないが,(ii) 鉛直波数スペクトルの歪みは大きく影響を与えることがわかった. 「低鉛直波数側にエネルギーを持った内部波場において, ファインスケール・パラメタリゼーションが過大評価傾向を示す」という結果が得られたが, これはTakahashi and Hibiya (2019) およびイギリス・アメリカの研究チームによって実施された大規模観測の結果とも整合的であった.

第148回横須賀大気海洋セミナー

日時
4月16日(火)14:00〜15:00
場所
横須賀本部 海洋研究棟3階セミナー室
発表者
井上龍一郎(GOORC)
タイトル
慶良間海裂における乱流観測と係留観測
要旨
2016年12月に白鳳丸によって行われた慶良間海裂周辺における観測の結果を紹介する。乱流計を用いた微細構造観測では、慶良間海裂入り口のシルにおいて、鉛直拡散係数で、外洋の一般的な値の1000倍以上の強い乱流が観測された。係留観測から得られた流速と水温変動の周期性から、この強い混合には潮汐が関与していると考えられた。そこで、慶良間海裂を模した2次元数値実験を行い、係留観測で卓越していた潮流の役割を調べた。その結果、慶良間海裂内のシルで発生する内部波は、海裂内に放射・砕波することによって、沖縄海盆底層水の更新のみならず、海裂内の北太平洋中層水の変質にも寄与しうることが明らかになった。