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第 221回 大気海洋物理学・気候力学セミナー のおしらせ
日 時： 9月 22日(木) 午前 09:30
場 所： 環境科学院 D棟2階 D201号室
発表者： 阿部泰人 （低温科学研究所/PD)
\\Hiroto Abe, (Post Doctoral Fellow)
題 目： the Hawaiian Lee Countercurrent の経年変動
\\Interannual variation of the Hawaiian Lee Countercurrent
発表者： Bo Qiu (Professor of Oceanography,
Department of Oceanography,
School of Ocean & Earth Science & Technology, University of Hawaii at Manoa）
題 目： Multi-Decadal Sea Level and Gyre
Circulation Variability in the Western Tropical Pacific Ocean
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the Hawaiian Lee Countercurrent の経年変動 \\Interannual variation of the Hawaiian Lee Countercurrent \\ (阿部泰人\\Hiroto Abe) 発表要旨 :
The Hawaiian Lee Countercurrent(以降HLCC)は，西向き流である北赤道海流に 逆行する流れである．本研究は，この表層流の時間変動(特に経年変動)につい て調査した．HLCCの西端に関し，過去の研究は日付変更線付近であると指摘して いるが，独自に作成した流速分布では西経173o付近であった．衛星海面高度計資料の 解析結果によると，HLCCの流軸位置に季節変動が見られた(夏季に南下，冬季 に北上)．この南北シフトを考慮すると，HLCCは夏季に強化，冬季に弱化していた． HLCCの南北シフトと強弱の季節変動は，ハワイ諸島西方の風応力場によって， 定性的に説明された．HLCCの経年変動もまた確認され，季節変動のそれと比べ大きかっ た．HLCCは，2000年に特に弱化し，1994年，1998年，1999年，2001年〜2005年に強 化していた．HLCCの経年変動は，ハワイ諸島西方の風応力場のそれと，力学的に対応してい なかった．一方，ハワイ諸島“東方”の風応力場変動によって励起された，大規模な海面 高度変動の西方伝播により，HLCCの強化，弱化が定性的かつ定量的に説明された．この事 実は，風応力場によって駆動されたシンプルな力学モデルと衛星海面高度計資料の解 析によって示された． Abstract : Temporal variations of the Hawaiian Lee Countercurrent (HLCC), an eastward surface current flowing against large-scale westward flow of the North Pacific subtropical circulation, are investigated with special reference to interannual variation. It is shown that from the climatological viewpoint, western termination of the HLCC is around 173oW, although past studies suggested it was around the International Date Line. Analysis using satellite altimeter data reveals a seasonal variation in meridional position of the current axis: north in winter and south in summer. By taking account of this meridional shift, it is shown that the strength of the HLCC is stronger in summer and weaker in winter. These meridional shift and strength variation are qualitatively explained by wind stress curl (WSC) field in the west of the Hawaii Islands. Interannual variation of the HLCC is also found in strength, which is much larger than that of the seasonal variation. The HLCC is strikingly weak in 2000, and strong in 1994, 1998, 1999 and during the period from 2001 to 2005. A comparison between the strengths of the HLCC and WSC, contrary to an expectation, does not show any dynamical consistency between them. Instead, it is found that strengthening and weakening of the HLCC is caused by westward propagation of large-scale sea surface height anomalies, which are originally induced by WSC in the east. This fact is supported by both a simple numerical model driven by wind stress data and satellite altimeter data.Multi-Decadal Sea Level and Gyre Circulation Variability in the Western Tropical Pacific Ocean \\ (Bo Qiu) 発表要旨 :
Sea level rise with the trend > 10 mm/yr has been observed in the tropical western Pacific Ocean over the 1993-2009 period. This rate is three times faster than the global mean value of the sea level rise. Analyses of the satellite altimeter data and repeat hydrographic data along 137E reveal that this regionally enhanced sea level rise is thermosteric in nature and vertically confined to a patch in the upper ocean above the 12 deg.C isotherm. Dynamically, this regional sea level trend is accompanied by southward migration and strengthening of the North Pacific Current (NEC) and North Pacific Countercurrent (NECC). Using a 1.5-layer reduced-gravity model forced by the ECMWF reanalysis wind stress data, the authors find that both the observed sea level rise and the NEC/NECC's southward migrating and strengthening trends are largely attributable to the upper ocean watermass redistribution caused by the surface wind stresses of the recently strengthened atmospheric Walker circulation. Based on the long-term model simulation, it is further found that the observed southward migrating and strengthening trends of the NEC and NECC began in the early 1990s. In the two decades prior to 1993, the NEC and NECC had weakened and migrated northward in response to a decrease in the trade winds across the tropical Pacific Ocean.
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