北太平洋における東方伝播する十年規模貯熱量シグナルの起源

AAS03-08発表要旨 / 日本地球惑星科学連合2012年大会（2012年5月20日～5月25日, 幕張メッセ国際会議場） / 日本惑星科学連合の許諾に基づき本文ファイルを掲

北太平洋における十年規模貯熱量シグナルの東方伝播とその起源

海洋表層貯熱量は年々～十年規模の自然変動の気候シグナルを長期にわたって記憶することから、これら変動の予測可能性の鍵を与えると考えられる。本研究では、海洋前線帯を表現できる大気海洋結合モデルの長期積分結果の解析から、北太平洋中緯度に於ける海洋表層貯熱量の十年規模変動は、亜寒帯前線帯に沿って密度補償する水温・塩分勾配と前線帯の十年規模南北変位に起因するという仮説を提案した。要旨 ; 2012年度日本海洋学会秋季大会（2012年9月13～9月17日, 東海大学

Dynamics of North Pacific oceanic heat content variability on decadal time-scale

Presentation S3-8752 abstract, PICES 2012 Annual Meeting (October 12?21, Hiroshima, Japan

Baiu Rainband Termination in Atmospheric and Coupled Atmosphere-Ocean Models

The baiu rainband is a summer rainband stretching from eastern China through Japan toward the northwestern Pacific. The climatological termination of the baiu rainband is investigated using the Japanese 25-yr Reanalysis (JRA-25), a stand-alone atmospheric general circulation model (GCM) forced with observed sea surface temperature (SST) and an atmosphere-ocean GCM (AOGCM). The baiu rainband over the North Pacific abruptly shifts northward and weakens substantially in early July in the atmospheric GCM (AGCM), too early compared to observations (late July). The midtroposphere westerly jet and its thermal advection explain this meridional shift of the baiu rainband, but the ocean surface evaporation modulates the precipitation intensity. In AGCM, deep convection in the subtropical northwestern Pacific sets in prematurely, displacing the westerly jet northward over the cold ocean surface earlier than in observations. The suppressed surface evaporation over the cold ocean suppresses precipitation even though the midtropospheric warm advection and vertically integrated moisture convergence are similar to those before the westerly jet's northward shift. As a result, the baiu rainband abruptly weakens after the northward shift in JRA-25 and AGCM. In AOGCM, cold SST biases in the subtropics inhibit deep convection, delaying the poleward excursion of the westerly jet. As a result, the upward motion induced by both the strong westerly jet and the rainband persist over the northwestern Pacific through summer in the AOGCM. Theresults indicate that the westerly jet and the ocean evaporation underneath are important for the baiu rainband, the latter suggesting an oceanic effect on this important phenomenon

秋の北極海と冬のオホーツク海に於ける海氷の十年規模共変動

日本気象学会2012年度秋季大会（2012年12月3日～5日, 北海道大学 学術交流会館） / 発表番号: P17

The effective use of shortwave penetration below the ocean surface in a MOM3-based ocean general circulation model

There are two problems with the shortwave penetration scheme used in Modular Ocean Model version 3 (MOM3): (i) the spatiotemporal variability of the thickness of the first layer resulting from the free surface is not considered, and (ii) shortwave irradiance penetrates the ocean bottom. Because both of these problems can cause artificial heat sources or sinks, their effects are evaluated in the present study using a MOM3-based ocean general circulation model. The first problem creates an artificial heat sink (source) in the regions of positive (negative) sea surface height (SSH) with a maximum amplitude greater than 10 W m-2 and decreases (increases) sea surface temperature (SST) by up to 0.3°C on the basis of annual mean. This change in SST leads to a reduction in global mean evaporation and, as a result, an increase in SSH, which enhances the artificial heat sink. After several years of integration, this positive feedback amplifies the effects of the first problem in cases of stand-alone ocean simulations forced by freshwater flux. The estimated artificial heat sink induced by the second problem reaches 50 W m-2, and the decrease in SST exceeds 1.0°C. However, the effects of this problem are restricted within shallow coastal areas and do not involve positive feedback

What controls equatorial Atlantic winds in boreal spring?

The factors controlling equatorial Atlantic winds in boreal spring are examined using both observations and general circulation model (GCM) simulations from the coupled model intercomparison phase 5. The results show that the prevailing surface easterlies flow against the attendant pressure gradient and must therefore be maintained by other terms in the momentum budget. An important contribution comes from meridional advection of zonal momentum but the dominant contribution is the vertical transport of zonal momentum from the free troposphere to the surface. This implies that surface winds are strongly influenced by conditions in the free troposphere, chiefly pressure gradients and, to a lesser extent, meridional advection. Both factors are linked to the patterns of deep convection. Applying these findings to GCM errors indicates, that, consistent with the results of previous studies, the persistent westerly surface wind bias found in most GCMs is due mostly to precipitation errors, in particular excessive precipitation south of the equator over the ocean and deficient precipitation over equatorial South America. Free tropospheric influences also dominate the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that the free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Ninos), the results imply that interannual variability in the region may rely far less on coupled air-sea feedbacks than is the case in the tropical Pacific

Multiple causes of interannual sea surface temperature variability in the equatorial Atlantic Ocean

The eastern equatorial Atlantic Ocean is subject to interannual fluctuations of sea surface temperatures, with climatic impacts on the surrounding continents. The dynamic mechanism underlying Atlantic temperature variability is thought to be similar to that of the El Nino/Southern Oscillation (ENSO) in the equatorial Pacific, where air-sea coupling leads to a positive feedback between surface winds in the western basin, sea surface temperature in the eastern basin, and equatorial oceanic heat content. Here we use a suite of observational data, climate reanalysis products, and general circulation model simulations to reassess the factors driving the interannual variability. We show that some of the warm events can not be explained by previously identified equatorial wind stress forcing and ENSO-like dynamics. Instead, these events are driven by a mechanism in which surface wind forcing just north of the equator induces warm ocean temperature anomalies that are subsequently advected toward the equator. We find the surface wind patterns are associated with long-lived subtropical sea surface temperature anomalies and suggest they therefore reflect a link between equatorial and subtropical Atlantic variability

北太平洋亜寒帯前線の南北変位に対する大気と海洋の応答: 大気海洋結合モデル CFES を用いた実験

近年の観測データを用いた研究から、風成海洋循環の変動に伴う黒潮続流域での海洋前線の南北シフトが、 海洋から大気への熱的な強制の変動を通じて、海盆ス ケールの大気循環変動に影響を与る可能性が示唆され ている (Frankignoul et al. 2011, Taguchi et al. 2012)。 しかし既往研究で採用された診断的な解析手法では、 海洋前線変動の大気への影響の因果関係を直接的に明 らかに出来ないため、大気大循環モデルに海洋前線ス ケールの海面水温 (SST) 偏差を与える感度実験が、国 内外で精力的に実施されている (例えば Okajima et al. 2014)。本研究では、このような海洋前線変動に対する 大気応答実験を大気海洋結合モデルを用いて行うこと により、より現実的な条件の下での海洋前線変動に対 する大気応答を調べ、究極的には、そのような大気応 答の海洋への再影響を評価することを目的とする。要旨 ; 2014年度日本海洋学会春季大会（2014年3月26日～3月30日, 東京海洋大学