Research

Modeling the influence of Tropical Sea Surface Temperature (SST) on Intertropical convergence zone (ITCZ) and summer monsoon during the Last Glacial Maximum (LGM) 

2010-YunnanConference-poster

地质历史中,地球轨道参数变化主导着低纬太阳辐射量和热带表层海水温度(SST)的分布与变化,而热带辐合带(ITCZ)和夏季风则受到SST梯度和分布格局的控制,是热带大气对低纬太阳辐射和SST异常强迫的响应。末次盛冰期(LGM)时期热带SST的重建存在很大不确定性:CLIMAP (Climate: Long-range Investigation, Mapping, and Prediction)计划于1976和1981年的重建不断受到后续研究的挑战。虽然这些新的热带SST重建资料本身仍存在很大差别与分歧,但至少为LGM时期热带SST的变化幅度和分布格局提供了大量新的数据支撑。我们收集了三套不同区域的浮游有孔虫转换函数重建SST:(1)Wang (1999)的热带西太平洋SST重建;(2)Mix等(1999)的赤道东太平洋和热带大西洋SST重建;(3)MARGO(Multiproxy approach for the reconstruction of the glacial ocean surface)计划(2004)的全球热带SST重建。然后将其内嵌到CLIMAP全球SST中,作为CAM3大气环流模式的边界条件,研究冰期条件下不同的SST分布格局对ITCZ和夏季风的影响,以揭示古气候数值模拟中由SST边界条件导致的气候不确定性。

The changes of earth’s orbital parameter dominate the distributions and variations of low latitude insolation and different patterns and gradients of SST, which further induce drastic responses in the tropical atmosphere (such as ITCZ and summer monsoon) through tropical convections and atmospheric teleconnections. But there are still large uncertainties in the tropical SST reconstructions during the Last Glacial Maximum (LGM). We collected three LGM SST datasets reconstructed by planktonic foramifera transfer functions in different tropical regions and inlayed them into global CLIMAP SST field as new boundary conditions of CAM3, a newest atmospheric general circulation model (AGCM). Through the sensitivity experiments of CAM3, we investigated the influences of different tropical SST patterns on ITCZ and summer monsoon, and revealed the LGM climate uncertainties due to the SST variations in paleoclimate modeling. We also used the SST and Sea ice datasets outputted from CCSM3’s LGM simulation as boundary conditions of CAM3, and compared this simulation of CAM3 to that of CCSM3.

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Solar forced transient evolution of Pacific upper water thermal structure during the Holocene in UViC_ESCM, an earth system model of intermediate complexity

2013-1-27-王跃-SCSdeepSea-poster

全新世以来大气层顶接收到的总太阳辐照度(TSI)呈现出百年至年代际时间尺度变化,这种由太阳发出的短波辐射总量变化受太阳活动本身控制,并被认为可以导致地球表层气候的显著响应,例如太平洋上层水体的热力结构可能在太阳活动最大值期呈现出类似La Nina的SST响应格局。我们利用中等复杂程度的地球气候系统模式(UViC_ESCM)进行瞬变加速模拟,结果发现在过去七千年以来的太阳活动驱动下,西太平洋暖池的三维热力结构在百年时间尺度上表现出增强放大型响应。TSI增大时, 暖池表层的SST变暖幅度要大于赤道东太平洋,导致热带太平洋纬向(由西向东)的SST梯度增强,次表层TWT在副热带北太平洋变幅大于赤道西太平洋,导致经向(由北向南)的TWT梯度增强。这两个温度梯度与TSI驱动在1100、769、500、357和208年周期上都显著相关。

The total shortwave radiation outputted by the solar is dominated by the solar activities. During the Holocene, small-magnitude variations of Total Solar Irradiance (TSI) at the top of atmosphere (TOA) can results in significant surface climate responses of the Earth at centennial-decadal timescales. And the Pacific upper water thermal structure may act as a transmitter of solar forcing with a La Nina-like SST pattern during solar maxima periods. In a transient accelerated simulation of UViC_ESCM, we identified enhanced linear responses of 3-dimensional Western Pacific Warm Pool to centennial solar activities (or TSI) forcing since 7 ka. During solar maximum periods, the thermal structures of the Pacific upper water in boreal winter are featured by larger magnitude of centennial fluctuations at surface western tropical Pacific and at subsurface subtropical North Pacific relative to other parts of Pacific. Related centennial changes of zonal and meridional thermal gradients in surface and subsurface Pacific show different spectrum distributions.

图片-6

(Wang Y. et al., 2013, CSB)

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Precessional forced paleoclimate evolutions in the Asian-Pacific-Indian Ocean regions

作为地球表层气候系统的能量来源,太阳短波辐射变化驱动着地质历史中的气候变迁。万年以上时间尺度上,地球轨道参数变化(特别是岁差)显著地调控地球接收到的太阳辐射的空间-季节分布格局。这种轨道周期上的太阳辐射异常不仅通过高纬冰盖的增长-缩小造成地球气候的冰期-间冰期旋回,而且直接导致中-低纬海-陆-气耦合过程的剧烈变动,例如亚洲季风强度变化、热带太平洋的厄尔尼诺-南方涛动(ENSO)的长周期响应等。岁差周期上的亚洲夏季风演变不仅与亚洲-太平洋之间的中-上对流层热力梯度(亚洲-太平洋涛动,APO)高度相关,还可能对应有热带印度洋的纬向偶极子模态(IOD)的岁差周期波动,而东亚冬季风也可能和中纬度北太平洋过程(北太平洋模态,NPM)密不可分,这些都是研究岁差周期上太平洋ENSO响应的前提。我们的研究侧重探索岁差辐射量变化驱动下,APO、IOD和NPM的长时间尺度演变特征,试图揭示亚印太区域的海陆气耦合过程对瞬变太阳辐射的响应方式与机制所在。

As the main energy source of the earth’s surface climate, shortwave radiation from the solar can induce climatic evolutions at geological history. At timescales longer than ten thousand years, the orbital parameters of the earth (especially the precession) can significantly modulate the spatial-seasonal patterns of solar radiation at the top of atmosphere. These solar radiation anomalies at orbital timescales not only resulted in the glacial-interglacial cycles of the earth through the advance-regression of ice-sheet at high latitudes, but also directly compelled drastic variations of air-land-ocean system at middle-low latitudes, in which the El Nino-Southern Oscillation (ENSO) of the tropical Pacific, the zonal dipole mode of the tropical Indian Ocean (IOD) and the Asian monsoon interact with the extratropical North Pacific mode (NPM) and the Asian-Pacific oscillation (APO) through “Atmospheric bridge” or “Oceanic tunnel”. We use different air-ocean coupled climate models of different complexities to investigate the paleoclimate evolutions in the Asian-Pacific-Indian Ocean regions at the precessional band, which can be listed as follows:

  • East Asian summer monsoon (EASM) and associated APO (Asian-Pacific Oscillation)

Firstly, in a 46-kyrs’ transient accelerated simulation with the NCAR Community Climate System Model version 3 (CCSM3) under orbital insolation forcing, the atmospheric results are analyzed to reveal the non-uniform evolution of Asian summer monsoon at precessional bands, including synchronous circulation/wind anomalies over East Asia and out-of-phase precipitation seesaw between the northern and the southern part of China, and their physical mechanism from extratropical mid-upper troposphere (named APO) (2012-7-2-王跃-poster-CESS2). Then we detailed the relative roles of land- and ocean-atmosphere interactions in the precessional forced paleo-APO and associated East Asian summer monsoon using a 300-kyrs’ transient accelerated simulation of the Community Earth System Model (CESM). In phased with a precession parameter minimum, the enhanced EASM is associated with a seesaw of the summer middle-upper tropospheric eddy temperature between Asia and the North Pacific (named positive paleo-APO), which is mainly a response of the land-atmosphere interaction to the increased boreal summer insolation.

我们利用过去46kyr/300kyr以来的轨道参数变化,加速驱动CCSM3/CESM)完全海气耦合模式,着重分析模式中夏季大气的瞬变响应,揭示了岁差周期上亚洲夏季风的非同步演化特征(亚非夏季风区的表层风场整体增强、东亚夏季降雨量呈现南北反位相变化),并发现上述岁差驱动的大气环流和夏季风降雨变化可以用热带以外中-上对流层的纬向热力梯度机制(即亚洲-太平洋涛动,APO)进行解释。单独CAM4模式(CESM的大气分量)的敏感性试验表明,岁差周期上东亚夏季风和古APO主要是陆地-大气相互作用对夏季辐射量增加的响应,海气相互作用的响应仅具有次级影响。

  • Relative Role of land- and ocean-atmospheric interactions in APO&EASM

In a 250-kyr transient simulation of the Community Earth System Model (CESM), we identified a precessional forced seesaw of the summer middle-upper tropospheric eddy temperature between Asia and the North Pacific as the paleo-APO (Asian-Pacific oscillation). The paleo-APO variability is out of phase with the precession parameter. Corresponding to a positive paleo-APO phase, both the subtropical anticyclonic circulation over the North Pacific and the East Asian summer monsoon (EASM) strengthen,  and there is less precipitation between the Yangtze and Yellow Rivers and more precipitation over southeastern and northern China during June-July-August (JJA).

The variations in the simulated paleo-APO and East Asian southerly wind at the precessional band agree well with the stalagmite δ18O reconstructions at the Dongge, Sanbao-Linzhu (S_L), and Hulu caves in China, which also implies that these geological proxies may well reflect the variability in the southerly wind over East Asia. Since summer rainfall is also used as an important climatic factor behind the stalagmite δ18O reconstructions (with more rainfall corresponding to more negative δ18O), we further compared the model rainfall with those cave δ18O proxies in the EASM region. The maximums of the simulated JJA rainfall are in good agreement with the negative peaks in δ18O at the Dongge cave and are out of phase with those at the S_L cave and Hulu caves. This result implies that at the precessional band, while the integrated water vapor (with lower δ18O values) transported by the enhanced southerly winds can be used to explain the negative δ18O shifts in the EASM region, amount of local precipitations may also largely contribute to the negative δ18O peaks at the Dongge cave. The relative contributions of precipitation and wind circulation to stalagmites δ18O in different EASM regions should be addressed in future work.

Equilibrium experiments with both CESM and CAM4 further show that the response of the land-atmosphere coupling system alone to the precessional insolation forcing dominates the variations in the paleo-APO and the associated EASM circulation and rainfall anomalies, whereas the effect of ocean-atmosphere interactions on the paleo-APO is secondary. Moreover, a positive phase of the precessional forced paleo-APO is closely associated with a zonal positive-negative-positive pattern of June-July-August SST in the tropical Pacific and a western cold-eastern warm pattern of SST in the extratropical North Pacific. This relationship between the paleo-APO and the tropical Pacific SST is different from that of the modern climate, which may indicate a different relationship between EASM and ENSO.

  • Indian Ocean Dipole

The Indian Ocean Dipole (IOD) is one of the most significant tropical air-sea coupled phenomenon with a weakening or strengthening of the climatological sea surface temperature (SST) gradient (western cold-eastern warm) across the basin of the Indian Ocean. A positive phase of IOD is normally characterized by negative SST anomalies in the equatorial southeastern Indian Ocean and positive SST anomalies in the equatorial western Indian Ocean. Using the CESM model outputs under the transient orbital insolation forcing since 300 ka, we identified an IOD-like SST’ pattern in the tropical Indian Ocean at precessional band and named it the paleo-IOD. In line with a minimum of the precessional parameter, a positive paleo-IOD state is characterized by a western warm-eastern cold seesaw of SST’ across the equatorial Indian Ocean from August to October. This zonal SST’ seesaw can extend to the subsurface ocean between 60 and 80 m and is associated with stronger upwelling in EIO. A comparison between the model Dipole Mode Index (DMI) and the paleoceanographic reconstructed DMI based on the UK37- SST proxy shows the good consistency at precessional band.

我们利用过去300kyrs的岁差太阳辐射量变化驱动CESM模式进行瞬变加速模拟,识别出岁差周期上(23kyrs)热带印度洋SST的纬向偶极子型异常格局,并将其命名为古IOD(paleo-IOD)。当古IOD平均状态进入岁差周期上的正位相时(例如10-8ka的岁差参数极小值期),北半球夏-秋季(8月-10月)赤道印度洋西部SST异常偏暖、降雨量增加,热带东印度洋SST异常偏冷、降雨量减少。这种热带印度洋表层的纬向热力跷跷板格局可以向下延伸至60m-80m水深的次表层大洋,并伴有热带东印度洋海洋上升流的显著增强。相关的北半球夏-秋季对流层大气环流异常表现为如下特征:西印度洋上升运动异常增强,热带东南印度洋下沉运动异常增强,赤道印度洋表层盛行东风异常。正位相的古IOD平均状态很大程度上源自夏季岁差辐射量增加所驱动的局地海气相互作用,同时冬季岁差辐射量减少也可以通过“海洋热记忆效应”影响到夏-秋季SST,从而增强与正位相的古IOD相关的纬向SST跷跷板格局。我们模拟的偶极子模态强度指数(DMI)与基于古海洋学UK37-SST指标重建的DMI在岁差周期上存在很好的一致性。

图片-1

  • East Asian winter monsoon and associated NPM (North Pacific mode)

In the modern climate, the extratropical North Pacific is characterized by a zonal seesaw of sea surface temperature (SST) during boreal winter and spring and this seesaw pattern is called the North Pacific mode (NPM). In a positive phase of NPM, there are negative anomalies of SST in the western-central part of the North Pacific and a horseshoe-like warming over the other parts of the North Pacific. The SST variability over the North Pacific associated with a positive phase of NPM may be forced by atmospheric anomalies with a enhanced Aleutian low and stronger westerly winds in midlatitudes. These anomalies on the interdecadal timescales are also called the Pacific Decadal Oscillation. We utilized the output of the transient accelerated simulation from CESM under the orbital insolation forcing since 300 ka, and found that in responses to both a decreased winter insolation and an increased summer insolation, extratropical SST exhibits an NPM-like positive phase with colder SST in the Northwest Pacific and warmer SST in the Northeast Pacific from November to April. And there is an enhanced Aleutian low at the surface and decreased southerly winds and rainfall/snowfall appear over the eastern coasts of the Asian continent, indicating a stronger East Asian winter monsoon. 2014-7-4-王跃-poster-CESS3

从CESM模式的瞬变加速模拟和平衡态模拟中,我们识别出热带以外北太平洋海气耦合系统对岁差周期上太阳辐射量变化的响应模态,并将其命名为北太平洋模态(NPM)。相对于岁差参数极大值期(如22ka和0ka),10-8ka时的岁差参数减小将导致北半球冬季辐射量减少、夏季辐射量增加,NPM呈正位相格局:12月至4月份热带以外北太平洋表层海水温度(SST)异常呈西冷-东暖的纬向跷跷板格局,同时冬季阿留申低压增强,对流层出现异常气旋环流。中纬度北太平洋上空的对流层扰动大气温度异常和扰动位势高度异常构成了准正压的冷-槽式响应结构。东亚大陆和西北太平洋冬季降雨量减少,指示东亚冬季风增强;东北太平洋和北美西部沿岸冬季降雨量增加。

NPO-Figure2-Pacific-SST-EOF1-summer

Upper Figure: EOF1 results of Pacific SST associated with NPM (replotted using outputs of 300 kyr’s CESM simulation, which is  an updated  version of the 46 kyr’s CCSM3 simulation in Wang Y. et al., 2014, JGR)

Lower Figure: Boreal winter atmospheric anomalies associated with a positive NPM phase (replotted using outputs of 300 kyr’s CESM simulation, which is  an updated  version of the 46 kyr’s CCSM3 simulation in Wang Y. et al., 2014, JGR)

  • Modoki-ENSO-like annual cycle in the tropical Pacific

Based on a transient simulation of the Community Earth System Model, we identified two anomalous ‘‘zonal triple-pole type’’ annual cycles in the equatorial Pacific sea surface temperature (SST), which were induced by precessional evolution of the summer-minus-winter insolation difference and the autumn-minusspring insolation difference, respectively. For example, due to the increased summer–winter insolation contrast, a zonal positive–negative–positive pattern of equatorial SST anomalies was detected after subtracting basin-scale summer SST warming. The positive SST anomalies were associated with anomalous upward air flows over the western Pacific and eastern Pacific, whereas the negative SST anomalies in the central Pacific were coupled with anomalous downward air flows, oceanic upwelling, and thermocline cooling. These central Pacific anomalies were due to multiple air–sea interactions, particularly zonal advection feedback and Bjerknes feedback. This anomalous annual cycle also included winter equatorial air–sea coupled anomalies with similar spatial patterns but opposite signs. The annual mean equatorial rainfall was significantly increased west of 1358E but decreased between 1358E and 1608Win response to the moderately intensified Walker circulation west of 1608W. The autumn–spring insolation contrast induced similar seasonal reversed anomalies during autumn and spring, but the annual means were only weakly enhanced for the Walker circulation and the rainfall anomalies had smaller magnitudes east of 1608E. These distinct responses of the annual mean climate indicated different seasonal biases in terms of the equatorial SST and associated Walker circulation anomalies due to forcing by the two seasonal insolation contrasts, and these findings had meaningful implications for paleoceanographic studies.

厄尔尼诺-南方涛动(El Nino-Southern Oscillation,ENSO)是热带太平洋年际尺度气候变化的最显著特征,对我国夏季风降雨有重要影响,例如1998年长江流域洪水的罪魁祸首就是一个超级El Nino事件。但热带太平洋具有两种类型的El Nino事件,前者的海表温度异常表现为冬春季的纬向跷跷板格局(西冷-东暖),变暖中心位于东太平洋,属于经典型El Nino;后者则更多体现为夏季的纬向三极格局(东、西两头冷、中间暖),变暖中心位于日界线附近,称为Modoki型El Nino。随着1970年后全球变暖加剧, Modoki型El Nino的出现次数更为频繁。这说明不同的外部驱动条件下,ENSO相关的SST季节循环特征也会“与时俱进”。由于地球运行轨道参数具有周期性变化,比如地球自转轴像陀螺一样转动,导致两分点(春分、秋分)/两至点(冬至、夏至)在黄道面上也绕太阳旋转,其位置岁岁有差,这就是我国晋代天文学家虞喜(于公元330年)最早发现的“岁差”,祖冲之于公元463年制定《大明历》时将岁差引入历法计算,属于世界首创。这种2.3万年循环一次的岁差导致地球“晒太阳”的姿势不断调整,同一纬度上不同季节接收到的太阳辐射量也随时间周期性改变(图1),本研究关注的重点就是太阳辐射量的季节循环变化将会如何影响热带太平洋的类ENSO型古气候?

Nouse-Figure-25ka-insolation中文图1岁差驱动的赤道太阳辐射量季节循环变化

本研究利用美国NCAR CESM模式进行30万年以来轨道辐射量驱动的瞬变加速模拟,通过对赤道太平洋表层海温(SST)的季节循环进行多维经验正交函数分解,获得两个主要的“纬向三极型” 异常空间模态(图2), 其时间序列演变分别受控于岁差周期上的夏-冬季辐射量梯度变化(至点岁差驱动)和秋-春季辐射量梯度变化(分点岁差驱动)。例如,夏-冬季辐射量梯度增大时,夏季赤道太平洋SST呈洋盆尺度整体增暖特征,减去洋盆平均值之后,呈现出纬向“正-负-正”异常格局,其中太平洋西部和东部的暖SST异常对应于大气异常上升运动,而赤道中太平洋的冷SST异常对应于大气异常下沉运动、海洋上升流增强、温跃层变冷;与此同时,冬季赤道太平洋海气耦合异常也具有类似的空间格局、符号与夏季相反;尽管年平均SST由冬季变化主导,年平均Walker环流却由夏季主导(在160ºW以西显著增强),赤道降雨量在135ºE以西显著增加、在135ºE-160ºW之间显著减少。秋-春季辐射量梯度驱动的赤道太平洋海气耦合响应集中于秋季和春季,空间格局类似、秋季异常与春季异常符号相反,尽管年平均SST由秋季异常主导,年平均Walker环流仅在160ºE以东略微增强(春季主导),降雨量异常在160ºW以西有小幅度增加、在160ºW以东有小幅度减少,与Walker环流变化一致。因此赤道太平洋年平均气候态对夏-冬季辐射量梯度和秋-春季辐射量梯度具有不同响应方式,指示出赤道太平洋SST和Walker环流的不同季节偏向性,对于未来古海洋替代指标重建研究有很好的指示意义。

Figure-1a-cesmorb-Pacific-SST-5S-5N-month-lon-climo-annual-cycle-shift

图2 (a)现代气候平均的赤道海表温度季节循环,(b)和(c)分别是至点岁差和分点岁差辐射量驱动的季节循环异常(相对于现代的差值)。

Figure-1b-cesmorb-Pacific-SST-5S-5N-month-lon-climo-annual-cycle-shift

图3 (a)、(b)和(c)分别是至点岁差辐射量驱动的夏季、冬季、年平均赤道大气Walker环流异常;(d)、(e)和(f)分别是分点岁差辐射量驱动的秋季、春季、年平均赤道大气Walker环流异常。

Figure-12-cesmorb-Pacific-rainfall-5S-5N-month-lon-annual-cycle-shift

图4 (a)至点岁差辐射量驱动的年平均赤道降雨量(黑线)和sst异常(红线);(b)分点岁差辐射量驱动的年平均赤道降雨量(黑线)和sst异常(紫线);(c)至点岁差辐射量驱动的赤道降雨量季节循环异常;(d)分点岁差辐射量驱动的赤道降雨量季节循环异常。