Impact of resolving the diurnal cycle in an ocean-atmosphere GCM. Part 2: A diurnally coupled CGCM
Impact of resolving the diurnal cycle in an ocean-atmosphere GCM. Part 2: A diurnally coupled CGCM
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DOI：
10.1007/s00382-008-0429-z
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发表时间：
2008-12-01
期刊：
影响因子：
4.6
通讯作者：
Cole, J.
中科院分区：
文献类型：
--
作者：
Bernie, D. J.;Guilyardi, E.;Cole, J.
耦合海洋大气环流模式(GCM)通常每24小时耦合一次，不包括来自上层海洋的日循环。以往的研究试图研究上层海洋的昼夜循环的作用，特别是海温的昼夜变化，所使用的模型无法解决感兴趣的过程。在这项研究的第一部分中，我们发展了一个高垂直分辨率的海洋GCM构型，该构型可以分辨上层海洋的日循环。在这项研究中，每隔3h将其耦合到大气GCM，以检查平均气候模拟的敏感性及其变异性对包括日海气耦合的影响。日循环的加入导致了热带地区平均海表面温度(SST)的增加，其中最强的信号出现在赤道太平洋，那里变暖从中西太平洋的0.2摄氏度增加到赤道东太平洋的0.3摄氏度以上。这种变暖很大程度上是由海温的日变化对日平均海温进行修正的直接结果。赤道太平洋变暖导致降水从热带辐合带(ITCZ)向赤道重新分配。在西太平洋，新几内亚和170度E之间的降水增加，最高可达1.2毫米/天，与气候学相比有所改善。太平洋副热带气旋强度增加了约10%，与本研究第一部分的结果一致，这是由于赤道发散的埃克曼流之间的动量交换和深部地转辐合的修改，有效地增强了热带太平洋对纬向风应力的动力响应。在太平洋信风减弱的春季，SST的大日循环加剧了赤道太平洋的季节变暖。当信风再次增强时，海洋动力响应的增加导致更强的赤道上升流。这两个过程都导致了耦合系统中更强的季节性流域尺度反馈，使热带太平洋部分的季节循环强度增加了约10%。这意味着上层海洋的日循环在维持热带太平洋海温和信风季节循环的基本状态和时间的海洋-大气耦合反馈中起着作用。计入日周期会导致MJO总体活动减少。降水合成表明，当解决了耦合的日循环时，MJO更强、更连贯，传播和不同相位在热带印度-太平洋地区和更大的提前期都更加明显。研究的第一部分表明，SST的日变化受到MJO的调制，从而增加了SST对MJO不同阶段的季节内响应。基于降水的SST可变性的合成在耦合模拟中证实了这一点。在MJO的时间尺度上(20-100天)，海洋和大气的热力耦合增加了，这是降水和海温合成中MJO强度和相干性改善的原因。这些结果表明，海-气相互作用的日循环对热带气候的一系列上尺度变率有深远的影响，因此，它是模拟的气候系统的一个重要特征，目前在最新的耦合模式中要么被忽视，要么解决得很差。
Coupled ocean atmosphere general circulation models (GCM) are typically coupled once every 24 h, excluding the diurnal cycle from the upper ocean. Previous studies attempting to examine the role of the diurnal cycle of the upper ocean and particularly of diurnal SST variability have used models unable to resolve the processes of interest. In part 1 of this study a high vertical resolution ocean GCM configuration with modified physics was developed that could resolve the diurnal cycle in the upper ocean. In this study it is coupled every 3 h to atmospheric GCM to examine the sensitivity of the mean climate simulation and aspects of its variability to the inclusion of diurnal ocean-atmosphere coupling.The inclusion of the diurnal cycle leads to a tropics wide increase in mean sea surface temperature (SST), with the strongest signal being across the equatorial Pacific where the warming increases from 0.2 degrees C in the central and western Pacific to over 0.3 degrees C in the eastern equatorial Pacific. Much of this warming is shown to be a direct consequence of the rectification of daily mean SST by the diurnal variability of SST. The warming of the equatorial Pacific leads to a redistribution of precipitation from the Inter tropical convergence zone (ITCZ) toward the equator. In the western Pacific there is an increase in precipitation between Papa new guinea and 170 degrees E of up to 1.2 mm/day, improving the simulation compared to climatology.Pacific sub tropical cells are increased in strength by about 10%, in line with results of part 1 of this study, due to the modification of the exchange of momentum between the equatorially divergent Ekman currents and the geostropic convergence at depth, effectively increasing the dynamical response of the tropical Pacific to zonal wind stresses.During the spring relaxation of the Pacific trade winds, a large diurnal cycle of SST increases the seasonal warming of the equatorial Pacific. When the trade winds then re-intensify, the increase in the dynamical response of the ocean leads to a stronger equatorial upwelling. These two processes both lead to stronger seasonal basin scale feedbacks in the coupled system, increasing the strength of the seasonal cycle of the tropical Pacific sector by around 10%. This means that the diurnal cycle in the upper ocean plays a part in the coupled feedbacks between ocean and atmosphere that maintain the basic state and the timing of the seasonal cycle of SST and trade winds in the tropical Pacific.The Madden-Julian Oscillation (MJO) is examined by use of a large scale MJO index, lag correlations and composites of events. The inclusion of the diurnal cycle leads to a reduction in overall MJO activity. Precipitation composites show that the MJO is stronger and more coherent when the diurnal cycle of coupling is resolved, with the propagation and different phases being far more distinct both locally and to larger lead times across the tropical Indo-Pacific. Part one of this study showed that that diurnal variability of SST is modulated by the MJO and therefore increases the intraseasonal SST response to the different phases of the MJO. Precipitation-based composites of SST variability confirm this increase in the coupled simulations. It is argued that including this has increased the thermodynamical coupling of the ocean and atmosphere on the timescale of the MJO (20-100 days), accounting for the improvement in the MJO strength and coherency seen in composites of precipitation and SST.These results show that the diurnal cycle of ocean-atmosphere interaction has profound impact on a range of up-scale variability in the tropical climate and as such, it is an important feature of the modelled climate system which is currently either neglected or poorly resolved in state of the art coupled models.