【作者】 蒋兴文；

【导师】 李建平；

【作者基本信息】 兰州大学 ， 气象学， 2011， 博士

【摘要】 本文利用多源观测资料,使用统计、合成、诊断和数值模拟等多种分析方法,对夏季风爆发过程的海气相互作用进行了系统的研究。首先分析了全球逐日SST和风场的季节变化,发现季风区经向最暖海表温度轴(海温暖轴)存在突然北跳的特征,并且海温暖轴的突然北跳可以触发夏季风爆发。然后,通过分析海洋混合层热收支及其影响因子的季节变化,发现海温暖轴的突然北跳是海-陆-气耦合系统对太阳辐射季节变化的响应。最后,以孟加拉湾夏季风爆发为典型例子,从气候平均、个例分析和年际变化的角度,深入研究海温暖轴突然北跳对夏季风爆发的影响。主要结论如下：(1)海表温度的季节变化及其对夏季风爆发的影响热带地区边界层环流主要受SST的影响,暖SST区域与南北风的辐合区和降水大值中心相对应。信风区,SST的季节变化表现为海温暖轴在赤道两侧两个局地SST暖中心之间来回跳动,或是海温暖轴在赤道地区较小范围内移动,它对区域SST经向梯度的影响较小,因此风的变化也小。季风区,SST的季节变化主要表现为海温暖轴的突然北跳、南撤和阶段性停滞,显著影响着区域SST的经向梯度及风的季节变化。研究发现西南风的爆发时间和降水中心突然从南半球北跳到北半球的时间晚于海温暖轴从赤道突然北跳到10°N以北的时间。不同纬向区域,海温暖轴突然北跳时间越早(晚),西南风爆发越早(晚)。海温暖轴的突然北跳主要通过两种方式影响夏季风的爆发,一是通过暖SST提高大气的不稳定性；二是通过改变区域SST梯度,使层辐合增强,边界层水汽增加。(2)海温暖轴突然北跳的成因夏季风爆发前,北半球中层副热带高压脊线位于15°N附近,受副高内部下沉气流的影响,15°N附近总云量最少,春季在这里形成了经向海表面短波辐射通量的大值区；随着冬季风的不断减弱,海表面风速逐渐减小,引起季风区海表面潜热通量逐渐减弱,海洋混合层逐渐变浅。春季,南亚大陆迅速增温,这一方面在对流层低层的阿拉伯海和孟加拉湾强迫出反气旋性环流,中心位于15°N附近,反气旋中心的低风速和负涡度强迫使这里成为经向海表面潜热通量、海洋混合层深度和混合层底卷入冷海水的低值区；另一方面,南亚作为整体与西北太平洋的海陆热力差强迫出南风气流,使西太平洋15°N附近的北风减弱,于是逐渐形成经向海表面潜热通量和海洋混合层深度的低值区。这些作用使得春季15。N附近的SST增温速度最大,导致15°N附近SST大于赤道SST的时间早于其南面区域SST大于赤道SST的时间,于是海温暖轴突然北跳。(3)孟加拉湾夏季风爆发的定义夏季风爆发过程中,孟加拉湾受多种来源气流影响,经向风和纬向风的扰动多,不太适合于判断夏季风的爆发。但是,可以利用孟加拉湾风向绝对转角或孟加拉湾西南面西风风速的突然增加来定义孟加拉湾夏季风的爆发。这两种方法定义的夏季风爆发时间的年际变化与使用热力学变量和对流变量定义的一致。受热带对流环流关系支配,利用孟加拉湾西南面西风风速定义夏季风爆发时间能更好的反映出对流的变化。(4)孟加拉湾夏季风爆发的基本特征孟加拉湾夏季风在24候爆发。夏季风爆发前,孟加拉湾SST的快速增加使海温暖轴从赤道突然北跳,SST在夏季风爆发时达到最大；夏季风爆发后SST降低,海温暖轴南撤回赤道。孟加拉湾夏季风爆发前,在赤道中印度洋存在较强的降水和涡旋对。孟加拉湾夏季风爆发后经圈环流发生明显的变化,南北半球相互作用增加。孟加拉湾夏季风爆发后其上空对流层中上层形成暖中心,引起经向气温最大值从南半球移到北半球,同时区域经向温度梯度反向。(5)海温暖轴突然北跳影响孟加拉湾夏季风爆发的年际变化特征孟加拉湾夏季风爆发时间与海温暖轴突然北跳的时间之间存在显著的正相关,海温暖轴的突然北跳均超前于夏季风爆发。海温暖轴的突然北跳主要是通过经向SST梯度,而非SST强度来影响孟加拉湾夏季风爆发的年际变化的。孟加拉湾夏季风爆发偏早(晚)年,海温暖轴突然北跳后夏季风迅速(缓慢)爆发。孟加拉湾夏季风爆发偏早年与偏晚年相比,偏早年前期在海洋大陆地区降水偏多,凝结潜热加热在赤道东印度洋激发出强的西风气流。该西风气流一方面可使赤道东印度洋SST偏低,引起海温暖轴突然北跳和孟加拉湾夏季风爆发偏早；另一方面,它可能是海温暖轴突然北跳后导致孟加拉湾夏季风迅速爆发的原因。(6)海温暖轴突然北跳和青藏高原加热在孟加拉湾夏季风爆发过程中的相对重要性CAM3的试验结果表明,相对于青藏高原春季的地表加热,海温暖轴的突然北跳在触发孟加拉湾夏季风爆发中起着更为重要的作用。海温暖轴突然北跳后,孟加拉湾低层将产生辐合,对流得以发展并在低层激发出气旋性环流,对流和环流之间存在正反馈作用。模式中海温暖轴突然北跳对夏季风爆发的影响特征与观测一致。受春季西风基本气流的影响,青藏高原加热异常主要影响高原下游地区,对孟加拉湾环流的影响较小更多还原

【Abstract】 Air-sea interactions during monsoon onset over the Bay of Bengal (BOB) have been investigated by utilizing observational data and many analysis methods, such as statistical analysis, diagnostic analysis and numerical simulation. First, the seasonal cycle of sea surface temperature (SST) and wind show that this is an abrupt northward jump (ANJ) of the meridional warmest SST (WSSTA), and it can trigger the monsoon onset. Second, the analysis of seasonal cycle of heat budget of the ocean mixed layer (ML), general circulations and thermal condition of sea, land, and atmosphere indicated that ANJ of WSSTA is a response of ocean-land-atmosphere coupled system to seasonal cycle of solar radiation. Finally, the BOB has been selected as an example to investigate the climatology, case and interannual variability of how the ANJ of WSSTA triggers the monsoon onset. The major conclusions can be summarized as follows:(1) Influence of the annual cycle of SST on the monsoon onsetIt is found that SST is a major driver of tropical circulations in the atmospheric boundary layer. The warm SST zone is corresponds to the zone of convergence of southerly and northerly winds and precipitation maximum. In the trade wind regions, the annual cycle of SST involves a shift in the warmest SST axis (WSSTA) between two local maxima on either sides of the equator, or slight movement of WSSTA north of the equator. Consequently, WSSTA has little effect on the regional meridional SST gradient and wind direction. However, in the monsoon regions, the annual cycle of SST is characterized by an ANJ of WSSTA, resulting in a marked change in regional meridional SST gradient and consequent onset of winds and rain. The onset of the southwesterlies and the shift of rainfall maximum from the Southern Hemisphere (SH) to Northern Hemisphere (NH) lags behind the ANJ of WSSTA from the equator to north of 10°N. The ANJ of the WSSTA triggers convection by two ways. One is by destabilizing the atmosphere; the other is moisture convergence in the atmospheric boundary layer due to the SST gradient.(2) Why is there an ANJ of WSSTA? The ridgeline of subtropical high is located around 15°N, where the sky is clear due to subsidence, leading to formation of meridional maximum of surface short wave radiation flux in spring. As the winter monsoon weakens, the surface wind speed slows down, accompanied by weak latent heat flux and shallow ML. In spring, the land over south Asia warms rapidly. This leads to the anticyclonic circulations over the BOB and the Arabia Sea forced by sub-continent scale land-sea contrast, and southerlies in the northwestern Pacific forced by the thermal contrast between southern Asian and northwestern Pacific. In the center of the anticyclonic circulations, wind speed is weak and vorticity is positive, leading to weak sea surface evaporation, weak entrainment of cold water at the base of ML, and shallow ML. These processes contribute to the largest increase in the rate of SST around 15°N. Thus the SST around the 15°N is warmer than the equator before the SST south of 15°N greater than the equator, the result is the ANJ of the WSSTA before the monsoon onset.(3) Definition of the monsoon onset over the BOBDuring the monsoon onset, the winds in the BOB originate from different regions. Thus, the reversal of zonal wind or merdional wind is not stable which makes it difficult to define the monsoon onset by wind components. However, the monsoon onset can be defined objectively by using wind absolute angle and the westerly component in the southwest of the BOB. There is high correlation between the monsoon onset dates defined in these two ways and the dates determined by thermodynamic variables and convection. The variability of the westerlies in the southwest of the BOB can well represent the variability of convection in the BOB.(4) Characteristics of the monsoon onset over the BOB The climatological onset of the monsoon in the BOB shows that the ANJ of the WSSTA is caused by an increase SST in pentad 22 in the BOB, leading to the monsoon onset after about 2 pentads. Maximum SST values occur just before the monsoon onset. SST decrease after the monsoon onset, associated with an abrupt southward retreat to the equator. There are intensified rainfall and twin vortices in the equatorial central Indian Ocean (10). The meridional circulations show distinct differences after the monsoon onset, and intensified the interaction between the NH and SH. The monsoon onset leads to the formation of warm center in middle and upper troposphere over the BOB, resulting in the meridional warmest air axis shift from the SH to the BOB and the resultant reversal of merdional temperature gradient.(5) Interannual variability of the monsoon onset over the BOB and its relationship with ANJ of WSSTAThere is a significant positive correlation between the monsoon onset over the BOB and the ANJ of WSSTA. The dates of ANJ of WSSTA are prior to the dates of the monsoon onset for all years, although with different lead time. The interannual variability of ANJ of WSSTA affect the monsoon onset mainly by the meridional SST gradient, rather than the magnitude of SST. The earlier the monsoon onset, the quicker the monsoon onset is after ANJ of WSSTA. Compared with the late onset, the early onset show more rainfall in the Maritime Continent before the monsoon onset, resulting in westerly anomalies in the equatorial eastern 10 excited by the release of latent heat. The anomalous westerlies lead to the earlier ANJ of the WSSTA by cooling the SST in the equator, and the rapid monsoon onset.(6) Relative importance of ANJ of the WSSA and the Tibetan Plateau heating in the monsoon onset over the BOBThe experiment results of Community Atmosphere Model (CAM3) indicate that ANJ of WSSTA plays a more important role in the monsoon onset over the BOB than the Tibetan Plateau (TP) heating. After the ANJ of the WSSTA, convections occurred due to air convergence in lower troposphere, accompanied by cyclonic circulations. There is a positive feedback process between circulation and convection. The characteristics of circulation and rainfall induced by the ANJ of the WSSTA in the CAM3 are the same as observation. Affected by the westerlies in spring, the impacts of the TP heating are concentrated on its downstream regions. Therefore, TP heating has limited effects on the monsoon onset over the BOB.更多还原