Surface and upper air meteorological features during onset phase of 2003 monsoon

The second campaign of the Arabian Sea Monsoon Experiment (ARMEX-II) was conducted in two phases viz., March–April and May–June 2003. In the present work, the buoy and ocean research vessel data collected during the second phase of ARMEX-II have been analysed to bring out the characteristic features of monsoon onset. The results have shown that the thermodynamical features such as build up of lower tropospheric instability and increased height of zero degree isotherm occurred about a week before the monsoon onset over Kerala and adjoining southeast Arabian Sea. There was a sharp fall in the temperature difference between 850 and 500 hPa, and the height of zero degree isotherm about 2–3 days before the monsoon onset. The flux of sensible heat was positive (sea to air) over south Arabian Sea during the onset phase. Over the Bay of Bengal higher negative (air to sea) values of sensible flux prevailed before the monsoon onset which became less negative with the advance of monsoon over that region. The pre-onset period was characterized by large sea surface temperature (SST) gradient over the Arabian Sea with rapid decrease towards north of the warm pool region. The buoy observations have shown that SST remained close to 30.5°C in the warm pool region during the pre-onset period in 2003 but only 2–3 degrees away (north of this region) SSTs were as low as 28.5–29°C. An interesting aspect of sea level pressure (SLP) variability over the Indian seas during the onset phase of summer monsoon 2003 was undoubtedly, the highest SLP in the warm pool region inspite of very high SSTs.     

南海海-气通量交换研究进展

1998年的"南海季风试验(SCSMEX)"已经过去10年了,SCSMEX启动的南海海-气通量试验研究也有10个年头.在SCSMEX和国家自然科学基金面上项目"南海季风爆发期近海面层通量观测和湍流结构的观测研究"支持下,10年来在西沙实施了3次(1998年、2000年、2002年)海-气通量观测试验,开展了试验资料分析研究,重点是西南季风爆发前后海-气通量交换过程研究,辐射通量、感热通量、潜热通量、动量通量随天气条件的变化研究,海-气通量日变化,通量交换系数以及通量变化对低层大气、上层海洋的影响研究.对10年来南海通量研究作一回顾,对未来的通量观测研究计划特别是2008"亚洲季风年"西沙通量观测提出一些建议.     

Impact of sea surface temperature in modulating movement and intensity of tropical cyclones

It is well recognized that sea surface temperature (SST) plays a dominant role in the formation and intensification of tropical cyclones. A number of observational/empirical studies were conducted at different basins to investigate the influence of SST on the intensification of tropical cyclones and in turn, modification in SST by the cyclone itself. Although a few modeling studies confirmed the sensitivity of model simulation/forecast to SST, it is not well quantified, particularly for Bay of Bengal cyclones. The present study is designed to quantify the sensitivity of SST on mesoscale simulation of an explosively deepening storm over the Bay of Bengal, i.e., Orissa super cyclone (1999). Three numerical experiments are conducted with climatological SST, NCEP (National Center for Environmental Prediction) skin temperature as SST, and observed SST (satellite derived) toward 5-day simulation of the storm using mesoscale model MM5. At model initial state, NCEP skin temperature and observed SST over the Bay of Bengal are 1–2°C warmer than climatological SST, but cooler by nearly 1°C along the coastline. Observed SST shows a number of warm patches in the Bay of Bengal compared with NCEP skin temperature. The simulation results indicate that the sea surface temperature has a significant impact on model-simulated track and intensity of the cyclonic storm. The track and intensity of the storm is better simulated with the use of satellite-observed SST.     

青藏高原林芝与四川盆地温江地区晴天辐射和能量平衡特征

分析了2008年青藏高原林芝地区与四川盆地温江地区无降水条件下地表辐射、 湍流通量和地表反照率的日变化及月际变化特征, 并探讨了季风过程对其产生的影响.结果表明: 林芝与温江地区地表辐射和湍流通量都具有明显的日变化和月际变化周期, 季风期受云的影响, 日循环规律变得不是非常规则.季风对林芝地区地表能量分配影响极大, 季风前感热通量占主导地位, 季风期和季风后(夏、 秋节)潜热通量是净辐射的主要消耗项; 温江地区全年潜热在净辐射的分布中占主导地位, 感热通量的作用和土壤热通量相当. 林芝地区年平均地表反照率为0.21, 温江地区年平均仅为0.14; 季风前(3-5月)、 季风中(6-7月)和季风后(8-9月), 林芝地区的地表反照率分别为0.20、 0.19和0.20, 温江地区的地表反照率分别为0.13、 0.11和0.14.     

Ocean-atmosphere interaction and synoptic weather conditions in association with the two contrasting phases of monsoon during BOBMEX-1999

Surface meteorological parameters acquired during the field phase experiment, BOBMEX-99, for the stationary periods (SP I and II) of the ship ORV Sagar Kanya over Bay of Bengal have been analysed. Active and weak monsoon conditions were observed during the first and the second phase of the experiment respectively over India as well as over the stationary ship location. The phase mean sea surface temperature (SST) is found to be the same in both the phases, however large differences have been observed in the phase mean values of wind speed, mean sea level pressure, latent heat and momentum fluxes at air-sea interface. Synoptic scale monsoon disturbances formed only during the period of strong north-south pressure gradient over the Bay region. Events of prominent fall in SST and in the upper 15 m ocean layer mean temperature and salinity values during typical rainfall events are cited. The impact of monsoon disturbances on ocean-atmosphere interface transfer processes has been investigated.     

Intensification of Aila (May 2009) due to a warm core eddy in the north Bay of Bengal

A very severe cyclonic storm ??Aila?? hit West Bengal on 26 May 2009. The storm intensified when it encountered with a warm core (SST?=?31°C) anti-cyclonic eddy (ACE4) in the north Bay of Bengal. The storm intensity increased by 43% due to this eddy, which is comparable with that (34%) obtained from a best fit line (derived from several numerical experiments over north-west Pacific Ocean). The shallow mixed layer of the large-scale ocean and deep mixed layer inside the eddy appear to be crucial parameters besides translation speed of the storm (Uh), ambient relative humidity and thermal stratification below mixed layer, in the storm intensification. From the eddy size and Uh, the eddy feedback factor is found to be about 0.4 (i.e. 40%), which is close to the above. Since there exists an inverse relationship between Uh and UOHC (upper ocean heat content), slow (fast) moving storms require high (low) UOHC. The warm ACE4 with a high UOHC of 149?kj/cm² (300% higher than the climatological value) and deep warm layer (D26?=?126?m) opposes the cooling induced by the storm and helps for the intensification of the storm through the supply of large enthalpy (latent?+?sensible) flux.     

Interaction of monsoon with northern hemisphere mid-latitude circulation

Fourier analysis has been used for the monthly mean northern hemispheric geopotential height for the levels 700 mb and 300 mb for the months April through August in bad monsoon years (1972, 74 and 79) and in years of good monsoon rainfall over India (1967, 73, 77). From the Fourier coefficients the transport of momentum and of sensible heat have been computed in wave number domain. Waves 1 to 3 show contrasting features during years of good monsoon and bad monsoon. Northward transport of momentum across subtropical latitudes is larger in good monsoon years, while northward transport of sensible heat is larger in bad monsoon years. In good monsoon years there is a large divergence of momentum in the subtropics while there is a large convergence of momentum in the mid-latitudes. In bad monsoon years there is a large divergence of sensible heat in the sub-tropics, but a large convergence in the mid-latitudes. These quantities show similar features in pre-monsoon (April to May) during good and bad monsoon years.     

Changes in summer monsoon precipitation over Hunan Province during 1952–2007: response to the west Pacific sea surface temperature and global warming

Using a historical database (1952–2007) of sea surface temperature (SST) from a subtropical high-controlled area (110°E–140°E, 15°N–35°N) of the west Pacific Ocean and the precipitation over Hunan Province of southeast China, we analyzed time series variations of precipitation in relation to the East Asian summer monsoon and a global warming setting. The results show that there has been a significant increase in SST of the subtropical high-controlled area in the recent 50 years. Although the increase in annual summer monsoon precipitation during the same period has been subtle over Hunan province, seasonal rainfall distribution has obviously changed, represented by a reduction in May, but a significant increase through June to August, especially in July. We suggest that the mechanism of seasonal redistribution of monsoon precipitation is primarily due to the increasing SST that delays the intrusion of the west Pacific Subtropical High, therefore leading to a postponing of migration of the East Asian summer monsoon rainfall belt inland and northward.     

Variations of Indian monsoon precipitation during the last 32 kyr reflected in the surface hydrography of the Western Bay of Bengal

Hydrography of the Bay of Bengal is highly influenced by the river runoff and rainfall during the southwest monsoon. We have reconstructed δ¹⁸Osw, sea surface salinity and sea surface temperature (SST) changes in the Bay of Bengal by using paired measurements of δ¹⁸O and Mg/Ca in a planktonic foraminifera species Globigerinoides ruber from core SK218/1 in the western Bay of Bengal in order to understand the rainfall variability associated with southwest monsoon over the past 32 kyr. Our SST reconstructions reveal that Bay of Bengal was ～3.2 °C cooler during the LGM as compared to present day temperature and a ～3.5 °C rise in SST is documented from 17 to 10 ka. Both SST and δ¹⁸Osw exhibit greater amplitude fluctuations during MIS 2 which is attributable to the variability of NE monsoon rainfall and associated river discharge into the Bay of Bengal in association with strong seasonal temperature contrast. On set of strengthening phase of SW monsoon was started during Bølling/Allerød as evidenced by the low δ¹⁸Osw values ～14.7 ka. δ¹⁸Osw show consistently lower values during Holocene (with an exception around 5 ka), which suggests that the freshening of Bay of Bengal due to heavy precipitation and river discharge caused by strong SW monsoon. Results of this study signify that the maximum fluctuations of the NE monsoon rainfall during MIS 2 appear to be controlled by the strong seasonality and boundary conditions.     

Variations in sea surface temperatures based on alkenones in Korea Plateau sediments of the East Sea (Sea of Japan) over the last 300,000 years

Variations in long chain alkenone-based sea surface temperature (SST) from a piston core (M04-PC1A) collected from the Korea Plateau in the East Sea (Sea of Japan) were investigated to understand paleoceanographic variations over the last 300,000 years. By combining sedimentological and geochemical proxies (the lithological marker of crudely laminated mud, alkenone SST, foraminiferal oxygen isotope values, and ¹⁴C age determination) and by comparison with previous works, we examined paleoceanographic variations back to Marine Isotope Stage (MIS) 8, approximately 300,000 years B.P. In particular, analysis of alkenones suggests that SSTs were about 8 °C and 11 °C lower during MIS 8 and MIS 6 than that in the present-day SST, respectively. Furthermore, SST was estimated to be 5 °C lower during the Last Glacial Maximum. These significant SST differences among MIS 8, 6, and 2 may be attributable to not only the formation of distinctive water masses, but also to differential alkenone synthesis under different environmental conditions. These results suggest that SSTs in the East Sea during the last three glacial periods (MIS 8, 6, and 2) were different, but rather were closely linked with regional oceanographic conditions overlapped with sensitive responses to the intensity of the East Asian monsoon. Surface-water freshening was a local paleoceanographic consequence that was imprinted in the core during MIS 2 and MIS 6, and potentially during MIS 4. Furthermore, alkenone-based SST data suggested that surface water circulation and biological productivity were strongly associated with the inflow of the Tsushima Warm Current during interglacial periods.     

Sea surface temperature variability over North Indian Ocean — A study of two contrasting monsoon seasons

Using the satellite derived sea surface temperature (SST) data for 1979 (bad monsoon) and 1983 (good monsoon), the SST variability for two contrasting monsoon seasons is studied. The study indicates that large negative anomalies off the Somali and Arabian coasts are associated with good monsoon rainfall over India. The strong monsoonal cooling in these regions can be attributed to strong low level winds and intense upwelling. The reappearance of 27°C isotherm off Somali coast in May/June coincides with the onset of southwest monsoon over India. Further, the influence of zonal anomaly of SST off Somalia Coast (SCZASST) and Central Indian Ocean Zonal Anomaly of SST (CIOZASST) with monsoon rainfall over India is brought out. The former is negatively related to the monsoon rainfall over western and central parts of India, whilst CIOZASST is positively related.     

珠峰北坡地区近地层大气湍流与地气能量交换特征

利用珠峰北坡曲宗地区连续一年的大气观测资料（2005年4月至2006年3月）,分析了珠峰北坡地区近地层大气湍流宏观统计特征和西南季风爆发前后地气能量交换特征。研究表明在珠峰北坡地区Monin Obukhov相似定律同样适用。拟合得到了珠峰北坡曲宗地区近地层无因次风速分量方差以及温度和湿度归一化标准差和静力学稳定度的函数关系。研究得出曲宗地区能量平衡各分量（净辐射通量、感热通量、潜热通量和土壤热通量）以及地面加热场具有明显的季节变化和日变化规律。尤其是在西南季风的影响下,曲宗地区感热通量和潜热通量在季风爆发前后具有明显相反的变化趋势。其它特征参数（波文比和地表反射率）在西南季风爆发前后的变化规律也十分明显。     

Meteorological fields variability over the Indian seas in pre and summer monsoon months during extreme monsoon seasons

In this study, the possible linkage between summer monsoon rainfall over India and surface meteorological fields (basic fields and heat budget components) over monsoon region (30‡E-120‡E, 30‡S30‡N) during the pre-monsoon month of May and summer monsoon season (June to September) are examined. For this purpose, monthly surface meteorological fields anomaly are analyzed for 42 years (1958-1999) using reanalysis data of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The statistical significance of the anomaly (difference) between the surplus and deficient monsoon years in the surface meteorological fields are also examined by Student’s t-test at 95% confidence level. Significant negative anomalies of mean sea level pressure are observed over India, Arabian Sea and Arabian Peninsular in the pre-monsoon month of May and monsoon season. Significant positive anomalies in the zonal and meridional wind (at 2 m) in the month of May are observed in the west Arabian Sea off Somali coast and for monsoon season it is in the central Arabian Sea that extends up to Somalia. Significant positive anomalies of the surface temperature and air temperature (at 2 m) in the month of May are observed over north India and adjoining Pakistan and Afghanistan region. During monsoon season this region is replaced by significant negative anomalies. In the month of May, significant positive anomalies of cloud amount are observed over Somali coast, north Bay of Bengal and adjoining West Bengal and Bangladesh. During monsoon season, cloud amount shows positive anomalies over NW India and north Arabian Sea. There is overall reduction in the incoming shortwave radiation flux during surplus monsoon years. A higher magnitude of latent heat flux is also found in surplus monsoon years for the month of May as well as the monsoon season. The significant positive anomaly of latent heat flux in May, observed over southwest Arabian Sea, may be considered as an advance indicator of the possible behavior of the subsequent monsoon season. The distribution of net heat flux is predominantly negative over eastern Arabian Sea, Bay of Bengal and Indian Ocean. Anomaly between the two extreme monsoon years in post 1980 (i.e., 1988 and 1987) shows that shortwave flux, latent heat flux and net heat flux indicate reversal in sign, particularly in south Indian Ocean. Variations of the heat budget components over four smaller sectors of Indian seas, namely Arabian Sea, Bay of Bengal and west Indian Ocean and east Indian Ocean show that a small sector of Arabian Sea is most dominant during May and other sectors showing reversal in sign of latent heat flux during monsoon season.     

2005年青藏高原唐古拉地区地表能量收支状况分析

利用青藏高原唐古拉地区2005年涡动系统和10 m气象塔数据资料,计算分析了该地区地表能量收支状况.结果表明: 感热与潜热均有明显的季节变化特征,感热春季较大,夏季有下降趋势;潜热夏秋季节较大,冬春季节较小;净辐射在冬春季节主要转化为感热,夏秋季节转化为潜热,这些主要受季风、活动层冻、融过程及净辐射变化的影响.Bowen比夏秋季节平均为0.7,冬春季节平均为3.4,变化范围为-1.0～17.9.另外,研究显示降雨对感热、潜热通量影响较大.     

Harmonic analysis of summer mean wind at 200 mbar level during contrasting monsoon years over India

Summer (June–August) mean zonal and meridional wind components at 200 mbar level are subjected to harmonic analysis for the years 1970, 1971, 1972 and 1979. It is found that the small scale disturbances are intense during normal monsoon years. The westerlies in the belt 10°S to 30°S are stronger during drought years. During normal monsoon years (1970, 1971) the northward transport of westerly momentum by wave number 1 at 19.6°N is large as compared to drought years (1972, 1979). The transport of westerly momentum by standing eddies is northward for all the years between 5°S and 28.7°N but large during the normal monsoon years.     

Identification of a surface layer structure and analysis of humidity data in two weather situations at Jodhpur (26°18′N, 73°04′E), India,during MONTBLEX 1990

The Monsoon Trough Boundary Layer Experiment held in 1990 was a multi-institutional effort to probe the atmospheric boundary layer over the monsoon trough over northern India. For this experiment, four micrometeorological towers were set up at four different locations along the normal position of the trough. One such tower of 30m height was located at Jodhpur (26‡18′N, 73‡04′E), Rajasthan. The fast and slow response data available during the experiment have been used in the present study to determine a suitable layer-structure of the surface layer for evaluation of sensible heat flux using the multilayer hypothesis of Kramm (1989).     

Interaction Between the Enso and Asian Monsoon Recorded in Dasuopu Ice Core From Himalayas

INTERACTION BETWEEN THE ENSO AND ASIAN MONSOON RECORDED IN DASUOPU ICE CORE FROM HIMALAYAS     

末次盛冰期西太平洋海温差异对亚洲夏季风影响的数值模拟

文章利用Zhao等的模拟结果,进一步研究了在末次盛冰期(LGM)情景下汪品先和CLIMAP两种重建海洋表面温度(SST)资料差异对亚洲夏季风的影响。模拟结果表明:在LGM情景下西太平洋海域SST资料的不同对模拟的亚洲夏季风有着十分重要的作用。夏季,与CLGM方案相比,在WLGM方案中,当热带西太平洋SST较暖时,印度地区的大气热量出现显著增加,大气热量的这种变化,使得南非高压、南印度洋经向Hadley环流加强,伴随着索马里越赤道气流加强,也导致了印度季风区纬向季风环流的加强,从而造成了印度夏季风增强、降水增多;与较暖的热带西太平洋相对应,澳大利亚高压和120°E附近越赤道气流减弱,东亚季风区20°N以南经向季风环流加强、20°N以北经向季风环流减弱,指示着一个强的南海夏季风和较弱东亚副热带大陆夏季风。     

Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal

We use daily satellite estimates of sea surface temperature (SST) and rainfall during 1998–2005 to show that onset of convection over the central Bay of Bengal (88–92°E, 14–18°N) during the core summer monsoon (mid-May to September) is linked to the meridional gradient of SST in the bay. The SST gradient was computed between two boxes in the northern (88–92°E, 18–22°N) and southern (82–88°E, 4–8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current. Convection over central bay followed the SST difference between the northern and southern bay (ΔT) exceeding 0.75°C in 28 cases. There was no instance of ΔT exceeding this threshold without a burst in convection. There were, however, five instances of convection occurring without this SST gradient. Long rainfall events (events lasting more than a week) were associated with an SST event (ΔT ≥ 0.75°C); rainfall events tended to be short when not associated with an SST event. The SST gradient was important for the onset of convection, but not for its persistence: convection often persisted for several days even after the SST gradient weakened. The lag between ΔT exceeding 0.75°C and the onset of convection was 0–18 days, but the lag histogram peaked at one week. In 75% of the 28 cases, convection occurred within a week of ΔT exceeding the threshold of 0.75°C. The northern bay SST, T _N , contributed more to ΔT, but it was a weaker criterion for convection than the SST gradient. A sensitivity analysis showed that the corresponding threshold for T _N was 29°C. We hypothesise that the excess heating (∼1°C above the threshold for deep convection) required in the northern bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient.     

Effect of land surface processes on the Tibetan Plateau’s past and its predicted response to global warming: an analytical investigation based on simulation results from the CMIP5 model

Complex interactions between the land surface and atmosphere and the exchange of water and energy have a significant impact on climate. The Tibetan Plateau is the highest plateau in the world and is known as “Earth’s third pole”. Because of its unique natural geographical and climatic characteristics, it directly affects China’s climate, as well as the world’s climate, through its thermal and dynamic roles. In this study, the BCCCSM1.1 model for the simulation results of CMIP5 is used to analyze the variation of the land surface processes of the Tibetan Plateau and the possible linkages with temperature change. The analysis showed that, from 1850 to 2005, as temperature increases, the model shows surface downward short-wave radiation, upward short-wave radiation, and net radiation to decrease, and long-wave radiation to increase. Meanwhile, latent heat flux increases, whereas sensible heat flux decreases. Except for sensible heat flux, the correlation coefficients of land surface fluxes with surface air temperature are all significant at the 99 % significance level. The model results indicate rising temperature to cause the ablation of ice (or snow) cover and increasing leaf area index, with reduced snowfall, together with a series of other changes, resulting in increasing upward and downward long-wave radiation and changes in soil moisture, evaporation, latent heat flux, and water vapor in the air. However, rising temperature also reduces the difference between the surface and air temperature and the surface albedo, which lead to further reductions of downward and upward short-wave radiation. The surface air temperature in winter increases by 0.93 °C/100 years, whereas the change is at a minimum (0.66 °C/100 years) during the summer. Downward short-wave and net radiation demonstrate the largest decline in the summer, whereas upward short-wave radiation demonstrates its largest decline during the spring. Downward short-wave radiation is predominantly affected by air humidity, followed by the impact of total cloud fraction. The average downward short-wave and net radiation attain their maxima in May, whereas for upward short-wave radiation the maximum is in March. The model predicts surface temperature to increase under all the different representative concentration pathway (RCP) scenarios, with the rise under RCP8.5 reaching 5.1 °C/100 years. Long-wave radiation increases under the different emission scenarios, while downward short-wave radiation increases under the low- and medium-emission concentration pathways, but decreases under RCP8.5. Upward short-wave radiation reduces under the various emission scenarios, and the marginal growth decreases as the emission concentration increases.