Nitrate in the atmospheric boundary layer of the tropical South Pacific: Implications regarding sources and transport

Weekly bulk aerosol samples collected at Funafuti, Tuvalu (8°30S, 179°12E), American Samoa (14°15S, 170°35W), and Rarotonga (21°15S, 159°45W), from 1983 through most of 1987 have been analyzed for nitrate and other constituents. The mean nitrate concentration is about 0.11 g m^–3 at each of these stations: 0.107±0.011 g m^–3 at Funafuti; 0.116±0.008 at American Samoa; and 0.117±0.010 at Rarotonga. Previous measurements of mineral aerosol and trace metal concentrations at American Samoa are among the lowest ever recorded for the near-surface troposphere and indicate that this region is minimally affected by transport of soil material and pollutants from the continents. Consequently, the nitrate concentration of 0.11 g m^–3 can be regarded as the natural level for the remote marine boundary layer of the tropical South Pacific Ocean. In contrast, over the tropical North Pacific which is significantly impacted by the transport of material from Asia and North America, the mean nitrate concentrations are about three times higher, 0.29 and 0.36 g m^–3 at Midway and Oahu, respectively. The major sources of the nitrate over the tropical South Pacific are still very uncertain. A very significant correlation between the nitrate concentrations at American Samoa and the concentrations of ²¹⁰Pb suggests that transport from continental sources might be important. This continental source could be lightning, which occurs most frequently over the tropical continents. A near-zero correlation with ⁷Be indicates that the stratosphere and upper troposphere are probably not the major sources. A significant biogenic source would be consistent with the higher mean nitrate concentrations, 0.16 to 0.17 g m^–3, found over the equatorial Pacific at Fanning Island (3°55N, 159°20W) and Nauru (0°32S, 166°57E). The lack of correlation between nitrate and nss sulfate at American Samoa does not necessarily preclude an important role for marine biogenic sources.     

Statistics of the tropopause inversion layer over Beijing

High resolution radiosonde data from Beijing, China in 2002 are used to study the strong tropopause inversion layer （TIL） in the extratropical regions in eastern Asia. The analysis, based on the tropopause- based mean （TB-mean） method, shows that the TIL over Beijing has similar features as over other sites in the same latitude in Northern America. The reduced values of buoyancy frequency in 13-17 km altitude in winter-spring are attributed to the higher occurrence frequency of the secondary tropopause in this season. In the monthly mean temperature profile relative to the secondary tropopause, there also exists a TIL with somewhat enhanced static stability directly over the secondary sharp thermal tropopause, and a 4 km thickness layer with reduced values of buoyancy frequency just below the tropopause, which corresponds to the 13-17 km layer in the first TB-mean thermal profile. In the monthly mean temperature profile relative to the secondary tropopause, a TIL also exists but it is not as strong.
For individual cases, a modified definition of the TIL, focusing on the super stability and the small distance from the tropopause, is introduced. The analysis shows that the lower boundary of the newly defined TIL is about 0.42 km above the tropopause, and that it is higher in winter and lower in summer; the thickness of the TIL is larger in winter-spring.     

博乐市２００５－２００９年地温变化特征

利用1955年-2003年NCEP＼NCAR再分析资料和美国Scripps海洋研究所环境数据分析中一0（JEDAC）提供的冬季热含量资料,采用小波分析、相关及合成分析等方法,分析了西太平洋暖池热含量变化特征及其与东亚冬季风关系。结果表明：西太平洋暖池热含量与东亚冬季风有着非常密切的联系,当西太平洋暖池热含量异常偏高时,对流层低层在菲律宾及以东洋面形成一个异常的气旋性环流,中国大陆上空形成一个异常的反气旋性环流,从而使得东亚冬季风在东南区加强,西北区减弱。     

西太平洋热带海域秋冬积云对流输送的初步研究

本文建立以考虑云侧边夹卷效应和微物理过程的一维半暖积云模式为基础的积云输送模式,并利用此模式和实测雷达资料,导出了计算西太平洋热带海域积云群体输送公式,对此进行了实际计算,还讨论了积云群体输送对大气状态的影响。     

西北太平洋热带气旋对东亚大气环流的影响

使用1960—2015年6—10月上海台风研究所(CMA-STI)整编的热带气旋最佳路径数据集和NCEP/NCAR再分析资料,借助相关性分析、典型相关分析、耦合场分离方法等统计学方法研究了热带气旋对东亚大气环流的影响。主要结论如下:热带气旋活动增强使得对流层高层温度增加,低层温度降低;对位势高度场的影响主要是使得位势高度降低,最大的影响区在500 hPa;分析东亚大气环流与热带气旋之间的耦合相互作用发现,大气环流和热带气旋相互反馈作用较强的区域往往也是热带气旋对大气环流独立影响较强的区域。两者相互耦合影响部分的解释方差(4%～7%)明显小于单方面由热带气旋引起的大气环流变化部分的解释方差(12%～18%)。     

Structure of the inhomogeneities of the fields of temperature,wind, and humidity in the tropical tropopause layer and lower stratosphere over West Africa (Burkina Faso, 2006)

Data are presented on some structure parameters of mesoscale turbulence mechanisms playing a significant role in the troposphere-stratosphere exchange in the tropical tropopause layer and in the lower stratosphere. The data are obtained in the framework of the international aircraft AMMA experiment in Burkina Faso (West Africa) in August 2006.     

Climatic analysis of tropopause during the northwestern Indian Ocean tropical cyclones

In this research, tropopause temperature (TT) and tropopause geopotential height (TGH) over the inner-core and environmental regions of all tropical cyclones (TCs) over the northwest of the Indian Ocean (NWIO) from 1990 to 2019 were investigated. To this aim, observational/analysis/reanalysis data and also simulated data from both historical and Representative Concentration Pathway 8.5 (RCP8.5) experiments of some global climate models (GCMs) from the Coupled Model Intercomparision Project (CMIP5) were used. Dynamical and thermo-dynamical environmental factors controlling TC, together with their correlation with different phases of some climatic indices were considered. Results indicated that the eastern part of the NWIO was more favorable for TC genesis and intensification.Lower-level stratospheric (upper-level tropospheric) cooling (warming) was detected over the NWIO during 1990−2019. Over the both inner-core and environmental regions of the NWIO TCs, the coldest tropopause occurred at a CS-Category and the warmest tropopause happened at the first stage of a VSCS event. Over the inner-core (environmental) region, the highest tropopause was detected at the first stage of a CS event (at the end of a VSCS life cycle). A significant majority of the used CMIP5 GCMs produced stratospheric cooling and tropospheric warming trends over the NWIO, similar to those obtained using ERA5 reanalysis dataset. Finally, the decreasing trend of TT over the both inner-core and environmental regions of NWIO TCs together with temperature decreasing trend obtained from the CMIP5 GCMs simulations suggest that the NWIO is prone to experience more TCs, especially the intense ones, in the future.     

热带西太平洋边界层的多层结构分析

本文主要利用第一次“中美热带西太平洋海气相互作用联合调查”所获得的海洋水文、气象及大气探测资料,分析了热带西太平洋暖水区边界层大气层结特征,指出热带西太平洋边界层大气具有多层结构和多层送温现象,边界层内各层厚度及气温、位温、混合比随高度的变化趋势如下:超绝热层位于地面以上50m左右,气温、位温、混合比迅速减小,系绝热不稳定层;第一层为混合层,平均厚度为351m,位温、混合比变化不明显;混合层上又是逆温层大多为30—100m厚,位温迅速增大,混合比迅速减小,系绝对稳定层;700—1000m为浅对流云活跃层,     

Mean structure of the tropical tropopause and its variability over the Indian longitude sector

The mean spatiotemporal variations in tropopause parameters over the tropics (±35°, in latitude) in the Indian monsoon region are examined using the upper air data for an extended period obtained from radiosonde and Radio Occultation measurements. In general, the altitude of cold point tropopause (CPT) is a minimum near the equator and increases with latitude on either side. While CPT over the entire southern tropical latitudes and northern equatorial region is cooler (higher) during boreal winter and warmer (lower) during boreal summer, the annual pattern of CPT-temperature reverses in the northern hemispheric off-equatorial region. The temperature of lapse rate tropopause (LRT) is always negatively correlated with its altitude. While the annual variation of LRT-temperature in tropics is always positively correlated with CPT-temperature, the annual variation of LRT-altitude differs mainly in the off-equatorial regions. While the altitude of the convective tropopause is positively correlated with CPT-altitude over the latitude region 20°S–5°N, they are negatively correlated at the north of 10°N. In general, the tropical tropopause layer (TTL) is very thin (~3 km) near the equator and its thickness increases with latitude on either side of the equator to reach a peak value (of ~6 km) around ±30°. A pronounced decrease in TTL-thickness observed over the northern off-equatorial region during the ASM period can be attributed to the manifestation of very deep convection over the land near the Head Bay-of-Bengal region. The TTL-lapse-rate (γ_TTL) is large in the equatorial region and decreases with increase in latitude. While γ_TTL in the northern hemispheric off-equatorial region is low during winter, it increases and becomes comparable to that over equatorial region during the ASM period. The annual variations in CPT parameters as well as the TTL- thickness are significantly modulated by quasi-biennial oscillation and the El Niño Southern Oscillation.     

Operational hindcasting of the tropical Pacific

Procedures have been implemented at the Climate Analysis Center of the National Meteorological Center (CAC/NMC) to provide montly hindcasts of oceanographic conditions in the tropical Pacific. A central component of this system is a primitive equation ocean general circulation model that was developed at the Geophysical Fluid Dynamics Laboratory (GFDL). This is forced with monthly mean fields for wind stress and net heat flux. Until recently the former were derived from ship reports available on the Global Telecommunication System (GTS). The heat fluxes are slightly modified climatological fluxes from Esbensen and Kushnir. To correct for errors in the simulations, thermal data in the upper 450 and surface-temperature data are assimilated montly.Numerical experiments were run to examine the sensitivity of the simulations to small changes in the stress fields. Variations of the drag coefficient by 15% result in differences in sea-surface temperature (SST) and subsurface thermal structure in the eastern Pacific that are comparable with the observed annual and interannual variability. Comparisons with simulations in which the wind stresses were derived from operational atmospheric analyses show sensitivities of the same magnitude. Comparisons of simulations forced either with these of ship-recorded winds to a run with data assimilation show that significant errors are found in both, especially in the off-equatorial regions. Consequently, until forcing fields are improved, accurate simulations will require the use of data assimilation.     

Sensitivity of the tropical Pacific Ocean to precipitation-induced freshwater flux

We have performed experiments using an ocean model to study the sensitivity of tropical Pacific Ocean to variations in precipitation induced freshwater fluxes. Variations in these fluxes arise from natural causes on all time scales. In addition, estimates of these fluxes are uncertain because of differences among measurement techniques. The model used is a quasi-isopycnal model, covering the Pacific from 40?°S to 40?°N. The surface forcing is constructed from observed wind stress, evaporation, precipitation, and sea surface temperature (SST) fields. The heat flux is produced with an iterative technique so as to maintain the model close to the observed climatology, but with only a weak damping to that climatology. Climatological estimates of evaporation are combined with various estimates of precipitation to determine the net surface freshwater flux. Results indicate that increased freshwater input decreases salinity as expected, but increases temperatures in the upper ocean. Using the freshwater flux estimated from the Microwave Sounding Unit leads to a warming of up to 0.6?°C in the western Pacific over?a case with zero net freshwater flux. SST is sensitive to the discrepancies among different precipitation observations, with root-mean-square differences in SST on the order of 0.2–0.3?°C. The change in SST is more pronounced in the eastern Pacific, with difference of over 1?°C found among the various precipitation products. Interannual variation in precipitation during El Niño events leads to increased warming. During the winter of 1982–83, freshwater flux accounts for about 0.4?°C (approximately 10–15% of the maximum warming) of the surface warming in the central-eastern Pacific. Thus, the error of SST caused by the discrepancies in precipitation products is more than half of the SST anomaly produced by the interannual variability of observed precipitation. Further experiments, in which freshwater flux anomalies are imposed in the western, central, and eastern Pacific, show that the influence of net freshwater flux is also spatially dependent. The imposition of freshwater flux in the far western Pacific leads to a trapping of salinity anomalies to the surface layers near the equator. An identical flux imposed in the central Pacific produces deeper and off-equatorial salinity anomalies. The contrast between these two simulations is consistent with other simulations of the western Pacific barrier layer formation.     

Two anomalous convective systems in the tropical western Pacific and their influences on the East Asian summer monsoon

根据经验正交函数(EOF)分解向外长波辐射资料,发现在热带西太平洋存在两个异常对流系统。除经典的菲律宾对流系统外(PC),在密克罗尼西亚群岛附近还存在另一个对流系统(MC)。MC是EOF分解的第一分量,解释的方差要大于PC。两个对流系统均受到热带海表温度异常的调制,其中PC与El Ni駉有关,而MC则与夏季中东太平洋海表温度异常有关。两个对流系统都能对东亚夏季风产生影响。一般而言,对流增强(减弱)时,西太平洋副高东退(西伸),强度减弱(增强)。PC减弱(增强)时,长江流域到日本的夏季风降水增加(减少)。与PC相比,MC对东亚大陆降水的影响较弱。此外,对流减弱的影响要大于对流增强的影响。     

Salinity minimum in the tropical Pacific

It is shown that the salinity minimum at subsurface depths of the tropical Pacific is a local phenomenon. Characteristics of the salinity minimum are relative by absolute values and variable in time. It appears and disappears in the intertropical convergence zone according to variability of the freshwater budget sign. The salinity minimum appears during the negative phase of the freshwater budget on the background of the previous freshening of the sea surface. The salinity minimum at intermediate depths in both hemispheres is a single phenomenon of climatic time scale. At present, it exists at intermediate layers in the arid zones of both hemispheres due to the negative phase of the freshwater budget. This minimum is related to the Earth climate system variability at the geological time scale. Differences in its properties in the Northern and Southern hemispheres reflect differences in the freshwater budget values and the duration of their influence at the geological time scale.     

Long-range transport of carbon monoxide from tropical ground to upper troposphere: a case study for South East Asia in October 1997

High concentrations of carbon monoxide (CO) were observed in October 1997 at the upper troposphere of the western tropical Pacific. Transport from the potential sources of CO due to biomass burnings in the tropics was investigated by using a global chemical transport model (CTM) driven by assimilated meteorological data provided from European Centre for Medium-Range Weather Forecasts (ECMWF). A CTM evaluation simulation using water vapor showed that the amount of vertical transport of moisture by large-scale flow was consistent with the precipitation predicted at the convective zone. A series of CTM simulations using 10-day emission periods of an artificial material with lifetime of 60 days indicated that vertical lifting of surface air at the Indonesian archipelago occurred in the concentrated convections west of Sumatra Island. No evidence was found that CO from the Amazon region or Africa significantly contributed to high concentrations in the western tropical Pacific. Transport formed a large-scale anvil below the tropopause by rapid vertical transport and by divergence flow. The average time required for the transport from Kalimantan and Sumatra Island to the point of high CO concentration was about 15 days. High concentrations at an altitude of 10 km in the Southern Hemisphere were transported by large-scale subsidence from the upper tropospheric maximum, which was presumably produced from the sources at Kalimantan and Sumatra Island. Estimated emissions of CO in September and October from Kalimantan and Sumatra were substantially larger than the previous estimates. Omission of chemical reaction was a possible problem for the overestimate, but not significant. The possible problems in the transport were incorrect CTM transport due to insufficient horizontal (2.5×2.5°) and vertical resolution of the CTM, and to inaccuracy in the wind fields at the upper part of the troposphere and a divergent flow pattern in the upper part of the troposphere.     

Further evidences of deep convective vertical transport of water vapor through the tropopause

A few years ago, we identified a deep convective transport mechanism, of water vapor through the tropopause, namely, storm top gravity wave breaking, such that tropospheric water substance can be injected into the lower stratosphere via this pathway. The main evidence presented previously was taken from the lower resolution AVHRR images of the storm anvil top cirrus plumes obtained by polar orbiting satellites. Recent observations have provided further supporting evidence for this important cross-tropopause transport mechanism. There are now many higher resolution satellite images, mainly from MODIS instrument, that show more definitely the existence of these plumes, many of which would probably be unseen by lower resolution images.Furthermore, a thunderstorm movie taken in Denver (USA) area during STEPS2000 field campaign and another thunderstorm movie taken by a building top webcam in Zurich also demonstrate that the jumping cirrus phenomenon, first identified by T. Fujita in 1980s, may be quite common in active thunderstorm cells, quite contrary to previous belief that it is rare. We have used a cloud model to demonstrate that the jumping cirrus is exactly the gravity wave breaking phenomenon that transports water vapor through the tropopause.These additional evidences provide increasing support that deep convection contributes substantially to the troposphere-to-stratosphere transport of water substance. This corroborates well with recent studies of the stratospheric HDO/H₂O ratio which is much highly than it would be if the transport is via slow ascent. The only explanation that can be used to interpret this observation at present is that water substance is transported through the tropopause via rapid vertical motion, i.e., deep convection.     

Decadal change in relationship between western North Pacific tropical cyclone frequency and the tropical Pacific SST

In this study, we examine the relationship between the number of tropical cyclones (TCs) in the western North Pacific and the tropical Pacific sea surface temperature (SST) during the main TC season (July–November) for the period of 1965–2006. Results show that there are periods when TC frequency and the tropical Pacific SST are well correlated and periods when the relationship breaks down. Therefore, decadal variation is readily apparent in the relationship between the TC frequency and the SST variations in the tropical Pacific. We further examine the oceanic and atmospheric states in the two periods (i.e., 1979–1989 vs. 1990–2000) when the marked contrast in the correlation between the TC frequency and the tropical Pacific SST is observed. Before 1990, the analysis indicates that oceanic conditions largely influenced anomalous TC frequency, whereas atmospheric conditions had little impact. After 1990, there the reverse appears to be the case, i.e., atmospheric conditions drive anomalous TC frequency and oceanic conditions are relatively unimportant. A role of atmosphere and ocean in relation to the TC development in the western North Pacific changes, which is consistent with the change of the correlations between the TC frequency and the tropical Pacific SST.