The importance of ship log data: reconstructing North Atlantic, European and Mediterranean sea level pressure fields back to 1750

Local to regional climate anomalies are to a large extent determined by the state of the atmospheric circulation. The knowledge of large-scale sea level pressure (SLP) variations in former times is therefore crucial when addressing past climate changes across Europe and the Mediterranean. However, currently available SLP reconstructions lack data from the ocean, particularly in the pre-1850 period. Here we present a new statistically-derived 5° × 5° resolved gridded seasonal SLP dataset covering the eastern North Atlantic, Europe and the Mediterranean area (40°W–50°E; 20°N–70°N) back to 1750 using terrestrial instrumental pressure series and marine wind information from ship logbooks. For the period 1750–1850, the new SLP reconstruction provides a more accurate representation of the strength of the winter westerlies as well as the location and variability of the Azores High than currently available multiproxy pressure field reconstructions. These findings strongly support the potential of ship logbooks as an important source to determine past circulation variations especially for the pre-1850 period. This new dataset can be further used for dynamical studies relating large-scale atmospheric circulation to temperature and precipitation variability over the Mediterranean and Eurasia, for the comparison with outputs from GCMs as well as for detection and attribution studies.     

Interannual horizontal heat advection in the surface mixed layer over the equatorial Pacific Ocean: assimilation versus TAO analysis

This study is motivated by an interest in obtaining a new automated classification scheme of daily circulation types suitable for use throughout Europe. The classification scheme is performed on 500 hPa geopotential height anomalies (NCEP Reanalysis data, 2.5°×2.5°). Nine grid points represent the study area. Five anticyclonic types (Anw, Ane, A, Asw and Ase) and seven cyclonic types (C, Cnnw, Cwnw, Cwsw, Cssw, Cse, Cne) are defined. Each of the circulation types has a distinct underlying synoptic pattern that produces the expected type and direction of flow over the study area. The classification scheme is applied to three different case studies in the Mediterranean Basin: Greece, Cyprus and central Italy. The precipitation percentage of the cyclonic type and the mean seasonal correlation coefficients for all circulation types are the two criteria used to evaluate the performance of the classification scheme. The ability of the HadAM3P general circulation model to reproduce the mean pattern and frequency of circulation types at the 500 hPa level in comparison to the NCEP dataset for the period 1960–1990 is also evaluated. The percentage of rainfall that corresponds to the cyclonic circulation types is greater than 85% for the three study regions. Furthermore, the correlation coefficients for the three classifications are very encouraging, for nearly all days of the study period. Compared to observations, the GCM is able to capture the mean patterns but not able to replicate exactly the observed variability of the circulation types over the three study regions.     

Boreal winter rainfall anomaly over the tropical indo-pacific and its effect on northern hemisphere atmospheric circulation in CMIP5 models

Experimental outputs of 11 Atmospheric Model Intercomparison Project （AMIP） models from phase 5 of the Coupled Model Intercomparison Project （CMIP5） are analyzed to assess the atmospheric circulation anomaly over Northern Hemisphere induced by the anomalous rainfall over tropical Pacific and Indian Ocean during boreal winter.The analysis shows that the main features of the interannual variation of tropical rainfall anomalies,especially over the Central Pacific （CP） （5°S-5°N,175°E-135°W） and Indo-western Pacific （IWP） （20°S-20°N,110°-150°E） are well captured in all the CMIP5/AMIP models.For the IWP and western Indian Ocean （WIO） （10°S-10°N,45°-75°E）,the anomalous rainfall is weaker in the 11 CMIP5/AMIP models than in the observation.During El Ni（n）o/La Ni（n）a mature phases in boreal winter,consistent with observations,there are geopotential height anomalies known as the Pacific North American （PNA） pattern and Indo-western Pacific and East Asia （IWPEA） pattern in the upper troposphere,and the northwestern Pacific anticyclone （cyclone） （NWPA） in the lower troposphere in the models.Comparison between the models and observations shows that the ability to simulate the PNA and NWPA pattern depends on the ability to simulate the anomalous rainfall over the CP,while the ability to simulate the IWPEA pattern is related to the ability to simulate the rainfall anomaly in the IWP and WIO,as the SST anomaly is same in AMIP experiments.It is found that the tropical rainfall anomaly is important in modeling the impact of the tropical Indo-Pacific Ocean on the extratropical atmospheric circulation anomaly.     

Main types of synoptic processes and circulation types generating heavy precipitation events in Armenia

The results of synoptic analysis and objective Lamb circulation classification scheme (LAMB) during the days with heavy precipitation events in Armenia are presented. An analysis of synoptic situations in the area of Armenia and Southern Caucasus for the researched period 2001–2009 has shown that there are seven types of synoptic process characteristic of days with heavy precipitation events in Armenia. Adoption of the LAMB made it possible to identify the typical circulation types for each type of the synoptic process singled out. The results suggest that the LAMB is able to recognize the typical pattern of the distribution of sea level pressure field for each type of the synoptic processes. However, the use of the LAMB is further complicated in Southern Caucasus which can be explained by the existence of regional peculiarities of atmospheric circulation associated with significant influence of the Caucasian ridge. The LAMB may be recommended as an extra tool for synoptic analysis as well as for developing of synoptic climatology and statistical downscaling methods for Armenia and Southern Caucasus.     

A Coupled General Circulation Model for the Tropical Pacific Ocean and Global Atmosphere

On the basis of Zeng’s theoretical design, a coupled general circulation model (CGCM) is developed with its characteristics different from other CGCMs such as the unified vertical coordinates and subtraction of the standard stratification for both atmosphere and ocean, available energy consideration, and so on. The oceanic component is a free surface tropical Pacific Ocean GCM between 30oN and 30oS with horizontal grid spacing of 1o in latitude and 2o in longitude, and with 14 vertical layers. The atmospheric component it a global GCM with low-resolution of 4o in latitude and 5o in longitude, and two layers or equal man in the vertical between the surface and 200 hPa. The atmospheric GCM includes comprehensive physical processes. The coupled model is subjected to seasonally-varying cycle. Several coupling experiments, ranging from straight forward coupling without flux correction to one with flux correction, and to so-called predictor-corrector monthly coupling (PCMC), are conducted to show the existence and final controlling of the climate drift in the coupled system. After removing the climate drift with the PCMC scheme, the coupled model is integrated for more than twenty years. The results show reasonable simulations of the annual mean and its seasonal cycle of the atmospheric and oceanic circulation. The model also produces the coherent interannual variations of the climate system, manifesting the observed El Ni?o / Southern Oscillation (ENSO).     

广西区域霾过程高低空大气环流型影响分析

利用1980—2015年NCEP/NCAR逐日再分析资料和广西80个地面气象站资料,采用Lamb-Jenkinson方法对广西区域霾过程的大气环流进行分型,并探讨高低空环流型配置与区域霾过程的关系。结果表明:当地面东南风型、高空西风型时,广西出现霾过程的频率最高;地面东风型配合高空西风型对年霾过程的贡献最大,地面东南风型配合高空西风型主要出现在冷暖气流转换的春季、地面低压型配合高空高压脊型出现在西太平洋副热带高压明显加强、地面处于弱低压环境场的夏季、地面东风型配合高空西风型出现在环流稳定平直的秋季、地面东风型配合高空高压脊型出现在环流经向度明显增强的冬季,霾过程具有冬季多发、夏季少发的特点。近36年来,地面东风型配合高空高压脊型是广西霾过程呈偏多趋势的主导环流型。     

近百年东亚冬季气温及其大气环流变化型态

利用最新20世纪近百年再分析气象资料,研究近百年东亚冬季气温变化型及其相关的大气环流型态.结果表明近百年内东亚冬季气温主要有两种变化型:第一是东亚西南与东北相反气温变化型,表现在40°N以南及105°E以西地区(西南地区)气温变化与40°N以北及105°E以东地区(东北地区)变化相反;第二是40°N以南气温一致变化型.与第一种气温变化型耦合的大气模态是500hPa欧亚型遥相关、西伯利亚高压及北大西洋涛动.当欧亚型遥相关负位相,北大西洋涛动正位相及西伯利亚高压减弱时,有利于蒙古和我国105° E以东的区域增温而我国西南地区和青藏高原降温,反之亦然.第二种气温变化型耦合大气模态是500hPa西太平洋型遥相关,北太平洋涛动.当西太平洋型遥相关及北太平洋涛动处于正位相时(北太平洋北负南正),东亚40°N以南地区增温,东亚40°N以北地区降温.耦合的大气模态的型态差异,影响各阶段气温的年际变化.近一百年中,欧亚型遥相关和北大西洋涛动在1984～2010期间的型态最显著,是20世纪80年代东亚显著增暖的原因之一.研究还发现20世纪中期后东亚气温的年际变化与极地环流的变化联系紧密,表现在西伯利亚高压范围东扩并与极地环流联系,也是近百年气温趋势上升的一个原因.     

Circulation over Bulgaria and its connection with NAO indices and Sea Surface Temperatures

Trends in atmospheric pressure, circulation and some relationships between North Atlantic Oscillation (NAO) indices, sea surface temperatures, and atmospheric circulation over Bulgaria are discussed in this article. Data for measured atmospheric pressure at stations Burgas, Pleven, and Sandanski are used. Information about atmospheric circulation over Bulgaria was obtained using sea level pressure and 700 hPa Omega (vertical velocity) reanalysis daily data for grid cells covering the territory of Bulgaria for the period 1948–2010. Zonal and meridional indices for Bulgaria were also calculated based on the data for sea level pressure. NAO index calculated by NOAA and NCAR is correlated with atmospheric pressure and circulation. A total of 12 areas in three major water basins influencing Bulgarian climate—North Atlantic, Mediterranean, and Black Seas—were studied. Main methods employed in the article are statistical—trend analysis, multiple linear regression, correlation, nonparametric tests, etc. There is no change in the mean values of atmospheric pressure over Bulgaria. Circulation over Bulgaria during the research period increases its anticyclonal patterns mainly due to the decrease of the number of cyclones. Dynamics in zonal and meridional indices for Bulgaria result in an increase of the northwest transport in the winter and an increase of the northeast transport in the summer. Cyclones over Bulgaria determine the values of atmospheric pressure. Influence of the NAO on atmospheric pressure and circulation is stronger in winter. Atmospheric processes, expressed by the number of cyclones and anticyclones, are most active in spring. Current trends are towards increasing of sea surface temperatures (SSTs) at all investigated places. Temporally, the effect of SSTs on the number of cyclones, anticyclones, zonal and meridional indices for Bulgaria during the different seasons comes with a delay of 1 to 3 months. Constructed multiple linear regression (MLR) models with predictors SSTs adequately describe the atmospheric circulation over Bulgaria. There is a clear pattern of SSTs distribution, which leads to a higher number of cyclones over Bulgaria in winter—lower than normal temperatures in the Aegean Sea and higher than normal in the Black Sea. A decrease in the difference of temperatures between the Gulf Stream and western colder parts leads to higher values of winter zonal transport over Bulgaria. Higher than normal temperatures in Black Sea lead to a higher number of cyclones in spring. Higher difference in temperatures of the North Atlantic leads to a stronger cyclogenesis and enhanced zonal transport, which affects autumn circulation over Bulgaria.     

Winter AO/NAO modifies summer ocean heat content and monsoonal circulation over the western Indian Ocean

This paper analyzes the possible influence of boreal winter Arctic Oscillation/North Atlantic Oscillation (AO/ NAO) on the Indian Ocean upper ocean heat content in summer as well as the summer monsoonal circulation. The strong interannual co-variation between winter 1000-hPa geopotential height in the Northern Hemisphere and summer ocean heat content in the uppermost 120 m over the tropical Indian Ocean was investigated by a singular decomposition analysis for the period 1979–2014. The second paired-modes explain 23.8% of the squared covariance, and reveal an AO/NAO pattern over the North Atlantic and a warming upper ocean in the western tropical Indian Ocean. The positive upper ocean heat content enhances evaporation and convection, and results in an anomalous meridional circulation with ascending motion over 5°S–5°N and descending over 15°–25°N. Correspondingly, in the lower troposphere, significantly anomalous northerly winds appear over the western Indian Ocean north of the equator, implying a weaker summer monsoon circulation. The off-equator oceanic Rossby wave plays a key role in linking the AO/NAO and the summer heat content anomalies. In boreal winter, a positive AO/NAO triggers a down-welling Rossby wave in the central tropical Indian Ocean through the atmospheric teleconnection. As the Rossby wave arrives in the western Indian Ocean in summer, it results in anomalous upper ocean heating near the equator mainly through the meridional advection. The AO/NAO-forced Rossby wave and the resultant upper ocean warming are well reproduced by an ocean circulation model. The winter AO/NAO could be a potential season-lead driver of the summer atmospheric circulation over the northwestern Indian Ocean.     

Current and future atmospheric circulation at 500 hPa over Greenland simulated by the CMIP3 and CMIP5 global models

The Greenland ice sheet is projected to be strongly affected by global warming. These projections are either issued from downscaling methods (such as Regional Climate Models) or they come directly from General Circulation Models (GCMs). In this context, it is necessary to evaluate the accuracy of the daily atmospheric circulation simulated by the GCMs, since it is used as forcing for downscaling methods. Thus, we use an automatic circulation type classification based on two indices (Euclidean distance and Spearman rank correlation using the daily 500 hPa geopotential height) to evaluate the ability of the GCMs from both CMIP3 and CMIP5 databases to simulate the main circulation types over Greenland during summer. For each circulation type, the GCMs are compared to three reanalysis datasets on the basis of their frequency and persistence differences. For the current climate (1961–1990), we show that most of the GCMs do not reproduce the expected frequency and the persistence of the circulation types and that they simulate poorly the observed daily variability of the general circulation. Only a few GCMs can be used as reliable forcings for downscaling methods over Greenland. Finally, when applying the same approach to the future projections of the GCMs, no significant change in the atmospheric circulation over Greenland is detected, besides a generalised increase of the geopotential height due to a uniform warming of the atmosphere.     

An assessment of the Jenkinson and Collison synoptic classification to a continental mid-latitude location

A weather-type catalogue based on the Jenkinson and Collison method was developed for an area in south-west Russia for the period 1961–2010. Gridded sea level pressure data was obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The resulting catalogue was analysed for frequency of individual types and groups of weather types to characterise long-term atmospheric circulation in this region. Overall, the most frequent type is anticyclonic (A) (23.3 %) followed by cyclonic (C) (11.9 %); however, there are some key seasonal patterns with westerly circulation being significantly more common in winter than summer. The utility of this synoptic classification is evaluated by modelling daily rainfall amounts. A low level of error is found using a simple model based on the prevailing weather type. Finally, characteristics of the circulation classification are compared to those for the original JC British Isles catalogue and a much more equal distribution of flow types is seen in the former classification.     

How well are daily intense rainfall events captured by current climate models over Africa?

The ability of state-of-the-art climate models to capture the mean spatial and temporal characteristics of daily intense rainfall events over Africa is evaluated by analyzing regional climate model (RCM) simulations at 90- and 30-km along with output from four atmospheric general circulation models (AGCMs) and coupled atmosphere–ocean general circulation models (AOGCMs) of the Climate Model Intercomparison Project 5. Daily intense rainfall events are extracted at grid point scale using a 95th percentile threshold approach applied to all rainy days (i.e., daily rainfall ≥1 mm day^?1) over the 1998–2008 period for which two satellite-derived precipitation products are available. Both RCM simulations provide similar results. They accurately capture the spatial and temporal characteristics of intense events, while they tend to overestimate their number and underestimate their intensity. The skill of AGCMs and AOGCMs is generally similar over the African continent and similar to previous global climate model generations. The majority of the AGCMs and AOGCMs greatly overestimate the frequency of intense events, particularly in the tropics, generally fail at simulating the observed intensity, and systematically overestimate their spatial coverage. The RCM performs at least as well as the most accurate global climate model, demonstrating a clear added value to general circulation model simulations and the usefulness of regional modeling for investigating the physics leading to intense events and their change under global warming.     

Comparison of principal component and cluster analysis for classifying circulation pattern sequences for the European domain

The study examines two methods for classification of daily pressure-pattern sequences (extended PCA and extended cluster analysis) and evaluates whether sequence classification is more suitable to describe surface air-temperature conditions in Europe. For this purpose, sequences of daily sea-level pressure fields are classified for each month of the year using a wide range of class numbers (2–40) and sequence lengths from 1 to 6 days. In each case, the classification is used to reconstruct temperature variability in 5°×5° grid-boxes throughout Europe for the period 1850–2003 in order to compare the circulation type based reconstruction to monthly observations for skill estimation. Results show that extended PCA leads to more dynamic pattern sequences for subordinated classes, but also that it is limited in skill, because classes of very low frequencies are generated. In contrast, extended cluster analysis offers higher skill levels for temperature downscaling. However, both methods can benefit from using sequences instead of single days for classification, especially for continental regions.     

西江流域面雨量与区域大气环流型关系

利用Lamb-Jenkinson大气环流分型方法,对西江流域1971—2015年逐日平均850 hPa和500 hPa高度场进行环流客观分型,分析流域降水天气环流型出现概率及主导环流型变化特征,探讨主导环流型对西江流域总面雨量和子流域面雨量的贡献率及环流型配置与降水的关系。结果表明:当850 hPa为西南风型、500 hPa为西风型时,流域出现降水天气的概率最大;850 hPa气旋型和500 hPa西风型对年总面雨量和各子流域面雨量的贡献率均为最大,且对东部子流域面雨量的贡献率大于西部子流域,850 hPa南风型与500 hPa反气旋型的环流配置是西部子流域秋季降水偏多的主导环流型配置;春季850 hPa气旋型与500 hPa西风型、夏季850 hPa气旋型与500 hPa西风型、秋季850 hPa南风型与500 hPa反气旋型、冬季850 hPa西南风型与500 hPa西风型的环流配置时,出现强降水天气的概率分别为18.7%,21.1%,4.0%和2.0%,即夏季最大,其次为春季,冬季最小。近45年,850 hPa气旋型、500 hPa西风型对流域年总面雨量的贡献率呈增加趋势,是西江流域面雨量呈偏多趋势的主导环流型。     

Numerical simulation by a stretched-coordinate Ocean General Circulation Model: A preliminary results

Summary ?A three-dimensional Ocean General Circulation Model has been developed in stretched coordinate from scratch. The same model has been used to perform some numerical experiments to simulate the basic circulation pattern and the model variability to atmospheric forcing. For numerical simulations 72 × 25 grid points in the horizontal directions and nine (10, 30, 75, 250, 500, 1000, 1500, 2000 and 3000 m) vertical levels are considered. The lateral boundaries are set at 60° N and 60° S. The basic focus of the paper is on the demonstration of the performance of the model and its assessment by employing appropriate forcing from the outputs of an atmospheric general circulation model. Hence, the model was forced with the forcing (wind and thermodynamic) derived from the ECMWF runs from the AMIP archives. The preliminary results show the realistic simulation of basic pattern of different fields. The model simulations show that the model is able to reproduce some of the general features of the ocean, such as surface currents, surface temperature and salinity, mass transport and meridional heat transport. It is also to be noted that the model is capable to capture the El-Ni?o and La-Ni?a type events. Received April 3, 2002; revised June 6, 2002; accepted July 24, 2002 Published online: February 20, 2003     

High latitude river runoff in a doubled CO2 climate

A global atmospheric model is used to calculate the monthly river flow for nine of the world's major high latitude rivers for the present climate and for a doubled CO₂ climate. The model has a horizontal resolution of 4° × 5°, but the model's runoff from each grid box is quartered and added to the appropriate river drainage basin on a 2° × 2.5° resolution. A routing scheme is used to move runoff from a grid box to its neighboring downstream grid box and ultimately to the mouth of the river. In a model simulation in which atmospheric carbon dioxide is doubled, mean annual precipitation and river flow increase for all of these rivers, increased outflow at the river mouths begins earlier in the spring, and the maximum outflow occurs approximately one month sooner due to an earlier snow melt season. In the doubled CO₂ climate, snow mass decreases for the Yukon and Mackenzie rivers in North America and for rivers in northwestern Asia, but snow mass increases for rivers in northeastern Asia.     

2009年初冬沪宁高速公路大雾过程成因分析

利用沪宁高速公路自动气象监测站(AMW)数据,结合NCEP/NCAR的1°×1°格点再分析资料,对2009年12月1—2日发生在沪宁高速公路上的大雾天气过程的能见度演变特征及其与相关气象要素的关系进行了分析,并对大雾形成机理进行了研究。结果表明:能见度与各气象要素之间均呈非线性关系,与相对湿度呈稳定的指数衰减关系,与温度及风速呈复杂的多项式关系;高空暖性高压脊和地面变性冷高压的高低空环流配置为雾的形成提供了逆温层结和近地面的弱风场条件;偏东气流和逆温层保证了水汽供应及在低层汇聚;雾区上空的热力因子和动力因子的分析证明了雾区大气层结状态的稳定性。     

THE DOUBLE-LAYER STRUCTURE OF THE HADLEY CIRCULATION AND ITS INTERDECADAL EVOLUTION CHARACTERISTICS

Based on the three-pattern decomposition of global atmospheric circulation(TPDGAC), this study investigates the double-layer structure of the Hadley circulation(HC) and its interdecadal evolution characteristics by using monthly horizontal wind field from NCEP/NCAR reanalysis data from 1948—2011. The following major conclusions are drawn: First, the double-layer structure of the HC is an objective fact, and it constantly exists in April,May, June, October and November in the Southern Hemisphere. Second, the double-layer structure is more obvious in the Southern than in the Northern Hemisphere. Since the double-layer structure is sloped in the vertical direction, it should be taken into consideration when analyzing the variations of the strength and location of the center of the HC.Third, the strength of the double-layer structure of the HC in the Southern Hemisphere consistently exhibits decadal variations with a strong, weak and strong pattern in all five months(April, May, June, October, and November), with cycles of 20-30 a and 40-60 a. Fourth, the center of the HC(mean position of the double-layer structure) in the Southern Hemisphere consistently and remarkably shifts southward in all the five months. The net poleward shifts over the 64 years are 5.18°, 2.11°, 2.50°, 1.79° and 5.76° for the five respective months, with a mean shift of 3.47°.     

Changes in atmospheric circulation over Europe detected by objective and subjective methods

Summary Changes in atmospheric circulation over Europe since 1958 were examined using both objective (modes of low-frequency variability and objective classification of circulation types) and subjective (Hess-Brezowsky classification of weather types) methods. The analysis was performed with an emphasis on the differences between the winter (DJF) and summer (JJA) seasons, and between objectively and subjectively based results. Majority of the most important changes in atmospheric circulation are same or similar for the objective and subjective methods: they include the strengthening of the zonal flow in winter since the 1960s to the early 1990s; the increase (decrease) in frequency of anticyclonic (cyclonic) types in winter from the late 1960s to the early 1990s, with a subsequent decline (rise); and the sharp increase in the persistence (measured by the mean residence time) of all groups of circulation types in winter around 1990 and of anticyclonic types in summer during the 1990s. Differences between the findings obtained using the objective and subjective methods may result from the intrinsically different approach to the classification (e.g. the Hess-Brezowsky weather types have a typical duration of at least 3 days while objective types typically last 1–3 days). Generally, changes in atmospheric circulation which have taken place since the 1960s were more pronounced in winter than in summer. The most conspicuous change seems to be the considerable increase in the persistence of circulation types during the 1990s, which may be also reflected in the increase in the occurrence of climatic extremes observed in Europe during recent years.     

东北地区降水与大气环流关系

利用Lamb-Jenkinson大气环流分型方法, 将1951—2004年逐日的海平面气压场分型, 得到27种不同的环流型, 研究了大气环流型与我国东北地区降水的关系。给出了8种出现频率最高的主要环流型出现的规律及它们平均的环流形势, 分析了8种主要环流型下东北地区降水异常分布状况, 并选出东北地区资料齐全、有代表性的9个站点进行了深入细致地分析, 成功地建立了降水与环流型出现频率的统计关系, 并用此重建了9个代表站54年的降水序列。结果表明:Lamb-Jenkinson大气环流分型方法可以很好地应用于我国东北地区, 由此划分的环流型符合实际情况; 8种主要环流型与降水的空间分布特征有很好的对应关系; 所建立的9个代表站降水距平回归方程能够解释近54年各站的大部分降水变化, 进一步证明了东北地区降水与大气环流之间的密切关系。