Icebergs in the Southern Ocean and Factors Defining Their Distribution

Using the data of ~58 000 ship observations of Antarctic icebergs in 1947–2014, the map of average summertime concentration of icebergs (namely, of their number within a circle with the radius of 15 nautical miles) in the Southern Ocean was constructed. The main features of the iceberg distribution are revealed, and their possible reasons are investigated. It is shown that in the open ocean sea currents play a key role in the iceberg distribution. Wind effects are pronounced when ocean currents are weak or absent. According to the authors’ estimates, wind plays a decisive role only in the formation of one wide quasimeridional tongue of high iceberg concentration in the Weddell Sea. It is difficult to assess the impact of Antarctic glaciers’ productivity on the iceberg distribution, because currents, wind, and breaking and jamming of icebergs in shallow water areas cause their rapid redistribution. The clear physical explanation of the main features of iceberg concentration distribution on the constructed map indicates that it provides a rather real pattern.     

The near‐surface circulation and exchange in the newfoundland grand banks region

Abstract

Analysis of 39 satellite‐tracked drifter records from the Newfoundland Grand Banks region has allowed maps of the mean and variable flows to be drawn. The variable currents are particularly large relative to the mean for the shelf, Flemish Cap and in the Newfoundland Basin. The ratio of the mean to variable flow is largest along the path of the Labrador Current. Drifters that either have been released on or migrate onto the Grand Banks remain therefor an average of 71 d. A statistical study of the effect of wind on drifter motion has shown that winds can only account for about 10% of current variability. This result is examined with consideration given to data noise, aliasing and non‐stationary conditions. Some drifters that were deployed in the Labrador Current moved onto the shelf and vice versa. These observations have been used to estimate the rate of exchange between the Current and the Grand Banks. Using this exchange rate in a box model, it is calculated that, over the iceberg season, 30% of the bergs will be in the Avalon Channel, 20% on the Grand Banks and 50% in the Labrador Current, in good agreement with the observed distribution. An alternative model based solely on advection is considered as well. The exchange model is also applied to the salinity budget for the Labrador Current with some success.     

New Zealand climate: The temperature record,historical data and some agricultural implications

Evidence is presented that suggests that annual and seasonal temperatures are in phase throughout the New Zealand region for climatic variation. Annual and seasonal temperature curves dating back to 1853 based on the longest instrumental records show a warming of over 1 °C between the early 1860's and the present day, with a 0.5 °C discontinuity since the mid 1940's. Although the warming is regional, the magnitude and timing of the fluctuations vary because of the country's high relief, and wide latitude range. These factors give differing exposure to the atmosphere and oceanic circulations, the resultant interaction of which allows the country to be divided into six temperature response areas which are homogeneous for fluctuations. The information of the last fifty years in these areas is compared. The story from the longer record is supported by phenomena documented from historical sources such as glacial variations, iceberg sightings, and severe snowstorm occurrences. The length and intensity of the growing season is examined in two climatically different areas of New Zealand to ascertain the agricultural significance of the described climate record.     

Modeling Iceberg Drift in the Barents Sea from Field Data

The iceberg drift model is developed and used for simulating the iceberg drift trajectory in the Barents Sea. The model is forced by hydrometeorological characteristics obtained from ship observations. Original techniques for retrieving the sea-level slope gradient and surface velocity of currents are proposed, implemented, and validated using independent data. Thus, additional data were calculated from field data in order to use the iceberg drift model with the full set of external forces. This allowed improving the iceberg trajectory simulation and assessing the contribution of all forces that affect the iceberg drift. The iceberg drift calculations demonstrate that the drift characteristics are extremely sensitive to all external effects and the model parameters; therefore, the quality of input hydrometeorological data essentially affects the simulation of real iceberg trajectories.     

A role for icebergs in the 8.2 ka climate event

We investigate the potential role of icebergs in the 8.2 ka climate event, using a coupled climate model equipped with an iceberg component. First, we evaluate the effect of a large iceberg discharge originating from the decaying Laurentide ice sheet on ocean circulation, compared to a release of an identical volume of freshwater alone. Our results show that, on top of the freshwater effect, a large iceberg discharge facilitates sea-ice growth as a result of lower sea-surface temperatures induced by latent heat of melting. This causes an 8% increased sea-ice cover, 5% stronger reduction in North Atlantic Deep Water production and 1°C lower temperature in Greenland. Second, we use the model to investigate the effect of a hypothetical two-stage lake drainage, which is suggested by several investigators to have triggered the 8.2 ka climate event. To account for the final collapse of the ice-dam holding the Laurentide Lakes we accompany the secondary freshwater pulse in one scenario with a fast 5-year iceberg discharge and in a second scenario with a slow 100-year iceberg discharge. Our experiments show that a two-stage lake drainage accompanied by the collapsing ice-dam could explain the anomalies observed around the 8.2 ka climate event in various climate records. In addition, they advocate a potential role for icebergs in the 8.2 ka climate event and illustrate the importance of latent heat of melting in the simulation of climate events that involve icebergs. Our two-stage lake drainage experiments provide a framework in the discussion of two-stage lake drainage and ice sheet collapse.     

On the deterioration of icebergs in the marginal ice zone

Abstract

During the Labrador Ice Margin Experiment (LIMEX) of March‐April 1989, the International Ice Patrol (IIP) of the United States Coast Guard deployed two satellite‐tracked TIROS Arctic Drifter (TAD) platforms on two medium‐sized tabular icebergs. The icebergs were drifting in sea ice of about 9/10 concentration east of Newfoundland. These deployments were part of an experiment to examine differential sea‐ice/iceberg motion during spring conditions near the ice margin. Sea‐ice concentration and movement data were collected concurrently by other LIMEX investigators.

The TADs, deployed on 11 March 1989, were tracked using the ARGOS data collection and location system carried on two NOAA polar‐orbiting satellites of the TIROS family. For two months following the deployment, IIP periodically attempted to relocate the icebergs during routine aerial iceberg patrols. One of the TADs stopped transmitting on 23 April 1989 probably because of a major calving event that resulted in the TAD being crushed. As of 24 April the drift rate of the other TAD nearly doubled compared with its drift rate prior to that date, indicating that it had fallen off the iceberg and was floating on water. By 24 April there was no sea ice near either of the two icebergs.

The TAD data provide a unique datasetfor modelling the deterioration of icebergs while they emerge out of the marginal ice zone and travel in open water. It is shown that a good knowledge of the environmental conditions, pariicularly water temperature and sea state, are critical to model successfully the deterioration and calving of the two icebergs.     

Small icebergs and iceberg fragments off Newfoundland: Relationships to deterioration mechanisms and the regional iceberg population

Abstract

Several sets of previously compiled data on iceberg size distributions in areas between North America and Greenland are compared and analyzed. The obtained results indicate that populations of icebergs with lengths L ≥ 20 m are well‐described by log‐normal and gamma distributions together with statistical parameters which are compatible with existing understanding of regional iceberg deterioration and change processes. A lesser amount of data on size distributions in Newfoundland icebergs with L < 20 m show occurrence probabilities which increase exponentially with decreasing iceberg length. Evidence is presented to show that these data are most consistent with the dominance of fracture processes in determining iceberg occurrence versus length relationships. Physical arguments are presented to suggest that the key fracture events are initiated in the larger icebergs by coincidences of wave‐generated bending stress maxima and randomly distributed structural flaws. Similar considerations and use of a simple sequential fracture model imply that size distributions in icebergs (and iceberg fragments) with L < 20 m are more consistent with the presence of an alternative, spontaneous failure mechanism also acting at randomly distributed structural defects. The implications of these results for forecasting and monitoring populations of small icebergs are discussed.     

Trends and Variability in Sea Ice and Icebergs off the Canadian East Coast

ABSTRACT

Seasonal time series of sea-ice area or extent in several regions along the east coast of Canada were compiled from several sources for the period 1901 to 2013 and compared with an index of ice extent off southwest Greenland, iceberg season length south of 48°N, air temperature, and other climate indices. Trends in winter ice area and iceberg season length are significant over the past 100 years and 30 years. Variability of winter ice area and iceberg season length is associated with a combination of the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) indices superimposed on a negative trend. Thus, large declines in ice area and iceberg season length in the 1920s and 1990s can be attributed to a decreasing NAO index and a shift to the positive phase of the AMO at the end of these decades. Ice extent in southern areas such as the Scotian Shelf is more strongly correlated with the Western Atlantic index than with the NAO. Ice area trends (in percent per decade) are larger in magnitude and account for twice as much of the variance in ice area for summer than for winter, with summer trends significant over 30-, 60- and 100-year periods. Sea-ice variability is generally consistent with air temperature variability in the various regions; in the 1930s, during the early twentieth-century warming period, ice anomalies were higher and temperature anomalies were lower along the coast of eastern Canada than along the coast of southwestern Greenland.     

Simulation of iceberg drift as a component of ice monitoring in the West Arctic

Discussed is the hydrodynamic model of iceberg drift. Presented are the examples ofits use in operational practice for predicting iceberg drift in the Kara Sea and for enhancing ice monitoring in the Barents Sea.     

ENSO事件指数与指标研究

根据 1 951～ 1 996年完整可靠的海面温度 ( SST)和南方涛动指数 ( SOI)资料 ,以中、东太平洋 SST异常变化为指标 ,定义了 ENSO事件 ,并在此基础上提出了以 ENSO事件过程海温距平累积值为指标的强度指数。此外 ,还定义了 ENSO事件的海 -气综合强度指数 ,并对 ENSO事件的长度、强度及爆发类型等进行了分类。     

太平洋海温异常对其上空环流影响的分析及数值试验

本文首先用《逐月全球气候资料》中的实测风资料对80年代的两次El Nino。事件(1982—1983年和1986—1987年)太平洋地区上空环流的变化情况进行了分析研究,然后用一个九层原始方程模式对其进行数值模拟试验. 观测资料分析及数值模拟结果皆表明,赤道东太平洋的这两次海温异常都使其上空的环流发生明显的变化,但由于两次海温异常的强度、范围和延续时间有所不同,对环流的影响也不相同.本文将简要地介绍这两次El Nio期间太平洋上空环流的演变特征及数值试验的结果.     

Differences in Atmospheric Boundary-Layer Characteristics Between Wet and Dry Episodes of the Indian Summer Monsoon

The differences and similarities in atmospheric boundary-layer (ABL) characteristics, in particular the ABL height, evolution and wind field, between two contrasting episodes of the Indian summer monsoon have been studied using measurements from wind profilers and an instrumented 50-m tower at Gadanki in India. The observed differences are discussed in light of various forcing mechanisms, in particular the effect of soil moisture on the surface energy balance and ABL. The differences in ABL height, its evolution and the wind field between episodes are quite pronounced. Wet episodes not only have a shallower ABL but also the growth is delayed by 1–4 h when compared with that for dry episodes. Abundant soil moisture during the wet episodes (a factor of two greater than during the dry episodes) reduces the buoyancy flux, and thereby not only limits the ABL height but also delays the commencement of ABL growth. The low-level jet (LLJ) is stronger during the dry episodes and has a larger diurnal range than during the wet episodes. The highest occurrence and magnitude of LLJ apparent at a height of 1.5 km during early morning hours shift progressively with height and time till the afternoon, following ABL evolution. The weaker LLJ during the wet episodes is attributed to its southward migration from its mean position (15 \(^{\circ }\) N). Larger signal-to-noise ratio and spectral width values are observed during the early night to midnight, compared to noon-time, when the ABL is buoyantly turbulent.     

Lower tropospheric transport over the Southern Ocean

Radon and particle concentrations obtained at Macquarie Island, halfway between Australia and Antarctica, and on Tasmania's north-west coast at Cape Grim during 1987 are examined. Four-day trajectories based on 12 hourly analyses over the Australian region are used to explore the transport of continental material across the Southern Ocean. This study suggests that at least 25% of the variance of radon concentration at Macquarie Island can be accounted for by transport from the Australian continent. Trajectories at intervals of one hour are used to demonstrate the agreement between episodes of trajectories passing over land and episodes of elevated radon and particle concentration. Wind roses of percentage probability of various levels of radon and particle concentration are compared with back trajectories for their accuracy in depicting episodes of continental air incursion.     

High temperature extremes in the Czech Republic 1961–2010 and their synoptic variants

Our research focuses on the analysis of extreme high maximum air temperature events (EXHTEs) in the Czech Republic in the period 1961–2010, their climatological characteristics, and on the identification of synoptic-scale circulation conditions conductive to them. EXHTEs are detected using the Weather Extremity Index (WEI) combining return periods of daily maximum air temperature, duration of events, and the extent of the affected area. We selected 37 EXHTEs as non-overlapping periods with the highest WEI. Some long EXHTEs were divided into several shorter synoptically homogeneous episodes. Using the two-level divisive clustering of 700 hPa air temperature and wind field anomalies, we obtained four main variants of synoptic-scale circulation conditions. The most frequent variant associated with extreme episodes is characterized by a westerly flow connected with a high pressure ridge extending northeastward from North Africa over Central Europe or with an anticyclone centered over the Central Mediterranean. The most extreme episodes occurred during the variant characterized by an easterly flow between a high pressure area to the northeast and a low pressure area to the southeast.     

Investigation of wet acidic deposition episodes capable of damaging red spruce in the Mt. Mitchell State Park

Cloud events whose wet deposition is comparable to that shown to trigger negative health symptoms in trees constitute environmentally significant episodes of wet deposition. They accounted for approximately one fourth of all cloud events that traversed the Mt. Mitchell State Park, NC, during the summer field seasons of 1986–1988 and 1993. The environmentally significant episodes of wet deposition most frequently occurred during the month of June. The June occurrence is equivalent to the frequency of acid mist treatments shown to initiate health related decline in Red Spruce. The slightly more than three seasons of data were analyzed for a possible relationship between cloudwater acidity, liquid water content, wind speed and wind direction. The data analysis shows: (i) A scatter plot of the liquid water flux versus the H⁺ ion flux differentiates between environmentally significant and non-environmentally significant cloud events of wet deposition. (ii) The synoptic and resulting local wind speed and direction are important in determining whether a cloud event will be environmentally significant or not, since they are related to the origin and the approach of the airflow into the Mt. Mitchell site. (iii) A characteristic 850 mb height field associated with the most frequently occurring environmentally significant cloud only event of wet deposition. Further investigation of wet acidic deposition episodes capable of damaging Red Spruce in the Mt. Mitchell State Park     

呼和浩特近59年降水极端事件分析

利用呼和浩特气象站1951—2009年逐日降水量资料,以年序列的第90个百分位,建立了日降水量极端气候事件的阈值,检测了近59年来呼和浩特逐日降水量极端事件的出现频率,分析了极端事件阈值和日数及降水量的年际、年代际和季节变化,结果显示:①呼和浩特日降水极端事件的阈值小,为10.6mm;全年极端事件出现的频次11d。②降水极端事件主要出现在4-10月,且8月最多。③近59年来呼和浩特全年降水极端事件及其降水量没有显著的增减变化趋势,但而进入21世纪后,极端降水事件及其降水量的变率加大,降水强度明显减小。     

Simulating Heinrich event 1 with interactive icebergs

During the last glacial, major abrupt climate events known as Heinrich events left distinct fingerprints of ice rafted detritus, and are thus associated with iceberg armadas; the release of many icebergs into the North Atlantic Ocean. We simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere–ocean model. In our model, dynamic-thermodynamic icebergs influence the climate through two direct effects. First, melting of the icebergs causes freshening of the upper ocean, and second, the latent heat used in the phase-transition of ice to water results in cooling of the iceberg surroundings. This cooling effect of icebergs is generally neglected in models. We investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off. At the peak of the simulated Heinrich event, icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects. The cause of this intriguing result must be sought in the involvement of a secondary mechanism: facilitation of sea-ice formation, which can disturb deep water production at key convection sites, with consequences for the thermohaline circulation. We performed additional sensitivity experiments, designed to explore the effect of the more plausible distribution of the dynamic icebergs’ melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid-latitude North Atlantic (NA) Ocean. The early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments, which must be a distribution-effect: the dynamically distributed icebergs quickly affect western NADW formation, which synergizes with direct sea-ice facilitation, causing an earlier sea-ice expansion and climatic response. Furthermore, compared to dynamic-thermodynamic icebergs, a homogeneous hosing overestimates the fresh water flux in the Eastern Ruddiman belt, causing a fresh anomaly in the Eastern North Atlantic, leading to a delayed recovery of the circulation after the event.     

‘Splendid rains have fallen’: links between El Niño and rainfall variability in the Kalahari, 1840–1900

This study uses a range of published and unpublished historical documentary sources to explore the nature of rainfall variability in the Kalahari Desert and adjacent hardveld regions of central southern Africa during the seventeen Pacific El Niño–Southern Oscillation (ENSO) episodes that occurred between 1840 and 1900. Documentary data are used in two ways. First, maps of relative annual rainfall levels are presented for each of the 12 single-year and five protracted ENSO episodes during the period, in order to identify the associated inter-annual rainfall variations. These suggest that the relationship between ENSO episodes and rainfall variability identified for the twentieth century, whereby warm events are frequently preceded by wetter conditions during the austral summer prior to the event year and succeeded by drought in the following summer, has broadly held for much of the last 160 years. This is despite the long-term fluctuations in precipitation and temperature which are known to have occurred over this period. Droughts are identified following at least thirteen of the 17 single-year and protracted ENSO episodes. Pre-ENSO wetter periods are less common, with only nine of the ENSO episodes preceded by above-normal rainfall. Second, the documentary data are analyzed in detail in order to reveal any evidence for high resolution intra-annual variations in the seasonal distribution of rainfall during ENSO events. Seasonal sequences of rainfall/drought appear to have closely followed contemporary patterns, with heavy rainfall commonly occurring late in the pre-ENSO year or early in the ENSO year(s), and drought at the start of the post-ENSO year. This relationship can be seen to hold most strongly for single-year ENSO warm events and for the first year of protracted events, but rainfall conditions were more variable during the later years of protracted events.     

Assessment of atmosphere-ocean general circulation model simulations of winter northern hemisphere atmospheric blocking

An assessment of six coupled Atmosphere-Ocean General Circulation Models (AOGCMs) is undertaken in order to evaluate their ability in simulating winter atmospheric blocking highs in the northern hemisphere. The poor representation of atmospheric blocking in climate models is a long-standing problem (e.g. D’Andrea et?al. in Clim Dyn 4:385–407, 1998), and despite considerable effort in model development, there is only a moderate improvement in blocking simulation. A modified version of the Tibaldi and Molteni (in Tellus A 42:343–365, 1990) blocking index is applied to daily averaged 500?hPa geopotential fields, from the ERA-40 reanalysis and as simulated by the climate models, during the winter periods from 1957 to 1999. The two preferred regions of blocking development, in the Euro-Atlantic and North Pacific, are relatively well captured by most of the models. However, the prominent error in blocking simulations consists of an underestimation of the total frequency of blocking episodes over both regions. A more detailed analysis revealed that this error was due to an insufficient number of medium spells and long-lasting episodes, and a shift in blocking lifetime distributions towards shorter blocks in the Euro-Atlantic sector. In the Pacific, results are more diverse; the models are equally likely to overestimate or underestimate the frequency at different spell lengths. Blocking spatial signatures are relatively well simulated in the Euro-Atlantic sector, while errors in the intensity and geographical location of the blocks emerge in the Pacific. The impact of models’ systematic errors on blocking simulation has also been analysed. The time-mean atmospheric circulation biases affect the frequency of blocking episodes, and the maximum event duration in the Euro-Atlantic region, while they sometimes cause geographical mislocations in the Pacific sector. The analysis of the systematic error in time-variability has revealed a negative relationship between the high-frequency variability of the transient eddies in the areas affected by blocking and blocking frequency. The blocking responses to errors in the low-frequency variability are different according to the region considered; the amplitude of the low-frequency variability is positively related to the blocking frequency and persistence in the Euro-Atlantic sector, while no such consistency is observed in the Pacific.     

Synoptic and local scale atmospheric circulation associated with air pollution episodes in an urban Mediterranean area

Air pollution episodes in urban coastal areas follow certain pre-determined patterns, being associated with certain local meteorological conditions and emission of primary pollutants. In this study, the synoptic and local scale atmospheric circulation that prevails during air pollution episodes in a coastal major city in Greece, Thessaloniki, is examined for a period of 15 years (1989–2004). The study signifies the importance of studying air pollution meteorological patterns between coastal areas with different terrain characteristics. For Thessaloniki, it was found that the episodes occur mainly during the cold period of the year, while four types of synoptic scale circulation were recognized (I, II, III, IV) and five patterns of the local scale circulation (A1, A2, B1, B2 and B3). The highest percentage of episodes is associated with the presence of an anticyclone over the northern Greece (types I and IV), being characterized by weak or very weak surface pressure gradient intensity, according to the position and extension of the anticyclone. Moreover, a temperature increase of at least 1°C during the previous 3 days is required in the lower troposphere. Consistent with the synoptic conditions, the development of the sea breeze plays a crucial role in the occurrence of the episodes, even in the cold period of the year, when the sea breeze can still develop with smaller frequency and intensity. Finally, it was found that a small number of episodes is related with the advection of polluted air masses from the industrial area in the northwest of the city and from the Eordaia area in the west, which is the largest lignite producing area of Balkans.