The Continuous Plankton Recorder survey and the North Atlantic Oscillation: Interannual- to Multidecadal-scale patterns of phytoplankton variability in the North Atlantic Ocean

At interannual to multidecadal time scales, much of the oceanographic and climatic variability in the North Atlantic Ocean can be associated with the North Atlantic Oscillation (NAO). While evidence suggests that there is a relationship between the NAO and zooplankton dynamics in the North Atlantic Ocean, the phytoplankton response to NAO-induced changes in the environment is less clear. Time series of monthly mean phytoplankton colour values, as compiled by the Continuous Plankton Recorder (CPR) survey, are analysed to infer relationships between the NAO and phytoplankton dynamics throughout the North Atlantic Ocean. While a few areas display highly significant (p < 0.05) trends in the CPR colour time series during the period 1948–2000, nominally significant (p < 0.20) positive trends are widespread across the basin, particularly on the continental shelves and in a transition zone stretching across the Central North Atlantic. When long-term trends are removed from both the NAO index and CPR colour time series, the correlation between them ceases to be significant. Several hypotheses are proposed to explain the observed variability in the CPR colour and its relationship with climate in the North Atlantic.     

Historical storminess and climate ‘see-saws’ in the North Atlantic region

The existence of a well-defined climate ‘see-saw’ across the North Atlantic region and surrounding areas has been known for over 200 years. The occurrence of severe winters in western Greenland frequently coincides with mild winters in northern Europe. Conversely, mild winters in western Greenland are frequently associated with cold winters across northern Europe. Whereas this ‘see-saw’ is normally discussed in terms of air temperature and pressure differences, here we explore how the climate ‘see-saw’ is reflected in records of historic storminess from Scotland, NW Ireland and Iceland. It is concluded that the stormiest winters in these regions during the last ca. 150 years have occurred when western Greenland temperatures have been significantly below average. In contrast, winters of reduced storminess have coincided with winters when air temperatures have been significantly above average in western Greenland. This reconstruction of winter storminess implies a relationship between chronologies of coastal erosion and the history of North Atlantic climate ‘see-saw’ dynamics with sustained winter storminess, and hence increased coastal erosion, taking place when the Icelandic low pressure cell is strongly anchored within the circulation of the northern hemisphere. Considered over the last ca. 2000 years, it would appear that winter storminess and climate-driven coastal erosion was at a minimum during the Medieval Warm Period. By contrast, the time interval from ca. AD 1420 until present has been associated with sustained winter storminess across the North Atlantic that has resulted in accelerated coastal erosion and sand drift.     

Geographical distribution of fish catches and temperature variations in the northeast Atlantic since 1945

Warming of the northeast Atlantic is expected to affect the location and productivity of fish stocks. It is examined whether variations in catches of cod, herring, mackerel, anchovy and sardines in the ICES statistical areas are related to variations in ocean temperature. Temperatures at certain locations along the Norwegian coast are taken as proxies for temperatures in the Norwegian Sea and the North Sea. It is found that the catches of cod in the North Sea are inversely correlated with temperature and that recruitment and catches of cod in the Norwegian Sea and the Barents Sea are positively related to temperature. There is also some indication of a positive correlation between temperature and the catches of mackerel in the North Sea and the Norwegian Sea, and between temperature and the catches of sardines in the North Sea.     

Assemblages of phytoplankton pigments along a shipping line through the North Atlantic and tropical Pacific

A set of phytoplankton pigment measurements collected on eight quarterly transects from France to New Caledonia is analyzed in order to identify the main assemblages of phytoplankton and to relate their occurrence to oceanic conditions. Pigment concentrations are first divided by the sum [monovinyl chlorophyll a plus divinyl chlorophyll a] to remove the effect of biomass, and second are normalized to give an equal weight to all pigments. The resulting 17 pigments × 799 observations matrix is then classified into 10 clusters using neural methodology. Eight out of these 10 clusters have a well marked regional or seasonal character, thus evidencing adapted responses of the phytoplankton communities. The main gradient opposes two clusters with high fucoxanthin and chlorophyll c₁₊₂ in the North Atlantic in January, April and July, to three clusters in the South Pacific Subtropical Gyre with high divinyl chlorophyll a, zeaxanthin and phycoerythrin. One of the clusters in the South Pacific Subtropical Gyre has relatively high zeaxanthin and phycoerythrin contents and dominates in November and February (austral summer), while another with relatively high divinyl chlorophylls a and b dominates in May and August (austral winter). The third one in the South Pacific is characterized by high carotene concentration and its occurrence peaks in February and May. In the equatorial current system, one cluster, rich in chlorophylls b and c₁₊₂, is strictly located in a narrow zone centred at the equator, while another with relatively high violaxanthin concentration is restricted to the high nutrient - low chlorophyll waters in only the southern part of the South Equatorial Current. One cluster with relatively high prasinoxanthin content has a spatial distribution spanning the entire South Equatorial Current. Two clusters have a ubiquitous distribution: one in the equatorial Pacific, the Carribbean Sea and the North Atlantic during summer has pigment concentrations close to the average of the entire dataset, and the other in the South Pacific Subtropical Gyre, the Carribbean Sea and the North Atlantic during autumn clearly has an oligotrophic character. Many of the differences between clusters are caused by diagnostic pigments of nano- or picoflagellates. While the space and time characteristics of the clusters are well marked and might correspond to differences in physical and chemical forcing, knowledge of the ecological requirements of these flagellates is generally lacking to explain how the variability of the environment triggers these clusters.     

Storminess and surges in the South-Western Approaches of the eastern North Atlantic: the synoptic climatology of recent extreme coastal storms

The climatology of intense winter cyclone events in the eastern North Atlantic responsible for high magnitude surge generation (top 1% of events) within the region of the South-Western Approaches to northwest France and southwest England is extracted from daily sea-level and 500-hPa level atmospheric pressure analyses. Cluster analysis yields discrete cyclone track regimes linked to upper airflow patterns being responsible for the generation of intense storms (central pressure at sea-level ≤990 hPa) which promote severe surge events ≥60 cm along the French coast of the South-Western Approaches. Fluctuations in storminess are strongly influenced by the southward intrusion and strengthening of the jet stream in mid-Atlantic. These occurrences are often associated with negative sea surface temperature (SST) anomalies near Newfoundland and a strengthening of the thermal gradient across the Atlantic well to the south of its normal position. Resultant cyclogenesis promotes storms displaying a delay in minimum central pressure attainment until well east of 14°W, encouraging enhancement of surge flow. Stepwise multiple regression analysis indicates that the most influential variables in promoting severe surge events in the South-Western Approaches are trans-Atlantic sea surface temperature gradients. The most important influence is the prevailing west–east sea surface temperature gradient during the month of the storm, followed by that for the prior month of the storm and thirdly, the north–south sea surface temperature gradient prevailing during the month of the event. Other influential variables reflect the character of the cyclone, storm duration, mean deepening rate of storm central pressure, the value of the outermost closed isobar around the storm centre, and the longitudinal position of the outermost closed isobar to the right of the cyclone track. In contrast, the North Atlantic Oscillation (NAO) is not very effective as a discriminator of surge activity. This may reflect the greater intra-month volatility of parameters used to derive the NAO index than of other monthly variables considered in the study. The lack of resolvability at the individual storm level probably also arises because of the monthly detailing of the NAO against storms of 2–3 days duration. This behavioural model of extreme storminess in the South-Western Approaches to northwest France provides the basis by which extremes of coastal susceptibility can be calibrated.     

北极径流增加对大西洋经向翻转环流影响的模拟研究

观测显示过去几十年间北极入海径流呈现增加趋势,CMIP5耦合模式预测表明21世纪北极入海径流仍会增加,在RCP8.5路径下,21世纪末北极入海径流量将会是1950年的1.4倍。本文利用冰-海耦合数值模式研究了北极径流增加对大西洋经向翻转环流的影响。基于两个数值实验的结果表明,如果北极入海径流按每年0.22%的速度（与RCP8.5路径下的速度相当）增加,大西洋经向翻转环流的强度在100、150和200年后将会分别减弱0.6（3%）、1.2（7%）和1.8（11%） Sv。北极入海径流增加导致大西洋经向翻转环流减弱的主要原因是,北极入海径流增加的淡水被输运到北大西洋后,会抑制北大西洋深层水的生成,这也会导致北大西洋深层水海水年龄的增加。     

气候变化对东北大西洋渔获物组成、多样性和营养级的影响

东北大西洋是世界上重要的捕捞海域,气候变化对该海域捕捞产生了重要的影响。本文基于联合国粮农组织所提供的1982−2016年东北大西洋渔获产量数据,对该海域渔获物组成、多样性、平均营养级及主成分变化特征进行时间序列上的分析,并结合东北大西洋海域气候、环境因子,应用广义可加模型探究渔获物组成与气候变化之间的关系。结果显示：渔获物多样性的变化总体上呈下降趋势,2002−2010年间处于较低水平;平均营养级在2002年之前呈平缓下降的趋势,2002年之后开始波动上升,相关性分析表明这两个指标与海域环境因子的变化较为相关。对渔获物组成进行主成分分析显示,第一主成份变化的方差解释率达到35.3%,且与海域气候、环境因素有较高的相关性,第一主成分变化能够较好地表征气候影响下渔获物组成变化的情况。广义可加模型分析结果显示,渔获物组成变化的影响因素按解释率由高到低分别为：海表温度、海平面高度、盐度、海冰和北大西洋涛动指数。该研究有助于认识气候变化对海洋渔业资源及其结构组成的影响。     

Results from a detailed aeromagnetic survey across the northeast Newfoundland margin, Part II: Early opening of the North Atlantic between the British Isles and Newfoundland

Poles of rotation for the North Atlantic have been derived from the results of a new aeromagnetic survey northeast of Newfoundland. Reconstruction of the North Atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the Atlantic from Flemish Cap and Goban Spur to the Azores-Gibraltar Fracture Zone. A group of similar anomalies has also been identified in the Bay of Biscay. North of Goban Spur and Flemish Cap, these anomalies follow the ocean-continent boundary. Poles of rotation derived for this anomaly show that it forms an isochron (100–110 m.y.) during the long Cretaceous normal polarity interval. The cause of this anomaly is not definite, but it may represent an increase in the magnetization of the crust during a limited time within the Cretaceous Magnetic Quiet Zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by Raymond and LaBrecque.The North Atlantic has also been reconstructed at the time of the initial opening in the region between Flemish Cap and the Charlie-Gibbs Fracture Zone, using inferred ocean-continent boundaries on the west and east sides: it has been shown that the entire region could not have saparated at one time, but that spreading between the British Isles and Newfoundland had to progress from south to north. Consequently, when active sea-floor spreading was taking place between Goban Spur and Flemish Cap (about 110 m.y.) the region to the north was still being stretched. The calculated amount of stretching as derived from the reconstructions (about 25%) agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region, and with the results of deep seismic studies. Active spreading in the north started about 100 m.y. ago.     

潮汐对瞬态气候响应模拟的影响研究

In this study, the impact of oceanic processes on the sensitivity of transient climate change is investigated using two sets of coupled experiments with and without tidal forcing, which are termed Exp_Tide and Exp_Control,respectively. After introducing tidal forcing, the transient climate response(TCR) decreases from 2.32 K to 1.90 K,and the surface air temperature warming at high latitudes decreases by 29%. Large ocean heat uptake efficiency and heat storage can explain the low TCR in Exp_Tide. Approximately 21% more heat is stored in the ocean in Exp_Tide(1.10×10~(24) J) than in Exp_Control(0.91×10~(24) J). Most of the large ocean warming occurs in the upper 1 000 m between 60°S and 60°N, primarily in the Atlantic and Southern Oceans. This ocean warming is closely related to the Atlantic Meridional Overturning Circulation(AMOC). The initial transport at mid-and high latitudes and the decline in the AMOC observed in Exp_Tide are both larger than those observed in Exp_Control. The spatial structures of AMOC are also different with and without tidal forcing in present experiments. The AMOC in Exp_Tide has a large northward extension. We also investigated the relationship between AMOC and TCR suggested by previous studies using the present experiments.     

Basin-scale distributions of stable nitrogen isotopes in the subtropical North Atlantic Ocean: Contribution of diazotroph nitrogen to particulate organic matter and mesozooplankton

New N inputs via biological N₂-fixation play a critical role in supporting oceanic primary production and influencing global biogeochemical cycles. Numerous studies have documented significant N₂-fixation rates in the North Atlantic, but relatively little is known of the pathways and fluxes of new N through planktonic food webs. Here, we report the natural abundance of ¹⁵N in, and contribution of diazotroph N (N_D) inputs to, suspended particles and mesozooplankton collected along two transects in the subtropical North Atlantic Ocean (STNA). Samples were collected in April-May 2000 along the two main transects to evaluate spatial trends of ¹⁵N within the Western Atlantic and across the basin. We found that N₂-fixation is a key component in supporting both primary and secondary productions throughout the STNA. N_D contribution was generally higher for suspended particles than for mesozooplankton, and we observed a high N_D contribution to suspended particles over large spatial scales in the western and central STNA. Mesozooplankton N_D incorporation suggests that diazotroph production supports oceanic food webs over longer timescales (e.g., weeks to months) than that of particle turnover (e.g., days). Larger mesozooplankton (1000-4000 ??m) generally incorporated more N_D than smaller mesozooplankton (250-1000 ??m), and thus may exert a stronger influence on an N_D movement within the water column. Spatial and vertical patterns of variation in mesozooplankton ??¹⁵N also suggest either broad geographical differences in an N_D contribution to secondary production, or temporal variations in N_D incorporation via mesozooplankton communities.