Climatic and limnological changes associated with the Younger Dryas in Atlantic Canada

Pollen, diatom and chironomid fossils from the sediments of a core from Brier Island Bog Lake, Nova Scotia were studied in an attempt to relate changes in microfossil composition to a climatic cooling in Atlantic Canada correlative with the European Younger Dryas ca. 10 to 11 ka. Our paleolimnological data were then compared to similar types of data from Splan Pond, New Brunswick to determine if there were any significant differences between a coastal and a more inland site. Nonarboreal pollen was dominant throughout the Brier Island core and the interval 10.0–11.0 ka did not show the typical decline in Picea and increases in tundra-like vegetation characteristic of many sites in Atlantic Canada. However, the limnological indicators did undergo marked changes in taxon composition. The chironomid assemblage was initially dominated by shallow-water, warm-adapted chironomid taxa followed by abundant Sergentia (a cold stenotherm) during 10–11 ka. Sergentia disappeared in the post Younger Dryas interval and the warm-adapted genera resumed dominance. Chironomid-inferred paleotemperature reconstructions revealed that at both Brier Island Bog Lake and Splan Pond, summer surface-water temperatures dropped abruptly to between 13 and 17°C during the 10–11 ka interval, suggesting that a cooler climate was present in Atlantic Canada correlative with the European Younger Dryas. Diatom assemblage changes during the same period corroborate the occurrence of limnological fluctuations.     

The impact of cold North Atlantic sea surface temperatures on climate: implications for the Younger Dryas cooling (11–10 k)

The sensitivity of global climate to colder North Atlantic sea surface temperatures is in vestigated with the use of the GISS general circulation model. North Atlantic ocean temperatures 18,000 B.P., resembling those prevalent during the Younger Dryas, were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11,000 B.P. The results show that with both 11,000 B.P. and present conditions the colder ocean temperatures produce cooling over western and central Europe, in good agreement with Younger Dryas paleoclimatic evidence. Cooling also occurs over extreme eastern North America, although the precise magnitude and location depends upon the specification of ocean temperature change in the western Atlantic. Despite the presence of increased land ice and colder ocean temperatures, the Younger Dryas summer air temperatures at Northern Hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters, chiefly precession, and atmospheric subsidence at the perimeter of the ice sheets.     

Variations of atmospheric 14C concentrations over the Alleroød-Younger Dryas transition

Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolution AMS ¹⁴C-dated records, Lake Go?ci??, and a floating Late Weichselian glacial varve chronology at the Allerød-Younger Dryas transition allowed us to assess in detail atmospheric Δ¹⁴C changes between late Allerød and early Preboreal. The combined data set shows a drastic rise in Δ¹⁴C during the first 200 years or so of Younger Dryas and the two following about 500 year-long ¹⁴C plateaux. Model experiments which included variations in the geomagnetic field, atmospheric CO₂ variations and a drastic reduction in North Atlantic Deep Water flux at the onset of Younger Dryas allowed to reproduce the distinct rise in Δ¹⁴C during the first 200 years of Younger Dryas fairly well. Also the drop in Δ¹⁴C at the Younger Dryas/Holocene boundary seems reasonably explained by changes in North Atlantic Deep Water circulation. However, the reason behind the anomalous behaviour of the Δ¹⁴C signal in the middle of Younger Dryas remains an open question.     

Synchroneity between marine and terrestrial responses to millennial scale climatic variability during the last glacial period in the Mediterranean region

Land-sea climatic proxies have been obtained from the Last Glacial section of IMAGES core MD95-2043 (western Mediterranean Sea). Vegetation and alkenone derived SST curves indicate rapid (~150 years) and synchronous terrestrial and marine climatic changes, paralleling the Dansgaard-Oeschger (D-O) climatic variability over Greenland. This frequency of climate change can be related to shifts between the two modes of operation of the North Atlantic Oscillation (NAO). Transfer functions applied to the pollen data indicate that there was an amplification of the climatic signal during Heinrich events (HEs) in comparison with other D-O stadials. The development and persistence of both Scandinavian and Atlantic Mobile Polar Highs over southwestern Europe may explain the extreme cooling (~10 °C) and dryness (400 mm) during Heinrich events 5 and 4 in the Mediterranean region. Comparison of the results of core MD95-2043 with similar climatic data from IMAGES core MD95-2042, located off Portugal, indicates that thermal and precipitation gradients occurred between the Mediterranean and the Atlantic sides of Iberia within HEs. HEs 4 and 5 are associated with more humid conditions in the Atlantic (by 200 mm) than in the Mediterranean site, as is the case at the present time. This comparison also illustrates the different behaviour of these areas during the D-O stadials. In contrast with the Mediterranean site, the Atlantic site shows similar precipitation and temperature drops for all the D-O stadials, including those related to the HEs. Here we propose the operation of different Mobile Polar Highs (MPH) as the driving mechanism for this difference in behaviour between the Atlantic and Mediterranean sides of Iberia. HEs are related to a stronger influence of the Scandinavian MPH, forcing a severe aridification and cooling of the full Iberian Peninsula. The Atlantic MPH may have been dominant during the other stadials, which would preferentially affect Southwestern Iberia.     

High frequency pulses of East Asian monsoon climate in the last two glaciations: link with the North Atlantic

Three loess sections in the Loess Plateau of China have been studied to characterize the variations of the East Asian monsoon climate in the Late Quaternary period. Paleo-weathering profiles based on two weathering indices reveal a series of spatially correlative intervals of lower weathering intensity (LW), in the last glacial and late penultimate glacial loess, indicating significantly cooler or/and drier conditions. Most of them are identified at or near the transitional boundaries between loess and soil units, suggesting that climatic conditions favorable for LW events tended to occur at or near major climatic boundaries. Twenty-eight radiocarbon and thermoluminescence measurements, combined with Kukla's magnetic susceptibility age model date these events at ∼14, ∼21, ∼29, ∼38, ∼52, ∼71, ∼135, and ∼145 ka BP, with ∼5–10 ka frequency inlaid within the orbitally induced ∼20-ka periodicity. The ages of the first six events in the last glacial period are therefore close to those of the coarse-grained Heinrich layers in the North Atlantic Ocean, which resulted from massive discharges of icebergs. The results indicate that the climate in the area affected by the East Asian monsoon has experienced high-frequency changes more or less synchronous with the Heinrich events. These high-frequency changes have been reported by Porter and An, based on the grain-size time series from the Luochuan loess section. Our data also reveal that similar events also occurred during the penultimate glaciation. The mechanisms linking the Heinrich events and the East Asian monsoon climate may be similar to that driving the Younger Dryas event which has been identified in a large number of records in China and in the surrounding oceans. Received: 16 October 1995 / Accepted: 10 May 1996     

Assessing the role of North Atlantic freshwater forcing in millennial scale climate variability: a tropical Atlantic perspective

This study analyzes a three-member ensemble of experiments, in which 0.1 Sv of freshwater was applied to the North Atlantic for 100 years in order to address the potential for large freshwater inputs in the North Atlantic to drive abrupt climate change. The model used is the GFDL R30 coupled ocean–atmosphere general circulation model. We focus in particular on the effects of this forcing on the tropical Atlantic region, which has been studied extensively by paleoclimatologists. In response to the freshwater forcing, North Atlantic meridional overturning circulation is reduced to roughly 40% by the end of the 100 year freshwater pulse. Consequently, the North Atlantic region cools by up to 8°C. The extreme cooling of the North Atlantic increases the pole-to-equator temperature gradient and requires more heat be provided to the high latitude Atlantic from the tropical Atlantic. To accommodate the increased heat requirement, the ITCZ shifts southward to allow for greater heat transport across the equator. Accompanying this southward ITCZ shift, the Northeast trade winds strengthen and precipitation patterns throughout the tropical Atlantic are altered. Specifically, precipitation in Northeast Brazil increases, and precipitation in Africa decreases slightly. In addition, we find that surface air temperatures warm over the tropical Atlantic and over Africa, but cool over northern South America. Sea-surface temperatures in the tropical Atlantic warm slightly with larger warm anomalies developing in the thermocline. These responses are robust for each member of the ensemble, and have now been identified by a number of freshwater forcing studies using coupled OAGCMs. The model responses to freshwater forcing are generally smaller in magnitude, but have the same direction, as paleoclimate data from the Younger Dryas suggest. In certain cases, however, the model responses and the paleoclimate data directly contradict one another. Discrepancies between the model simulations and the paleoclimate data could be due to a number of factors, including inaccuracies in the freshwater forcing, inappropriate boundary conditions, and uncertainties in the interpretation of the paleoclimate data. Despite these discrepancies, it is clear from our results that abrupt climate changes in the high latitude North Atlantic have the potential to significantly impact tropical climate. This warrants further model experimentation into the role of freshwater forcing in driving climate change.     

The atmospheric winter circulation during the Younger Dryas stadial in the Atlantic/European sector

The Younger Dryas (YD) stadial signified an interruption of the warming during the transition from the last glacial to the present interglacial. The mechanism responsible for this cooling is still uncertain, so valuable information concerning climate variability can be obtained by numerical simulation of the YD climate. We performed four experiments on the Younger Dryas climate with the Hamburg atmospheric general circulation model. Here we use the results of these experiments, which differed in prescribed boundary conditions, to characterize the atmospheric winter circulation during the YD stadial in the North Atlantic/European sector. The 10 year means of the following variables are presented: sea level pressure, 500 hPa geopotential heights and 200 hPa winds. In addition, we used daily values to calculate an index to assess the occurrence of blocking and strong zonal flow and to compute storm tracks. Our results show that the YD cooling in Europe was present with a strong and stable westerly circulation without blocking. This is in conflict with an earlier study suggesting frequent easterly winds over NW-Europe. In our experiments the sea-ice cover in the North Atlantic Ocean was the crucial factor forcing this specific YD circulation. Moreover, the jet stream over the North Atlantic was strengthened considerably, causing an enhanced cyclonic activity over the Eurasian continent. The YD winter circulation was different from the circulation found in most simulation studies on the Last Glacial Maximum, since no glacial anticyclones were present and no split of the jet stream occurred. Received: 1 November 1995 / Accepted: 29 May 1996     

Surface temperature in NW Europe during the Younger Dryas: AGCM simulation compared with temperature reconstructions

 During the Younger Dryas (YD) the climate in NW Europe returned to near-glacial conditions. To improve our understanding of climate variability during this cold interval, we compare an AGCM simulation of this climate, performed with the ECHAM model, with temperature reconstructions for NW Europe based on geological and paleoecological records. Maps for the mean winter, summer and annual temperature are presented. The simulated winters are consistent with reconstructions in the northern part of the study area. A strong deviation is noted in Ireland and England, where the simulation is too warm by at least 10 °C. It appears that the N Atlantic was cooler than prescribed in the YD simulation, including a southward expansion of the sea-ice margin. The comparison for the summer shows a too warm continental Europe in the simulation. Supposedly, these anomalously warm conditions are caused by the AGCM’s response to the prescribed increased summer insolation. The region of maximum summer cooling is similar in both the simulation and reconstruction, i.e., S Sweden. We suggest that this is due to the local cooling effect of the Scandinavian ice sheet. Compared to the present climate a considerable increase of the annual temperature range is inferred, especially for regions close to the Atlantic Ocean. Received: 20 November 1996 / Accepted: 8 July 1997     

Pacific and Atlantic influences on Mesoamerican climate over the past millennium

A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Ni?o/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Ni?a and warm Atlantic SSTs prevailed, and the 1980–1990s when El Ni?o and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Ni?o and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.     

The variable link between PNA and NAO in observations and in multi-century CGCM simulations

The link between the Pacific/North American pattern (PNA) and the North Atlantic Oscillation (NAO) is investigated in reanalysis data (NCEP, ERA40) and multi-century CGCM runs for present day climate using three versions of the ECHAM model. PNA and NAO patterns and indices are determined via rotated principal component analysis on monthly mean 500?hPa geopotential height fields using the varimax criteria. On average, the multi-century CGCM simulations show a significant anti-correlation between PNA and NAO. Further, multi-decadal periods with significantly enhanced (high anti-correlation, active phase) or weakened (low correlations, inactive phase) coupling are found in all CGCMs. In the simulated active phases, the storm track activity near Newfoundland has a stronger link with the PNA variability than during the inactive phases. On average, the reanalysis datasets show no significant anti-correlation between PNA and NAO indices, but during the sub-period 1973?C1994 a significant anti-correlation is detected, suggesting that the present climate could correspond to an inactive period as detected in the CGCMs. An analysis of possible physical mechanisms suggests that the link between the patterns is established by the baroclinic waves forming the North Atlantic storm track. The geopotential height anomalies associated with negative PNA phases induce an increased advection of warm and moist air from the Gulf of Mexico and cold air from Canada. Both types of advection contribute to increase baroclinicity over eastern North America and also to increase the low level latent heat content of the warm air masses. Thus, growth conditions for eddies at the entrance of the North Atlantic storm track are enhanced. Considering the average temporal development during winter for the CGCM, results show an enhanced Newfoundland storm track maximum in the early winter for negative PNA, followed by a downstream enhancement of the Atlantic storm track in the subsequent months. In active (passive) phases, this seasonal development is enhanced (suppressed). As the storm track over the central and eastern Atlantic is closely related to the NAO variability, this development can be explained by the shift of the NAO index to more positive values.     

Assessing the sensitivity of the North Atlantic Ocean circulation to freshwater perturbation in various glacial climate states

A striking characteristic of glacial climate in the North Atlantic region is the recurrence of abrupt shifts between cold stadials and mild interstadials. These shifts have been associated with abrupt changes in Atlantic Meridional Overturning Circulation (AMOC) mode, possibly in response to glacial meltwater perturbations. However, it is poorly understood why they were more clearly expressed during Marine Isotope Stage 3 (MIS3, ~60?C27?ka BP) than during Termination 1 (T1, ~18?C10?ka BP) and especially around the Last Glacial Maximum (LGM, ~23?C19?ka BP). One clue may reside in varying climate forcings, making MIS3 and T1 generally milder than LGM. To investigate this idea, we evaluate in a climate model how ice sheet size, atmospheric greenhouse gas concentration and orbital insolation changes between 56?ka BP (=56k), 21k and 12.5k affect the glacial AMOC response to additional freshwater forcing. We have performed three ensemble simulations with the earth system model LOVECLIM using those forcings. We find that the AMOC mode in the mild glacial climate type (56k and 12.5k), with deep convection in the Labrador Sea and the Nordic Seas, is more sensitive to a constant 0.15?Sv freshwater forcing than in the cold type (21k), with deep convection mainly south of Greenland and Iceland. The initial AMOC weakening in response to freshwater forcing is larger in the mild type due to an early shutdown of Labrador Sea deep convection, which is completely absent in the 21k simulation. This causes a larger fraction of the freshwater anomaly to remain at surface in the mild type compared to the cold type. After 200?years, a weak AMOC is established in both climate types, as further freshening is compensated by an anomalous salt advection from the (sub-)tropical North Atlantic. However, the slightly fresher sea surface in the mild type facilitates further weakening of the AMOC, which occurs when a surface buoyancy threshold (?0.6?kg?m^?3 surface density anomaly to the 56k reference state) is stochastically crossed in the Nordic Seas. While described details are model-specific, our results imply that a more northern location of deep convection sites during milder glacial times may have amplified frequency and amplitude of abrupt climate shifts.     

Paleoclimatic inferences from glacial fluctuations on Svalbard during the last 20 000 years

The climate history of western Spitsbergen, Svalbard is deduced from variations of glaciers during the last 20 000 years. A major depression of the regional equilibrium line altitude (ELA) occurred during the Late Weichselian glacial maximum (18000–13000y ago) when low summer temperatures may have caused year-round snow accumulation on the ground. This rapid expansion of the glaciers also indicates nearby moisture sources, suggesting partly open conditions in the Norwegian Sea during the summers. A rapid glacial retreat around 13 000–12 500 y BP was caused by a sudden warming. During the Younger Dryas the ELA along the extreme western coast of Spitsbergen was not significantly lower than at present. In contrast to Fennoscandia, the British Isles and the Alps, there is no evidence for readvance of local glaciers during Younger Dryas on western Spitsbergen. This difference is attributed to a much dryer climate on Spitsbergen and probably only slight changes in sea surface temperatures. In addition, summer melting in this high arctic area is more sensitive to orbitally increased insolation. Around 10 000 y BP another rapid warming occurred and during early and mid Holocene the summer temperatures were significantly higher than at present. A temperature decline during the late Holocene caused regrowth of the glaciers which reached their maximum Holocene position during the last century.Contribution to Clima Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

The impact of El Ni?o on South American summer climate during different phases of the Pacific Decadal Oscillation

In austral summer, the observed El Ni?o (EN) events during warm Pacific Decadal Oscillation (PDO) phases (PDO(+)) exhibited large anomalous upper level wave patterns in response to larger Sea Surface Temperature (SST) anomalies in the Equatorial Pacific and Atlantic Oceans compared with SST anomalies in EN events during cold PDO phases (PDO(?)). The precipitation anomalies in PDO(+) EN are increased over Southeastern South America (SESA) associated with the intensification of the moisture flux convergence in this region. The PDO(?) EN events exhibit positive precipitation anomalies only over southern SESA, while negative anomalies were observed in the north. Downward motion and anomalous divergence over central eastern Brazil may have contributed to the weakening of the northwesterly moisture flux convergence associated with the South American Low Level Jet (SALLJ) over the subtropics. The extratropical cyclones showed higher frequency and lower central pressures in southern Brazil, Uruguay, northeastern Argentina, and Southwest Atlantic Ocean during the PDO(+) EN events compared with the PDO(?) EN events. Such increase in the frequency and intensity of cyclogenesis cases seems to be in accordance with the anomalous moisture flux convergence over the SESA and associated reduction in the Sea Level Pressure observed during PDO(+) EN events. In order to investigate the impact of a canonical El Ni?o event over South America under different PDO phases, two numerical experiments were done with an Atmospheric General Circulation Model. Global SST and ice sea fields average over years characterized by (a) PDO(+) and (b) PDO(?) were considered as climatologically fields, and a composite of anomalies of SST of all El Ni?o events observed in 1950?C1999 was added in the region 20oS?C20oN;120oW?C175oW of both ??climatologies.?? The differences in experiments suggest that a canonical EN may produce significant different anomalous atmospheric patterns associated with distinct PDO climatologies. The more significant differences are simulated over extreme northern and eastern Brazil. Additional numerical experiments isolating the observed variability of SST over several oceanic basins during different PDO phases will be conducted to study their particular role on the South American climate.     

Interannual and intraannual variations in total precipitable water over the North Atlantic from satellite microwave measurements

Based on the long-term data of satellite microwave radiometers, the estimates are obtained of interannual and intraannual variations in monthly mean values of total precipitable water in the atmosphere over the North Atlantic from 1988 to 2011, in particular, in the regions characterized by the maximum rate of heat and moisture exchange between the ocean and atmosphere such as the Gulf Stream, Newfoundland, and Norway-Greenland energy-active zones. Long-term trends in total precipitable water in the atmosphere over these regions are estimated. The variations in total precipitable water in the atmosphere in 2010 are noted which were caused by oil spills in the Gulf of Mexico (the spring of 2010) and by the severe drought in the European part of Russia (the summer of 2010).     

Low-latitude control of interhemispheric sea-surface temperature contrast in the tropical Atlantic over the past 21 kyears: the possible role of SE trade winds

To stimulate a discussion on the role of tropical atmospheric circulation versus thermohaline circulation changes for tropical Atlantic sea-surface temperature (SST) variations, we present a record of the SST contrast (SST) between the tropical northwest and southeast Atlantic from the Last Glacial Maximum to the Late Holocene. The SST was calculated from two alkenone-derived SST records; one from the Caribbean Sea and the other from the Angola Basin. Changes in the cross-equatorial SST were then compared with an abundance record of Florisphaera profunda from the equatorial Atlantic, which is indicative of SE trade-wind induced variations in thermocline depth in the equatorial divergence zone. This comparison implies that the Last Glacial Maximum, the Younger Dryas, and the Mid to Late Holocene were periods of strong SE trade winds, which led to an intense upwelling-related cooling in the southeast Atlantic and concurrently enhanced advection of warm tropical South Atlantic waters into the western tropical Atlantic. Accordingly, a coupled ocean-atmospheric process has probably created a dipole-like SST distribution pattern in the tropical Atlantic during these three distinct climatic periods. In contrast, Heinrich Event 1, the Bølling-Allerød, and the Early Holocene were intervals of weakened SE trade winds, causing a warming in the southeast Atlantic. However, synchronous warming in both regions during Heinrich Event 1 can be partially attributed to a weakening of thermohaline overturning which caused a reduced northward heat transport from the low-latitude to the high-latitude North Atlantic.     

新仙女木事件及全新世早中期降温事件——来自洱海湖泊沉积的记录

Three cold events (the Younger Dryas, 9.4 ka cal BP, 5.8 ka cal BP) since the 13 ka cal BP in Erhai (EH) Lake catchment, Yunnan Province, were analyzed using the Total Organic Carbon (TOC) series of the EH core. By comparison of the EH core, Qinghai Lake core and Guliya ice core, differences of these cold events were determined. Erhai Lake's responses to the global cold events were lagged in time and weakened in intensity in comparison with Qinghai Lake's. The latitude location of Erhai Lake and the obstruction of Tibetan Plateau may in part explain the differences. However, the remarkable cold event of 8.2 ka cal BP in the Guliya ice core was absent in the records of Erhai Lake and Qinghai Lake. Power spectrum analysis of the TOC proxy series shows that there were three kinds of millennial cycles, i.e. 5 ka, 2.3 ka, and 1.5 ka, in climate changes in Erhai Lake, which reveal the responses of climate to suborbit cycles.     

North Atlantic climate responses to perturbations in Antarctic Intermediate Water

Recent observations suggest Antarctic Intermediate Water (AAIW) properties are changing. The impact of such variations is explored using idealised perturbation experiments with a coupled climate model, HadCM3. AAIW properties are altered between 10 and 20°S in the South Atlantic, maintaining constant potential density. The perturbed AAIW remains subsurface in the South Atlantic, but as it moves northwards, it surfaces and interacts with the atmosphere leading to density anomalies due to heat exchanges. For a cooler, fresher AAIW, there is a significant decrease in the mean North Atlantic sea surface temperature (SST), of up to 1°C, during years 51?C100. In the North Atlantic Current region there are persistent cold anomalies from 2,000?m depth to the surface, and in the overlying atmosphere. Atmospheric surface pressure increases over the mid-latitude Atlantic, and precipitation decreases over northwest Africa and southwest Europe. Surface heat flux anomalies show that these impacts are caused by changes in the ocean rather than atmospheric forcing. The SST response is associated with significant changes in the Atlantic meridional overturning circulation (MOC). After 50?years there is a decrease in the MOC that persists for the remainder of the simulation, resulting from changes in the column-averaged density difference between 30°S and 60°N. Rather than showing a linear response, a warmer, saltier AAIW also leads to a decreased MOC strength for years 51?C100 and resulting cooling in the North Atlantic. The non-linearity can be attributed to opposing density responses as the perturbed water masses interact with the atmosphere.     

Characteristics of summertime daily rainfall variability over South America and the South Atlantic Convergence Zone

Summary ?This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ). During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced) rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes. High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events. Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002     

Link between the North and South Atlantic during the Heinrich events of the last glacial period

High resolution benthic oxygen isotope records combined with radiocarbon datings, from cores retrieved in the North, Equatorial, and South Atlantic are used to establish a reliable cronostratigraphy for the last 60 ky. This common temporal framework enables us to study the timing of the sub-Milankovitch climate variability in the entire surface Atlantic during this period, as reflected in planktonic oxygen isotope records. Variations in sea surface temperatures in the Equatorial and South Atlantic reveal two warm periods during the mid-stage 3 which are correlated to the warming observed in the North Atlantic after Heinrich events (HL) 5 and 4. However, the records show that the warming started about 1500 y earlier in the South Atlantic. A zonally averaged ocean circulation model simulates a similar north-south thermal antiphasing between the latitudes of our coring sites, when pertubated by a freshwater flux anomaly. We infer that the observed phase relationship between the northern and the southern Atlantic is related to periods of reduced NADW production in the North Atlantic, such as during HL5 and HL4. Received: 24 November 1998 / Accepted: 16 May 1999     

Projected Changes in Surface Air Temperature and Surface Wind in the Gulf of St. Lawrence

Abstract

The impacts of climate change on surface air temperature (SAT) and winds in the Gulf of St. Lawrence (GSL) are investigated by performing simulations from 1970 to 2099 with the Canadian Regional Climate Model (CRCM), driven by a five-member ensemble. Three members are from Canadian Global Climate Model (CGCM3) simulations following scenario A1B from the Intergovernmental Panel on Climate Change (IPCC); one member is from the Community Climate System Model, version 3 (CCSM3) simulation, also following the A1B scenario; and one member is from the CCSM4 (version 4) simulation following the Representative Concentration Pathway (RCP8.5) scenario. Compared with North America Regional Reanalysis (NARR) data, it is shown that CRCM can reproduce the observed SAT spatial patterns; for example, both CRCM simulations and NARR data show a warm SAT tongue along the eastern Gulf; CRCM simulations also capture the dominant northwesterly winds in January and the southwesterly winds in July. In terms of future climate scenarios, the spatial patterns of SAT show plausible seasonal variations. In January, the warming is 3°–3.5°C in the northern Gulf and 2.5°–3°C near Cabot Strait during 2040–2069, whereas the warming is more uniform during 2070–2099, with SAT increases of 4°–5°C. In summer, the warming gradually decreases from the western side of the GSL to the eastern side because of the different heat capacities between land and water. Moreover, the January winds increase by 0.2–0.4?m?s^?1 during 2040–2069, related to weakening stability in the atmospheric planetary boundary layer. However, during 2070–2099, the winds decrease by 0.2–0.4?m?s^?1 over the western Gulf, reflecting the northeastward shift in northwest Atlantic storm tracks. In July, enhanced baroclinicity along the east coast of North America dominates the wind changes, with increases of 0.2–0.4?m?s^?1. On average, the variance for the SAT changes is about 10% of the SAT increase, and the variance for projected wind changes is the same magnitude as the projected changes, suggesting uncertainty in the latter.