North Atlantic Variability Research Articles

Causes of Enhanced SST Variability over the Equatorial Atlantic and Its Relationship to the Atlantic Zonal Mode in CMIP5

A spurious band of enhanced sea surface temperature (SST) variance (SBEV) is identified over the northern equatorial Atlantic in the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model, version 2.1. The SBEV is especially pronounced in boreal spring owing to the combined effect of both anomalous atmospheric thermal forcing and oceanic vertical upwelling. The SBEV is a common bias in phase 5 of the Coupled Model Intercomparison Project (CMIP5), found in 14 out of 23 models. The SBEV in CMIP5 is associated with the atmospheric thermal forcing and the oceanic vertical upwelling, similar to GFDL CM2.1. While the tropical North Atlantic variability is only weakly correlated with the Atlantic zonal mode (AZM) in observations, the SBEV in CMIP5 produces conditions that drive and intensify the AZM variability via triggering the Bjerknes feedback. This partially explains why AZM is strong in some CMIP5 models even though the equatorial cold tongue and easterly trades are biased low.

A last millennium perspective on North Atlantic variability: exploiting synergies between models and proxy data

The North Atlantic is a key region for decadal prediction as it has experienced significant multi-decadal variability over the observed period. This variability, which is thought to be intrinsic to the region, can potentially modulate, either by amplifying or mitigating, the global warming signal from anthropogenic greenhouse emissions. For example, studies suggest that the North Atlantic contributed to the recent hiatus period between 1998 and 2012, by triggering an atmospheric response which impacted on the eastern tropical Pacific (e.g. McGregor et al., 2014). The subpolar North Atlantic is also a major CO2 sink, and therefore of great importance for the global carbon cycle (Perez et al., 2013). One of the key players in the North Atlantic region is the Atlantic Meridional Overturning Circulation (AMOC), which is associated with sinking due to deep water formation in the Labrador and Nordic Seas. The AMOC is the primary control of the poleward heat transport in the Atlantic region. Therefore, the AMOC is associated with important climate impacts, and plays an active role in various feedback mechanisms with, for example, sea ice (Mahajan et al., 2011) and the atmospheric circulation (Gastineau and Frankignoul, 2012). The AMOC has exhibited abrupt variations in the past (e.g. the last glacial period, Rahmstorf, 2002) and could experience a major slowdown in the future due to the combined effect of surface warming and Greenland ice sheet melting on deep water formation (Bakker et al., 2016). The possibility of such a shutdown has stimulated considerable international efforts to observe and reconstruct the past AMOC changes. Only by understanding its natural variability will we be able to detect and anticipate an anthropogenic impact on the AMOC. Decadal modulations are also found in other large-scale modes of climate variability, such as the North Atlantic Oscillation (NAO) (Stephenson et al., 2000), the Subpolar Gyre strength (SPG) (Hakkinen and Rhines, 2004) and the Atlantic Multidecadal Variability (AMV) (Enfield et al., 2001), which have all been linked with widespread climate impacts over the surrounding continents. Modelling studies suggest that all these modes interact with the AMOC (Gastineau and Frankignoul, 2012; Hatun et al., 2005; Knight et al., 2005) but the exact interrelationships are complex and remain to be disentangled. Also to be determined are the underlying mechanisms responsible for the decadal and centennial AMOC modulations, with different climate models showing different key drivers (Menary et al., 2015a). Similarly, the exact impact of the natural external forcings (e.g. volcanic aerosols, solar irradiance) on the variability of these different largescale climate modes still remains unclear.

Observed and Simulated Fingerprints of Multidecadal Climate Variability and Their Contributions to Periods of Global SST Stagnation

AbstractThis study investigates spatiotemporal features of multidecadal climate variability using observations and climate model simulation. Aside from a long-term warming trend, observational SST and atmospheric circulation records are dominated by an almost 65-yr variability component. Although its center of action is over the North Atlantic, it manifests also over the Pacific and Indian Oceans, suggesting a tropical interbasin teleconnection maintained through an atmospheric bridge. An analysis shows that simulated internal climate variability in a coupled climate model (CSIRO Mk3.6.0) reproduces the main spatiotemporal features of the observed component. Model-based multidecadal variability includes a coupled ocean–atmosphere teleconnection, established through a zonally oriented atmospheric overturning circulation between the tropical North Atlantic and eastern tropical Pacific. During the warm SST phase in the North Atlantic, increasing SSTs over the tropical North Atlantic strengthen locally ascending air motion and intensify subsidence and low-level divergence in the eastern tropical Pacific. This corresponds with a strengthening of trade winds and cooling in the tropical central Pacific. The model’s derived component substantially shapes its global climate variability and is tightly linked to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This suggests potential predictive utility and underscores the importance of correctly representing North Atlantic variability in simulations of global and regional climate. If the observations-based component of variability originates from internal climate processes, as found in the model, the recently observed (1970s–2000s) North Atlantic warming and eastern tropical Pacific cooling might presage an ongoing transition to a cold North Atlantic phase with possible implications for near-term global temperature evolution.

Effect of future climate change on the coupling between the tropical oceans and precipitation over Southeastern South America

El Nino, the Indian Ocean Dipole (IOD), and the Tropical North Atlantic variability (TNA) can influence southeastern South America (SESA) rainfall, particularly during springtime. A recent study has shown that during the last century, there were two synchronization periods (1930s and 1970s) in which different tropical oceans interacted among themselves and, in turn, induced precipitation anomalies over SESA. In this study, we evaluate how this collective influence of the tropical oceans on SESA precipitation observed during the twentieth century, can change in the next century as result of anthropogenic forcing. To do so, we use the output of seven different CMIP5 models and construct a network using as nodes the indices of the Nino3.4, the TNA, the IOD and rainfall over SESA. After evaluating their skill in representing the observed network statistics during the twentieth century, we study changes in the network under RCP4.5 and 8.5 global warming scenarios in the twenty-first century. Focusing on the grand CMIP5 ensemble mean, results suggest that an anthropogenic forcing would increase the number of synchronization periods per century, their time length, and the connectivity between nodes (with the exception of TNA and IOD in RCP8.5). The stronger connectivity of SESA precipitation with the tropical oceans in both scenarios suggest an increase of the oceanic influence on rainfall over SESA as a result of anthropogenic forcing, which would enhance its seasonal predictability. However, these results have to be taken with caution because there is a large disparity in model behavior and thus a large uncertainty in conclusions suggested from the grand ensemble mean.

Evidence of a decadal solar signal in the Amazon River: 1903 to 2013

AbstractIt has been shown that tropical climates can be notably influenced by the decadal solar cycle; however, the relationship between this solar forcing and the tropical Amazon River has been overlooked in previous research. In this study, we reveal evidence of such a link by analyzing a 1903–2013 record of Amazon discharge. We identify a decadal flow cycle that is anticorrelated with the solar activity measured by the decadal sunspot cycle. This relationship persists through time and appears to result from a solar influence on the tropical Atlantic Ocean. The amplitude of the decadal solar signal in flow is apparently modulated by the interdecadal North Atlantic variability. Because Amazonia is an important element of the planetary water cycle, our findings have implications for studies on global change.

Winter Extreme Flux Events in the Kuroshio and Gulf Stream Extension Regions and Relationship with Modes of North Pacific and Atlantic Variability

Abstract Boreal winter (November–March) extreme flux events in the Kuroshio Extension region (KER) of the northwestern Pacific and the Gulf Stream region (GSR) of the northwestern Atlantic are analyzed and compared, based on NCEP Climate Forecast System Reanalysis (CFSR), NCEP–NCAR reanalysis, and NOAA Twentieth Century Reanalysis data, as well as the observationally derived OAFlux dataset. These extreme flux events, most of which last less than 3 days, are characterized by cold air outbreaks (CAOs) with an anomalous northerly wind that brings cold and dry air from the Eurasian and North American continents to the KER and GSR, respectively. A close relationship between the extreme flux events over KER (GSR) and the Aleutian low pattern (ALP) [east Atlantic pattern (EAP)] is found with more frequent occurrence of the extreme flux events during a positive ALP (EAP) phase and vice versa. A further lag-composite analysis suggests that the ALP (EAP) is associated with accumulated effects of the synoptic winter storms accompanied by the extreme flux events and shows that the event-day storms tend to have a preferred southeastward propagation path over the North Pacific (Atlantic), potentially contributing to the southward shift of the storm track over the eastern North Pacific (Atlantic) basin during the ALP (EAP) positive phase. Finally, lag-regression analyses indicate a potential positive influence of sea surface temperature (SST) anomalies along the KER (GSR) on the development of the extreme flux events in the North Pacific (Atlantic).

Attributing observed Greenland responses to natural and anthropogenic climate forcings

We attribute climate variability in four independent reconstructions of Greenland-average temperature and precipitation over the twentieth century. The reconstructions exhibit substantial differences in the timing and amplitudes of climate variations. Linear, empirical models of Greenland-average temperature and precipitation variations on multi-decadal timescales are established from a suite of Community Climate System Model 3 simulations of the preindustrial millennium. They are compared against observational reconstructions after being tested against simulations of the industrial and future periods. Empirical estimates of variations over the industrial and future periods are correlated at greater than 0.95 with simulated values. Greenhouse gas increases account for the majority of the temperature and precipitation increases after the mid-1900s. In contrast to the simulations, observed temperatures and precipitation do not increase until the mid-1990s. Thus, the empirical models over-predict the response to greenhouse gases over the twentieth century. We conclude that CCSM3 is not capturing processes that are proving important to Greenland surface conditions. Furthermore, modes of North Atlantic variability exhibit opposite relationships with some observations compared with the simulations. In those cases, reversing the sign of this component of variability yields significant correlations between the estimated and observed accumulation values.

North Atlantic variability driven by stochastic forcing in a simple model

ABSTRACTThis study investigates the mechanisms driving North Atlantic (NA) variability using a simple model that incorporates the time evolution of interactive upper ocean temperature anomalies, horizontal (Gyre, Ψg) and vertical (meridional overturning circulation, Ψm) circulation. The model is forced with multicentury long synthetic time series of external stochastic forcing that captures key statistical properties of observations such as the range of fluctuations and persistence of processes. The simulated oceanic response may be viewed as a delayed response to a cumulative atmospheric forcing over an interval defined by the system damping properties. Depending on the choice of parameters, the model suggests either compensatory mechanism (Ψm and Ψg are anti-correlated) or amplification mechanism (Ψm and Ψg are positively correlated). The compensatory mechanism implies that an increase of heat supplied by an anomalously strong Ψg would be balanced by a decrease of heat provided by a weaker Ψm and vice versa. The amplification mechanism suggests that both Ψm and Ψg maintain the heat budget in the system compensating its damping properties. Some evidence for these mechanisms is found in a global climate model. Further investigations of NA variability mechanisms are important as they improve understanding of how the NA climate system functions.

Tibetan Plateau summer precipitation: covariability with circulation indices

Relations between Tibetan Plateau precipitation and large-scale climate indices are studied based on the Standardized Precipitation Index (SPI) and the boreal summer season. The focus is on the decadal variability of links between the large-scale circulation and the plateau drought and wetness. Analysis of teleconnectivity of the continental northern hemisphere standardized summer precipitation reveals the Tibetan Plateau as a major SPI teleconnectivity center in south-eastern Asia connecting remote correlation patterns over Eurasia. Employing a moving window approach, changes in covariability and synchronizations between Tibetan Plateau summer SPI and climate indices are analyzed on decadal time scales. Decadal variability in the relationships between Tibetan Plateau summer SPI and the large-scale climate system is characterized by three shifts related to changes in the North Atlantic, the Indian Ocean, and the tropical Pacific. Changes in the North Atlantic variability (North Atlantic Oscillation) result in a stable level of Tibetan Plateau summer SPI variability; the response to changes in tropical Pacific variability is prominent in various indices such as Asian monsoon, Pacific/North America, and East Atlantic/Western Russia pattern.

Rapid 20th century warming in the Caribbean and impact of remote forcing on climate in the northern tropical Atlantic as recorded in a Guadeloupe coral

We have generated 104-year long (1895–1999) monthly δ 18O and Sr/Ca time series from a fast-growing Diploria strigosa coral core drilled off Guadeloupe Island, Lesser Antilles. Coral Sr/Ca reliably records interannual to decadal surface air temperature (SAT) variations in the region and shows a pronounced warming of approximately 1.5 °C since 1950, with the strongest warming (1.2 °C) occurring since 1975. This warming is also evident in SAT measured at Guadeloupe, which ends in 1951. Thus, our Sr/Ca series extends the air temperature record by 56 years. We find that the past few decades are the warmest years over the entire period of record. The accelerated warming since 1950 is accompanied by a pronounced decrease in regional precipitation. This dampens the warming signal indicated by coral δ 18O, which is too low (only 0.7–0.8 °C since 1951). Consistently, δ 18O sw estimated from the coral proxies also shows a strong decrease since 1950. Our data suggest an inverse relationship between SAT and precipitation (i.e. warmer and drier) for the latter half of the 20th century with the strongest trends since the mid-1970s. This is consistent with recent observational and model data, which report that while over the tropical oceans rainfall has increased due to an increase in sea surface temperatures, precipitation over land regions is reduced. A continuation of this warming and drying trend over Caribbean land regions would have severe societal consequences, especially in the context of anthropogenic warming. The El Niño Southern-Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are the two major climate modes affecting large-scale SST variability in the northern tropical Atlantic. Both Sr/Ca and δ 18O show a close relationship to ENSO and the NAO. A quantitative comparison between extremes in mean March–May coral δ 18O and the Nino3 and NAO indices imply that climate variability in the northern tropical Atlantic is mainly forced by tropical Pacific and North Atlantic variability. Spectral analysis suggests that the relative importance of ENSO and the NAO is frequency dependent, with ENSO dominating at interannual, and the NAO dominating at interdecadal time scales.

100‐year mass changes in the Swiss Alps linked to the Atlantic Multidecadal Oscillation

Thirty new 100‐year records of glacier surface mass balance, accumulation and melt in the Swiss Alps are presented. The time series are based on a comprehensive set of field data and distributed modeling and provide insights into the glacier‐climate linkage. Considerable mass loss over the 20th century is evident for all glaciers, but rates differ strongly. Glacier mass loss shows multidecadal variations and was particularly rapid in the 1940s and since the 1980s. Mass balance is significantly anticorrelated to the Atlantic Multidecadal Oscillation (AMO) index assumed to be linked to thermohaline ocean circulation. We show that North Atlantic variability had a recognizable impact on glacier changes in the Swiss Alps for at least 250 years.

Implications of Both Statistical Equilibrium and Global Warming Simulations with CCSM3. Part II: On the Multidecadal Variability in the North Atlantic Basin

Abstract The nature of the multidecadal variability in the North Atlantic basin is investigated through detailed analysis of multicentury integrations performed using the low-resolution version of the Community Climate System Model, version 3 (CCSM3), a modern atmosphere–ocean coupled general circulation model. Specifically, the results of control simulations under both preindustrial and present-day perpetual seasonal cycle conditions are compared to each other and also to the results of five simulations with increasing CO2 concentration scenarios. In the absence of greenhouse gas–induced warming, the meridional overturning circulation (MOC) variability is shown to be dependent on the details of the simulation. In the present-day control simulation, the MOC is characterized by a broad spectrum of low frequencies, whereas, in preindustrial control simulations, MOC variability is characterized either by a well-defined periodicity of 60 yr or by a broad spectrum of low frequencies. In all the control simulations, the MOC appears to respond with a delay of 10 yr to synchronous temperature and salinity anomalies in the deep water formation sites located in the subpolar gyre, but salinity dominates the density anomalies. The explanation of the modeled MOC periodicity is therefore sought in the creation of these density anomalies. The influence of increased sea ice coverage under cold/preindustrial conditions is shown to modify the salinity variability, but it is not a sufficient condition for the support of the MOC periodicity. Instead, its source appears to be a modified subpolar gyre circulation resulting from interaction with the bottom bathymetry, which is able to sustain strong coupling between the horizontal and overturning circulations. Based on the global warming analyses, for the simulations initialized from the cold/preindustrial statistical equilibrium run, the North Atlantic variability continues to be dominated by strong coupling between the horizontal and overturning circulations if the imposed forcing is weak. More generally, the delayed response of the MOC to surface density anomalies in the deep water formation regions is preserved under weak forcing.

Rapid climatic variability in the west Mediterranean during the last 25 000 years from high resolution pollen data

Abstract. High-temporal resolution pollen record from the Alboran Sea ODP Site 976, pollen-based quantitative climate reconstruction and biomisation show that changes of Mediterranean vegetation have been clearly modulated by short and long term variability during the last 25 000 years. The reliability of the quantitative climate reconstruction from marine pollen spectra has been tested using 22 marine core-top samples from the Mediterranean. The ODP Site 976 pollen record and climatic reconstruction confirm that Mediterranean environments have a rapid response to the climatic fluctuations during the last Termination. The western Mediterranean vegetation response appears nearly synchronous with North Atlantic variability during the last deglaciation as well as during the Holocene. High-resolution analyses of the ODP Site 976 pollen record show a cooling trend during the Bölling/Allerød period. In addition, this period is marked by two warm episodes bracketing a cooling event that represent the Bölling-Older Dryas-Allerød succession. During the Holocene, recurrent declines of the forest cover over the Alboran Sea borderlands indicate climate events that correlate well with several events of increased Mediterranean dryness observed on the continent and with Mediterranean Sea cooling episodes detected by alkenone-based sea surface temperature reconstructions. These events clearly reflect the response of the Mediterranean vegetation to the North Atlantic Holocene cold events.

North Atlantic climate variability from a self‐organizing map perspective

North Atlantic variability in general, and the North Atlantic Oscillation (NAO) in particular, is a long‐studied, very important but still not well‐understood problem in climatology. The recent trend to a higher wintertime NAO index was accompanied by an additional increase in the Azores High not coupled to changes in the Icelandic Low, as shown by a self‐organizing maps (SOMs) analysis of monthly mean DJF mean sea level pressure data from 1957 to 2002. SOMs are a nonlinear tool to optimally extract a user‐specified number of patterns or icons from an input data set and to uniquely relate any input data field to an icon, allowing analyses of occurrence frequencies and transitions complementary to principal component analysis (PCA). SOMs analysis of ERA‐40 data finds a North Atlantic “monopole” roughly colocated with the mean position of the Azores High, as well as the well‐known NAO dipole involving the Icelandic Low and the subtropical high. Little trend is shown in December, but the Azores High increased along with the NAO in January and February over the study interval, with implications for storminess in northwestern Europe. In short, our SOM‐based analyses of winter MSLP have both confirmed prior knowledge and expanded it through the relative ease of use and power with nonlinear systems of the SOM‐based approach to climatological analysis.

Homoclinic bifurcations in the quasi-geostrophic double-gyre circulation

The wind-driven double-gyre circulation in a rectangular basin goes through several dynamical regimes as the amount of lateral friction is decreased. This paper studies the transition to irregular flow in the double-gyre circulation by applying dynamical systems methodology to a quasigeostrophic, equivalent-barotropic model with a 10-km resolution. The origin of the irregularities, in space and time, is the occurrence of homoclinic bifurcations that involve phase-space behavior far from stationary solutions. The connection between these homoclinic bifurcations and earlier transitions, which occur at larger lateral friction, is explained. The earlier transitions, such as pitchfork and asymmetric Hopf bifurcation, only involve the nonlinear saturation of linear instabilities, while the homoclinic bifurcations are associated with genuinely nonlinear behavior. The sequence of bifurcations—pitchfork, Hopf, and homoclinic—is independent of the lateral friction and may be described as the unfolding of a singularity that occurs in the frictionless, Hamiltonian limit of the governing equations. Two distinct chaotic regimes are identified: Lorenz chaos at relatively large lateral friction versus Shilnikov chaos at relatively small lateral friction. Both types of homoclinic bifurcations induce chaotic behavior of the recirculation gyres that is dominated by relaxation oscillations with a well-defined period. The relevance of these results to the mid-latitude oceans’ observed low-frequency variations is discussed. A previously documented 7-year peak in observed North-Atlantic variability is shown to exist across a hierarchy of models that share the gyre modes and homoclinic bifurcations discussed herein.

Climate teleconnections at monthly time scales in the Ligurian Sea inferred from satellite data

The existence and spatial distribution of possible teleconnections between the South Pacific and North Atlantic oceans and the Ligurian Sea (North-western Mediterranean) are investigated in the present paper. Teleconnections are searched by cross-correlating monthly spatio-temporal time series of 1.1 km resolution sea surface temperature (SST), and a 22.2 km resolution sea level anomaly (SLA), measured from satellite from March 1993 to August 1999, with two indices characterising the South Pacific and the North Atlantic variability: the Southern Oscillation (SO) and the North Atlantic Oscillation (NAO) indices, respectively. Concerning the variability induced by the North Atlantic Ocean, it is shown that it mostly influences the SLA field in the Ligurian Sea. Specifically, relevant anti-correlations between SLA and North Atlantic variability have been found in all the Ligurian sub-basin. As expected by geographical proximity, the effects of North Atlantic on the SLA field in the Ligurian Sea are instantaneous at monthly time scales. Instead, correlations between SST and NAO Index are found at time lag τ = 1 month in the southern part of the basin highlighting the memory of the ocean related to their heat capacity. Significant anti-correlations between SO Index and the SST field in the Ligurian Sea, were obtained at time lag τ = 4 months in the coastal areas of the sub-basin. Results also indicate that the impact of teleconnections in the area studied is not geographically uniform.

The North Atlantic variability structure, storm tracks, and precipitation depending on the polar vortex strength

Abstract. Motivated by the strong evidence that the state of the northern hemisphere vortex in boreal winter influences tropospheric variability, teleconnection patterns over the North Atlantic are defined separately for winter episodes where the zonal wind at 50hPa and 65° N is above or below the critical velocity for vertical propagation of zonal planetary wave 1. We argue that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the polar vortex, while this is not the case at sea level. If the polar vortex is strong, there exists one meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the polar vortex is weak, there exist two such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong polar vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results suggest that it is important to include the state of the polar vortex strength in any study of the variability over the North Atlantic.

Summer Sea Surface Temperature Conditions in the North Atlantic and Their Impact upon the Atmospheric Circulation in Early Winter

The origin of the so-called summer North Atlantic ‘‘Horseshoe’’ (HS) sea surface temperature (SST) mode of variability, which is statistically linked to the next winter’s North Atlantic Oscillation (NAO), is investigated from data and experiments with the CCM3 atmospheric general circulation model (AGCM). Lagged observational analyses reveal a linkage between HS and anomalous rainfall in the vicinity of the Atlantic intertropical convergence zone. Prescribing the observed anomalous convection in the model generates forced atmospheric Rossby waves that propagate into the North Atlantic sector. The accompanying perturbations in the surface turbulent and radiative fluxes are consistent with forcing the SST anomalies associated with HS. It is suggested that HS can therefore be interpreted as the remote footprint of tropical atmospheric changes. The ARPEGE AGCM is then used to test if the persistence of HS SST anomalies from summer to late fall can feed back to the atmosphere and have an impact on the next winter’s North Atlantic variability. Observed HS SST patterns are imposed in the model from August to November. They generate a weak but coherent early winter response projecting onto the NAO and therefore reproduce the observed HS‐NAO relationship obtained from lagged statistics. Changes in the simulated upper-level jet are associated with the anomalous HS meridional SST gradient and interact with synoptic eddy activity from October onward. The strength and position of the transients as a function of seasons are hypothesized to be of central importance to explain the nature, timing, and sign of the model response. In summary, the present study emphasizes the importance of summer oceanic and atmospheric conditions in both the Tropics and extratropics, and their persistence into early winter for explaining part of the NAO’s lowfrequency variability.

Last Interglacial conditions in southern Europe: evidence from Ioannina, northwest Greece

A new record combining isotopic and palynological results over the interval 133–111 ka BP at 100–200 year resolution from a long lacustrine sequence at Ioannina, northwest Greece, is presented. The sequence provides an opportunity to examine the nature of climate variability during the Last Interglacial in southern Europe where information has hitherto been relatively limited. The record shows that the frequency and amplitude of changes during the transitional late glacial and late interglacial phases were markedly higher than that of the full interglacial interval. These differences are probably a reflection of the relative size of ice sheets in the circum-North Atlantic and associated with ice-rafting events and climatic perturbations during these periods. During intervals of increased ice volume, it appears that North Atlantic variability has a significant downstream impact, dominating the climate signal in northwest Greece. During intervals of minimum ice volume, there may be a decoupling between the North Atlantic system and continental climates with other factors, such as insolation changes, becoming more important. The length of the Last Interglacial at Ioannina defined by the presence of forest is here estimated to be ca. 15 ka, in agreement with recent results from Portugal, but in conflict with estimates of ca. 10 ka for the duration of the Eemian in northwest Europe. In the absence of independent confirmation for these estimates, however, these differences and associated implications remain unresolved.

Influence of the North Atlantic Oscillation on the Canary Islands Precipitation

The aim of this paper is to investigate the relationship between the Canary Island rainfall and the Atlantic large-scale circulation, characterized by the North Atlantic oscillation (NAO) index. The Canary Islands are located in the Atlantic subtropical belt under the direct influence of the Azores high and the trade winds. Their steep orography makes the islands very sensitive to small variations in a synoptic situation, thus providing an excellent natural observatory for the North Atlantic variability associated with changes in pressure patterns. A significant relationship between rainfall and the NAO is found for five of the seven Canary Islands. In order to characterize the physical mechanisms involved, a set of automatic objective techniques for identification and detection of disturbances is applied to the 1000- and 500-hPa geopotential from the NCEP–NCAR reanalysis for the period from 1955 to 1998. This method allows for the identification and detection of four main synoptic systems—either at the surface or at 500 hPa—that are responsible for 80% of the precipitation over the Canary Islands: Atlantic surface lows (ASLs), 500-hPa lows (UALs), 500-hPa troughs over the Canary Islands (TROs), and deep Atlantic lows (DALs), which affect the entire troposphere. Three of the detected disturbances (ASLs, TROs, and DALs) exhibit a dipolar structure with a low pressure center over 35°N associated with a positive anomaly at higher latitudes, resembling the NAO negative pattern. The analysis of the variability of the disturbance occurrence and the changes in their associated anomaly patterns shows that deeper and more frequent ASLs and TROs affect the Canary Islands during the negative phase of the NAO. However, UAL disturbances are less frequent, and DALs do not exhibit significant variations with the NAO phase. The standard deviation of the 2.5–8-day bandpass-filtered geopotential height shows that the maximum variability associated with the NAO occurs over the Canary Island area, confirming its sensitivity to NAO variations. The study provides a comprehensive view of the mechanisms involved in the precipitation generation over the Canary Islands, documenting a sensitivity to the NAO influences for a group of islands that have been poorly studied so far.