Interannual and Decadal Variability of Landfalling Tropical Cyclones in the Southeast Coastal States of the United States

The interannual variability of the Atlantic tropical cyclone(TC)frequency is well known.Separately,recent studies have also suggested that a much longer,multidecadal(40-60 year)trend might be emerging from the recent increase in Atlantic TC activity.However,the overall structure of the intrinsic frequencies(or temporal modes)of Atlantic TC activity is not yet known.The focus of this study is to systematically analyze the intrinsic frequencies of Atlantic TC activity using hurricane and tropical storm landfall data collected along the southeast coast(SEC)of the United States.Based on an Empirical Mode Decomposition (EMD) analysis of the frequency of landfall TCs along the SEC from 188701999,we have found that Atlantic TC activity has four primary,temporal modes.The interannual and multidecadal modes reported in the published literature are two such modes.After identifying all primary modes,the relative importance of each mode and its physical cause can be analyzed.For example,the most energetic mode is the interannual mode(2-7 year period).This mode is known to be associated with the 2-7 year El Nino/La Nino cycle.The average number of annual landfalling TCs along the SEC decreased by 24% during El Nino years,but did not show significant increase during weak and moderate La Nina years.However,intense La Nina years were generally associated with more than average landfalling TCs along the SEC.The effects of El Nino and La Nina also became more significant when only hurricanes were considered.The significance of the effects of El Nino and La Nina on landfalling TCs and hurricanes in different US southeast coastal states showed significant differences.     

Modelling the dominant climate signals around southern Africa

In this study, statistical techniques are employed to decompose climate signals around southern Africa into the dominant temporal frequencies, with the aim of modelling and predicting area-averaged rainfall. In the rainfall time series over the period 1900–1999, the annual cycle accounts for 83% of variance. Residual spectral energy cascades from biennial (42%) to interannual (20%) to decadal bands (3%). Regional climate signals are revealed through a multi-taper singular value decomposition analysis of sea surface temperature and sea level pressure fields over the Atlantic and Indian Oceans, in conjunction with southern Africa rainfall. Rossby wave action in the South Indian Ocean dominates the biennial scale variability. El Niño-Southern Oscillation (ENSO) and related Indian Ocean dipole patterns are important for interannual variability. Significant sea temperature and pressure fluctuations occurring 6–12 months prior to rainfall contribute biennial and interannual indices to a multi-variate model that demonstrates useful predictive skill.     

Remotely forced variability in the tropical Atlantic Ocean

An ensemble of eight hindcasts has been conducted using an ocean-atmosphere general circulation model fully coupled only within the Atlantic basin, with prescribed observational sea surface temperature (SST) for 1950–1998 in the global ocean outside the Atlantic basin. The purpose of these experiments is to understand the influence of the external SST anomalies on the interannual variability in the tropical Atlantic Ocean. Statistical methods, including empirical orthogonal function analysis with maximized signal-to-noise ratio, have been used to extract the remotely forced Atlantic signals from the ensemble of simulations. It is found that the leading external source on the interannual time scales is the El Niño/Southern Oscillation (ENSO) in the Pacific Ocean. The ENSO signal in the tropical Atlantic shows a distinct progression from season to season. During the boreal winter of a maturing El Niño event, the model shows a major warm center in the southern subtropical Atlantic together with warm anomalies in the northern subtropical Atlantic. The southern subtropical SST anomalies is caused by a weakening of the southeast trade winds, which are partly associated with the influence of an atmospheric wave train generated in the western Pacific Ocean and propagating into the Atlantic basin in the Southern Hemisphere during boreal fall. In the boreal spring, the northern tropical Atlantic Ocean is warmed up by a weakening of the northeast trade winds, which is also associated with a wave train generated in the central tropical Pacific during the winter season of an El Niño event. Apart from the atmospheric planetary waves, these SST anomalies are also related to the sea level pressure (SLP) increase in the eastern tropical Atlantic due to the global adjustment to the maturing El Niño in the tropical Pacific. The tropical SLP anomalies are further enhanced in boreal spring, which induce anomalous easterlies on and to the south of the equator and lead to a dynamical oceanic response that causes cold SST anomalies in the eastern and equatorial Atlantic from boreal spring to summer. Most of these SST anomalies persist into the boreal fall season.

---

The nonlinear association between ENSO and the Euro-Atlantic winter sea level pressure

A nonlinear projection of the tropical Pacific sea surface temperature anomalies (SSTA) onto the Northern Hemisphere winter sea level pressure (SLP) anomalies by neural networks (NN) was performed to investigate the nonlinear association between El Niño-Southern Oscillation (ENSO) and the Euro-Atlantic winter climate. While the linear impact of ENSO on the Euro-Atlantic winter SLP is weak, the NN projection reveals statistically significant SLP anomalies over the Euro-Atlantic sector during both extreme cold and warm ENSO episodes, suggesting that the Euro-Atlantic climate mainly responds to ENSO nonlinearly. The nonlinear response, mainly a quadratic response to the SSTA, reveals that regardless of the sign of the SSTA, positive SLP anomalies are found over the North Atlantic, stretching from eastern Canada to Europe (with anomaly center located just northwestward of Portugal), and negative anomalies centered over Scandinavia and Norwegian Sea, consistent with the excitation of the positive North Atlantic Oscillation pattern.     

Coupled variability and air-sea interaction in the South Atlantic Ocean

A total of 52 years of data (1949–2000) from the NCEP/NCAR reanalysis are used to investigate mechanisms involved in forcing and damping of sea surface temperature (SST) variability in the South Atlantic Ocean. Organized patterns of coupled ocean–atmosphere variability are identified using EOF and SVD analyses. The leading mode of coupled variability consists of an SST pattern with a strong northeast–southwest gradient and an SLP monopole centered at 15°W, 45°S. The anomalous winds associated with this monopole generate the SST pattern through anomalous latent heat flux and mixed layer deepening. Other heat flux components and anomalous Ekman transport play only a secondary role. Once established, the SST pattern is attenuated through latent heat flux. The higher SST modes are also induced by anomalous winds and destroyed by latent heat flux. It thus appears that the coupled variability in the South Atlantic Ocean consists of atmospheric circulation anomalies that induce SST anomalies through anomalous latent heat fluxes and wind-induced mixed layer deepening. These SST anomalies are destroyed by latent heat flux with no detectable systematic feedback onto the atmospheric circulation. Atmospheric variability in the South Atlantic is found to be largely independent of that elsewhere, although there is a weak relation with ENSO (El Niño-Southern Oscillation).     

Interannual modulation of subtropical Atlantic boreal summer dust variability by ENSO

Dust variability in the climate system has been studied for several decades, yet there remains an incomplete understanding of the dynamical mechanisms controlling interannual and decadal variations in dust transport. The sparseness of multi-year observational datasets has limited our understanding of the relationship between climate variations and atmospheric dust. We use available in situ and satellite observations of dust and a century-length fully coupled Community Earth System Model (CESM) simulation to show that the El Niño-Southern Oscillation (ENSO) exerts a control on North African dust transport during boreal summer. In CESM, this relationship is stronger over the dusty tropical North Atlantic than near Barbados, one of the few sites having a multi-decadal observed record. During strong La Niña summers in CESM, a statistically significant increase in lower tropospheric easterly wind is associated with an increase in North African dust transport over the Atlantic. Barbados dust and Pacific SST variability are only weakly correlated in both observations and CESM, suggesting that other processes are controlling the cross-basin variability of dust. We also use our CESM simulation to show that the relationship between downstream North African dust transport and ENSO fluctuates on multidecadal timescales and is associated with a phase shift in the North Atlantic Oscillation. Our findings indicate that existing observations of dust over the tropical North Atlantic are not extensive enough to completely describe the variability of dust and dust transport, and demonstrate the importance of global models to supplement and interpret observational records.     

Sea surface temperature associations with the late Indian summer monsoon

Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.     

Low-frequency variability and CO2 transient climate change. Part 3. Intermonthly and interannual variability

Components of interannual, intermonthly, and total monthly variability of lower troposphere temperature are calculated from a global coupled ocean-atmosphere general circulation model (GCM) (referred to as the coupled model), from the same atmospheric model coupled to a nondynamic mixedlayer ocean (referred to as the mixed-layer model), and from microwave sounding unit (MSU) satellite data. The coupled model produces most features of intermonthly and interannual variability compared to the MSU data, but with somewhat reduced amplitude in the extratropics and increased variability in the tropical western Pacific and tropical Atlantic. The relatively short 14-year period of record of the MSU data precludes definitive conclusions about variability in the observed system at longer time scales (e.g., decadal or longer). Different 14-year periods from the coupled model show variability on those longer time scales that were noted in Part 1 of this series. The relative contributions of intermonthly and interannual variability that make up the total monthly variability are similar between the coupled model and the MSU data, suggesting that similar mechanisms are at work in both the model and observed system. These include El Niño-Southern Oscillation (ENSO)-type interannual variability in the tropics, Madden-Julian Oscillation (MJO) type intermonthly variability in the tropics, and blocking-type intermonthly variability in the extratropics. Manifestations of all of these features have been noted in various versions of the model. Significant changes of variability noted in the coupled model with doubled carbon dioxide differ from those in our mixed-layer model and earlier studies with mixed-layer models. In particular, in our mixed-layer model intermonthly and interannual variability changes are similar with a mixture of regional increases and decreases, but with mainly decreases in the zonal mean from about 20°S to 60°N and near 60°S. In the coupled model, intermonthly and interannual changes of variability with doubled CO₂ show mostly increases of tropical interannual variability and decreases of intermonthly variability near 60°N. These changes in the tropics are related to changes in ENSO, the south Asian monsoon, and other regional hydrological regimes, while the alterations near 60°N are likely associated with changes in blocking activity. These results point to the important contribution from ENSO seen in the coupled model and the MSU data that are not present in the mixed-layer model.     

Using El Niño/Southern Oscillation Climate Data to Predict Rice Production in Indonesia

Despite the strong signal of El Niño/Southern Oscillation (ENSO) events on climate in the Indo-Pacific region, models linking ENSO-based climate variability to seasonal rice production and food security in the region have not been well developed or widely used in a policy context. This study successfully measures the connections among sea surface temperature anomalies (SSTAs), rainfall, and rice production in Indonesia during the past three decades. Regression results show particularly strong connections on Java, where 55% of the country's rice is grown. Two-thirds of the interannual variance in rice plantings and 40% of the interannual variance in rice production during the main (wet) season on Java are explained by year-to-year fluctuations in SSTAs measured 4 and 8 months in advance, respectively. These effects are cumulative; during strong El Niño years, production shortfalls in the wet season are not made up later in the crop year. The analysis demonstrates that quantitative predictions of ENSO's effects on rice harvests can provide an additional tool for managing food security in one of the world's most populous and important rice-producing countries.     

Dynamics of the Indian monsoon and ENSO relationships in the SINTEX global coupled model

This paper uses recent gridded climatological data and a coupled general circulation model (GCM) simulation in order to assess the relationships between the interannual variability of the Indian summer monsoon (ISM) and the El Niño-Southern Oscillation (ENSO). The focus is on the dynamics of the ISM-ENSO relationships and the ability of the state-of-the-art coupled GCM to reproduce the complex lead-lag relationships between the ISM and the ENSO. The coupled GCM is successful in reproducing the ISM circulation and rainfall climatology in the Indian areas even though the entire ISM circulation is weaker relative to that observed. In both observations and in the simulation, the ISM rainfall anomalies are significantly associated with fluctuations of the Hadley circulation and the 200 hPa zonal wind anomalies over the Indian Ocean. A quasi-biennial time scale is found to structure the ISM dynamical and rainfall indices in both cases. Moreover, ISM indices have a similar interannual variability in the simulation and observations. The coupled model is less successful in simulating the annual cycle in the tropical Pacific. A major model bias is the eastward displacement of the western North Pacific inter-tropical convergence zone (ITCZ), near the dateline, during northern summer. This introduces a strong semiannual component in Pacific Walker circulation indices and central equatorial Pacific sea surface temperatures. Another weakness of the coupled model is a less-than-adequate simulation of the Southern Oscillation due to an erroneous eastward extension of the Southern Pacific convergence zone (SPCZ) year round. Despite these problems, the coupled model captures some aspects of the interannual variability in the tropical Pacific. ENSO events are phase-locked with the annual cycle as observed, but are of reduced amplitude relative to the observations. Wavelet analysis of the model Niño34 time series shows enhanced power in the 2–4 year band, as compared to the 2–8 year range for observations during the 1950–2000 period. The ISM circulation is weakened during ENSO years in both the simulation and the observations. However, the model fails to reproduce the lead-lag relationship between the ISM and Niño34 sea surface temperatures (SSTs). Furthermore, lag correlations show that the delayed response of the wind stress over the central Pacific to ISM variability is insignificant in the simulation. These features are mainly due to the unrealistic interannual variability simulated by the model in the western North Pacific. The amplitude and even the sign of the simulated surface and upper level wind anomalies in these areas are not consistent with observed patterns during weak/strong ISM years. The ISM and western North Pacific ITCZ fluctuate independently in the observations, while they are negatively and significantly correlated in the simulation. This isolates the Pacific Walker circulation from the ISM forcing. These systematic errors may also contribute to the reduced amplitude of ENSO variability in the coupled simulation. Most of the unrealistic features in simulating the Indo-Pacific interannual variability may be traced back to systematic errors in the base state of the coupled model.     

A statistical study of the regional impact of deforestation on climate in the LMD GCM

The present study uses the general circulation model of the Laboratoire de Météorologie Dynamique (LMD-GCM) coupled to the land-surface, vegetation model SECHIBA. The impact of deforestation on climate is discussed. Replacing tropical forests by degraded pastures changes albedo, the roughness length and the hydrological properties of the surface. The experiment was carried out over eleven years using the observed sea surface temperature from 1978 to 1988, which includes two major El Niño events. The discussion of the results in this study is limited to the regional impact of deforestation. The changes found for the surface fluxes in Amazonia, Africa and Indonesia are examined in detail and compared in order to understand the impact on temperature. Special attention is paid to feedback mechanisms which compensate for the surface changes and to the statistical significance of these results within the tropical variability of climate. It is shown that the relatively small regional impact of deforestation in this study is statistically significant and largely independent of the El Niño-Southern Oscillation phenomenon.This paper was presented at the Second International Conference on Modelling of Global Climate Change and Variability (Hamburg, September 1992)     

A brief introduction to the issue of climate and marine fisheries

Climatic variability has profound effects on the distribution, abundance and catch of oceanic fish species around the world. The major modes of this climate variability include the El Niño-Southern Oscillation (ENSO) events, the Pacific Decadal Oscillation (PDO) also referred to as the Interdecadal Pacific Oscillation (IPO), the Indian Ocean Dipole (IOD), the Southern Annular Mode (SAM) and the North Atlantic Oscillation (NAO). Other modes of climate variability include the North Pacific Gyre Oscillation (NPGO), the Atlantic Multidecadal Oscillation (AMO) and the Arctic Oscillation (AO). ENSO events are the principle source of interannual global climate variability, centred in the ocean–atmosphere circulations of the tropical Pacific Ocean and operating on seasonal to interannual time scales. ENSO and the strength of its climate teleconnections are modulated on decadal timescales by the IPO. The time scale of the IOD is seasonal to interannual. The SAM in the mid to high latitudes of the Southern Hemisphere operates in the range of 50–60 days. A prominent teleconnection pattern throughout the year in the Northern Hemisphere is the North Atlantic Oscillation (NAO) which modulates the strength of the westerlies across the North Atlantic in winter, has an impact on the catches of marine fisheries. ENSO events affect the distribution of tuna species in the equatorial Pacific, especially skipjack tuna as well as the abundance and distribution of fish along the western coasts of the Americas. The IOD modulates the distribution of tuna populations and catches in the Indian Ocean, whilst the NAO affects cod stocks heavily exploited in the Atlantic Ocean. The SAM, and its effects on sea surface temperatures influence krill biomass and fisheries catches in the Southern Ocean. The response of oceanic fish stocks to these sources of climatic variability can be used as a guide to the likely effects of climate change on these valuable resources.     

Multidecadal variability of summer temperature over Romania and its relation with Atlantic Multidecadal Oscillation

We investigate the multidecadal variability of summer temperature over Romania as measured at 14 meteorological stations with long-term observational records. The dominant pattern of summer temperature variability has a monopolar structure and shows pronounced multidecadal variations. A correlation analysis reveals that these multidecadal variations are related with multidecadal variations in the frequency of four daily atmospheric circulation patterns from the North Atlantic region. It is found that on multidecadal time scales, negative summer mean temperature (TT) anomalies are associated with positive sea level pressure (SLP) anomalies centered over the northern part of the Atlantic Ocean and Scandinavia and negative SLP anomalies centered over the northern part of Africa. It is speculated that a possible cause of multidecadal fluctuations in the frequency of these four patterns are the sea surface temperature (SST) anomalies associated to the Atlantic Multidecadal Oscillation (AMO). These results have implications for predicting the evolution of summer temperature over Romania on multidecadal time scales.     

IMPACTS OF DIFFERENT KINDS OF ENSO ON LANDFALLING TROPICAL CYCLONES IN CHINA

Interannual variability of landfalling tropical cyclones(TCs) in China during 1960-2010 is investigated.By using the method of partial least squares regression(PLS-regression),canonical ENSO and ENSO Modoki are identified to be the factors that contribute to the interannual variability of landfalling TCs.El Ni o Modoki years are associated with a greater-than-average frequency of landfalling TCs in China,but reversed in canonical El Ni o years.Significant difference in genesis locations of landfalling TCs in China for the two kinds of El Ni o phases occurs dominantly in the northern tropical western North Pacific(WNP).The patterns of low-level circulation anomalies and outgoing longwave radiation(OLR) anomalies associated with landfalling TC genesis with different types of El Ni o phases are examined.During canonical El Ni o years,a broad zonal band of positive OLR anomalies dominates the tropical WNP,while the circulation anomalies exhibit a meridionally symmetrical dipole pattern with an anticyclonic anomaly in the subtropics and a cyclonic anomaly near the tropics.In El Ni o Modoki years,a vast region of negative OLR anomalies,roughly to the south of 25°N with a strong large-scale cyclonic anomaly over the tropical WNP,provides a more favorable condition for landfalling TC genesis compared to its counterpart during canonical El Ni o years.For more landfalling TCs formed in the northern tropical WNP in El Ni o Modoki years,there are more TCs making landfall on the northern coast of China in El Ni o Modoki years than in canonical El Ni o years.The number of landfalling TCs is slightly above normal in canonical La Ni a years.Enhanced convection is found in the South China Sea(SCS) and the west of the tropical WNP,which results in landfalling TCs forming more westward in canonical La Ni a years.During La Ni a Modoki years,the landfalling TC frequency are below normal,owing to an unfavorable condition for TC genesis persisting in a broad zonal band from 5°N to 25°N.Since the western North Pacific subtropical high(WNPSH) in La Ni a Modoki years is located in the westernmost region,TCs mainly make landfall on the south coast of China.     

Air–Sea Flux Estimates And The 1997–1998 Enso Event

Bulk formulae for wind stress, sensible and latent heat flux are presented that are suitable for strong mesoscale events such as westerly wind bursts that contribute to the El Niño-Southern Oscillation (ENSO). Their exchange coefficients for heat and momentum have a simple polynomial dependence on wind speed and a linear dependence on air–sea temperature difference. The accuracy of these formulae are validated with respect to air–sea fluxes estimated using the standard algorithm adopted by the Tropical Ocean-Global AtmosphereCoupled-Ocean Atmosphere Response Experiment (TOGA COARE). The comparison ismade for observations from 96 Tropical Atmosphere Ocean (TAO) array and National Oceanographic Data Center (NODC) moorings in the equatorial and North Pacific Ocean spanning years 1990–1999. The bulk formulae are shown to have very small median root–mean-square differences with respect to the TOGA COARE estimates: 0.003 N m^-2, 1.0 W m^-2, and 10.0 W m^-2 for the wind stress, sensible heat flux, and latent heat flux, respectively.The variability of air–sea fluxes during the 1997–1998 ENSO is also examined, along with a possible relationship between air–sea fluxes and surface ocean mixed layer depth (MLD). The wind stress and latent heat flux during the 1997 El Niño are found to be greater in the warm pool of the western Pacific than in the central Pacific where the ENSO is most clearly seen. These differences disappear upon the start of La Niña. The MLD in the equatorial Pacific is found to be moderately correlated to air–sea fluxes just before the start of the 1998 La Niña and poorly correlated otherwise.     

What Drives the Decadal Variability of Global Tropical Storm Days from 1965 to 2019?

The tropical storm day(TSD)is a combined measure of genesis and lifespan.It reflects tropical cyclone(TC)overall activity,yet its variability has rarely been studied,especially globally.Here we show that the global total TSDs exhibit pronounced interannual(3-6 years)and decadal(10 years)variations over the past five-to-six decades without a significant trend.The leading modes of the interannual and decadal variability of global TSD feature similar patterns in the western Pacific and Atlantic,but different patterns in the Eastern Pacific and the Southern Indian Ocean.The interannual and decadal leading modes are primarily linked to El Ni?o-Southern Oscillation(ENSO)and Pacific Decadal Oscillation(PDO),respectively.The TSDs-ENSO relationship has been steady during the entire 55-year period,but the TSDs-PDO relationship has experienced a breakdown in the 1980 s.We find that the decadal variation of TSD in the Pacific is associated with the PDO sea surface temperature(SST)anomalies in the tropical eastern Pacific(PDO-E),while that in the Atlantic and the Indian Ocean is associated with the PDO SST anomalies in the western Pacific(PDO-W).However,the PDO-E and PDO-W SST anomalies are poorly coupled in the 1980 s,and this"destructive PDO"pattern results in a breakdown of the TSDs-PDO relationship.The results here have an important implication for seasonal to decadal predictions of global TSD.     

The signature of ENSO in the Northern Hemisphere midlatitude seasonal mean flow and high-frequency intraseasonal variability

Summary The impact of pronounced positive and negative sea surface temperature (STT) anomalies in the tropical Pacific associated with the El Niño/Southern Oscillation (ENSO) phenomenon on the atmospheric circulation in the Northern Hemisphere extratropics during the boreal winter season is investigated. This includes both the impact on the seasonal mean flow and on the intraseasonal variability on synoptic time scales. Moreover, the interaction between the transient fluctuations on these times scales and the mean circulation is examined. Both data from an ensemble of five simulations with the ECHAM3 atmospheric general circulation model at a horizontal resolution of T42 each covering the period from 1979 through 1992 and operational analyses from ECMWF for the corresponding period are examined. In each of the simulations observed SSTs for the period of investigation are given as lower boundary forcing, but different atmospheric initial conditions are prescribed.The simulations with ECHAM3 reveal a distinct impact of the pronounced SST-anomalies in the tropical Pacific on the atmospheric circulation in the Northern Hemisphere extratropics during El Niño as well as during La Niña events. These changes in the atmospheric circulation, which are found to be highly significant in the Pacific/North American as well as in the Atlantic/European region, are consistent with the essential results obtained from the analyses. The pronounced SST-anomalies in the tropical Pacific lead to changes in the mean circulation, which are characterized by typical circulation patterns. These changes in the mean circulation are accompanied by marked variations of the activity of the transient fluctuations on synoptic time scales, that are changes in both the kinetic energy on these time scales and the atmospheric transports of momentum and heat accomplished by the short baroclinic waves. The synoptic disturbances, on the other hand, play also an important role in controlling the changes in the mean circulation associated with the ENSO phenomenon. They maintain these typical circulation patterns via barotropic, but counteract them via baroclinic processes.The hypothesis of an impact of the ENSO phenomenon in the Atlantic/European region can be supported. As the determining factor the intensification (reduction) of the Aleutian low and the simultaneous reduction (intensification) of the Icelandic low during El Niño and during La Niña events respectively, is identified. The changes in the intensity of the Aleutian low during the ENSO-events are accompanied by an alteration of the transport of momentum caused by the short baroclinic waves over the North American continent in such a way that the changes in the intensity of the Icelandic low during El Niño as well as during La Niña events are maintained.With 16 Figures     

Interannual and interdecadal oscillation patterns in sea level

Relative sea-level height (RSLH) data at 213 tide-gauge stations have been analyzed on a monthly and an annual basis to study interannual and interdecadal oscillations, respectively. The main tools of the study are singular spectrum analysis (SSA) and multi-channel SSA (M-SSA). Very-low-frequency variability of RSLH was filtered by SSA to estimate the linear trend at each station. Global sea-level rise, after postglacial rebound corrections, has been found to equal 1.62±0.38 mm/y, by averaging over 175 stations which have a trend consistent with the neighboring ones. We have identified two dominant time scales of El Niño-Southern Oscillation (ENSO) variability, quasi-biennial and low-frequency, in the RSLH data at almost all stations. However, the amplitudes of both ENSO signals are higher in the equatorial Pacific and along the west coast of North America. RSLH data were interpolated along ocean coasts by latitudinal intervals of 5 or 10 degrees, depending on station density. Interannual variability was then examined by M-SSA in five regions: eastern Pacific (25°S–55°N at 10° resolution), western Pacific (35°S–45°N at 10°), equatorial Pacific (123°E–169°W, 6 stations), eastern Atlantic (30°S, 0°, and 30°N–70°N at 5°) and western Atlantic (50°S–50°N at 10°). Throughout the Pacific, we have found three dominant spatio-temporal oscillatory patterns, associated with time scales of ENSO variability; their periods are 2, 2.5–3 and 4–6 y. In the eastern Pacific, the biennial mode and the 6-y low-frequency mode propagate poleward. There is a southward propagation of low-frequency modes in the western Pacific RSLH, between 35°N and 5°S, but no clear propagation in the latitudes further south. However, equatorward propagation of the biennial signal is very clear in the Southern Hemisphere. In the equatorial Pacific, both the quasi-quadrennial and quasi-biennial modes at 10°N propagate westward. Strong and weak El Niño years are evident in the sea-level time series reconstructed from the quasi-biennial and low-frequency modes. Interannual variability with periods of 3 and 4–8 y is detected in the Atlantic RSLH data. In the eastern Atlantic region, we have found slow propagation of both modes northward and southward, away from 40–45°N. Interdecadal oscillations were studied using 81 stations with sufficiently long and continuous records. Most of these have variability at 9–13 and some at 18 y. Two significant eigenmode pairs, corresponding to periods of 11.6 and 12.8 y, are found in the eastern and western Atlantic ocean at latitudes 40°N–70°N and 10°N–50°N, respectively.     

Variability of the Atlantic meridional overturning circulation in the last millennium and two IPCC scenarios

The variability of the Atlantic meridional overturning circulation (AMOC) is investigated in several climate simulations with the ECHO-G atmosphere-ocean general circulation model, including two forced integrations of the last millennium, one millennial-long control run, and two future scenario simulations of the twenty-first century. This constitutes a new framework in which the AMOC response to future climate change conditions is addressed in the context of both its past evolution and its natural variability. The main mechanisms responsible for the AMOC variability at interannual and multidecadal time scales are described. At high frequencies, the AMOC is directly responding to local changes in the Ekman transport, associated with three modes of climate variability: El Ni?o-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the East Atlantic (EA) pattern. At low frequencies, the AMOC is largely controlled by convection activity south of Greenland. Again, the atmosphere is found to play a leading role in these variations. Positive anomalies of convection are preceded in 1?year by intensified zonal winds, associated in the forced runs to a positive NAO-like pattern. Finally, the sensitivity of the AMOC to three different forcing factors is investigated. The major impact is associated with increasing greenhouse gases, given their strong and persistent radiative forcing. Starting in the Industrial Era and continuing in the future scenarios, the AMOC experiences a final decrease of up to 40% with respect to the preindustrial average. Also, a weak but significant AMOC strengthening is found in response to the major volcanic eruptions, which produce colder and saltier surface conditions over the main convection regions. In contrast, no meaningful impact of the solar forcing on the AMOC is observed. Indeed, solar irradiance only affects convection in the Nordic Seas, with a marginal contribution to the AMOC variability in the ECHO-G runs.     

Nonlinear interdecadal changes of the El Niño-Southern Oscillation

Nonlinear interdecadal changes in the El Niño-Southern Oscillation (ENSO) phenomenon are investigated using several tools: a nonlinear canonical correlation analysis (NLCCA) method based on neural networks, a hybrid coupled model, and the delayed oscillator theory. The leading NLCCA mode between the tropical Pacific wind stress (WS) and sea surface temperature (SST) reveals notable interdecadal changes of ENSO behaviour before and after the mid 1970s climate regime shift, with greater nonlinearity found during 1981–99 than during 1961–75. Spatial asymmetry (for both SST and WS anomalies) between warm El Niño and cool La Niña events was significantly enhanced in the later period. During 1981–99, the location of the equatorial easterly anomalies was unchanged from the earlier period, but in the opposite ENSO phase, the westerly anomalies were shifted eastward by up to 25°. According to the delayed oscillator theory, such an eastward shift would lengthen the duration of the warm events by up to 45%, but leave the duration of the cool events unchanged. Supporting evidence was found from a hybrid coupled model built with the Lamont dynamical ocean model coupled to a statistical atmospheric model consisting of either the leading NLCCA or CCA mode.