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Preindustrial control simulations with the third Hadley Centre Global Environmental Model, run in the Global Coupled configuration 3.1 of the Met Office Unified Model (HadGEM3‐GC3.1) are presented at two resolutions. These are N216ORCA025, which has a horizontal resolution of 60 km in the atmosphere and 0.25° in the ocean, and N96ORCA1, which has a horizontal resolution of 130 km in the atmosphere and 1° in the ocean. The aim of this study is to document the climate variability in these simulations, make comparisons against present‐day observations (albeit under different forcing), and discuss differences arising due to resolution. In terms of interannual variability in the leading modes of climate variability the two resolutions behave generally very similarly. Notable differences are in the westward extent of El Niño and the pattern of Atlantic multidecadal variability, in which N216ORCA025 compares more favorably to observations, and in the Antarctic Circumpolar Current, which is far too weak in N216ORCA025. In the North Atlantic region, N216ORCA025 has a stronger and deeper Atlantic Meridional Overturning Circulation, which compares well against observations, and reduced biases in temperature and salinity in the North Atlantic subpolar gyre. These simulations are being provided to the sixth Coupled Model Intercomparison Project (CMIP6) and provide a baseline against which further forced experiments may be assessed. Plain Language Summary: In this paper, we present the latest computer models of the joint atmosphere and ocean system. These models were developed at the U.K. Met Office Hadley Centre. They are designed to simulate the climate of the past, present, and future and to be used in scientific analysis and decision making. In this study, the are intended to simulate a continuous preindustrial state, to provide a reference level for future experiments and analysis. We present two resolutions of the same model, where the resolution is analogous to the number of pixels on, for example, a smartphone display. We find that the model with greater resolution also simulates many aspects of the global climate better than the model with lower resolution. These include El Niño, sea surface temperature variability in the Atlantic Ocean, and the depth of the AMOC in the North Atlantic. However, in other aspects, such as the strength of the major current circling Antarctica, this version is worse. Key Points: Preindustrial control simulations for CMIP6 with HadGEM3‐GC3.1 are presented using two model resolutionsOur evaluation focuses on interannual variability in key climate indicesThe high‐resolution model shows improvements in ENSO, AMV, and in the depth structure of the AMOC. However, the ACC strength is worse [ABSTRACT FROM AUTHOR]

This paper explores climate variability of the lower troposphere and boundary layer in the tropical Atlantic sector through a series of modeling simulations with a diagnostic primitive equation model. The focus is on the role that realistic diabatic heating and its vertical placement as well as surface temperature have in inducing/ reinforcing the local monthly wind circulation, the role that thermal and momentum transients play in the Tropics, the potential for feedbacks, and the way through which other basins influence the tropical Atlantic region. NCEP-NCAR reanalysis data for the period 1958-93 are used to provide forcing and model verification. In the first part of the paper local effects are considered. It is found that the most important terms controlling anomalous surface winds over the ocean are boundary layer temperature gradients and diabatic heating anomalies at low levels (below 780 mb). Anomalous diabatic heating at mid- and upper levels (430-690 mb) contributes to the near-surface circulation poleward of 158 over the warm hemisphere. Anomalous diabatic heating over the African continent influences zonal winds well into the ocean. It is found that the anomalies of surface latent heat flux induced by the interhemispheric distribution of anomalies provide positive feedback on both sides of the equator, in the deep Tropics and west of 208W. It provides negative feedback off the northwest coast of Africa. In the second part the relative importance of remote forcing is considered. It is found that anomalous heating associated with interhemispheric gradients of surface temperature in the tropical Atlantic influence winds in the northern extratropics in a wavelike pattern during boreal spring. Anomalous heating associated with equatorial anomalies of surface temperature influence winds in the southern extratropics in a wavelike pattern during boreal summer. In contrast, the influence of heating in the midlatitudes is confined to the northern subtropics.... [ABSTRACT FROM AUTHOR]

This paper aims to describe the spatial-temporal variability in catch of the main fishery resources of the Amazon River and floodplain lakes of the Lower Amazon, as well as relating the Catch per Unit of Effort with anomalies of some of the Amazon River, atmosphere and Atlantic Ocean system variables, determining the influence of the environment on the Amazonian fishery resources. Finfish landings data from the towns and villages of the Lower Amazon for the fisheries of three sites (Óbidos, Santarém and Monte Alegre), were obtained for the period between January 1993 and December 2004. Analysis of variance, detrended correspondence analysis, redundancy analysis and multiple regression techniques were used for the statistical analysis of the distinct time series. Fisheries production in the Lower Amazon presents differences between the Amazon River and the floodplain lakes. Production in the Amazon River is approximately half of the one of the floodplain lakes. This variability occurs both along the Lower Amazon River region (longitudinal gradient) and laterally (latitudinal gradient) for every fishing ground studied here. The distinct environmental variables alone or in association act differently on the fishery stocks and the success of catches in each fishery group studied here. Important variables are the flooding events; the soil the sea surface temperatures; the humidity; the wind and the occurence of El Niño-Southern Oscillation events. Fishery productivity presents a large difference in quantity and distribution patterns between the river and floodplain lakes. This variability occurs in the region of the Lower Amazon as well as laterally for each fishery group studied, being dependent on the ecological characteristics and life strategies of each fish group considered here. [ABSTRACT FROM AUTHOR]

This paper investigates the Atlantic Ocean influence on equatorial Pacific decadal variability. Using an ensemble of simulations, where the ICTPAGCM ('SPEEDY') is coupled to the NEMO/OPA ocean model in the Indo-Pacific region and forced by observed sea surface temperatures in the Atlantic region, it is shown that the Atlantic Multidecadal Oscillation (AMO) has had a substantial influence on the equatorial Pacific decadal variability. According to AMO phases we have identified three periods with strong Atlantic forcing of equatorial Pacific changes, namely (1) 1931-1950 minus 1910-1929, (2) 1970-1989 minus 1931-1950 and (3) 1994-2013 minus 1970-1989. Both observations and the model show easterly surface wind anomalies in the central Pacific, cooling in the central-eastern Pacific and warming in the western Pacific/Indian Ocean region in events (1) and (3) and the opposite signals in event (2). The physical mechanism for these responses is related to a modification of the Walker circulation because a positive (negative) AMO leads to an overall warmer (cooler) tropical Atlantic. The warmer (cooler) tropical Atlantic modifies the Walker circulation, leading to rising (sinking) and upper-level divergence (convergence) motion in the Atlantic region and sinking (rising) motion and upper-level convergence (divergence) in the central Pacific region. [ABSTRACT FROM AUTHOR]

In this paper the optimal precursors for wintertime Eurasian blocking onset are acquired by solving a nonlinear optimization problem whose objective function is constructed based on a blocking index with a triangular T21, three-level, quasi-geostrophic global spectral model. The winter climatological state is chosen as the reference basic state. Numerical results show that the optimal precursors are characterized by a baroclinic pattern with a westward tilt with height, which are mainly located upstream of the blocking region. For an optimization time of 5 days, these perturbations are mainly localized over the Northeast Atlantic Ocean and continental Europe. With the extension of the optimization time to 8 days, these perturbations are distributed more upstream and extensively in the zonal direction. Wave spectrum analysis reveals that the optimal precursors are composed of not only synoptic-scale (wave numbers 5-18) waves, but planetary-scale (wave numbers 0-4) waves as well. The synoptic-scale optimal precursors are mainly located in the mid-latitude area, while the planetary-scale optimal precursors focus primarily on the high-latitude region. The formation of a strong planetary-scale positive blocking anomaly is accompanied by the reinforcement of synoptic-scale perturbations and further fragmentation into two branches, in which the northern branch is generally stronger than the southern one. The eddy forcing arising from the self-interaction of synoptic-scale disturbances is shown to be crucial in triggering the dipole blocking anomaly, and the planetary-scale optimal precursor provides the initial favorable background conditions for blocking onset. [ABSTRACT FROM AUTHOR]

In this paper a detailed global climatology of wind-sea and swell parameters, based on the 45-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) wave reanalysis is presented. The spatial pattern of the swell dominance of the earth''s oceans, in terms of the wave field energy balance and wave field characteristics, is also investigated. Statistical analysis shows that the global ocean is strongly dominated by swell waves. The interannual variability of the wind-sea and swell significant wave heights, and how they are related to the resultant significant wave height, is analyzed over the Pacific, Atlantic, and Indian Oceans. The leading modes of variability of wind sea and swell demonstrate noticeable differences, particularly in the Pacific and Atlantic Oceans. During the Northern Hemisphere winter, a strong north--south swell propagation pattern is observed in the Atlantic Ocean. Statistically significant secular increases in the wind-sea and swell significant wave heights are found in the North Pacific and North Atlantic Oceans. [ABSTRACT FROM AUTHOR]

This review paper discusses the physical basis and the potential for decadal climate predictability over the Atlantic and its adjacent land areas. Many observational and modeling studies describe pronounced decadal and multidecadal variability in the Atlantic Ocean. However, it still needs to be quantified to which extent the variations in the ocean drive variations in the atmosphere and over land. In particular, although a clear impact of the Tropics on the midlatitudes has been demonstrated, it is unclear if and how the extratropical atmosphere responds to midlatitudinal sea surface temperature anomalies. Although the mechanisms behind the decadal to multidecadal variability in the Atlantic sector are still controversial, there is some consensus that some of the longer-term multidecadal variability is driven by variations in the thermohaline circulation. The variations in the North Atlantic thermohaline circulation appear to be predictable one to two decades ahead, as shown by a number of perfect model predictability experiments. The next few decades will be dominated by these multidecadal variations, although the effects of anthropogenic climate change are likely to introduce trends. Some impact of the variations of the thermohaline circulation on the atmosphere has been demonstrated in some studies so that useful decadal predictions with economic benefit may be possible. [ABSTRACT FROM AUTHOR]

The question of the impact of the Atlantic on North American (NA) seasonal prediction skill and predictability is examined. Basic material is collected from the literature, a review of seasonal forecast procedures in Canada and the United States, and some fresh calculations using the NCEP–NCAR reanalysis data. The general impression is one of low predictability (due to the Atlantic) for seasonal mean surface temperature and precipitation over NA. Predictability may be slightly better in the Caribbean and the (sub)tropical Americas, even for precipitation. The NAO is widely seen as an agent making the Atlantic influence felt in NA. While the NAO is well established in most months, its prediction skill is limited. Year-round evidence for an equatorially displaced version of the NAO (named ED_NAO) carrying a good fraction of the variance is also found. In general the predictability from the Pacific is thought to dominate over that from the Atlantic sector, which explains the minimal number of reported Atmospheric Model Intercomparison Project (AMIP) runs that explore Atlantic-only impacts. Caveats are noted as to the question of the influence of a single predictor in a nonlinear environment with many predictors. Skill of a new one-tier global coupled atmosphere–ocean model system at NCEP is reviewed; limited skill is found in midlatitudes and there is modest predictability to look forward to. There are several signs of enthusiasm in the community about using “trends” (low-frequency variations): (a) seasonal forecast tools include persistence of last 10 years’ averaged anomaly (relative to the official 30-yr climatology), (b) hurricane forecasts are based largely on recognizing a global multidecadal mode (which is similar to an Atlantic trend mode in SST), and (c) two recent papers, one empirical and one modeling, giving equal roles to the (North) Pacific and Atlantic in “explaining” variations in drought frequency over NA on a 20 yr or longer time scale during the twentieth century. [ABSTRACT FROM AUTHOR]

Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program. [ABSTRACT FROM AUTHOR]

A thorough analysis of the stability of the uncoupled Rooth interhemispheric three-box model of thermohaline circulation (THC) is presented. The model consists of a northern high-latitude box, a tropical box, and a southern high-latitude box, which correspond to the northern, tropical, and southern Atlantic Ocean, respectively. Restoring boundary conditions are adopted for the temperature variables, and flux boundary conditions are adopted for the salinity variables. This paper examines how the strength of THC changes when the system undergoes forcings that are analogous to those of global warming conditions by applying the equilibrium state perturbations to the moisture and heat fluxes into the three boxes. In each class of experiments, using suitably defined metrics, the authors determine the boundary dividing the set of forcing scenarios that lead the system to equilibria characterized by a THC pattern similar to the present one from those that drive the system to equilibria with a reversed THC. Fast increases in the moisture flux into the northern high-latitude box are more effective than slow increases in leading the THC to a breakdown, while the increases of moisture flux into the southern high-latitude box strongly inhibit the breakdown and can prevent it, as in the case of slow increases in the Northern Hemisphere. High rates of heat flux increase in the Northern Hemisphere destabilize the system more effectively than low ones; increases in the heat fluxes in the Southern Hemisphere tend to stabilize the system. [ABSTRACT FROM AUTHOR]

Although skillful seasonal hurricane forecasts for the Atlantic basin are now a reality, large gaps remain in our understanding of observed variations in the distribution of activity within the hurricane season. The month of August roughly spans the first third of the climatologically most active part of the season, but activity during the month is quite variable. This paper reports on an initial investigation into forecasting year-to-year variability of August tropical cyclone (TC) activity using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset. It is shown that 55%–75% of the variance of August TC activity can be hindcast using a combination of 4–5 global predictors chosen from a 12-predictor pool with each of the predictors showing precursor associations with TC activity. The most prominent predictive signal is the equatorial July 200-mb wind off the west coast of South America. When this wind is anomalously strong from the northeast during July, Atlantic TC activity in August is almost always enhanced. Other July conditions associated with active Augusts include a weak subtropical high in the North Atlantic, an enhanced subtropical high in the northwest Pacific, and low pressure in the Bering Sea region. The most important application of the August-only forecast is that predicted net tropical cyclone (NTC) activity in August has a significant relationship with the incidence of U.S. August TC landfall events. Better understanding of August-only TC variability will allow for a more complete perspective of total seasonal variability and, as such, assist in making better seasonal forecasts. [ABSTRACT FROM AUTHOR]

A predictability analysis developed within a general framework of linear stochastic dynamics in a companion paper is applied to a simple coupled climate model of tropical Atlantic variability (TAV). The simple model extends the univariate stochastic climate model of Hasselmann by including positive air–sea feedback and heat advection by mean ocean currents. The interplay between the positive air–sea feedback and the negative oceanic feedbacks gives rise to oscillatory coupled modes. The relationship between these coupled modes and the system's predictability is explored for a wide range of coupled regimes. It is shown that the system's predictability cannot be simply determined from oscillatory behavior of the dominant coupled mode when coupling is weak. The predictable dynamics of the weakly coupled system depend upon the interference among many coupled modes and the spatial structures of the stochastic forcing. Using the simple model as an example, the concept of optimizing the predictability of a linear stochastic system is illustrated. The implication of these results for the predictability of weakly coupled climate systems in the real world is also discussed. [ABSTRACT FROM AUTHOR]

Abstract: The ARAMIS project (Altimétrie sur un Rail Atlantique et Mesures In Situ) provides a long term survey (2002–2008) for thermohaline structures in the tropical Atlantic Ocean. Here, these data are used to address questions regarding the existence and formation of barrier layers. The analysis presented which uses different criteria to calculate isothermal layer depth and mixed layer depth, shows the sensitivity of different methods to a variety of variables. A 0.5°C criterion for the isothermal layer and its equivalent in density change for the mixed layer are found to be the most adaptable due to their robustness. Barrier layers observed with ARAMIS data sets are in good agreement with climatologies and in-situ Argo floats along the route. Three systems of barrier layers are observed during the voyages. Their variability and formation as described in previous works concerned with the tropical Atlantic are analyzed. Different processes such as subduction, frontal activity, and precipitation are needed to explain their presence. [Copyright &y& Elsevier]

The large-scale context for oceanic subduction is examined in the region bounded by 10°N and 40°N and 10°W and 40°W using data collected from June 1991 to June 1993. In contrast to climatology, it was found that over the entire region the ocean gained heat, ranging from 10–30 W m–2 to the north to 40–50 W m–2 to the south. Steady tradewinds dominated the southeastern half of the region. To the northwest of the axis of these tradewinds was a broad region of downward Ekman pumping with a 2-yr mean of about 50 m yr–1. Wind-driven surface currents and a seasonal cycle in mixed layer temperature and depth were found that reflected the influence of local forcing. The interior circulation was consistent with the clockwise circulation of the eastern end of the subtropical gyre, though eddy variability dominated the northern part of the region. Due to Ekman convergence the winter mixed layer in the central and southwestern parts of the region was as deep as to the north; however, it was also warm. This deep, warm mixed layer in the center of the region aided the subduction of water from the northeast by providing a protective cap; isopycnals that outcropped to the north passed well below the base of this mixed layer. [Copyright &y& Elsevier]

Deals with the inflow of warm and saline Atlantic water to the Nordic seas, an important factor for climate, ecology and biological production in Northern Europe. Large-scale features of Atlantic inflow; Atlantic inflow in the Svinoy section; Transport estimates in the Svinoy section; Seasonality.

This paper constitutes the second part of the analysis of the behaviour of centres of action above the Atlantic since 1881. The present study examines the applicability of the generated time series of the central pressure and geographical location of the Iceland Low (IL), Azores High (AH), Inter Tropical Convergence Zone (ITCZ) and South Atlantic Subtropical High (SSM) to the description of the surface circulation and its influence on the climate anomalies above, the Atlantic and adjacent land areas of' in the period 1950--1989. Using the normalized anomalies of the pressure gradient between the cores of the AH and IL (AH--IL gradient) as a NAO (North Atlantic Oscillation) index, it can be shown that this index is a practicable tool for the detection of characteristic spatial anomaly patterns in different climate variables above the North Atlantic and surrounding land areas. The 'strong' NAO (the AH--IL gradient above 'normal') in winter leads to a warming above northern and central Europe as well as to the developmentof the dipole-like anomaly patterns in the wind speed, sea-surface temperature, surface and tropospheric temperature and other variables above the western and eastern North Atlantic. The composite analysis for the 'weak' NAO reveals inverse patterns. The application of the characteristic parameters of the ITCZ and of the two subtropical highsas predictors of the rainfall variability in northeast Brazil and the Sahel for the period 1950--1983 shows that for the most part, thesevariations can be explained by the anomalies of the latitude position and intensity of the ITCZ. [ABSTRACT FROM AUTHOR]