Greenland ice-core data have revealed large decadal climate variations over the North Atlantic that can be related to a major source of low-frequency variability, the North Atlantic Oscillation. Over the past decade, the Oscillation has remained in one extreme phase during the winters, contributing significantly to the recent wintertime warmth across Europe and to cold conditions in the northwest Atlantic. An evaluation of the atmospheric moisture budget reveals coherent large-scale changes since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and parts of Scandinavia have generally experienced wetter than normal conditions.

Climate influences a variety of ecological processes. These effects operate through local weather parameters such as temperature, wind, rain, snow, and ocean currents, as well as interactions among these. In the temperate zone, local variations in weather are often coupled over large geographic areas through the transient behavior of atmospheric planetary-scale waves. These variations drive temporally and spatially averaged exchanges of heat, momentum, and water vapor that ultimately determine growth, recruitment, and migration patterns. Recently, there have been several studies of the impact of large-scale climatic forcing on ecological systems. We review how two of the best-known climate phenomena-the North Atlantic Oscillation and the El Niño-Southern Oscillation-affect ecological patterns and processes in both marine and terrestrial systems.

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The North Atlantic Oscillation (NAO) is one of the most prominent and recur rent patterns of atmospheric circulation variability. It dictates climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during boreal winter, so variations in the NAO are important to society and for the environment. Understanding the processes that govern this variability is, therefore, of high priority, especially in the context of global climate change. This review, aimed at a scientifically diverse audience, pro vides general background material for the other chapters in the monograph, and it synthesizes some of their central points. It begins with a description of the spatial structure of climate and climate variability, including how the NAO relates to other prominent patterns of atmospheric circulation variability. There is no unique way to define the spatial structure of the NAO, or thus its temporal evolution, but several common approaches are illustrated. The relationship between the NAO and variations in surface temperature, storms and precipitation, and thus the econ omy, as well as the ocean and ecosystem responses to NAO variability, are described. Although the NAO is a mode of variability internal to the atmosphere, indices of it exhibit decadal variability and trends. That not all of its variability can be attributed to intraseasonal stochastic atmospheric processes points to a role for external forcings and, perhaps, a small but useful amount of predictability. The surface, stratospheric and anthropogenic processes that may influence the phase and amplitude of the NAO are reviewed. 1. INTRODUCTION Over the middle and high latitudes of the Northern Hemisphere (NH), especially during the cold season months (November-April), the most prominent and recurrent pattern of atmospheric variability is the North Atlantic Oscillation (NAO). The NAO refers to a redistribution of atmospheric mass between the Arctic and the subtropical Atlantic, and swings from one phase to another produce large changes in

Decadal and interannual variations in the global atmosphere and ocean are investigated using the global sea surface temperature (SST), sea-level pressure, surface winds, precipitation and Northern Hemisphere 500hPa geopotential height data for 30 to 50 years. The most dominant interannual variation in the global atmosphere and ocean is associated with El Nino and Southern Oscillation (ENSO) phenomena. Two dominant decadal modes are obtained. The first mode mainly dominates over the entire Pacific Ocean and changed abruptly in the late 1970s. The spatial pattern of the tropical SST after 1978 contains large positive anomalies over most of the tropical ocean, particularly over the eastern Pacific. Negative SST anomalies are found in the mid-latitudes over the North and the South Pacific. The sea-level pressure in the tropics shows negative anomalies in the eastern Pacific, but positive anomalies in the western Pacific and Indian Oceans. There exist westerly anomalies in the equatorial western-central Pacific, indicating the weakening of the tropical Walker circulation. The surface precipitation increased in the tropical central-eastern Pacific where large increases of SST exist. Composite and singular value decomposition (SVD) analyses demonstrate that the Pacific/North American (PNA) pattern was strengthened and associated with this first decadal mode. It is suggested that the enhanced tropical convection due to the increase of SST over the tropical central-eastern Pacific may strengthen the PNA pattern. The second decadal mode has dominant SST anomalies mainly in the Northern Hemisphere mid-latitudes. The Northern Hemisphere 500hPa height field associated with this mode shows a dominance of a north-south seesaw between middle latitudes and higher latitudes. A north-south dipole structure in the western Atlantic is especially noticeable. Large positive anomalies extending from East Asia to the North Pacific are also significant. The second mode has a time scale of about 15 years and changed its sign from negative to positive around 1987. Linear trends for SST, sea-level pressure, surface winds and 500hPa height are also investigated. Significant positive trends of SST are found over the most tropical regions and negative SST trends exist in the North Pacific. There exist large trends of sea-level pressure and surface winds over the North Atlantic. Easterlies are strengthened over most of the tropics. The PNA pattern in the 500hPa height field has tended to intensify during the last 50 years.

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Variability of the North Atlantic Oscillation and the Tropical Atlantic dominate the climate of the North Atlantic sector, the underlying ocean and surrounding continents on interannual to decadal time scales. Here we review these phenomena, their climatic impacts and our present state of understanding of their underlying cause. Copyright