Robert Dickson

Long-Term Coordinated Changes in the Convective Activity of the North Atlantic

The North Atlantic is a peculiarly convective ocean. The convective renewal of intermediate and deep waters in the Labrador Sea and Greenland/Iceland Sea both contribute significantly to the production and export of North Atlantic Deep Water, thus helping to drive the global thermohaline circulation, while the formation and spreading of 18-degree water at shallow-to-intermediate depths off the US eastern seaboard is a major element in the circulation and hydrographic character of the west Atlantic. For as long as time-series of adequate precision have been available to us, it has been apparent that the intensity of convection at each of these sites, and the hydrographic character of their products have been subject to major interannual change, as shown by Aagaard (1968), Clarke et al (1990), and Meincke et al (1992) for the Greenland Sea, in the OWS BRAVO record from the Labrador Sea, (eg Lazier,1980 et seq.), and at the PANULIRUS / Hydrostation “S” site in the Northern Sargasso off Bermuda (eg Jenkins, 1982, Talley and Raymer, 1982). This paper reviews the recent history of these changes showing that the major convective centres of the Greenland- and Labrador Seas are currently at opposite convective extrema in our postwar record, with vertical exchange at the former site limited to 1000 m or so, but with Labrador Sea convection reaching deeper than previously observed, to over 2300 m. As a result, Greenland Sea Deep Water has become progressively warmer and more saline since the early ‘70’s due to increased horizontal exchange with the Arctic Ocean through Fram Strait, while the Labrador Sea Water has become progressively colder and fresher over the same period through increased vertical exchange; most recently, convection has become deep enough there to reach into the more saline NADW which underlies it, so that cooler, but now saltier and denser LSW has resulted.

The Arctic Ocean Response to the North Atlantic Oscillation

The climatically sensitive zone of the Arctic Ocean lies squarely within the domain of the North Atlantic oscillation (NAO), one of the most robust recurrent modes of atmospheric behavior. However, the specific response of the Arctic to annual and longer-period changes in the NAO is not well understood. Here that response is investigated using a wide range of datasets, but concentrating on the winter season when the forcing is maximal and on the postwar period, which includes the most comprehensive instrumental record. This period also contains the largest recorded low-frequency change in NAO activity—from its most persistent and extreme low index phase in the 1960s to its most persistent and extreme high index phase in the late 1980s/early 1990s. This longperiod shift between contrasting NAO extrema was accompanied, among other changes, by an intensifying storm track through the Nordic Seas, a radical increase in the atmospheric moisture flux convergence and winter precipitation in this sector, an increase in the amount and temperature of the Atlantic water inflow to the Arctic Ocean via both inflow branches (Barents Sea Throughflow and West Spitsbergen Current), a decrease in the late-winter extent of sea ice throughout the European subarctic, and (temporarily at least) an increase in the annual volume flux of ice from the Fram Strait.

The Ocean's Response to North Atlantic Oscillation Variability

The North Atlantic Oscillation (NAO) is the dominant mode of atmospheric variability in the North Atlantic Sector. Basin scale changes in the atmospheric forcing significantly affect properties and circulation of the ocean. Part of the response is local and rapid (surface temperature, mixed-layer depth, upper ocean heat content, surface Ekman transport, sea ice cover). However, the geostrophically balanced large-scale horizontal and overturning circulation can take several years to adjust to changes in the forcing. The delayed response is non-local in the sense that waves and the mean circulation communicate perturbations at the air-sea interface to other parts of the Atlantic basin. A delayed and non-local response can potentially give rise to oscillatory behavior if there is significant feedback from the ocean to the atmosphere. We conjecture that, on decadal and longer time scales, changes in the oceans heat storage and transport should have an increasingly important impact on the climate. Finally, changes in the ocean circulation and distribution of heat and freshwater will also alter ventilation rates and pathways. Thus we expect a change in the net uptake of gases (e.g., O 2 , CO 2 ), altered nutrient balance, and changes in the dispersion of marine life. We review what is known about the oceanic response to changes in NAO-induced forcing from combined theoretical, numerical experimentation and observational perspectives.

The Biological Response in the Sea to Climatic Changes

Publisher Summary This chapter examines the response of animals to climatic change on a world-wide scale. In terms of populations, the response to climatic change is the prime environmental agent. Individuals are affected by a gamut of environmental factors, but the populations have evolved to resist them insofar as they can. The evolution of migration has resulted in a larval drift between spawning ground and nursery ground, where, presumably, the opportunities for larval survival, in terms of food, have been maximized for very long periods. Thus, the match/mismatch mechanism is really part of the pattern of migration; it should be absent from populations that do not migrate and from those that may not experience lack of food. The chapter attempts to describe the principal climatic change makers that are at work and identifies some characteristic time and space scales of climatic variation. Some recent trends of climatic and hydrographic variation are also described. Set against this background of environmental change, the chapter reviews those changes in marine populations that are thought to be attributable to some variation in the physical environment. These various elements of change are brought together in an attempt to cast light on the processes involved.

Atlantic Climate Variability and Predictability: A CLIVAR Perspective

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.

Satellite Evidence of Enhanced Upwelling Along the European Continental Slope

Abstract TIROS-N AVHRR imagery is used to describe a persistent but localized band of upwelling which follows the contours of the European continental slope from the Porcupine Seabight (southwest of Ireland) to the Bay of Biscay. Its persistent occurrence, its close association with the upper part of the slope, and the northward broadening of the upwelling region are shown to be consistent with recently published theory (Killworth, 1978) concerning the enhancement of upwelling by interaction between slope topography and Kelvin (or other) waves propagating along the slope. Some limited evidence of enhanced biological productivity is also described.

Environmental Influences on the Survival of North Sea Cod

The problem of relating indices of recruitment to the physical environment of a species is, as Larkin (1970, p. 9) has pointed out, that “virtually any set of stock-recruit data is sufficiently variable to inspire untestable hypotheses about the effects of trends in environments, especially with the wealth of meteorological and oceanographic data that can be mined for real and fortuitous correlations”. Consequently, any observed relationship between recruitment and the environment should be examined critically, and any conclusions reached should be considered as tentative, at least until they can be tested against predicted results.

The Thermocline Response to Transient Atmospheric Forcing in the Interior Midlatitude North Pacific 1976–1978

Abstract The Ekman pumping mechanism for altering the depth of the main thermocline in response to wind stress curl is tested in the central midlatitude North Pacific. According to this mechanism, the depth of the main thermocline should decrease under cyclonic wind stress curl and increase under anticyclonic wind stress curl. For the two years 1976–78, temperature measurements from an XBT measurement program between North America and Japan have allowed the monthly thermal structure to be measured over an area 30–50°N, 130–170°W, accompanied with synoptic estimates of wind stress curl. Working with anomalous estimates that deviate from the normal seasonal cycle, the month-to-month secular change in the depth of the main thermocline during the nine months of each year from February to October is found to have responded to the anomalous wind stress curl according to what was expected from the Ekman pumping mechanism. The expected and observed secular changes in the thermocline depth for these times of the y...

On the Sources of North Atlantic Deep Water

Abstract Because the volumetric census of deep and bottom water in the North Atlantic Ocean consists of three isolated linear ridges along which heat and salt flow through the main volumetric mode (and point of intersection), it is possible to deduce the expected ratio of heat flux and ratio of salt fluxes measured in the Denmark Strait overflow off Greenland and in the Antarctic Bottom Water near the equator. The weakly stratified layers of upper North Atlantic Deep Water fall on the nearly linear ridge at temperatures above that of the mode. There is an incompatibility between observed ratio and deduced ratio. It is predicted that a remeasurement of the flux of Antarctic Bottom Water near the equator will show that the previous determination of 4°N is unrepresentatively low.