Decadal-scale Trends Research Articles

Decadal-scale Trends Across Several Marine Trophic Levels in the Southern Benguela Upwelling System off South Africa

Decadal-scale Trends Across Several Marine Trophic Levels in the Southern Benguela Upwelling System off South Africa

Decadal-scale Trends Across Several Marine Trophic Levels in the Southern Benguela Upwelling System off South Africa

Abstract Whereas zooplankton in most eastern boundary current systems (EBCSs) studied to date has declined in terms of biomass over the past 5 decades, it has increased 100-fold in terms of numerical abundance in the southern Benguela region, thereby undergoing a displacement in species composition through time. These long-term changes in coastal zooplankton off South Africa have previously been hypothesized to be mediated by both “bottom-up” and “top-down” control mechanisms, in contrast to other EBCSs and marine ecosystems in general which have traditionally been viewed to be structured from below. From a “bottom-up” perspective, the increase in zooplankton abundance is a long-term biological response to intensified coastal upwelling, common to EBCSs, and its associated processes of nutrient enrichment and increased phytoplankton production and biomass. On the other hand, such increase in zooplankton and the accompanying shift in its community structure are thought to result from a “cascading” trophic e...

Reduced sensitivity of recent tree-growth to temperature at high northern latitudes

Tree-ring chronologies that represent annual changes in the density of wood formed during the late summer can provide a proxy for local summertime air temperature1. Here we undertake an examination of large-regional-scale wood-density/air-temperature relationships using measurements from hundreds of sites at high latitudes in the Northern Hemisphere. When averaged over large areas of northern America and Eurasia, tree-ring density series display a strong coherence with summer temperature measurements averaged over the same areas, demonstrating the ability of this proxy to portray mean temperature changes over sub-continents and even the whole Northern Hemisphere. During the second half of the twentieth century, the decadal-scale trends in wood density and summer temperatures have increasingly diverged as wood density has progressively fallen. The cause of this increasing insensitivity of wood density to temperature changes is not known, but if it is not taken into account in dendroclimatic reconstructions, past temperatures could be overestimated. Moreover, the recent reduction in the response of trees to air-temperature changes would mean that estimates of future atmospheric CO2 concentrations, based on carbon-cycle models that are uniformly sensitive to high-latitude warming, could be too low.

GLACIAL VARVE THICKNESS AND 127 YEARS OF INSTRUMENTAL CLIMATE DATA: A COMPARISON

Annually laminated sediments (glacial varves) from Lake Silvaplauna, a High Alpine proglacial lake in the Central Swiss Alps, were compared with glacier monitoring data and instrumental climate data from 1864 to 1990. Long-term and short-term responses to climatic change as well as anthropogenic influence can be traced separately in the varve succession. Economic development in the lake catchment has resulted in higher autochthonous production in recent years. Autochthonous components contribute around 10% to the total amount of sediment accumulated annually since 1960 but their contribution is negligible before this date. Decadal-scale varve thickness trends correlate with glacier size-variations. A stepwise, running multiple regression analysis demonstrates that interannual changes in varve thickness are strongly correlated with changes in mean summer temperatures, but cannot be sufficiently explained without considering summer precipitation and the number of days with snow per year. The wide range of observed correlation coefficients reveals the sensitivity of the archive to temporal variability of the climatic forcing factors and makes the development of transfer functions ambiguous.

Low-Frequency Ocean-Atmosphere Variability across the Tropical Western Pacific

Abstract Oceanic and atmospheric data from the tropical western Pacific are analyzed to describe decadal-scale trends during the last 20 years, and these low-frequency trends are compared with shorter-term Southern Oscillation-related variations. Regional indices of western and central equatorial Pacific SST exhibit significant upward trends in recent decades. The decadal variability in the tropical Pacific is large enough relative to interannual variability to significantly affect the interpretation of standardized SST anomaly indices used to monitor Southern Oscillation phenomena. Specific humidity in the tropical western Pacific boundary layer exhibits a statistically significant upward trend consistent with previously published results based on a shorter data record. The convective instability of the tropical troposphere is increasing, but two indices related to precipitation show no evidence of a trend. These trends cannot be explained as an aggregate of the effects of more frequent El Nino warm even...

Large-Scale Atmospheric Forcing of Recent Trends toward Early Snowmelt Runoff in California

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate cause of the trend toward warmer winters in California is a concurrent, long-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishable from natural atmospheric variability. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central North Pacific Ocean that is teleconnected to California temperatures. Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures.

Lead variability in the western North Atlantic Ocean and central Greenland ice: Implications for the search for decadal trends in anthropogenic emissions

As Patterson and coworkers have shown, most of the lead in the modern ocean and atmosphere is of anthropogenic origin. Reductions in the utilization of leaded gasoline over the past two decades should decrease lead deposition from the atmosphere in remote locations. The search for trends in Pb deposition within a single decade is bedeviled by large-amplitude short-term variability due to the inherent noisiness of the atmosphere/ocean system. We find that, over the course of a year, lead concentrations in the surface waters of the western North Atlantic Ocean are variable (factor of 2), and in the snow deposited in central Greenland, highly variable (order of magnitude). In the western North Atlantic, 210Pb normalization minimizes this problem because 210Pb and Pb sources are spatially correlated and continental 210Pb emissions are constant. It is clear the Pb in surface waters of the western North Atlantic has decreased by a factor of 4 during the 1980s. 210Pb normalization does not help in the Arctic because stable Pb and 210Pb are not spatially correlated. Because of the order-of-magnitude variability in Greenland snow Pb linked to annual cycles, any discontinuous time series is likely to be affected by the phenomenon of aliasing. Aliasing makes it difficult to determine if there is a trend in Pb deposition in central Greenland during the 1980s; present evidence suggests that the reduction in Pb concentration in snow during the 1980s is less than a factor of two; certainly quite a bit less than observed in the western North Atlantic and less than the factor of >7 reduction in leaded gasoline utilization in the United States during the decade. Although we expect that decadal-scale trends in the 1970s and 1980s are in fact occurring due to the phasing out of leaded gasoline, the reported magnitude of decadal-scale trends should be regarded with some reservation until confirmed by independent samplings.

Climatic variability of temperature and humidity over the tropical western Pacific

The vertical structures of recent decadal‐scale trends of temperature and humidity over the tropical western Pacific Ocean are documented using daily data at 16 pressure levels (from the surface to 300 mbar) from four radiosonde stations. Both temperature and specific humidity have increased throughout the troposphere since the mid‐1970 's. The specific humidity increase is sufficiently large that relative humidity also shows positive trends at all tropospheric levels above the surface despite the warming trend. The specific humidity trend is greatest near the surface and decreases with height, whereas the temperature trend increases with height above the boundary layer. Specific humidity in the tropics is shown to be a sensitive indicator of long‐term climate change, because the vertical structure of the specific humidity trend (largest near the surface) is distinctly different from the vertical structure of the seasonal cycle or interannual variability of specific humidity variability, which exhibit peak amplitudes in mid‐troposphere. Thus short‐term climate variability of humidity is a poor proxy for long‐term climate change. Reproduction of the difference in vertical structures of long‐term and short‐term specific humidity fluctuations would provide a good test of the fidelity of model projections of climate change.