The mineral component of pelagic sediment is brought to the deep sea by transport in the wind. Extraction and analysis of this dust allows estimation of the past aridity of the eolian source region, via flux determinations, and of the intensity of the transporting winds, from grain size data. These two parameters, the grain size and mass flux of dust, vary independently. There are three significant sources of modern dust, eastern and central Asia, northwest Africa, and Arabia, all in the northern hemisphere. As the rainfall associated with the Intertropical Convergence Zone is an effective barrier to southerly transport of dust, the northern hemisphere is an order of magnitude more “dusty” than is the southern, an asymmetry that has characterized most of the Cenozoic. Eolian flux data show that most of the northern hemisphere was more arid during glacial maxima, with 3 to 5 times as much dust transported during glacial stages than during interglacials; only northwestern South America varied in the opposite sense. The periodicity of Quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the Milankovitch cycles of orbital variability. Wind intensities vary on a shorter time scale than the general 100-kyr cycles of glaciation and general aridity. Eolian grain sizes display forcing at precessional (19 and 23 kyr), tilt (41 kyr), and at approximately 30 kyr periodicities. As a result the generalization that winds are uniformly stronger during glacial times is not valid. Whole-Cenozoic records show that the largest change in dust flux, an order of magnitude increase, occurred in the northern hemisphere and reflects continental drying associated with the late Pliocene onset of northern hemisphere glaciation. Southern hemisphere eolian records show no sign of paleoclimatic changes in the late Pliocene. The most important change in Cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the Paleocene-Eocene boundary. Before then, latest Cretaceous and Paleocene winds were essentially as strong as those of the late Cenozoic. This shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 Ma.

The paleoclimatic record provided by eolian deposition in the deep sea: The geologic history of wind

The evidence and implications of polar ice during the Mesozoic

. The early Eocene "equable climate problem", i.e. warm extratropical annual mean and above-freezing winter temperatures evidenced by proxy records, has remained as one of the great unsolved problems in paleoclimate. Recent progress in modeling and in paleoclimate proxy development provides an opportunity to revisit this problem to ascertain if the current generation of models can reproduce the past climate features without extensive modification. Here we have compiled early Eocene terrestrial temperature data and compared with climate model results using a consistent and rigorous methodology. We test the hypothesis that equable climates can be explained simply as a response to increased greenhouse gas forcing within the framework of the atmospheric component of the Community Climate System Model (version 3), a climate model in common use for predicting future climate change. We find that, with suitably large radiative forcing, the model and data are in general agreement for annual mean and cold month mean temperatures, and that the pattern of high latitude amplification recorded by proxies can be largely, but not perfectly, reproduced.

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The early Eocene equable climate problem revisited

Twenty-two monthly statistical quantities make up this dataset of grids which describes the atmospheric general circulation at eleven vertical levels across the troposphere and stratosphere over a 32-year period. These gridded analyses, created from upper air soundings across the globe, span the period from May 1958 to December 1989. The statistical parameters, which are useful for comparing climate model simulations to actual observations, include monthly means and variances of horizontal and vertical wind, temperature, geopotential height, moisture, and horizontal wind divergence. There are also covariance terms describing the relationship between wind, temperature, moisture, and geopotential height. In general, the analyses near the surface of the earth (900mb and below) tend to be less reliable than those of the higher levels because the individual rawinsonde reports are often incomplete due to missing 1000mb reports.

Curt Covey

Papers

The role of ocean heat transport in climatic change