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Variability of Climate (Decadal Variability - Oceans: Atlantic) -- Summary
Earth's climate is forever changing, on one time scale or another.  Over the past million years, the 100,000-year glacial-interglacial cycle has been the most pronounced of these climatic periodicities; but there is also evidence for millennial-scale, centennial-scale and decadal-scale climatic oscillations.  Our understanding of these cycles, including their causes, is crucial to the global warming debate; for the reality and consistency of these phenomena suggests that the warming of the past century or so may have been nothing more than the natural "next phase" of climatic change following on the heels of the Little Ice Age.  In this summary, we thus examine various types of decadal-scale variations in water properties of the Atlantic Ocean.

Marchitto et al. (1998) studied cadmium/calcium ratios in benthic foraminifera shells contained in sediment cores retrieved from the Bahama Banks region of the Northwest Providence Channel that connects the North Atlantic basin (Sargasso Sea) to the Florida Straits.  The evidence obtained in this investigation demonstrated, in their words, that "periods of enhanced intermediate-water production alternate with periods of enhanced deep-water formation on both orbital and millennial timescales," and that "analogous dynamics operate in the modern North Atlantic on much shorter (decadal) timescales."

This work adds to the mounting body of evidence that demonstrates there are relatively short period cycles in oceanic circulation characteristics that operate within intermediate length cycles that occur within much longer cycles.  This complex nested cyclical behavior of the oceans must bear some relationship (either causative, responsive or both) to a similar complex nested cyclical behavior of climate that cannot be ignored when interpreting historical climatic changes.  Simply put, there are a variety of reasons why the earth may or may not be warming at the present time.  The most recent temperature trend, in and of itself, tells one very little about the reason(s) for its existence or the length of time it may persist.  Studies like this one, which identify past repeated behavior of various oceanic properties, along with similar investigations in a number of other fields, are needed to make this crucial determination.

In a similar type of investigation set in the southern Caribbean, Black et al. (1999) conducted a high-resolution study of sediments deposited there over the past 825 years.  This record revealed substantial variability of both a decadal and centennial nature, which suggests that climate regime shifts are a natural aspect of Atlantic variability; and relating these features to other records of climate variability, Black et al. concluded that "these shifts may play a role in triggering changes in the frequency and persistence of drought over North America."

Another finding of note revealed by this study was the strong correspondence that exists between the up-and-down changes in North Atlantic climate and similar changes in 14C production rate, which is accepted as a valid measure of variable solar activity.  Black et al. say this finding "suggests that small changes in solar output may influence Atlantic variability on centennial time scales," and, we would add, decadal time scales too.  The results of this study thus add to the growing store of knowledge that illustrates that variability is the name of the game when it comes to earth's climate.  It shows that cyclical climate change is natural, and that one of the major drivers of this natural cyclical change is the cyclical variability of various phenomena occurring on or within the sun.

In a somewhat different type study conducted in the northeast Atlantic, Bacon (1998) used hydrographic data to create a 40-year history of deep-water outflow from the Nordic seas to the Atlantic Ocean, comparing it with a concomitant history of polar air temperature.  Over the prior four decades, these data demonstrated that the deep-water current approximately doubled and then returned to flows characteristic of the mid- to late-1950s, and that outflow rates were correlated with the mean air temperature of the three winters that preceded them, with high outflows being associated with low air temperatures, and vice versa.

In a still different type of study that reveals yet other types of decadal oceanic variability, Elsner et al. (2001) used data for annual U.S. hurricane numbers and average sea surface temperature (SST) anomalies for the region bounded by 6°N to 6°S latitude and 90°W to 180°W longitude (called the "cold tongue index" or CTI) to see if there is a connection between the number of hurricanes that hit the eastern coast of the United States each year and the presence or absence of El Niņo conditions.  Based on data for the period 1901-2000, they found that "when CTI values indicate below normal equatorial SSTs, the probability of a U.S. hurricane increases."  Or as they describe the relationship in another place, "the annual count of hurricanes is higher when values of the CTI are lower (La Niņa events)."  Also, they determined that the North Atlantic Oscillation is "an additional important factor in explaining U.S. hurricane activity on the decadal scale after accounting for ENSO."

The few studies reviewed above provide a glimpse into the cyclical world of oceanic and climatic change that combine to define the environmental history of the earth. Until we thoroughly understand how and why these cycles operate as they do, we will not be able to confidently predict the future ... nor even properly interpret the past.

Bacon, S.  1998.  Decadal variability in the outflow from the Nordic seas to the deep Atlantic Ocean.  Nature 394: 871-874.

Black, D.E., Peterson, L.C., Overpeck, J.T., Kaplan, A., Evans, M.N. and Kashgarian, M.  1999.  Eight centuries of North Atlantic Ocean atmosphere variability.  Science 286: 1709-1713.

Elsner, J.B. Bossak, B.H. and Niu, X.F.  2001.  Secular changes to the ENSO-U.S. Hurricane Relationship.  Geophysical Research Letters 28: 4123-4126.

Marchitto Jr., T.M., Curry, W.B. and Oppo, D.W.  1998.  Millennial-scale changes in North Atlantic circulation since the last glaciation.  Nature 393: 557-561.

Last updated 27 April 2005