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Spatial Pattern and Mechanism of Heat Content Change in the North Atlantic
5 Feb, 2008 04:30 pm
Parts of the Earth?s climate system are clearly changing through anthropogenic forcing: a rise in surface and atmospheric temperatures, a rise in sea level, the decline in summer sea ice in the Arctic and the reduction in many glaciers are all clearly documented by the IPCC report.
However, climate scientists have long understood that climate variability can be attributed not only to human influence, but also to natural variability. For instance, the El Niño/La Niña cycle in the Pacific is attributed to the natural variability of the ocean and atmosphere. As the Pacific climate shifts between an El Niño and La Niña state, rainfall patterns, air temperatures and winds over a wide swath of the globe are affected.
Ocean temperatures also change with the cycling of the El Niño/La Niña system. Currently, the Pacific is in a La Niña state and the temperatures of the surface waters of the eastern Pacific are anomalously cool; that is they are cooler than the climatological average. During an El Niña state, these temperatures are anomalously warm.
The Pacific basin does not hold a monopoly on natural climate variability. In the North Atlantic, scientists have noted changes in the large scale wind patterns over the course of decades and have termed these changes the North Atlantic Oscillation. Our recent study in Science focuses on the effect of these atmospheric wind changes on the underlying ocean temperatures. Our study was conducted in two parts: an analysis of historical hydrographic data that has been collected by ships over the past fifty years and an analysis of output from model simulations of the North Atlantic Ocean.
Our data analysis involved comparing temperature data over the full depth of the water column over the last 50 years: 65000 stations in the period 1950-1970 and 98000 stations in the period 1980-2000. The overall heat content change was found from the difference in the heat stored for each period. We found, in general agreement with past studies, that the heat stored in the North Atlantic Ocean has changed between 1950-1970 to 1980-2000, with a gain of heat in the latter period.
While a gain of heat is expected from anthropogenic forcing, the pattern of this gain in heat is more complex than initially expected: heat was gained in the tropics and mid-latitudes, but heat was lost at higher latitudes. In other words, over the fifty years from 1950 to 2000, some parts of the North Atlantic had warmed, other parts had cooled. These regional changes are much larger than the background average change, which led us to suspect that the ocean was responding to more than just anthropogenic warming.
To aid our interpretation of the ocean data, we conducted circulation model experiments driven by realistic surface air-sea fluxes and winds for each period. These experiments revealed that much of the heat content changes over the North Atlantic were associated with a wind-induced redistribution of heat together with a background input of heat in the tropics. This wind-induced change has altered over the last 40 years, associated with the North Atlantic Oscillation, which alters on interannual and decadal timescales. Thus, though some parts of the climate system are certainly changing through anthropogenic forcing, the North Atlantic changes in heat content show a more complicated response than expected, a response that we believe is largely driven by natural variability. While our study results do not preclude the influence of anthropogenic warming on the North Atlantic heat content changes, it appears that decadal variability in the North Atlantic is strong enough, at present, to mask any background warming trend.
Our research is not intended to be a referendum as to whether greenhouse warming is happening, since anthropogenic warming is almost certainly occurring given the wide range of global signals showing change. Rather, our work provides a cautionary note for the investigation and interpretation of climate signals in the ocean. Our work also highlights the need for long-term monitoring of the ocean’s temperature field. Only with such monitoring will oceanographers be able to unravel the influence of anthropogenic forcing and natural variability on the ocean’s temperatures.
Susan Lozier, Susan Leadbetter, Richard G. Williams, et al. "The Spatial Pattern and Mechanisms of Heat Content Change in the North Atlantic." Science Express, 3 January 2008. Abstract availabe here.