The climate associated with global oceans undergoes natural variability with many time scales. A recent report in the Journal Science identifies atmospheric blocking over the North Atlantic as a major factor in such variability on time scales as short as a week. The correspondence between blocked westerly winds and warm ocean can be seen in recent decadal episodes, particularly from 1996 to 2010. It also describes much longer time scale Atlantic multidecadal ocean variability (AMV), including the extreme northern warming of the 1930s to 1960s, which predates the rise of human CO2 emissions that alarmists are convinced is causing all recent global warming.
Circulation of the waters in the Atlantic Ocean and the atmosphere above it influence regions far beyond the immediate surrounding area. Naturally, scientists have attempted to understand and predict the often dramatic year-to-year variability of the North Atlantic Oscillation (NAO), but modeling the associated ocean-atmosphere interaction has not yielded satisfactory results let alone accurate long-term predictions. Long-term warming and cooling cycles, referred to as Atlantic multidecadal variability (AMV), have traditionally been attributed to variability in the ocean’s overturning circulation—the famous ocean conveyor belt. In the November 4, 2011, issue of Science, Sirpa Häkkinen, Peter B. Rhines and Denise L. Worthen provide an alternative explanation for this variation based on atmospheric circulation patterns.
Atmospheric blocking over the northern North Atlantic, which involves isolation of large regions of air from the westerly circulation for 5 days or more, influences fundamentally the ocean circulation and upper ocean properties by affecting wind patterns. Winters with clusters of more frequent blocking between Greenland and western Europe correspond to a warmer, more saline subpolar ocean. The correspondence between blocked westerly winds and warm ocean holds in recent decadal episodes (especially 1996 to 2010). It also describes much longer time scale Atlantic multidecadal ocean variability (AMV), including the extreme pre–greenhouse-gas northern warming of the 1930s to 1960s. The space-time structure of the wind forcing associated with a blocked regime leads to weaker ocean gyres and weaker heat exchange, both of which contribute to the warm phase of AMV.
Though there have been recent discoveries that cast doubt on our understanding of the meridional overturning current (MOC), few climate scientists would discount the impact that the northward flow of warm water in the Atlantic has on climate. When studies have focused on atmospheric circulation they have usually concentrated on fixed spatial patterns such as the NAO, a back and forth oscillation in pressure between Iceland and the Azores. In contrast, Häkkinen et al.suggest that the ocean circulations can be forced by atmospheric conditions anywhere over the Atlantic—conditions called atmospheric blocks.
“Blocking occurs when the high-latitude jet stream develops large, nearly stationary meanders, essentially breaking Rossby waves with precursors upwind, over North America,” the report states. “These trap an air mass equatorward of an anticyclonic pressure ridge.” This is illustrated in the image below, from a perspective article by Tim Woollings of the Department of Meteorology, University of Reading.
In the normal weather pattern (A), the jet stream carries air from North America across the Atlantic to Europe. The wind shear on the flanks of the jet acts to maintain the ocean gyre circulations. During a blocking weather pattern (B), the strong westerly winds of the jet stream are diverted north and south around a dramatic reversal of streamlines. Häkkinen et al. propose that such short-term blocking events influence the Atlantic ocean’s circulation on time scales of several decades.
According to developing theory, the interannual variations of the NAO are governed by the dynamics of short-term weather events such as blocking. Häkkinen et al. take this idea a step farther and suggest that blocks over different parts of the Atlantic may have the same effect on AMV. “The possibility of coupled interaction of atmosphere with AMV seems likely, given the long-period variability of blocking reported here and in the even longer paleoclimate time series,” the authors report.
Häkkinen et al.’s results are based on the recent 20th-Century Reanalysis project (20CR), an international effort to produce a comprehensive global atmospheric circulation dataset spanning the twentieth century, assimilating only surface pressure reports and using observed monthly sea-surface temperature and sea-ice distributions as boundary conditions. This project provides 56 different versions of recent climate variability, each consistent with the available observations. From the reanalysis data indices of wind-related climate variability are calculated using empirical orthogonal function analysis of the winter (December to March) wind-stress curl.
In statistics and signal processing, the method of empirical orthogonal function (EOF) analysis is a decomposition of a signal or data set in terms of orthogonal basis functions which are determined from the data. It is the same as performing a principal components analysis on the data, except that the EOF method finds both time series and spatial patterns. According to a highly recommended text on the subject by I.T. Jolliffe: “The central idea of principal component analysis (PCA) is to reduce the dimensionality of a data set consisting of a large number of interrelated variables, while retaining as much as possible of the variation present in the data set. This is achieved by transforming to a new set of variables, the principle components, which are uncorrelated, and which are ordered so that the first few retain most of the variation present in all of the original variables.”
The December through March wind-stress curl variability from the 20th century atmospheric reanalysis based on EOF analysis is shown in the figure above. EOF1 (top) represents 22.3% of the wind-stress curl variability and has its centers of action displaced north-south relative to the subpolar ocean gyre. EOF2 (bottom), with 15.6% of the variance, has centers of action coinciding with the subpolar and northern subtropical ocean gyres. See the article for further discussion.
If you are getting the idea that this is work of a highly complex and technical nature you are spot on. Needless to say, there have been some criticisms of both the methodology employed, mostly centered around uncertainty in some of the reanalysis data, and of the interpretation of the results. Quoting from the perspective by Woollings:
In regions and periods of good data coverage, the ensemble members agree remarkably well, but they diverge where data are sparse. Häkkinen et al. do not use this information on uncertainty, and it seems an essential next step to use this, and perhaps other methods, to investigate how robust the reported long-term changes in blocking are.
From an ocean perspective, the theory fuels an ongoing debate over the relative importance of wind stress forcing and buoyancy changes associated with air-sea fluxes for North Atlantic variability. For example, other studies suggest that the delayed response to buoyancy forcing played a central role in some of the recent changes described by Häkkinen et al.
So do atmospheric currents drive the AMV and its associated changes in ocean currents? Or is it the other way around? As with most things involving Earth’s magnificently complex climate system there is no simple answer. Ocean and atmosphere are so intimately connected that their interaction has defied analysis and certainly confounded attempts to model it accurately. This is not the first time that the boundary between ocean and atmosphere has surprised scientists. Consider how important the interaction between air and water is, the mind-boggling complexity of what happens at that interface, and it becomes obvious that claims of a deep understanding by scientists regarding the future climate are laughable. This science is no more settled than a wind tossed sea.
Be safe, enjoy the interglacial and stay skeptical.