One of the first challenges we need to address in this course is this: what do we mean when we use the phrases 'decadal variability' or 'the decadal timescale'?
As you'll see, there is not a strong consensus about the meaning of these terms. Often, scientists adopt their own definitions as it suits their particular research question. For our first exercise, I'd like you to present a brief (one or two-sentence) description of 'decadal variability' as defined by a particular source.
I'd like you to include your response as a comment to this blog entry. Please choose your own unique source that has not been cited by another student (so the earlier you complete this assignment, the more freedom you'll have to choose).
Please add your contribution prior to our second class meeting (9:30 AM on September 13). I'll get you started with the first entry!
Ault and St. George (The magnitude of decadal and multidecadal variability in North American precipitation, Journal of Climate, 2010) examined the strength of decadal and multidecadal variability in seasonal and annual precipitation across North America. They defined 'decadal variability' as the amount of variance situated at frequencies between 10 and 20 years.
Mann and Park (Global-scale modes of surface temperature variability on interannual to century timescales, Journal of Geophysical Research, 1994) characterized global temperature variations across frequency bands ranging from interannual to centennial. The authors identified a "quasi-decadal" mode of temperature variability with a periodicity of 10-12 years.
Hurrell (Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation, Science, 1995) analyzed the decadal behavior of sea level pressure anomalies over two points in the North Atlantic. The author defined decadal variability as a persistent change from the previous mode lasting a decade.
Knutson and Manabe (Model Assesment of Decadal Variability and Trends in the Tropical Pacific Ocean, NOAA/Geophysical Fluid Dynamics Laboratory, 1997) explored trends in seas surface temperature of the Pacific Ocean using a global model of the oceans and the atmosphere. The defined decadal variability as a pattern greater than seven years but subtracting the century-scale signal.
Latif and Barnett (1995) explain two different types of decadal variability that have been reported in the literature. They describe a type of abrupt climate shift with decades of one anomaly followed by decades of an anomaly of the opposite sign, such as precipitation in the Sahel or North Pacific SSTs. On the other hand, strictly oscillatory variations such a North Pacific SSTs have been mentioned as another mode of decadal climate variability. The authors state that these two types are often the same cyclic process. ENSO was originally thought to be a sudden shift but once enough cycles were observed, it became clear that it was a cyclic process.
Bitz and Battisi (Interannual to Decadal Variability in Climate and the Glacier Mass Balance in Washington, Western Canada and Alaska, 1999) explored climatalogical factors influencing mass balance of six maritime glaciers along the northwestern coast of North America. The authors defined decadal variability simply in relation to the PDO (20-30 years) and GR (many years to decades) indices.
Sutton and Allen (Decadal predictability of North Atlantic sea surface temperature and climate, Nature, 1997) analyzed monthly mean SST compiled from shipboard observations and investigated the decadal timescale of North Atlantic SST variability associated with the advective propagation along the Gulf Stream/ the North Atlantic Current (NAC). They suggested that decadal variability, namely decadal fluctuations, has a time scale of 12-14 years in tropical North Atlantic SST and thus can be predicted in advance.
Trenberth and Hurrell (Decadal atmosphere-ocean variations in the Pacific, 1994) investigated the changes in the Pacific over a "decade-long" period from 1976 to 1988 as well as the longer record, and repeatedly refer to decadal timescale and variation without ever truly defining what they meant with the term. They focused on that 12 year frame but also looked at the longer record, and in the analysis used filters to remove periods less than 10 years.