How Long Will the Naturally Caused Drought Persist in the U.S.?


Anthony Watts recently published a post about the current drought in the U.S. titled To NCDC: We Haven’t Seen an El Nino since 2009/10, What Do You Expect? It reminded me and other persons (see Don B’s comment here) of Roger Pielke, Sr.’s post last year Perspective On The Hot and Dry Continental USA For 2012 Based On The Research Of Judy Curry and Of McCabe Et Al 2004. Roger, Sr. initially referred to a presentation by Judith Curry (Climate Dimensions of the Water Cycle). Judith discussed the presentation in her blog post here. Her presentation included a group of drought maps from McCabe et al (2004).

McCABE ET AL (2004)

McCabe et al (2004) Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States is an examination of the impacts of the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation on drought in the United States.  Full paper is here.   The abstract reads:

More than half (52%) of the spatial and temporal variance in multidecadal drought frequency over the conterminous United States is attributable to the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). An additional 22% of the variance in drought frequency is related to a complex spatial pattern of positive and negative trends in drought occurrence possibly related to increasing Northern Hemisphere temperatures or some other unidirectional climate trend. Recent droughts with broad impacts over the conterminous U.S. (1996, 1999-2002) were associated with North Atlantic warming (positive AMO) and northeastern and tropical Pacific cooling (negative PDO). Much of the long-term predictability of drought frequency may reside in the multidecadal behavior of the North Atlantic Ocean. Should the current positive AMO (warm North Atlantic) conditions persist into the upcoming decade, we suggest two possible drought scenarios that resemble the continental-scale patterns of the 1930s (positive PDO) and 1950s (negative PDO) drought.

My Figure 1 is Figure 5 from McCabe et al (2004). The two right-hand maps indicate drought conditions (in red) during positive AMO conditions. Map c is for positive AMO and positive PDO conditions, and map d indicates positive AMO with negative PDO conditions.

Figure 1

Figure 1

The AMO went positive in the mid-1990s, and it is still positive. Considering that its “cycle” varies from 50 to 80 years, it’s difficult to tell when an AMO cycle peaks and starts its decline, so we may easily have another couple of decades of positive AMO-induced drought in store. Refer to Figure 2, which is a comparison of the Atlantic Multidecadal Oscillation Index data from the NOAA ESRL (here), and the Pacific Decadal Oscillation Index (here). The AMO data has been standardized (divided by its standard deviation) to put it in the same format as the PDO index data. And both datasets have been smoothed with 121-month filters, which is one of the standard formats provided by the ESRL for their AMO data.

Figure 2

Figure 2

Looking at the two datasets since 1979 smoothed with 13-month filters, Figure 3, the PDO has cycled quite strongly between positive and negative values since 1998. I’ve also included standardized NINO3.4 sea surface temperature data in the graph. The NINO3.4 data are a commonly used index for the strength, frequency and duration of El Niños (positive spikes) and La Niñas (negative spikes). Much of the yearly variability in the PDO data are, of course, responses to El Niño and La Niña events, as are many of the AMO wiggles. The AMO rise may have slowed somewhat, again difficult to tell, but the PDO data has been trending toward cold PDO conditions. This additional long-term variation in the PDO data is caused by changes in wind patterns and the interdependent changes in sea level pressure in the North Pacific.

Figure 3

Figure 3

In their FAQ webpage here, NOAA answers in response to the question “Can we predict the AMO?” (my boldface):

We are not yet capable of predicting exactly when the AMO will switch, in any deterministic sense. Computer models, such as those that predict El Niño, are far from being able to do this. What is possible to do at present is to calculate the probability that a change in the AMO will occur within a given future time frame. Probabilistic projections of this kind may prove to be very useful for long-term planning in climate sensitive applications, such as water management.

Based on the results of McCabe et al (2004), drought conditions will likely cycle between maps c and d in their Figure 5 (my Figure 1) until the AMO decides to shift. Refer again to the closing sentence of the McCabe et al (2004) abstract:

Should the current positive AMO (warm North Atlantic) conditions persist into the upcoming decade, we suggest two possible drought scenarios that resemble the continental-scale patterns of the 1930s (positive PDO) and 1950s (negative PDO) drought.

McCabe et al (2004) has been cited more than 400 times by other peer-reviewed papers.


Another paper that describes the influence of the Atlantic Multidecadal Oscillation on precipitation in the United States is Enfield et al (2001) The Atlantic multidecadal oscillation and its relation to rainfall and river flows in the continental U.S. The abstract of Enfield et al (2001) reads (my boldface):

North Atlantic sea surface temperatures for 1856-1999 contain a 65-80 year cycle with a 0.4 _C range, referred to as the Atlantic Multidecadal Oscillation (AMO) by Kerr [2000]. AMO warm phases occurred during 1860- 1880 and 1940-1960, and cool phases during 1905-1925 and 1970-1990. The signal is global in scope, with a positively correlated co-oscillation in parts of the North Pacific, but it is most intense in the North Atlantic and covers the entire basin there. During AMO warmings most of the United States sees less than normal rainfall, including Midwest droughts in the 1930s and 1950s. Between AMO warm and cool phases, Mississippi River outflow varies by 10% while the inflow to Lake Okeechobee, Florida varies by 40%. The geographical pattern of variability is influenced mainly by changes in summer rainfall. The winter patterns of interannual rainfall variability associated with El Niño- Southern Oscillation are also significantly changed between AMO phases.

Enfield et al (2001) has been cited more than 700 times.


The draft of the upcoming NCADAC Climate Assessment Report was released for comment earlier this year. The NCADAC stands for “National Climate Assessment and Development Advisory Committee”. From that NCADAC webpage (my boldface):

The NCADAC, whose members are available here (and in the report), was established under the Department of Commerce in December 2010 and is supported through the National Oceanic and Atmospheric Administration (NOAA). It is a federal advisory committee established as per the Federal Advisory Committee Act of 1972. The Committee serves to oversee the activities of the National Climate Assessment. Its members are diverse in background, expertise, geography and sector of employment. A formal record of the committee can be found at the NOAA NCADAC website.

The National Climate Assessment is discussed here (my boldface):

The National Climate Assessment (NCA) is being conducted under the authority of the Global Change Research Act (GCRA) of 1990. The GCRA requires a report to the President and the Congress every four years that integrates, evaluates, and interprets the findings of the U.S. Global Change Research Program (USGCRP). The Act requires assessment of the effects of global change (both human-induced and natural) on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity. The time periods for analysis include current conditions as well as projections of major trends for the subsequent 25 to 100 years.

The full draft of the NCADAC report is a large file (147MB) and it’s available here.

Drought is logically one of the topics of the NCADAC draft report. The word drought appears more than 400 times in it. References to drought begin with the alarmist speculations in the opening paragraph of the Executive Summary:

Climate change is already affecting the American people. Certain types of weather events have become more frequent and/or intense, including heat waves, heavy downpours, and, in some regions, floods and droughts. Sea level is rising, oceans are becoming more acidic, and glaciers and arctic sea ice are melting. These changes are part of the pattern of global climate change, which is primarily driven by human activity.

The references to drought end with the last two sentences of the final page of text:

Extreme summer ice retreat also appears to be increasing the persistence of associated mid-latitude weather patterns, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves (Francis and Vavrus 2012). However, the combination of interannual variability and the small sample of years with extreme ice retreat make it difficult to identify a geographically consistent atmospheric response pattern in the middle latitudes.

The NCADAC’s politically motivated attribution of extreme weather events to human activity, of course, contradicts the findings of the IPCC’s Special Report. As Roger Pielke, Jr., reported in his blog post A Few Comments on the IPCC SREX Report:

Most importantly, the IPCC should be congratulated for delivering a message that cannot have been comfortable to deliver.  The IPCC has accurately reflected the scientific literature on the state of attribution with respect to extreme events — it is not there yet, not even close, for events such as floods, hurricanes, tornadoes, bushfires and on other topics there remain enormous uncertainties.  That is just the way that it is, so that is indeed what the IPCC should have reported.

Droughts are one of those other topics.


The data-based analyses like Enfield et al (2001) and McCabe et al (2004) found the Atlantic Multidecadal Oscillation to be a major contributor to drought conditions in the United States, so one would think the Atlantic Multidecadal Oscillation would be a frequent topic of discussion in the NCADAC report. It was mentioned once, and that reference had nothing to do with drought.


Under the heading of “29. Research Agenda for Climate Change Science” and subheading of “Research Goal 1 – Deepen understanding of the climate system, feedbacks, and impacts” the NCADAC draft report states:

High priority research needs include…

…Improved understanding of the interactions of climate change and natural variability at multiple time scales, including seasonal to decadal changes (and consideration of the El Niño Southern Oscillation, Pacific Decadal Oscillation, North Atlantic Oscillation, etc.), extreme events (hurricanes, droughts, and floods), potential changes in ocean circulation related to climate change, and the global transfer of heat laterally and toward the poles;

That of course raises a couple of fundamental questions: If research is needed to improve the “understanding of the interactions of climate change and natural variability at multiple time scales”, how then is the NCADAC so certain that weather, which we’ve experienced before and will continue to experience in the future, is now being forced by manmade greenhouse gases to the point where carbon dioxide has become the primary cause of extreme weather, including drought, in the United States? Shouldn’t natural variability have been studied already so that researchers were confident about its influences? Better yet, shouldn’t existing data-based studies of the influences of natural variability on U.S. climate be at least acknowledged in the NCADAC report?

Not too surprisingly, the draft of the NCADAC report does not mention McCabe et al (2004) or Enfield et al (2001).


The recent drought conditions in the United States may persist for another few decades based on the mode of the Atlantic Multidecadal Oscillation. If the drought does last for another couple of decades, it would impose a significant load on water resources in the United States, which are already being strained in some parts of the country. The scientists behind the NCADAC apparently have no interest in existing research into the impacts on drought of known modes of natural variability. Instead they elect to promote a political agenda. No surprise there.

At the minimum, shouldn’t the NCADAC report include a  warning to the country about the possibility of a naturally caused persistent drought over the next few decades? A warning that would allow state and federal agencies to attempt to adapt and take measures to help the people of this country live with the upcoming water deficits?

Climate science since the 1980s has been dominated by government-funded, computer-generated speculation about the assumed hypothetical impacts of manmade greenhouse gases, not about the factors that actually drive climate.

If the vast majority of the funding is going to the agenda-driven, carbon dioxide-centered research, is it any wonder the majority of the papers and their authors support the hypothesis of human-induced global warming and climate change? If my income and well-being depended on that government funding, I too would be peddling the evils of carbon dioxide just like all the other snake-oil salesmen.

About Bob Tisdale

Research interest: the long-term aftereffects of El Niño and La Nina events on global sea surface temperature and ocean heat content. Author of the ebook Who Turned on the Heat? and regular contributor at WattsUpWithThat.
This entry was posted in Atlantic Multidecadal Oscillation, Climate Assessment Report Faults, Drought. Bookmark the permalink.

6 Responses to How Long Will the Naturally Caused Drought Persist in the U.S.?

  1. Dan Pangburn says:

    Four papers on line, that you may find of interest, provide some eye-opening insight on the cause of change to average global temperature. The papers are straight-forward calculations using readily available data up to May, 2013.

    The first one is ‘Global warming made simple’ at . It shows, with simple thermal radiation calculations, how a tiny change in the amount of low altitude clouds could account for half of the average global temperature change in the 20th century, and what could have caused that tiny cloud change. (The other half of the temperature change is from net average natural ocean oscillation which is dominated by the PDO)

    The second paper is ‘Natural Climate change has been hiding in plain sight’ at . This paper presents a simple equation that calculates average global temperatures since they have been accurately measured world wide (about 1895) with an accuracy of 90%, irrespective of whether the influence of CO2 is included or not. The equation uses a proxy which is the time-integral of sunspot numbers. A graph is included which shows the calculated trajectory overlaid on measurements.

    A third paper, ‘The End of Global Warming’ at expands recent (since 1996) measurements and includes a graph showing the growing separation between the rising CO2 and not-rising average global temperature.

    The fourth paper exposes some of the mistakes that have been made by the ‘Consensus’ and the IPCC

  2. Pingback: On Muller et al (2013) “Decadal variations in the global atmospheric land temperatures” | Bob Tisdale – Climate Observations

  3. Pingback: On Muller et al (2013) “Decadal variations in the global atmospheric land temperatures” | Watts Up With That?

  4. Ian Cooper says:

    Hi Bob, in relation to figure 2 I am curious to know what the number is for the PDO at the first quarter of this year thanks? Or is it too early to determine that yet?

  5. Bob Tisdale says:

    Hi Ian: The PDO data is updated monthly and there’s data available through May 2013:

  6. Ian Cooper says:

    Thanks for that.

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