Spring temperatures and rainfall in 6 US southwest states linked to naturally occurring North Atlantic Oscillation per peer reviewed study-Journal of Climate, April 2015

Naturally occurring North Atlantic Oscillation affects Southwest US temperature extremes and rainfall in March-June period. Study includes California, Nevada, Arizona, New Mexico, Utah, and Colorado:
 
April 2015, "On the relationship between the North Atlantic Oscillation and early warm season temperatures in the southwestern US," Journal of Climate, American Meteorological Society

"Abstract 
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We report here that the North Atlantic Oscillation (NAO), which has been known to directly affect winter weather conditions in western Europe and the eastern United States of America, is also linked to surface air temperature over the broad southwestern United States (SWUS) region encompassing California, Nevada, Arizona, New Mexico, Utah, and Colorado in the early warm season. We have performed monthly timescale correlations and composite analyses using three different multi-decadal temperature datasets. Results from these analyses reveal that NAO-related upstream circulation positively affects not only the means, but also the extremes of the daily maximum and minimum temperatures in SWUS. This NAO effect is primarily linked with the positioning of upper-tropospheric anti-cyclones over the western US that are associated with development of the positive NAO phase, through changes in low-tropospheric wind directions as well as suppression of precipitation and enhanced short-wave radiation at the surface. The effect is observed in SWUS only during the March-June period because the monthly migration of anti-cyclones over the western US follows the migration of the NAO center over the subtropical Atlantic Ocean. The link between the SWUS temperatures and NAO has been strengthened in the last 30-year period (1980-2009) compared to the previous 30-year period (1950-1979). In contrast to the NAO-SWUS temperature relationship, the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) show only marginal correlation strengths in several limited regions for the same 60-year period.".

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"Corresponding author address: Seung Hee Kim, Center of Excellence in Earth Systems Modeling and Observations, Chapman University, Orange, CA, 92866. E-mail: sekim@chapman.edu" via Hockey Schtick
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About (North) Atlantic Multidecadal Oscillation (AMO) from NOAA:

"Frequently Asked Questions About the

Atlantic Multidecadal Oscillation (AMO)

1. What is the AMO?
2. How much of the Atlantic are we talking about?
3. What phase are we in right now?
4. What are the impacts of the AMO?
5. How does the AMO affect rainfall and droughts?
6. How does the AMO affect Florida?
7. How important is the AMO when it comes to hurricanes - in other words - is it one of the biggest drivers? Or Just a minor player?
8. Does the AMO influence the intensity or the frequency of hurricanes (which)?
9. If the AMO affects hurricanes - what drives the AMO?
10. Can we predict the AMO?
11. Is the AMO a natural phenomenon, or is it related to global warming?

What is the AMO?

The AMO is an ongoing series of long-duration changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time and a difference of about 1°F between extremes. These changes are natural and have been occurring for at least the last 1,000 years.

How much of the Atlantic are we talking about?

Most of the Atlantic between the equator and Greenland changes in unison. Some area of the North Pacific also seem to be affected.

What phase are we in right now?

Since the mid-1990s we have been in a warm phase.
 
What are the impacts of the AMO?

The AMO has affected air temperatures and rainfall over much of the Northern Hemisphere, in particular, North America and Europe. It is associated with changes in the frequency of North American droughts and is reflected in the frequency of severe Atlantic hurricanes. It alternately obscures and exaggerates the global increase in temperatures due to human-induced global warming.
 
How does the AMO affect rainfall and droughts?

Recent research suggests that the AMO is related to the past occurrence of major droughts in the Midwest and the Southwest. When the AMO is in its warm phase, these droughts tend to be more frequent and/or severe (prolonged?). Vice-versa for negative AMO. Two of the most severe droughts of the 20th century occurred during the positive AMO between 1925 and 1965: The Dustbowl of the 1930s and the 1950s drought. Florida and the Pacific Northwest tend to be the opposite - warm AMO, more rainfall.

How does the AMO affect Florida?

The AMO has a strong effect on Florida rainfall. Rainfall in central and south Florida becomes more plentiful when the Atlantic is in its warm phase and droughts and wildfires are more frequent in the cool phase. As a result of these variations, the inflow to Lake Okeechobee - which regulates South Florida’s water pply - changes by 40% between AMO extremes. In northern Florida the relationship begins to reverse - less rainfall when the Atlantic is warm.

How important is the AMO when it comes to hurricanes - in other words - is it one of the biggest drivers? Or Just a minor player?

During warm phases of the AMO, the numbers of tropical storms that mature into severe hurricanes is much greater than during cool phases, at least twice as many. Since the AMO switched to its warm phase around 1995, severe hurricanes have become much more frequent and this has led to a crisis in the insurance industry.

Does the AMO influence the intensity or the frequency of hurricanes (which)?

The frequency of weak-category storms - tropical storms and weak hurricanes - is not much affected by the AMO. However, the number of weak storms that mature into major hurricanes is noticeably increased. Thus, the intensity is affected, but, clearly, the frequency of major hurricanes is also affected. In that sense, it is difficult to discriminate between frequency and intensity and the distinction becomes somewhat meaningless. 

If the AMO (in part) affects hurricanes - what drives the AMO?

Models of the ocean and atmosphere that interact with each other indicate that the AMO cycle involves changes in the south-to-north circulation and overturning of water and heat in the Atlantic Ocean. This is the same circulation that we think weakens during ice ages, but in the case of the AMO the changes in circulation are much more subtle than those of the ice ages. The warm Gulf Stream current off the east coast of the United States is part of the Atlantic overturning circulation. When the overturning circulation decreases, the North Atlantic temperatures become cooler..

 
Can we predict the AMO?

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.

Is the AMO a natural phenomenon, or is it related to global warming?

Instruments have observed AMO cycles only for the last 150 years, not long enough to conclusively answer this question. However, studies of paleoclimate proxies, such as tree rings and ice cores, have shown that oscillations similar to those observed instrumentally have been occurring for at least the last millennium. This is clearly longer than modern man has been affecting climate, so the AMO is probably a natural climate oscillation. In the 20^th century, the climate swings of the AMO have alternately camouflaged and exaggerated the effects of global warming, and made attribution of global warming more difficult to ascertain."

 

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