Thursday, May 28, 2015

An Extremely Important Change In A Major Climate Input

Following up on the post immediately below, "Active Atlantic Hurricane Period That Began in 1995 May be Over: NOAA".
The flip of the Atlantic Multidecadal Oscillation back to the cool phase after 20 years in the warm phase is a very, very big deal.

First up, the current Sea Surface Temperature anomaly map from Unisys:
Current Sea Surface Temperature Anomaly Plot
The area off the west coast bulge of Africa is the hurricane Main Development Region (MDR), home to the Cape Verde-type hurricanes, typically the largest and most intense because of their long run before encountering land. If the SST's are warm. Big 'if'.

The warm area off South America's bulge at the equator is the El Niño region, extending back to Papua New Guinea.

The area of warm water off the west coast of North America, technically known as 'the blob' (seriously) is part of the measurement area of the Pacific Decadal Oscillation.

What the blob and the El Niño regions are doing right now is giving up heat to the air above and warming the atmosphere.

Next up,  NOAA's 2015 Atlantic Hurricane Season Outlook  issued: 27 May 2015
...3. Multi-decadal fluctuations in Atlantic hurricane activity
Atlantic hurricane seasons exhibit extended periods lasting 25-40 years of generally above-normal or below-normal activity. For example, a high-activity era for Atlantic hurricanes began in 1995. Seasons during 1995-2014 averaged about 14.7 named storms, 7.6 hurricanes, and 3.5 major hurricanes, with an ACE index of 142% of the median. NOAA classifies 12 of the 20 seasons since 1995 as above normal, with eight being very active (i.e., hyperactive defined by ACE > 165% of median). Only three seasons since 1995 were below normal (1997, 2009, and 2013). 

In contrast, the preceding low-activity era of 1971-1994 (Goldenberg et al. 2001) averaged 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes, with an ACE index of only 74% of the median. One-half of the seasons during this period were below normal, only two were above normal (1980, 1989), and none were hyperactive. 

These multi-decadal fluctuations in hurricane activity result almost entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms that first develop in the MDR. The AMO, and its associated changes in the strength of the west African monsoon system, is the climate pattern responsible for this multi-decadal variability. 

The high-activity era for Atlantic hurricanes that began in 1995 reflected a transition to the warm phase of the AMO and to an enhanced west African monsoon system as shown (Bell and Chelliah 2006). This climate pattern produces stronger hurricane seasons by creating conducive atmospheric conditions in the MDR, including 1) reduced vertical wind shear, 2) weaker easterly trade winds, 3) a more conducive configuration of the African easterly jet (i.e. increased cyclonic shear), 4) warm, moist, unstable air, and 5) reduced sinking motion. 

However, the atmospheric conditions expected during ASO 2015 (i.e. stronger vertical wind shear, enhanced sinking motion, increased atmospheric stability) contrast with these active-era patterns. Following two relatively quiet hurricane seasons (2013 and 2014 (Bell et al. 2014, 2015), along with the current projection of Atlantic SST anomalies onto the cold phase of the AMO, debate has surfaced as to whether we are still in this high-activity era.
Finally, from our May 2014 post "A Very Good Drought Prediction":
...The AMO is currently showing negative readings. This is probably a precursor to a full scale flip of the warm phase that began in 1995 but may not take hold for a year or two.
The PDO is currently showing positive readings but in this case it doesn't mean much. A look at the values from the early 2000's also shows the sign flipping from positive to negative and back.
We'll take a deeper look at the anomalous anomalies when the coming El Nino becomes established.

Historic and current AMO readings
Historic and current PDO readings
AMO January-April  2015  0.012  0.016  -0.109  -0.051
PDO January April   2015  2.45    2.30     2.00      1.44

For what this all means, besides hurricanes, take drought in North America as just one area of study. It's going to get wetter:

North American drought frequency

Current conditions +PDO/-AMO.
Much more to come.