Different Season, Same Result

The Summer is over and Fall arrives today, but the pattern that produced above normal rainfall over much of Monmouth County in the summer is here to stay for the Fall.

Rainfall amounts going back the past 90 days or back to June 23, 2011 have averaged well above normal (50% to 100%) for much of Monmouth County with the exception of extreme southeastern Monmouth County where rainfall amounts were only slightly above normal (10% to 50%).  For much of western, central, and northern Monmouth County, this summer received rainfall amounts that ranged from six to fourteen inches above normal, which is an amazing amount of rainfall while southeastern Monmouth County ranged from one to six inches above normal.

So obviously this summer has been very wet, so will the rainy pattern continue into the Fall?

The answer is yes and this pattern might be getting more amplified as well.  The common denominators of the late Summer pattern when much of this rainfall developed between late July and now, are not only still in place but intensifying.  This pattern developed due to the interaction of three important features, the negative PDO in the northern Pacific, the developing La Nina in the tropical Pacific, and the positive AMO in the Atlantic.

The PDO or Pacific Decadial Oscillation is a 30 to 40 year oscillation that leads to very warm northern Pacific Ocean temperatures (positive) or very cold northern Pacific Ocean Temperatures (negative).  When the PDO goes into a negative phase, a body of cold water develops in the eastern Gulf of Alaska and down the western coast of North and central America.  This process is extremely important because when the PDO goes negative, the process has a major impact on the strength and orientation of the Polar Jet stream over the Pacific and North America and has a very important impact on the orientation of La Niña as well!  When the PDO is negative, as it is now and will be for several years, the Polar jet stream is stronger over North America with a trough frequent over the Eastern United States.  This aspect of the pattern is not only not going to change but is in a state of intensification.

This leads me to the Tropical Pacific where a new La Niña is developing.  This La Niña is clearly an east based La Niña so far which means the coldest waters are focused towards South America rather than towards the date line or 180 degrees Longitude, which has a major impact on the trough axis over the central Pacific.  For the past 60 days, the state of the tropical Pacific ENSO (El Niño Southern Oscillation) has gone from neutral (early summer) to La Nina (late summer to now), which an extremely important observation.  Since La Nina has started to develop, the potential for powerful rain storms have grown.  This again is due to the orientation of La Niña as an east based La Niña leads again to more frequent troughs in the East and enhanced mid and upper level disturbances.  This La Niña is strengthening and the impacts from this La Niña should increase in influence over the Eastern United States through the next 90 days and beyond.

The Atlantic Multidecadial Oscillation or AMO is another ocean oscillation that lasts roughly 20 to 30 years.  When the AMO is negative the Atlantic Ocean sea surface temperature anomalies run below normal, which leads to less tropical development and a more progressive northern Atlantic pattern.  The influences of the AMO leads to near normal precipitation values in the Plains and below to near normal precipitation values in the Eastern United States.  However, when the AMO is positive or in a warm phase, the opposite is true.  The Plains end up with significantly below normal precipitation departures and at times serious droughts while the East coast has above normal precipitation values.  This is in part due to an increase in tropical low pressure development and a more amplified northern Atlantic pattern that tends to slow the departure of low pressure systems along the East coast.  Sound familiar?  The AMO is expected to remain positive over the next 20 years as well.

So as you can see, the main driving forces between the above normal rainfall amounts this late Summer are not going any where.  If anything, they are intensifying and likely will lead to even more major rainfall events that the one developing today.