In looking the summer, we usually focus on factors like ENSO state and strength, levels of solar activity, the modulation of both by the Quasi-biennial Oscillation (QBO), the multidecadal ocean cycles (PDO and AMO), volcanic activity and antecedent conditions, primarily soil moisture.
To show how ENSO and antecedent conditions alone have helped the last two summers, let’s start back in January 2007, when El Nino conditions prevailed.
During the spring (May 2007 shown) cold water started to develop in the eastern tropical Pacific and by the end of summer La Nina had developed.
Historically La Nina summers coming in the heels of an El Nino produce the warmest and droughty summers. This is because El Nino winters are often dry across the north though they can be stormy in the Deep South from California to Texas and Florida and up the east coast. La Ninas tend to be drier in the summer, which augments the winter dryness. Lack of soil moisture means a hot summer.1988 was one such classic example.
Averaging all developing La Nina summers post El Nino shows this.
Given the slow start to the La Nina and the fact that it was mainly in the east until the end of summer limited the warming some although it generally fit the pattern.
By mid-winter La Nina was roaring at full strength.
The summers after a La Nina winter (2nd year La Ninas as most La Ninas come on in the spring) tend to be very different – much cooler in the central than onset year La Ninas.
The coolness relates to the fact that La Nina winters and spring are stormy across the north with heavy snows, heavy spring snowmelt and heavy spring rains and severe weather. This means high soil moisture which retards the warm-up to summer.
CPC’s soil moisture models performs best in the spring for the late spring and summer when soil moisture or lack of soil moisture has the greatest effect on temperatures.
Last summer performed exactly as second year La Ninas and the soil moisture models suggested with coolest temperatures centered in the central. Chicago and Rockford did not reach 90F until September 1.
This showed up very well in the CDD anomaly chart for the season.
The La Nina weakened during the summer but only slightly and mainly in the eastern Pacific. La Nina came back though strongly in the fall peaking in late December
The La Nina has again weakened as the waters in the eastern and central tropical Pacific warmed. Most of the 22 climate ENSO models from April compiled by IRI show LA Nina giving way to La Nada (neutral) or El Nino.
What does a second year La Nina winter giving way to El Nino look like?
The summers were again generally cool.
Best Match Summers
When you factor in the other factors like PDO, AMO, solar, QBO, we get the best matches that reflect a similar cool summer scenario.
Hot Weather Northeast In April
Temperatures topped 90F on April 26th in New York City. 90F is not uncommon in April in the northeast and we find ironically it occurs in years with normal to cool summers like 1976.
High Latitude Volcano Summers
This year like last year we had an Alaska volcano blow, this time Mt. Redoubt, with numerous eruptions as high as 65,000 feet introducing sulfur dioxide (transformed to sulfate aerosols) to the polar stratosphere.
We found for high latitude volcanoes, the summers tend to be biased cool in the lower 48 states.
So in summary, slicing through the data in many different ways, we get a tendency for a normal to cool summer for much of the United States.