A leading mode of climate variability affecting U.S. (and global) climate especially the precipitation regime is the Pacific decadal oscillation (PDO). During the past several months the PDO has become neutral after the historically intense cool phase of 2010-13 followed by an even more impressive warm phase of 2014-16 (Fig. 1).
Fig. 1: The PDO regimes since 2010 has been dramatically cool or warm until later 2017 when the neutral phase formed. PDO data provided by Dr. Nate Mantua, NOAA Southwest Fisheries Science Center.
During the lengthy 2010-13 cool phase of the PDO annual U.S. precipitation averaged drier than normal across California and Texas/Louisiana (and vicinity) while much of the northern tier states were wetter than normal (Fig. 2). In the several years that followed the PDO reversed to the warm phase producing wetter than normal climate on an annual basis across much of the U.S. (Fig. 3). Of course other factors affected the U.S. precipitation regime during these periods such as ENSO and the North Atlantic SSTA regime. However, the cooler PDO (implying below normal evaporation rates across the eastern Pacific to fuel storm tracks) historically produces a drier U.S. climate (especially southern states) while storm tracks off the Pacific carry more precipitation from a warmer northeast Pacific Ocean when PDO is warm.
Fig. 2: U.S. precipitation anomalies during the 2010-13 long-term cool phase of the PDO.
Fig. 3: U.S. precipitation anomalies during the 2014-16 long-term warm phase of the PDO.
Given the history (since 1950) of long duration warm episodes of the PDO what should we expect in 2018? The analogs suggest no intense PDO returning and neutral phase remaining in-place (Fig. 4). Interestingly, the analog years produced a dry national annual climate most focused on the West U.S., Texas and East-Central U.S. (Fig. 5). During the past 90 days as PDO has become steadily neutral the U.S. precipitation scheme has been mostly drier than normal (Fig. 6). Evidence to support a drier than normal national climate in 2018 is beginning to evolve.
Fig. 4: The PDO analog years suggest neutral phase for 2018. PDO data provided by Dr. Nate Mantua, NOAA Southwest Fisheries Science Center.
Fig. 5: U.S. precipitation anomalies during the PDO analog years projected for 2018 indicate that nationally the climate is drier than normal.
Fig. 6: The 90-day percent of normal precipitation analysis across the U.S.