Fig. 1: 12-month (DEC-17 to NOV-18) historical precipitation anomaly rankings.
Discussion: The wettest 12-month period on record for North Carolina, Maryland, West Virginia and Pennsylvania has occurred (Fig. 1). The entire northeast quadrant of the U.S. has observed MUCH ABOVE normal precipitation (top 10% wettest years). Most of the U.S. east of the Continental Divide has been wetter than normal.
The moisture source region for U.S. storms, particularly across the eastern half of the nation is the Gulf of Mexico, East Pacific and West Atlantic Ocean(s). The amount of water in the atmosphere collected by passing storms to rinse out over the U.S. land mass is more abundant when the ocean surface is warmer.
In 2018 sea surface temperature anomalies (SSTA) have been much warmer than normal in the Gulf of Mexico, subtropical East Pacific and western North Atlantic basin (Fig. 2). More recently, the tropical East Pacific has turned much warmer than normal due to a developing El Nino.
The warm SSTA in the regions described are a leading contributor to the wet pattern observed the past 12 months especially in the eastern U.S. Interestingly, there is no readily available index to describe Gulf of Mexico, subtropical Pacific Ocean or western North Atlantic basin SSTA regimes. Conventional indices such as Pacific decadal oscillation or Atlantic multi-decadal oscillation are more ocean basin-wide indices not recognizing the large regional variance observed in 2018.
Regional SSTA regimes are extremely important in the climate change era. From a climate diagnostic point of view, climate change is defined as the slow-down in the upper atmosphere of prevailing jet stream winds that cause our weather patterns to form and move west-to-east in the middle latitudes. The slow-down is caused by less thermal gradient in the upper atmosphere in-between the tropics and the warmer polar region caused by a constricting polar ice cap.
The slower jet stream means increased susceptibility to amplifying ridge and trough patterns bringing stronger and longer-lasting storms (causing flooding) or sunny/dry patterns causing drought (and sometimes flash drought).
The oceans are responding similarly as “blobs” of warm water are emerging outside of regular process such as El Nino southern oscillation (ENSO), PDO and AMO influences.
The historic wet weather pattern affecting the East U.S. the past 12 months was inspired to a large degree by ocean warming outside of the conventional monitoring indices (ENSO, PDO and AMO). Close monitoring of these regional ocean “blobs” is required to help explain and predict climate.
Fig. 2: Global sea surface temperatures for the past 3 months identify anomalous warm regions contributing to U.S. climate outside of the developing El Nino. The warm zones identified have been present all year (prior to the developing El Nino). The anomalous warm ocean contributes increased moisture to prevailing storm tracks causing increased precipitation amount.