|2018-19 La Niña Event|
|ENSO Temp. Change||-1.3°C|
By early 2018, easterly trade winds in the central and eastern Pacific Ocean resulted in a rapid collapse of the 2017-18 El Niño event. The El Niño dissipated by mid-April as Oceanic Nino Index (ONI) values fell to neutral levels. The positive Pacific Decadal Oscillation (PDO) signature that had existed since 2014 began to degrade in early 2018, allowing a cooler and more stable environment to develop throughout most of the Pacific Ocean. Strong negative subsurface anomalies began to develop in the central Pacific, some as extreme as -6°C. The Climate Prediction Center (CPC) issued a La Niña watch in April 2018, noting that the "strongest La Niña of the 21st century" could develop in the Pacific due to a sharp reversal of the PDO pattern present throughout most of recent years.
By June, ONI values fell below zero, indicating the onset of La Niña. La Niña officially developed in July as ONI values reached -0.5°C. As a result of the forming La Niña, unusually heavy rains occurred in India and Pakistan, with some severe flooding in spots. The La Niña continued to strengthen into the late summer and fall, with the ONI peaking at -1.3°C in the November-December-January timeframe. The La Niña began to weaken by early 2019 due to intermittent westerly wind bursts in the Pacific of warmer water. The event quickly dissipated by April/May/June 2019, and was followed with ENSO-Neutral conditions the following year.
This La Niña event resulted in the return of a "classic positive AMO" pattern in the Atlantic Ocean not seen since 2011. As a result, the Atlantic Ocean saw many more strong tropical cyclones in the deep tropics, with two systems acquiring major hurricane status east of 60W and south of 25N. The event adversely affected the Pacific hurricane season, which was dominated by dry air, wind shear, and cooler than normal sea surface temperatures.