El Nino and La Nina An important atmospheric variation that has an average period of three to seven years. Goes between El Nino, Neutral, and La Nina (ENSO cycle, El Nino Southern Oscillation) Has large influence both in the tropics and midlatitudes. Main source of forecast skill beyond a few weeks.

An Important Measure is the Temperature in the Tropical Pacific

Niño Region SST Departures (oC) Recent Evolution The latest weekly SST departures are: Niño 4 -0.1ºC Niño 3.4 -0.8ºC Niño 3 -1.1ºC Niño 1+2 -1.2ºC

Central and Eastern Pacific Upper-Ocean (0-300 m) Weekly Average Temperature Anomalies Negative subsurface temperature anomalies were present through December 2016. Positive anomalies with large fluctuations in amplitude were present from mid-January through mid-July 2017. Since mid-July, anomalies decreased and have remained negative.

Sub-Surface Temperature Departures in the Equatorial Pacific In the last two months, negative subsurface temperature anomalies have persisted across the central and eastern Pacific Ocean. This figure cannot be updated at this time. Most recent pentad analysis Recently, the strongest negative anomalies are between 160ºW-80ºW.

Why do we care? The circulations in the midlatitudes are substantially different in El Nino, Neutral, and La Nina years. Since the temperature of the tropical Pacific changes relatively slowly, this gives some meteorologist some insights into the weather over the next several months.

El Nino – weak Aleutian High

La Nina – strong Aleutian High

CPC/IRI Probabilistic ENSO Outlook Updated: 9 November 2017 La Niña conditions are predicted to continue (~65%-75% chance) at least through the Northern Hemisphere winter 2017-18, with a 51% chance of continuation through February-April 2018.

IRI/CPC Pacific Niño 3.4 SST Model Outlook The multi-model averages predict La Niña to persist into early 2018. Figure provided by the International Research Institute (IRI) for Climate and Society (updated 17 November 2017).

SST Outlook: NCEP CFS.v2 Forecast (PDF corrected) Issued: 26 November 2017 The CFS.v2 ensemble mean (black dashed line) favors La Niña through March-May 2018.

Snow and La Nina Typically more snow in La Nina years--but no guarantees. This is a statistical relationship. Major impact after January 1

Implication for this year: higher than normal probability for lowland snow in Seattle

U. S. Seasonal Outlooks December 2017– February 2018 The seasonal outlooks combine the effects of long-term trends, soil moisture, and, when appropriate, ENSO. Precipitation Temperature