Michael J. McPhaden NOAA/Pacific Marine Environmental Laboratory Seattle, Washington 1 st Alexander von Humboldt International Conference The El Niño Phenomenon and its Global Impacts Guayaquil, Ecuador May 16-20, 2005 Evolution of the El Niño

Last Winter’s Weather! Seattle Southern California

January weather delivered classic El Niño performance 2 February 2005 “In more than half of the areas measured, the January snowpack was the lowest in 28 years.” “Record-high temperatures were recorded on Jan. 18, 19 and 23 — with Jan. 19 hitting a springlike 62 degrees.” “The average high in January was 47.4 degrees, compared to a norm of 45 degrees. ”

El Nino/Normal Every few years, the trade winds weaken…

El Nino/Normal Every few years, the trade winds weaken…

NINO3.4 and SOI, Darwin Tahiti NINO-3.4

Niño3.4 SST

Peak Phase Janowiak et al (2003) rainfall, Quikscat wind velocity relative to ERS climatology Reynolds et al (2003) SST, Quikscat wind stress relative to ERS climatology

TAO/TRITON ATLAS Mooring TAO/TRITON: A U.S./Japan collaboration

Current Conditions Near normal warm season conditions prevail

Current Conditions  Thermocline slopes down to west because of trade wind forcing.  Weak residual warm anomaly in the upper thermocline of the eastern Pacific in wake of a Kelvin wave passage.  Cold subsurface ( m) temperature anomalies; may indicate trend towards La Niña cooling.

May 2003-May 2005 Episodic westerly wind forcing and Kelvin wave responses

May 2003-May 2005 May-June 2003 Wind burst arrests cooling

May 2003-May 2005 “Oceanic and atmospheric conditions indicate that a transition to La Niña is already underway.” NOAA/NCEP 19 May 2003

May 2003-May 2005 “El Niño conditions are expected to develop during the next 3 months.” NOAA/NCEP 5 August 2004 June-July 2004 Westerly Wind Burst

May 2003-May 2005 February-March 2005 wind burst temporarily increases E. Pac. SST

May 2003-May 2005 “ENSO-neutral conditions are expected to prevail during the northern summer (June-August) in spite of recent increases in SST anomalies…” NOAA/NCEP 5 May 2005

Evolution: March 2004-Present QuickTime signal

Evolution: March 2004-Present QuickTime

 Westerly wind bursts associated with Madden- Julian Oscillation (MJO) convective events and, in February 2005, with tropical cyclone Percy. MJO Convection Indian | Pacific | Atlantic Mar 2004 May 2005 cloudy/wet clear/dry Cloudiness & Rainfall (OLR, 5°N-5°S)  Convection near the date line highly variable from month to month throughout the El Niño.

Westerly Wind Bursts  Amplitude and phase of ENSO

Upper Ocean Heat Content and El Niño (Recharge Oscillator Theory)  Build up of excess heat content along equator is a necessary precondition for El Niño to occur.  Magnitude of warm SST anomalies is related to the magnitude of the heat content buildup. Prior to El Niño  Slow evolution of upper ocean heat content is the fuel that drives year to year variations associated with El Niño. Wyrtki, 1985; Cane et al, 1986; Jin, 1997

Upper Ocean Heat Content and El Niño (Recharge Oscillator Theory)  Build up of excess heat content along equator is a necessary precondition for El Niño to occur.  Magnitude of warm SST anomalies is related to the magnitude of the heat content buildup.  The time between El Niños is determined by the time to recharge.  El Niño purges excess heat to higher latitudes, which terminates the event El Niño follows so soon after El Nino because heat not discharged from equatorial band in 2003?

El Niño Precipitation Anomalies (DJF) over the U.S.: Typical vs TypicalDecember 2004-February x 4-5 x normal 60% normal ( 3rd driest Jan- Feb in 50 yrs)

Atmospheric Circulation Changes During El Niño Changes in tropical rainfall patterns affect the global atmospheric circulation via “teleconnections” Horel and Wallace, 1981 Heavy rain Pacific-North American (PNA) Pressure Pattern Subtropical Jet Stream in NH Splits, Southern Branch Shifts South and Intensifies

Geopotential Height and Circulation at 500 mb for December 2004-February 2005 Atmospheric Winds at 500 mbHeight of the 500 mb Surface

Decadal Modulation of ENSO Interaction with Pacific Decadal Oscillation?

PDO positive phase Change in 1998?

Changes in Background Affect ENSO Stability and Frequency Fedorov and Philander, 2000 Shift after 1998 to colder conditions in eastern Pacific could imply weaker, more damped El Niños in which stochastic forcing is relatively more prominent and predictability is lower.

How Do We Define El Niño? “Weak warm-episode (El Niño) conditions are expected to continue into early 2005.” NOAA/NCEP Advisory 9 December 2004 Nino3.4 SST “Under the current oceanographic and meteorological conditions, the occurrence of ‘El Niño’ is not expected in the next two months.” ERFEN Advisory 15 December 2004 Nino1+2 SST

Summary  Unusual warming in the central equatorial Pacific met one definition for the occurrence of an El Niño event.  One of the weakest events in the past 50 years.  Weak and variable convection near date line indicates weak coupling to the atmosphere. As a result, teleconnections to higher latitudes were weak.  No persistent strong warming in the eastern equatorial Pacific or along the west coast of South America.  Trade wind weakening characterized by episodic westerly wind forcing; onset of El Niño linked to a westerly wind burst in June-July  The event followed the El Niño by only 2 years, perhaps because no intervening period of low equatorial heat content.

Issues  Why did the atmosphere not couple strongly to ocean in the central Pacific near the date line?  Why did no strong warm SST anomalies develop in the eastern equatorial Pacific or along the west coast of South America?  Why did the relationship between equatorial upper ocean heat content SST anomalies break down during the event?  What is the relationship between the PDO and El Niño? Are the and El Niños the first in a series of lower amplitude El Niños compared to the previous 25 years? Are ENSO stability characteristics changing as a result of interaction with the PDO?  How can we improve the predictability of El Niño in the face of strong episodic wind forcing?  How should we define El Niño?

EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION “Based on the recent evolution of SST anomalies and on a majority of the statistical and coupled model forecasts, it seems most likely that weak warm episode (El Niño) conditions will continue to weaken during the next three months and that ENSO-neutral conditions will prevail during the northern summer.” Niño3.4 Region Spring Predictability Barrier >>> El Nino Threshold

Effects of Westerly Wind Bursts on SST  Westerly wind bursts cool the western Pacific, and warm the central and eastern Pacific Ocean.  Nonlinear processes can rectify these tendencies into lower frequency variations.  Spatial structure resembles “optimal perturbations” in some coupled models of ENSO.

Typical El Niño Temperature and Precipitation Anomalies over the U.S. (December-February) TemperaturePrecipitation

El Niño Temperature Anomalies (DJF) over the U.S.: Typical vs TypicalDecember 2004-February 2005

Geopotential Height and Circulation at 500 mb for December 1997-February 1998 Height of the 500 mb SurfaceAtmospheric Winds at 500 mb

Geopotential Height at 500 mb for December 2004-February 2005 Arctic Oscillation??

Western U. S. Snow Pack “Snow pack in the [Seattle’s] Watersheds is 15% of normal.” King5 TV News 3 Mar 2005 “…federal and state officials are likely to declare a drought emergency as early as next week…” Seattle P-I 5 Mar 2005