Part I. Mike McPhaden--What We Know and What is Unresolved: An Observational Perspective Part II. David Battisti--What We Know and What is Unresolved: A Theoretical and Modeling Perspective Sleeping Lady Mountain Retreat Leavenworth, WA 19 September 2005 ENSO Dynamics and Global Impacts (“ A Benign Problem” )

Sleeping Lady Mountain Retreat Leavenworth, WA 19 September 2005 Part I. What We Know and What is Unresolved: An Observational Perspective

El Niño: Global Impacts Fatalities: 23,000 Economic Losses: $36 Billion

El and La El Niño is often followed by or preceded by La Niña: an unusual cooling of the tropical Pacific Upwelling zonesWestern Pacific “warm pool” El Niño happens roughly every 2-7 years, lasts months, and peaks at the end of the calendar year

El Nino/Normal

Anomalous Geostrophic Divergence Every few years, the trade winds weaken…

El Nino/Normal Anomalous Geostrophic Convergence

Feedbacks Ocean-Atmosphere Feedback Loops During El Niño WindsSST Fast Positive Feedback Warms

Feedbacks Ocean-Atmosphere Feedback Loops During El Niño WindsSST Fast Positive Feedback Warms Thermocline Depth Slow Negative Feedback Cools

Feedbacks Ocean-Atmosphere Feedback Loops During La Niña WindsSST Fast Positive Feedback Cools Thermocline Depth Slow Negative Feedback Warms

NINO3.4 and SOI, NINO-3.4 Darwin Tahiti

NINO3.4 and SOI, Darwin Tahiti NINO-3.4 SOI and NINO3.4 Correlation=-0.9 lag)

NINO3.4 and SOI, Darwin Tahiti NINO-3.4 El Niño/Southern Oscillation (ENSO): Warm phase (El Niño) // Cold Phase (La Niña)

Thermocline Depth (20°C)

 Build up of excess heat content along equator is a necessary precondition for El Niño to occur.  The time between El Niños is determined by the time to recharge.  El Niño purges excess heat to higher latitudes, which terminates the event. Upper Ocean Heat Content and El Niño (Recharge Oscillator Theory*) *Wyrtki, 1985; Cane et al, 1986; Jin, 1997

Niño3.4 SST

Janowiak et al (2003) rainfall & ERS wind velocity Reynolds et al (2003) SST & ERS wind stress Peak Phase, 1997

Peak Phase, 2004 Janowiak et al (2003) rainfall & Quikscat wind velocity relative to ERS climatology Reynolds et al (2003) SST & Quikscat wind stress relative to ERS climatology DRY

Processes Affecting Equatorial SST Enhanced Surface Heat Fluxes Zonal Advection Suppressed Upwelling

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

Global Impacts Impacts on Global Weather Patterns El Niño shifts the probability of droughts, floods, heat waves, and extreme weather events in large regions of the globe.

Tropical Storms  El Niño tends to suppress formation of Atlantic hurricanes.  El Niño tends to increase intensity and geographic range of Pacific hurricanes.  Opposite tendencies occur during La Niña. Impacts on Tropical Storms

Global Impacts El Niño shifts the probability of droughts, floods, heat waves, and extreme weather events in large regions of the globe.  Magnitude of impacts scales with magnitude of Pacific SST anomalies  La Niña impacts roughly opposite to those of El Niño Impacts on Global Weather Patterns

Global Impacts El Niño shifts the probability of droughts, floods, heat waves, and extreme weather events in large regions of the globe. Washington State: For 9 El Niños between Expected by Chance  6 warm winters (~2°F increase)3  2 neutral winters3  1 cold winter3 Impacts on Global Weather Patterns

Global Impacts Social and Economic Consequences El Niño can affect life, property, and economic vitality due to weather related hazards.

El Niño: Global Impacts Fatalities: 23,000 Economic Losses: $36 Billion

El Niño: U.S. Impacts Negative –189 Fatalities –$4-5 billion in economic losses Positive –850 lives saved –$20 billion in economic gains

Weather Noise and ENSO Stability  If ENSO is a freely oscillating instability of the ocean-atmosphere system governed by basin scale dynamics (Schopf & Suarez, 1988: Battisti & Hirst, 1989), weather “noise” is not essential but introduces irregularity.  If ENSO is a stable or weakly damped oscillator, external forcing in the form of weather noise is essential for initiation and development of warm events (Penland & Sardeshmukh, 1995; Moore & Kleeman, 1999; Kessler, 2002). Mean thermocline depth  Stability characteristics determined by strength of ocean- atmosphere coupling and may vary decadally with changing background conditions (Kirtman and Schopf, 1998; Fedorov and Philander, 2000) Fedorov & Philander, 2000 A= s; B= s

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

Current Conditions Near normal conditions prevail

Current Conditions  Thermocline slopes down to west because of trade wind forcing.  Cold subsurface ( m) temperature anomalies may indicate trend towards La Niña cooling.

Weather Noise and Stochastic Forcing Episodic westerly wind forcing and downwelling Kelvin wave responses

Weather Noise and Stochastic Forcing June-July 2004 Westerly Wind Burst BEFORE: “It is…likely that ENSO-neutral conditions will continue for the next 3 months (through August 2004).” NOAA/NCEP 10 June 2004 AFTER: “El Niño conditions are expected to develop during the next 3 months.” NOAA/NCEP 5 August 2004

Westerly Wind Bursts  Amplitude and Phase of ENSO

Westerly Wind Bursts  Amplitude and Phase of ENSO 29°C Stochastic forcing not entirely random

Effects of Westerly Wind Bursts on Equatorial SST  Westerly wind bursts cool the western Pacific, and warm the central and eastern Pacific Ocean via processes similar to those that operate on longer time scales (Shinoda & Hendon, 1998; Zhang, 2001; McPhaden, 2002).  Nonlinear processes can rectify short time scale variations into lower frequency changes (Lukas and Lindstrom, 1991; Kessler et al, 1995; Kessler and Kleeman, 2000; Waliser et al, 2003)  Spatial structure resembles “optimal perturbations” in some coupled models of ENSO (Moore and Kleeman, 1999). Enhanced Surface Heat Fluxes Zonal Advection Suppressed Upwelling

Sleeping Lady Mountain Retreat Leavenworth, WA 19 September 2005 Part II. What We Know and What is Unresolved: A Theoretical and Modeling Perspective

ENSO and Models/Theory: Resolved Issues ENSO is the result of coupled atmosphere-ocean physics in the tropical Pacific. –Ocean models must be forced by Southern Oscillation to produce ‘El Ninos’; –Atmosphere models must be forced by ‘El Nino’ SST to produce the Southern Oscillation. ENSO is a true mode of the coupled system. ENSO is a strong function of the climatological mean state. –The annual cycle acts to coordinate the ENSO mode to peak at the end of the calendar year. ENSO is predictable 12 months in advance. ENSO affects are teleconnected from the tropical Pacific by well-understood atmosphere and ocean dynamics: –Atmospheric impact is nearly global in extent.

Examples from an intermediate atmosphere/ocean model Some models of the tropical Pacific Atmosphere and Ocean system have realistic ENSO cycles

The processes that affect SST during an ENSO cycle are time and space dependant Horizontal advection, vertical mixing, entrainment and surface heat fluxes are all important for ENSO Nino3 Nino1 SST Tendency Time ( model years)

Basic elements of ENSO in observations and models The “delayed oscillator physics” and “recharge oscillator” are complimentary toy-model descriptions of the ENSO mode. Ocean Adjustment & Bjerknes

ENSO is a true eigenmode of the coupled atmosphere/ocean system in the Pacific The Bjerknes Mechanism and the Ocean Dynamics are features of the ENSO mode

Few Global Climate models have realistic ENSOs First EOF of tropical Pacific SST (all IPCC ‘04 models) Models w/ realistic ENSO space/time variability Global Climate models can produce realistic ENSO variability: - Eg, the high resolution GFDL tropical Pacific-global atmosphere model (Philander et al 1992); the paleo-CSM. Unfortunately, none of the current generation global climate models have realistic ENSO variability. Why? - lousy mean states in the tropical Pacific (why?); too low resolution in equatorial ocean; etc. Only one model used in the last IPCC assessment simulated ENSO variability that did not violate the robust observational constraints.

The ENSO mode (cont). ENSO exists because of the structure in the annual average state of the atmosphere/ocean system in the tropical Pacific, and … ENSO tends to peak at the end of the calendar year because of the annual cycle in the in the basic state. Spectrum of the pure ENSO mode Thompson and Battisti 2001

ENSO as a linear, stochastic system Results from a linear coupled atmosphere-ocean model forced by white noise Observed Model

ENSO peaks at the end of the calendar year because the of the annual cycle in the mean state Observation Model

ENSO is Predictable Skill depends on knowing where you are on the ENSO mode The long-lead time for skillful forecast is due to long period of the ENSO mode Presently, empirically based forecast models are at least as skillful as coupled GCMS

ENSO is predictable Empirical models skillfully predict the state of ENSO 12 months in advance. Skill Persistence Empirical Models

Skill depends on the start month. Empirical Model Persistence These plots are from an empirical model using tropical Pacific SST (1981-present). For this month’s forecast, see ENSO is predictable March Starts September Starts

Why is ENSO irregular? The Seasonal Footprinting Mechanism ENSO is variable, in part, because the mode interacts with the annual cycle. The Seasonal Footprinting Mechanism accounts for about 1/4 to 1/3 of ENSO variance. Other factors responsible for irregularity? - Westerly wind bursts? Vimont et al. 2001, 02, 03

ENSO: Unresolved Issues Is the ENSO mode stable or unstable in the present climate? (consensus - stable) –If stable, what are the sources of energy for ENSO? –Does stability matter for predictability? What is the limit of predictability? –24 months? 12 months? How does ENSO affect the mean state? Will ENSO change due to Global Warming? What is the cause of the decadal ENSO-like variability in the present ( ) climate?

ENSO: Unresolved Issues The leading pattern of variability in the tropical Pacific has an ENSO like pattern in SST and atmosphere circulation. Something new, or the debris that results from averaging over past ENSO events? (see Vimont 2005 if you aren’t already convinced)

ENSO and Models: Resolved Issues ENSO is the result of coupled atmosphere-ocean physics in the tropical Pacific. –Ocean models must be forced by Southern Oscillation to produce ‘El Ninos’; –Atmosphere models must be force by ‘El Nino’ SST to produce the Southern Oscillation. ENSO is a true mode of the coupled system. ENSO is a strong function of the climatological mean state. –The annual cycle acts to coordinate the ENSO mode to peak at the end of the calendar year. ENSO is predictable 12 months in advance. ENSO affects are teleconnected from the tropical Pacific by well-understood atmosphere and ocean dynamics: –Atmospheric impact is nearly global in extent.

ENSO: Unresolved Issues Is the ENSO mode stable or unstable in the present climate? (consensus - stable) –If stable, what are the sources of energy for ENSO? –Does stability matter for predictability? What is the limit of predictability? –24 months? 12 months? How does ENSO affect the mean state? Will ENSO change due to Global Warming? What is the cause of the decadal ENSO-like variability in the present ( ) climate?

Extratropical forcing of ENSO

Skill depends on the start month. Empirical Model Persistence March Starts September Starts These plots are from an empirical model using tropical Pacific SST (1981-present). For this month’s forecast, see ml ENSO is predictable