Eocene ENSO

Matthew HuberPurdue UniversityHuberm@purdue.edu

Breckenridge 2003, PaleoWorking GroupAll rights reserved by author as applicable.

A trend in tropical Pacific SSTs over the past 50 years, consisting of enhanced warming in the east relative to the west, resembles El Nino

Many coupled GCM results suggest that the response of the tropical Pacific to increased CO2 looks much like modern EL Nino events.

The results of these simulations support the conclusions drawn from most other GCM studies that future anthropogenic climate change will favor El Nino-like anomalies

What can proxy records of past climate tell us?

• So apparently higher greenhouse gas concentrations push the tropical Pacific to a mean state with a much reduced to zero thermocline tilt (Summarized by Vavrus and Liu)

• Same with a wide open Indonesian Seaway (Molnar and Cane)

• Also with a warm deep ocean (which is caused by higher greenhouse gas concentrations or from other causes) (Sun, Fedorov and Philander)

• Also an open Isthmus of Panama (Chaisson and Ravelo)• Is this true and is it really important?• How might we test this?

Heat convergence into the

ocean• “warm” colors indicates where the atmosphere heats the ocean, “cool” colors where the ocean heats the atmosphere

Major climatic, ecological, geological events in past 60 million years and CO2

• NCAR’s CLIMATE SYSTEMS MODEL CSM v1.4– A suite of simulations using an accelerated, iterative spinup, 3-10,000 deep water years of

integration, last >350 years in synchronously coupled mode

– Atmospheric GCM:CCM3 v. 3.6.6 • T31 (~3.75°x3.75°x18 levels)

– Land Component: LSM v.1.2• vegetation+land surface exchange with atmosphere

– Ice Component: CSIM 2.2.9• Flato and Hibler cavitating fluid rheology

– Ocean GCM: NCOM v.1.5• KPP vertical mixing, Gent-McWilliams isopycnal eddy mixing• ~.9° tropical and 1.8° polar meridional res. 3.6° zonal res. 25 vertical levels.

– Flux Coupler: v.4.0.5• Conservatively interpolates, calculates/passes fluxes between components.

– Experiments• Present day validation case: “degraded” modern conditions including simplified

bathymetry. Preindustrical pCO2 [280 ppm].• Eocene Cases: Early Eocene topography, vegetation (Sewall et al., 2000) and bathymetry

(Huber et al., 2002). 560 and 1120 ppm pCO2 (Pearson and Palmer, 2000; Singha and Stott, 1994; Royer et al., 2001).

– Initial Conditions• Two sets of Eocene initial conditions, one set begins with zonally constant HIGRAD

Eocene SSTs fit to a deep ocean temperature of 6°C and globally constant salinity. 2nd case begins with LOGRAD SST and deep ocean temperature=10°C.

cpl ice

ocn

lnd

atm

Mean annual SSTs and ocean currents (Eocene, 560 ppm)

AMJ

OND

Mean annual SST anomaly

(Eocene - Modern)

1120 ppm CO2

560 ppm CO2

North-South

temperature transect

(anomaly) • Eocene (560

ppm) minus modern

• Eocene (1120 ppm) minus modern

Heat convergence into the

ocean• “warm” colors indicates where the atmosphere heats the ocean, “cool” colors where the ocean heats the atmosphere

-4

-3

-2

-1

0

1

2

3

4

-80 -40 0 40 80

Pre Industrial APre Industrial B

Eocene BEocene A

Eocene CEocene D

NCAR CCSM

NCEP-T&C 2001NCEP-T&C 2001 w/corrections

Ocean Heat Transport

PW

latitude

Ocean heat transport

• The equatorial strip is cooled via upwelling, this same mechanism pumps heat into ocean which is then released in the Kuroshio and Southern Ocean

• This “heat pump” may be an important feature in past warm climates in general

• See papers by Sun and Liu and Philander and Pierrehumbert’s “Sleeping Dragon” paper

• Ocean heat transport is near modern values

• Is there any way to verify our predictions?

http://pmel.noaa.gov

If there wereSuch a thermoclineTilt in the mean State, theory tellsUs there shouldHave been ENSO

A Time Series of Eocene Quasi-Niño Index from CCSM

5.1 4.63.3 2.3-2.2 period (yrs)

B Spectra of Eocene Quasi-Niño Index from CCSM

frequency (cycles/month)

Ripepe et al, 1991

• The “heat pump” may provide a mechanism for diminishing tropical warming with higher greenhouse gas concentrations, provided that a Walker Cell is maintained (and strengthened)

• Records of Eocene ENSO variability from Wyoming and Germany varved lake sediments provide preliminary confirmation of a “cold tongue” in the mean state and suggest the robustness presence of an ocean “heat pump”.

• Tropical results– Pacific extratropics (subpolar/polar gyres) much warmer than tropics.

• Pacifc/Indian Ocean SST proxies (Zachos et al 1994, Bralower et al 1995, Pearson et al 2001) can be matched by model.

• The SST gradient maps into a much warmer/poorly stratified abyss, a sharp discontinuity, and a tropical/upper ocean that has a complex spatial pattern of temperature change

– The Walker Cell--Zonal Thermocline tilt does not weeken• Western Pacific warm pool warms and deepens, EEP cools.

• Because of warm pool -cold tongue pattern and a sharp thermocline with cool water underneath, ENSO exists.

– Why doesn’t the Tropical Pacific warm more?• Is it an ocean upwelling driven (ocean thermostat?)response?

• Or a cloud driven (cloud thermostat?) response?

• There are usually 3 options in interpreting the complex coupled interactions in a CGCM– Sensitivity Studies (Stratus cloud forcing)

– Compare with results from a simple model (two column Nilsson and Emanuel atmosphere coupled to Sun ocean)

– Punt!

Equatorial Temperatures from

Simple Model

Caballero and Huber, in prep