Modeling the MOC Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA The views described here are solely those of the presenter and not of GFDL/NOAA/DOC or any other agency or institution. erg/blogs/julian/images/gr eatoceanconveyor.jpg

Modeling the MOC Outline Role of MOC in maintaining mean climate –Northward Heat Transport –Northward Salt transport Role of MOC in Abrupt Climate Change –Unforced –Forced Predictability

How well do models simulate the T, S structure in the ocean? AR4 ensemble mean error Temperature IPCC WGI Chapter

AR4 ensemble mean error Salinity 90N 90S IPCC WGI Chapter 8 Suppl. Material EQ PSU +1.0

Role of MOC in Heat Transport

Impact of MOC on Climate (SAT) AOGCMs EMICs MOC “on” minus “off” o C Conclusion – MOC warms NH, locally large values, MOC cools SH

Impact of MOC on Climate Zonally averaged Precipitation differences (mm/day) Red lines control Blue lines difference Dashed – EMICs Solid - AOGCMs Conclusion – MOC on => ITCZ toward north

Impact of MOC on Climate Salinity MOC “on” minus “off” (PSU)

Role of MOC in maintaining mean climate North Atlantic saltier than without MOC –Rest of world ocean surface more fresh Northern Hemisphere warmer than without –Particularly the N Atlantic –Role of atmosphere mixing heat

Ocean MOC Role in Abrupt Climate Changes Unforced –Hall and Stouffer Nature Forced –Idealized (Hosing ) - Stouffer et al. JoC 2006

Transient An Anomalous Event (Unforced)

Maximum Negative Anomaly

Maximum Positive Anomaly

Surface Air Temperature Decadal Mean Difference

Surface variables/THC

Summary of Physical Mechanism

Unforced MOC variability If model if “realistic” –Can we predict this event? –Complicating GHG changes –Possible explanation of some parts of paleo- record

Experimental Design Manabe Climate Model “MCM” R30 AOGCM coupled model Idealized Water hosing –1 Sv for 100 years –After 100 years, stop hosing - allow recovery Case 1: Hosing 50N to 70N in Atlantic Case 2: Hosing south of 60S in Southern Ocean Compare to long control integration

Simulated Global MOC SH Index box NH Index box

Atlantic THC Response SV Years Atlantic THC does not respond in a seesaw-like manner NH Hosing – NH THC shuts down

SH THC Response SV Years SH Hosing – SH THC weakens. SH THC does not shut down

SAT Difference map SH hosing K Years hosing minus control

Surface Salinity Response Years Years NH SH NH SSS anomaly – Intense and confined SH SSS anomaly – Weaker and spreads PSUPSU

Differences in Sea Surface Salinity (PSU) Southern Freshwater Escape 25 years 100 years Hosing minus Control

Sea Surface Temperature Response Years Years NH SH Response more symmetrical than SSS Magnitude also becoming more similar K

Surface Air Temperature Response Years Years NH SH Response remarkably symmetrical ( first 100 yrs ) Magnitude very similar K

Precipitation Response Years Years NH SH Response very symmetric Magnitude very similar ITCZ shifts toward warmer hemisphere Cm/ day

Hosing Experiment Summary Symmetrical Atmospheric Response Much less symmetry in ocean Why? –Strong Circum-Antarctica winds –Northward flowing surface waters –Freshwater “escapes” into other basins Far a field impacts Less local impacts

MOC Predictability Unforced Changes –Are MOC predictable? –How long into future –Manifest in surface changes? Forced - GHG increase –Does MOC weaken? –How much? –Likelihood of complete shutdown?

Predictability Investigations just starting Perfect model experiments Use ICs from long control –Ocean ICs unchanged from control –Atmosphere ICs shifted in time by day or 2 Probably “best case” for predictability –No model errors –Ocean ICs perfectly known

Predictability of Atlantic Meridional Overturning Circulation (AMOC) in GFDL CM2.1 Climate Model

Are past ocean observations good enough to constrain MOC? Past ocean observations mainly XBTs –Temperature only –Upper 700 m or so Since 2003 or so – ARGO –T, S –Upper 2 km

Can Observations constrain the MOC? Need ARGO and atmospheric data to constrain MOC Other research suggests this may be too pessimistic.

A AR4 WG1 Assessment:  MOC very likely to weaken  MOC shutdown very unlikely MOC and Forced Climate Change

Why does MOC slow down as GHG increase? Role of surface fluxes –Heat fluxes –Water fluxes

Design of Partially Coupled Experiments Gregory et al GRL Run control and 1% per year CO2 increase experiment –Save out surface fluxes Use water fluxes from control in 1% run –TRAD_CH2O Use water fluxes from 1% in control –CRAD_TH2O Isolates role of heat and water fluxes

Summary of Partially Coupled Experiment Conclusions: 1. Heat fluxes changes always weaken MOC 2. Water fluxes changes mixed, but usually weaken MOC 3. Response to heat flux changes fairly uniform

Warming greatest over land and at most high northern latitudes and least over Southern Ocean and parts of the North Atlantic Ocean Weakening of MOC contributes to minimum in cooling in N Atlantic => smaller climate change => a positive impact? Surface Warming Pattern A1B, relative to IPCC WGI SPM

Summary: Ocean’s Role in Abrupt Climate Change Unforced –Possible to have large abrupt climate changes in AOGCMs Forced –Idealized Allows easy study of climate response Application to paleo-data and future climate changes –GHG increase Predictability of MOC changes

Questions Basic Issues –Why does the MOC exist? –How much mixing is there in the ocean? Do models have too much/too little? Impacts of mixing on the MOC What are the physical processes? –What is the role of ocean eddies? –How does MOC changes impact biology and associated changes in atmospheric pCO2?

Questions Variability –Are observations good enough to constrain MOC? –Are the MOC changes predictable? Time scale? Does it matter for where people live? Paleo-data tests? –What is role of MOC variability/changes in tropical Atlantic SST/hurricane changes? Future –Is future weakening of the MOC “bad”? –Interactions with Greenland/Antarctic ice melting

Thank you