Southern Ocean Centennial Variability Impact on North Atlantic Sea Level

OS31C-1748

Torge Martin 1, Mojib Latif 2,3, and Wonsun Park 2 1 Polar

Introduction

Science Center, Appl. Phys. Lab., U. of Washington, Seattle, WA;

2 Helmholtz

Centre for Ocean Research Kiel (GEOMAR), Kiel, Germany;

Southern Ocean Deep Convection Driving MCV

The Kiel Climate Model (KCM) simulates pronounced multi-centennial variability (MCV) in, amongst others, global surface air temperature, Southern Hemisphere sea ice extent, the heat content of the Southern Ocean, and the strength of the Atlantic Meridional Overturning Circulation (see below). The presence of MCV in North Atlantic sea level hints at Northern-SouthernHemisphere teleconnections. The MCV is driven by quasi-periodic oscillations of deep convection in the Atlantic Sector of the Southern Ocean. This internal variability has the potential to mask global warming signals.

2.  Weddell Gyre supports accumulation of heat and determines location of deep convection and

concentration

potential temperature anomaly

Convective regime

Oceanic heat loss and sea ice retreat associated with a convection event.

1.  M u l t i - c e n t e n n i a l variability (MCV) is found in sea surface temperature records of the Southern Ocean (SSO index, right) in KCM and observations (Smith et al., 2008) 2.  Tasmanian tree ring data (Cook et al., 2000) confirm presence of centennial surface temperature variability

mixed layer depth

3.  Maximum of SSO index coincides with Weddell Polynya of the 1970s!

3.  The heat that triggers the deep convection is provided by deep waters from North Atlantic exported to Southern Ocean

Conclusions 1

4.  Weddell Sea open ocean deep convection regulates formation rate of Antarctic Bottom Water (AABW) in the model

!  Self-sustained oscillations of Southern Ocean deep convection driving multi-centennial variability are found in a complex

5.  Northward AABW transport reaches maximum at end of convective regime (right, labelled “on”).

global climate model (Martin et al., 2012).

6.  AABW decline after convection shutdown causes increase of southward export of North Atlantic Deep Water (NADW)

(Latif et al., 2012, subm.)!

Multi-centennial variability in the Atlantic Meridional Overturning Circulation:!

!  Centennial variability is present in Tasmanian tree ring data. MCV potentially masks current global warming signals arising

7.  Northward propagation of surface salinity anomaly (0.5 psu) causes reduction of NADW formation ~80 years later

from anthropogenic emissions. (Latif et al., 2012, subm.)

MCV in North Atlantic Sea Level cm

Sea level height (model diagnostic)

MCV

30 cm / 100 years

gray shading marks periods w/o deep convection in the Southern Ocean

The Kiel Climate Model

1.  Multi-centennial variability (MCV) present in northern North Atlantic sea level in KCM (left) 2.  Maximum of sea surface height (SSH) variation occurs about 80 years after Southern Ocean convection shutdown (above) 3.  AMOC maximum at 30˚N peaks at about same time 4.  SSH variations amount to about ±30 cm/century in center of subpolar gyre, which is same order of magnitude as current rise in global average sea level of 3 mm/year 5.  SSH variability is steric: MCV in upper ocean temperature (100-500 m) and mid to deep ocean salinity (1500-3000 m)

The KCM is a global coupled general circulation model (Park et al., 2009) with

•  ECHAM5 atmosphere, T31L19 (~3.75°) •  NEMO (OPA9-LIM2) ice-ocean model, 2° grid (0.5° in the tropics), 31 levels

•  Millennial control simulation with constant 20th century GHG forcing, no flux correction, no anomaly coupling Corresponding Author Torge Martin [email protected]

University, Kiel, Germany

Observed Southern Ocean SST MCV

1.  Recurring deep convection in the Weddell Sea driven by the accumulation of heat at mid-depth and moderated by the local sea ice cover impacts the oceanic heat content in the Atlantic and Indian sectors of the Southern Ocean (left)

sea ice thickness

3 Kiel

References Cook, E. R., and others (2000) Warm-season temperatures since 1600 BC reconstructed from Tasmanian tree rings …, Clim. Dyn., 16(2), 79–91, doi: 10.1007/s003820050006. !

6.  S S T a n d S S S anomaly propagate northward only to 40-45˚N (right) 7.  Do local processes dominate MCV signal? 8.  SSH also linked to AMOC strength at 30˚N (very right)

Conclusions 2 !  MCV present in northern North Atlantic sea surface height in KCM !  Timescale hints at Southern Ocean teleconnection involving AMOC variations driven by AABW production and northward propagating surface properties !  Further analysis on interaction of local (northern North Atlantic) and remote (Southern Ocean) processes necessary including atmospheric mechanisms

Latif, M., T. Martin, and W. Park (2012) Southern Ocean Sector Centennial Climate Variability and Recent Decadal Trends, J. Clim., submitted. Martin, T., W. Park, and M. Latif (2012) Multi-centennial variability controlled by Southern Ocean convection in the Kiel Climate Model, Clim. Dyn., doi: 10.1007/s00382-012-1586-7, accepted

Park, W., N. Keenlyside, M. Latif, A. Ströh, R. Redler, E. Roeckner, and G. Madec (2009) Tropical Pacific Climate and Its Response to Global Warming in the Kiel Climate Model, J. Clim., 22(1), 71–92 Smith, T.M., R.W. Reynolds, T. C. Peterson, and J. Lawrimore (2008) Improvements to NOAA's Historical Merged Land-Ocean Surface Temperature Analysis (1880-2006), J. Clim., 21, 2283-2296