Atmospheric response to North
Pacific SST
The role of model resolution and synoptic SST variability
Frontal-scale Air-sea Interaction Workshop
Boulder, CO, USA
August 6, 2013
Guidi Zhou, Mojib Latif, Wonsun Park*, Richard Greatbatch
GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
*presenting
Outline
 Background and motivation
 Observed atmospheric response to reemergent SST
 AGCM response
 Sensitivity to background SST and horizontal
resolution
Background and motivation
 Extra-tropical SST experiences annual reemergence [Alexander et al. 1999…]
 The atmosphere responds to extra-tropical SST anomalies [Czaja & Frankignoul
1999, 2002…]
 Response is small compare to Tropical response and internal atm. variability
[Palmer & Sun 1985; Kushnir et al. 2002…]
 But potentially persistent due to the large thermal inertia of the oceanic mixed
layer [Kushnir et al. 2002…]
 Transient eddies are crucial [Peng & Whitaker 1999…]
 Oceanic fronts are important for anchoring the storm-track [Minobe et al.
2008; Taguchi et al. 2009…]
Observed atmospheric link to
reemergent SST anomalies
 ERSST 80-year wintermean data
 Remove ENSO from NP
SST using linear
regression (PC1*)
 Lagged regression of SST
on PC1* (PC1* leads)
 SST reemergence found
in NP
Regressed SST on NP SST PC1* (ENSO removed)
Observed atmospheric link to
reemergent SST anomalies
 ERSLP 80-year winter-mean
data
 Remove ENSO from NP SST
PC using linear regression
(PC1*)
 Lagged regression of SLP on
PC1* (PC1* leads)
 Significant links between
SLP and PC1* of previous
winters
 Deepened Aleutian Low
Regressed SLP on NP SST PC1* (ENSO removed)
Observed atmospheric link to
reemergent SST anomalies
Regressed SST on NP SST PC1* (ENSO removed)
Regressed SLP on NP SST PC1* (ENSO removed)
AGCM response to NP SSTa
Experimental setup
 Model: ECHAM5
 Resolution: T213 (~0.56°) L31
 Background SST: NOAA-OI-Daily SST (time-varying 1981-1990,
to preserve the fronts)
 SST anomaly forcing: +/- lag-0 regression pattern in NP
 4-month (NDJF) integrations
 Initialized Nov.1, +/- 10 winters each
 Mean (DJF) differences between +/- SSTa experiments studied
AGCM response to NP SSTa
SST anomaly (doubled)
PDO-like anomaly pattern
(K)
The relative position of the SST
anomaly with respect to the full SST
Positive PDO-like SSTa pattern would reduce the front
Contour: winter-mean climatological SST (°C)
Shading: PDO-like SSTa (°C)
AGCM response to NP SSTa
response characteristics
Deepened Aleutian Low
(hPa)
AGCM response to NP SSTa
response characteristics
Upward heat fluxes reinforce the SSTa in some regions
(W/m2)
AGCM response to NP SSTa
response characteristics
Zonally averaged over NP
equiv. barotropic
accelerated jet stream
Geopotential height (m)
U-velocity (m/s)
deep convection
Vertical velocity (Pa/s)
AGCM response to NP SSTa
eddy forcing
Consider 40°N (over cold SSTa)
conv. eddy vorticity flux
upper div., surface conv. conv. eddy momentum flux
div. eddy vorticity flux (1/s2)
divergence (1/s)
div. eddy zonal momentum
flux (m/s2)
AGCM response to NP SSTa
T213 Response summary
Characteristics
 Equivalent barotropic low 
 Upward heat flux reinforcing
the SSTa in some regions
 Accelerated jet stream
 Deep convection

 Deep diabatic heating
 Enhanced storm-track

Diagnostics (over cold SSTa)
Convergence of eddy vorticity flux is balanced by
divergence in circulation, resulting in ascent at
mid-levels and low-level convergence. The
vortex stretching associated produces the
surface low.
Convergence of eddy momentum flux reinforces
the jet.
Consistent with Kushnir et al. 2002...
High vs. low resolutions
T213- response
T31- response
Geopotential height (m)
Geopotential height (m)
Influence of synoptic SST variability
Sea surface temperature (K)
Snapshot, Jan 01, 1982.
Fine spatial/temporal
structures on T213
Synoptic SST variability
only in NP
ctrl, NP SSTa experiments
Climatological state, Jan 01
Smoothed spatial/temporal
structures on T213
ctrl, NP SSTa experiments
Observed daily varying SST vs.
climatological SST as background
T213-daily varying North
Pacific background SST
significant eddy-mediated response
T213-climatological
background SST
no significant response
Summary
 North Pacific atmosphere response to underlying SST
anomalies in observations
 This can be simulated only with higher resolution of AGCM,
e.g. T213
 Temporal resolution is also important in this aspect
 The response is eddy-mediated
In order to obtain a realistic response to
midlatitude SST anomalies one needs to
resolve the synoptic SST variability. This
requires employing sufficiently high
horizontal resolution.
Guidi Zhou
gzhou@geomar.de
Zonal mean SST gradient
Zonal (120°E-160°W) mean