atmospheric response to north pacific sst the role of model resolution and synoptic sst variability...
TRANSCRIPT
Atmospheric response to North Pacific SST
The role of model resolution and synoptic SST variability
Guidi Zhou, Mojib Latif, Wonsun Park*, Richard Greatbatch
GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany*presenting
Frontal-scale Air-sea Interaction WorkshopBoulder, CO, USAAugust 6, 2013
Background and motivation Observed atmospheric response to re-
emergent SST AGCM response Sensitivity to background SST and
horizontal resolution
Outline
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…]
Background and motivation
Observed atmospheric link to reemergent SST anomalies
Regressed SST on NP SST PC1* (ENSO removed)
ERSST 80-year winter-mean data
Remove ENSO from NP SST using linear regression (PC1*)
Lagged regression of SST on PC1* (PC1* leads)
SST reemergence found in NP
Observed atmospheric link to reemergent SST anomalies
Regressed SLP on NP SST PC1* (ENSO removed)
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
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
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
Experimental setup
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
Contour: winter-mean climatological SST (°C)Shading: PDO-like SSTa (°C)
Positive PDO-like SSTa pattern would reduce the front
AGCM response to NP SSTa
response characteristics
Deepened Aleutian Low
(hPa)
AGCM response to NP SSTa
response characteristics
(W/m2)
Upward heat fluxes reinforce the SSTa in some regions
AGCM response to NP SSTa
response characteristics
Geopotential height (m)
equiv. barotropic
U-velocity (m/s) Vertical velocity (Pa/s)
accelerated jet stream deep convection
Zonally averaged over NP
AGCM response to NP SSTa
eddy forcing
div. eddy vorticity flux (1/s2)
conv. eddy vorticity flux
divergence (1/s) div. eddy zonal momentumflux (m/s2)
upper div., surface conv.conv. eddy momentum flux
Consider 40°N (over cold SSTa)
AGCM response to NP SSTa
Characteristics Diagnostics (over cold SSTa)
T213 Response summary
Equivalent barotropic low Upward heat flux
reinforcing the SSTa in some regions
Accelerated jet stream Deep convection Deep diabatic heating Enhanced storm-track
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
T31- response
Geopotential height (m) Geopotential height (m)
T213- response
Influence of synoptic SST variability
Snapshot, Jan 01, 1982.Fine spatial/temporal structures on T213Synoptic SST variability only in NP
ctrl, NP SSTa experimentsClimatological state, Jan 01 Smoothed spatial/temporal structures on T213
ctrl, NP SSTa experiments
Sea surface temperature (K)
Observed daily varying SST vs. climatological
SST as backgroundT213-daily varying
North Pacific background SST
T213-climatological background
SST
no significant responsesignificant eddy-mediated response
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
Summary
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 [email protected]
Zonal mean SST gradient
Zonal (120°E-160°W) mean