Mechanisms involved in the amplification of the 11-yr solar cycle signal in the tropical Pacific
Stergios Misios and Hauke Schmidt
Max-Planck-Institute for Meteorology
IMPRS-ESM
Hamburg, Germany

The detection of solar signals in the equatorial Pacific is difficult.
• Oscillations of different time scales with partly large amplitudes exist ( ENSO, decadal or multidecadal oscillations ).
• Shortness of quality-controlled observations ( start in 1880 but the spatial coverage in Pacific is poor before 1950 ).
• Choice of analysis technique may lead to different conclusions
( composites vs. multiple regression ).
(e.g. White et al. (JGR, 1997) and Roy and Haigh (ACP, 2010) have analyzed signals positively correlated with solar activity.)

Observed solar (?) signals
SSN-MAX SSN-MIN
(K)
A La Niña-like response was identified in peak MAX years of SSNs (e.g. van
Loon et al., JGR, 2007).
BUT
No El Niño-like response is observed in MIN years of SSNs.

Observed solar (?) signals
Composite Multiple regression
Not only composites but multiple regression shows cooling in east Pacific (e.g.
Tung and Zhou, 2010,JAS)
BUT
Not consistent spatial patterns

Observed solar (?) signals
Basin-wide warming in data from 1955 onwards
White et al., 1997, JGR

Model Description – Simulation setup
Middle Atmosphere version of ECHAM5/MPIOM
Resolution: T31L90/GR30L40
Detailed stratospheric dynamics: internal QBO
Present-day greenhouse gas concentrations
140-yr CONTROL simulation
Coupled ENSEMBLE
11 realizations
1955-2006 solar spectral irradiances (Lean et al., 2000, GRL)
Solar induced ozone concentration changes from HAMMONIA (Schmidt et al., 2010, JGR)
Uncoupled ENSEMBLE
9 realizations
Setup as in the coupled simulations
Climatological SST and SIC boundary forcing from CONTROL

Data analysis
Deseasonalized ensemble mean annual values from 1955-2006.
Two methods:
Regression analysis onto annual F10.7
Statistical significance with t-test
Multichannel Singular Spectrum Analysis
[Ghil et al., Rev. Geoph., 2002]
Two oscillatory modes of 10.9 yr period are isolated.
Attention: ENSO in MAECHAM5/MPIOM is stronger than observed.

Time evolution of SST anomalies at the
Equator
(K)
ENSO hides a basin-wide solar warming/cooling

Regression of unfiltered and filtered SSTs onto
F10.7
Pacific-wide warming of ~0.1 K/100sfu.
The signal is stronger over the Pacific than in Indian and Atlantic oceans.

Atmosphere-Ocean responses at the Equator
Contours: Climatology
Shading: Anomalies
Weaker Walker circulation. Its western branch shifts eastward.
Reduced equatorial surface winds.
Thermocline shoaling in the western sector and thus negative anomalies in ocean temperature.
Reduced upwelling of cold water warms the surface
The average thermocline depth along the
Pacific is reduced.

Eastward shift of deep convection precipitation regressed onto time-lagged F10.7
Lag 0
Time
(years)
Lag -5
(sun leading precipitation by 5 years) (mm/day)

Comparison of simulations with and without ocean coupling
MAECHAM5/MPIOM MAECHAM5 only
Reduced surface easterlies occur independently of the ocean coupling.
Walker circulation weakens with higher temperatures (cf. Held and Soden, 2006;
Vecchi and Soden, 2007).

We find no explanation for a negative response in peak years. We can not rule out that solar forcing triggers some ENSO phase but the “signal” might also be accidental.
Simulations with MAECHAM-MPIOM demonstrate a Pacific-wide warming with increased solar activity.
The signal is weak but comparable to the analysis of White et al., (JGR, 1997).
The temperature signal in the tropical
Pacific is enhanced by a dynamical response of the ocean.
The weakening of the Walker circulation surface easterlies and the eastward displacement are simulated independently of the ocean coupling.
Summary
Observed
(White et al., JGR, 1997)
Simulated
(Misios and Schmidt, J. Climate, submitted)