Review of Seasonal Footprinting Mechanism for El Nino

Notes

Vimont, J., Battisti, D., Hirst, A., 2001. Footprinting: A seasonal connection between the tropics and mid-latitudes. Geophysical Research Letters, Vol. 28, https://doi.org/10.1029/2001GL013435.

Abstract: Connection between midlatitude and tropical Pacific is identified in CSIRO CGCM, involving a seasonal coupling between winter mid-latitude atmospheric circulation anomalies and summer equatorial wind stress anomalies. Summer tropical atmosphere responds to subtropical SSTAs generated by mid-latitude atmospheric variability during previous winter.

IMAV = Intrinsic Mid-Latitude Atmospheric Variability
produces equatorial zonal wind stress anomalies and the tropics adjust through coupled dynamics, producing an equatorially symmetric ENSO-like pattern of variability.
IMAV-equatorial zonal wind stress anomalies connection identified by requiring IMAV to be independent of tropical forcing, to ensure IMAV not caused by tropical variability.
They seek a structure that covaries with tropical zonal wind stress anomalies – defined by applying singular value decomposition (SVD) analysis to mid-latitude SLP and tropical zonal wind stress anomalies.
Regression maps of annually averaged SLP, zonal wind stress, and SST: a dipole SLP pattern across the Pacific w/ anomalous high and low SLP pattern across the Pacific… anomalous high and low SLP to the north and south of 50N.
Regression maps establish an IMAC and tropical ENSO connection but alone is insufficient evidence to conclude IMAV influences tropical ENSO-like variability.
Casuality can be determined by examining the lagged correlation between DYN-SLP and the cold tongue (CT) index.
Because the atmospheric bridge hypothesis is a sumultaneous connection, the null hypothesis is the automatic lagged correlation between DYN-SLP and the CT index: given that DYN-SLP is correlated with the CT index at zero lag and the CT index is autocorrelated with itself at lag tau, DYN-SLP should be automatically correlated with the CT index at lag tau.
Lagged correlation peaks when DYN-SLP leads CT index by one year.
Tropical zonal wind stress anomalies associated with IMAV do not extend to the equator by atmospheric processes alone, signifying an essential role of the oceanic ML.
Mechanism linking winter and summer variability revealed by examining winter and summer SST and heat flux maps.
During winter: pos tropical SSTAs collocated with downward net surface heat flux, IMAV forces winter SSTAs.
During summer: SST pattern is nearly identical to previous winter's SST pattern, persist into summer. But relationship between SST and sfc heat flux is reversed during summer. Ocean is forcing the atmosphere during summer months.
Summary: SST footprint imparted onto ocean via changes in surface heat flux during winter.
Important caveat:

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Vimont, J., Wallace, J., Battisti, D. 2003. Seasonal Footprinting Mechanism in the Pacific: Implications for ENSO. Journal of Climate, Volume 16, , 2668-2675.

Abstract: Midlatitude atmospheric variability is identified as an effective component of the stochastic forcing of ENSO, in which the tropical atmosphere is forced during the spring and summer by SST anomalies generated by midlatitude variability during the previous winter. Seasonal Footprinting Mechanism (SFM) has a strong relationship with ENSO, which may enhance ENSO predictability.

During winter, North Pacific Oscillation (NPO) imparts an SST footprint onto the ocean via changes in sfc heat flux. SST footprint persists into late spring and summer seasons.
In CSIRO CGCM, SFM accounts for 40% of interannual variability and 70% of interdecadal variability.
A statistically significant NPO-like pattern is indeed observed to precede ENSO by 1 yr.
Methods
Applied SVD analysis to winter midlatitude SLP, summer NH tropical zonal wind stress, and winter tropical SST (+11 months from SLP data, lagged).
Southern lobe of dipole SLP pattern in North Pacific indicates weakened trades throughout the subtropics, via a reduction of upward latent heat flux, reduced frictional convergence, reduced cloudiness, an increase in downward solar radiation in the Tropics.
NPO variability reproduces these characteristics.
Strength and location of summer ITCZ changes in response to underlying SST footprint…. Shifted northward and strengthened due to pos SSTAs in northern subtropics.
ENSO may be linearly stable given strong correlation between NPO-like SLP anomalies and tropical SST anomalies 1 yr later. Underscores the importance of midlatitude atmospheric variability as a potential leading contributor to the stochastic forcing of ENSO
The tropics must be 'set-up' for an El Nino event-- not every strong NPO event results in an El Nino. Western warm pool strength factors into this. A strong equatorial Pacific SST/SSH gradient is unstable.

Points to mid-latitude variability as having a potential role in the sustained El Nino during a nuclear winter, since the NPO certainly changes phase.

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Chen, S., Chen, W., Yu, B., Graf, H. 2013. Modulation of the seasonal footprinting mechanism by the boreal spring Arctic Oscillation. Geophysical Research Letters, Volume 40, Issue 24, https://doi.org/10.1002/2013GL058628.

NPO (North Pacific Oscillation) may be able to force an El Nino event during the following winter via the seasonal footprinting mechanism (SFM).
The springtime AO has a significant modulation effect on the connection between NPO and El Nino.
+AO during sprint and a +NPO during winter can result in El Nino warming anomalies via SFM.
However, when the AO is negative during spring there is no relationship.
The AO role is in changing the underlying SST footprinting over the subtropical northeastern Pacific.
NPO mechanism
when the southern lobe of the NPO exhibits a negative anomaly, the trade winds weaken over the central and eastern subtropical Pacific… SST warming is caused by decreased wind speeds.

WES feedback involves three different processes sensitive to the mean state and seasonal cycle:
(i) wind variations generate changes in surface evaporation
(ii) surface evaporation alters the underlying SST
(iii) the resulting SST anomalies generate atmospheric circulation anomalies that, if WES feedback is positive, should resemble the original wind variations