The Pacific Meridional Mode: The Pacific Meridional Mode: Diagnostics and ImpactsDiagnostics and Impacts

Dan Vimont Department of Atmospheric and

Oceanic SciencesCenter for Climatic Research

University of Wisconsin, Madison

John Chiang Department of Geography &

Berkeley Atmospheric Sciences Center

University of California, Berkeley

Climate Diagnostics and Prediction WorkshopClimate Diagnostics and Prediction WorkshopOctober 21, 2004October 21, 2004

Madison, WIMadison, WI

October 21, 2004 Climate Diagnostics and Prediction Workshop

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The Pacific Meridional Mode Analogies between the Atlantic and

Pacific Modeling the tropical response to mid-

latitude forcing The spatial structure of decadal ENSO-

like variability Conclusions

October 21, 2004 Climate Diagnostics and Prediction Workshop

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The Pacific Meridional Mode Analogies between the Atlantic and

Pacific Modeling the tropical response to mid-

latitude forcing The spatial structure of decadal ENSO-

like variability Conclusions

October 21, 2004 Climate Diagnostics and Prediction Workshop

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The Atlantic Meridional Mode Dominant statistical

mode of tropical Atlantic interannual to decadal variability: Meridional SST

gradient Cross-gradient

boundary-layer flow towards warmer water

ITCZ shift towards warmer hemisphere

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Tropical Mean States: SST

Cold tongue weighted toward east

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Tropical Mean States: Precip.

Mean ITCZ along cold tongue’s northern edge

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Tropical Seasonal Cycle

Atlantic:

Pacific:

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Pacific Meridional Mode: Motivation Each basin possesses similar mean states and

seasonal cycle: cold tongue to the east, similar seasonal cycles of ITCZ and cold tongue (Mitchell and Wallace, 1992)

Model studies without a thermocline-SST feedback produce meridional variability as the dominant mode (e.g. Xie and Saito, 2001)

Similar evidence for mid-latitude forcing of tropical variability in each basin (Curtis and Hastenrath, 1995; Nobre and Shukla; 1996, Xie and Tanimoto; Czaja et al., 2002; Vimont et al., 2001, 2003a, b)

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Observational Analysis Data: NCEP reanalysis

10m winds and SST; CPC merged precipitation

Defined over regions (in the Pacific and Atlantic) with similar mean states

Best fit linear regression to CTI (ENSO) removed from data

Method: Maximum Covariance Analysis (SVD analysis): defines patterns between two fields that are strongly coupled

MCA applied to 10m winds and SST

Data regressed onto SST expansion coefficient

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Analogous Meridional Modes

Leading statistically coupled mode for SST and 10m winds

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Analogous Meridional Modes

Precipitation regressed on SST time series from leading MCA mode

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Temporal evolution

Variations occur on many time scales, including interannual and decadal

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Temporal Evolution

Wind time index has maximum variance during boreal winter (NDJF)

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Temporal Evolution

SST time series has maximum variance during boreal spring (MAM)

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Temporal Evolution

Lag correlation peaks when wind time series leads SST time series

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Meridional Modes A “meridional mode” of tropical ocean-

atmosphere variability is identified in the Pacific. The Pacific meridional mode resembles the Atlantic

meridional mode in both spatial and temporal structure. The strong similarity between the two basins suggests that

the meridional modes arise from analogous processes

October 21, 2004 Climate Diagnostics and Prediction Workshop

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The Pacific Meridional Mode Analogies between the Atlantic and

Pacific Modeling the tropical response to

mid-latitude forcing The spatial structure of decadal ENSO-

like variability Conclusions

October 21, 2004 Climate Diagnostics and Prediction Workshop

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Analogous Meridional Modes

(i) Trade wind relaxation(ii) Up-gradient flow

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CCM3 Response to Meridional Mode SST

CCM forced by meridional mode SST: Up-gradient flow reproduced

Relaxed subtropical trades not reproduced

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Source of subtropical winds

Trades relax in response to NAO in the Atlantic, and NPO in the Pacific

North Pacific

Oscillation

North Atlantic

Oscillation

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Meridional Mode Evolution Wintertime fluctuations in the strength of the

subtropical trade winds affect the subtropical SST through surface heat fluxes

In the deep tropics, the atmosphere responds by producing surface winds that blow towards the warmer SST

Model simulation: we will force a coupled model with heat flux anomalies associated with the NPO during winter, then allow the model to freely evolve.

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CCM3.10 Experiments

NPO heat flux imposed during winter months: NDJFM

Ensemble simulations allow investigation of coupled variability (forced by NPO) without prohibitively long model integrations

Fluxes generated

by ATM and SOM

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CCM3.10 Coupled ResponseWinter

NPO heat flux forces SST anomalies

SummerCoupled response alters and prolongs

tropical SST anomaly

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Coupled response: WES feedback

SST anomaly amplifies slightly during winter (imposed forcing)

Latent heat flux continues to amplify SST anomaly during summer (after imposed forcing is shut off)

Coupled WES feedback enhances persistence and amplitude of SST anomalies

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Model Results The meridional mode can be excited by

mid-latitude atmospheric variability Wintertime variations in the trade wind strength

associated with the NAO or NPO alter subtropical SSTs through changes in surface heat fluxes

The tropics respond to these SST anomalies during spring and summer

The coupled WES feedback increases the amplitude and persistence of the tropical response

October 21, 2004 Climate Diagnostics and Prediction Workshop

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The Pacific Meridional Mode Analogies between the Atlantic and

Pacific Modeling the tropical response to mid-

latitude forcing The spatial structure of decadal

ENSO-like variability Conclusions

October 21, 2004 Climate Diagnostics and Prediction Workshop

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Interannual & Decadal EOFs

Highpass-filtered and lowpass-filtered EOFs reproduce ENSO and ENSO-like variability

(Zhang et al., 1997)

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Reconstructed data What happens if we reconstruct the lowpass-filtered data using

ONLY the highpass-filtered spatial information? Start by projecting the highpass-filtered EOFs (interannual spatial

information) onto the unfiltered data:

Next, apply a lowpass filter to these reconstructed pseudo-PC’s, and reconstruct the data using a subset of these pseudo-PC’s combined with the highpass-filtered EOFs:

€

z iR = Xe i

HP

€

XLPR = z iLPRe i

HP

i=1,2,3,4∑

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Reconstructed data Result: XLPR contains decadal temporal information only, and

interannual spatial information only. Next, perform EOF/PC anlaysis on XLPR.

If decadal processes are responsible for generating the meridionally broadened structure of ENSO-like decadal variability, then the EOFs of XLPR should not have an ENSO-like structure

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EOFs of reconstructed SST

EOF1 of the reconstructed SST reproduces the ENSO-like structure

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HP EOF3: ENSO

“leftovers”

Components of EOFLPR

Contributions from the interannual EOFs

HP EOF1: ENSO

Three HP EOF’s contribute to EOFLPR.Each HP EOF has a known relationship to ENSO.

HP EOF4: ENSO

precursor

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Components of EOFLPR

Temporal Relationships with ENSO

Lagged correlation between peak of

ENSO (PPC1) and PPC3 or PPC4:

PPC4 leads ENSO by 1-12

months

ENSO leads PPC3 by 1-12

months

October 21, 2004 Climate Diagnostics and Prediction Workshop

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The Pacific Meridional Mode Analogies between the Atlantic and

Pacific Modeling the tropical response to mid-

latitude forcing The spatial structure of decadal ENSO-

like variability Conclusions

October 21, 2004 Climate Diagnostics and Prediction Workshop

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Pacific Meridional Mode: Conclusions

A meridional mode of variability is identified in the Pacific The Pacific meridional mode has very similar spatial and

temporal characteristics as its Atlantic counterpart The Pacific and Atlantic meridional modes evolve via

coupled processes in the ITCZ - Cold Tongue region Both the Pacific and Atlantic meridional modes can be

excited by mid-latitude forcing in their respective Northern Basins (perhaps Southern?)

Model results indicate that positive coupled feedbacks enhance the meridional mode persistance and amplitude

The strong similarities between the Pacific and Atlantic meridional modes suggest that the modes are “real”

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Pacific Meridional Mode: Conclusions

The Pacific meridional mode may be an important contributor to interannual ENSO and decadal ENSO-like variability Meridional mode variability tends to precede ENSO by 2-4

seasons The spatial structure of decadal ENSO-like variability is well

reproduced as an average over ENSO precursors, the peak of an ENSO event, and ENSO “leftovers”. This suggests that decadal ENSO-like variability is realized through processes associated with the interannual ENSO cycle

Seasonality is very important Our understanding of climate variability is enhanced by an

understanding of the seasonal cycle