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Impacts of Atlantic Multidecadal Variability: seasonal mean climate and ENSO Rowan Sutton Dan Hodson, Buwen Dong 1. Walker Institute for Climate System Research, University of Reading 2. National Centre for Atmospheric Science – Climate Outline

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impacts of atlantic multidecadal variability seasonal mean climate and enso
Impacts of Atlantic Multidecadal Variability: seasonal mean climate and ENSO

Rowan Sutton

Dan Hodson, Buwen Dong

1. Walker Institute for Climate System Research,

University of Reading

2. National Centre for Atmospheric Science – Climate

outline
Outline
  • Proximal impacts on seasonal mean climate inferred from atmospheric GCM experiments – mainly JJA
  • Remote coupled responses in Indo-Pacific region impacts on ENSO
  • Outstanding issues

fast

A little slower

slide3

Atlantic Ocean Forcing of North American and European Summer Climate

Sutton & Hodson, Science, 309, 115-118, 1 July 2005

slide4
Enfield et al, 2001: Correlation between North Atlantic Sea Surface Temperatures and U.S. summer rainfall
  • Also significant changes in river flow and drought frequency (Mccabe et al, 2004), including potential contribution to 1930s dustbowl (Schubert et al, 2004)
our study
Our study
  • Previous evidence linking Atlantic Ocean to variations in N. American summer rainfall mainly circumstantial

Questions:

  • Could we identify/clarify causality?
  • Might there be other impacts, possibly in other regions?

Methodology:

  • Analysis of simulations with an atmosphere model forced with realistic and idealised variations in sea surface temperature & comparison with observations
slide6

CoolWarm Cool

AMO Index: North Atlantic SST (0-60N; 75-7.5W), low pass filtered

Regression of annual mean SST on AMO index

Analyse epoch differences between warm and cool phases

observed jja anomalies 1931 60 1961 90

~0.6hPA

1.5hPa

Observed JJA Anomalies (1931-60)-(1961-90)

sea level pressure precipitation (mm/day) land surf. air temp (oC)

  • SLP and SAT white where not significant at 90% level
  • Negative precipitation anomalies over Mexico & central US ; also South America
  • Positive temperature anomalies over US 0.5-0.75oC
atmosphere model simulations
Atmosphere Model Simulations

Model:

  • Hadley Centre HadAM3 general circulation model
  • 2.5 x 3.75 degrees, 19 levels
  • Observed Global SST:
    • Surface boundary conditions: observational HadISST SST and sea ice data 1871-1999
    • No variations in external forcings such as greenhouse gases (to isolate role of oceans)
    • Ensemble of 6 simulations differing only with respect to atmospheric initial conditions

B. Idealised North Atlantic SST

Simulations:

jja anomalies 1931 60 1961 90

~0.6hPA

0.5hPa

JJA Anomalies (1931-60)-(1961-90)

sea level pressure precipitation (mm/day) land surf. air temp (oC)

Ensemble mean anomalies from HadAM3 simulations forced with HadISST

  • Comparison suggests changes in the oceans are responsible for main atmospheric signals
isolating the role of the north atlantic
Isolating the role of the North Atlantic

Idealised North Atlantic SST patterns

NA

TNA

XNA

  • Patterns from regression on AMO index (x4)
  • HadAM3 forced with +/- anomaly patterns
  • 20 yr expts for NA, 10 yr for TNA, XNA

Additional experiments with the HadAM3 model:

slide11

~0.6hPa

~0.5hPa

JJA anomalies

NA+ - NA- C20C Observations

slide12

SLP anomalies for western European region (30W-25E; 40-60N)

obs

0

SLP anomalies for North American region (130W-70W; 15-45N)

obs

0

response to tropical and midlatitude sst anomalies
Response to tropical and midlatitude SST anomalies
  • TNA response Gill-like? (note also a tropic-wide signal)
  • XNA response baroclinic “downstream low”

XNA+ - XNA- TNA+ - TNA-

response of gill model to diabatic heating vectors low level horizontal flow

200 hPa streamfunction

Contours: vertical velocity

Response of Gill Model to diabatic heatingVectors: low level horizontal flow

Contours: pressure

JJA

importance of the atlantic over a longer period

r=-0.69

Eur. Obs slp

Importance of the Atlantic over a longer period

r=-0.52

  • AMO index accounts for significant fraction of variance in all cases (28-66%)
  • Other influences (e.g. Pacific) relevant to U.S. especially

U.S. Obs slp

r=-0.67

Eur. Model slp

r=-0.81

U.S. model slp

response to actual change in atlantic sst jja 1951 60 1961 90
Response to actual change in Atlantic SST JJA (1951-60)-(1961-90)

Obs

Response to global SST

Response to observed Atlantic SST change (30S-80N)

Impact of SST anoms outside Atlantic larger

Stronger impacts on South American precipitation

conclusions 1
Conclusions 1
  • During the twentieth century, interdecadal variability of North Atlantic SST had an important role in modulating boreal summerclimate – certainly in North America and possibly in Europe too.
  • Focussed here on time mean anomalies, but changes in frequency of extreme events (droughts, heat waves) likely to be most important for impacts – see Mccabe et al, 2004; Cassou et al, 2005
  • Implications for interpretation of past climate records – e.g. results suggest that change in North Atlantic in 1960s caused a cooling of U.S. & European summer climate; a further change may have contributed to recent warming.
a few words on other seasons
A few words on other seasons

DJF MAM

  • Large seasonal cycle in climate impacts
  • In all seasons response is strongest (highest signal-to-noise) in the tropics [but recall no forcing north of 70oN]
  • Important for interpretaion of proxy records
  • Comparison with twentieth century obs suggests Atlantic influence most important in JJA and SON

Surface air temperature

JJA SON

  • Reduction in ASO vertical shear consistent with Gill response

For more information: Climate response to basin-scale

warming and cooling of the North Atlatnic Ocean,

R. Sutton & D. Hodson, J. Climate, 2006, in press

atlantic impacts on the pacific
Atlantic impacts on the Pacific
  • Changes in the Atlantic Ocean can influence remote basins:
    • Oceanic teleconnections (mediated by Kelvin and Rossby waves)
    • Atmospheric teleconnections (Have seen that Atlantic SST anomalies can induce remote responses in Indo-Pacific)
  • Potential impacts on seasonal mean climate and interannual variability – including ENSO
slide21
Remote impacts of changes in the Thermohaline Circulation: oceanic versus atmospheric teleconnections
  • Extensive literature on oceanic adjustment to changes in the Atlantic THC (e.g. Kawase, 1987; Goodman, 2001; Johnson and Marshall, 2002)
  • Much more limited literature on the adjustment of the coupled ocean-atmosphere system (Dong and Sutton, 2002; Zhang and Delworth, 2005)
  • Timescale for oceanic teleconnections is much longer (many decades – set by Rossby wave propagation).
  • Dong and Sutton argued that atmospheric teleconnections dominate initial response to a rapid change in the THC. May also dominate equilibrium response.
enhancement of enso variability by a weakened atlantic thc in a coupled gcm b dong r sutton
Enhancement of ENSO variability by a weakened Atlantic THC in a coupled GCM (B. Dong & R. Sutton)

THC hosing experiment:

1 Sv (or 0.1Sv) applied for 100 years uniformly over the North Atlantic 50-70oN in HadCM3 model

change in annual mean state
Change in Annual Mean State
  • response over eastern and central Pacific broadly consistent between uncoupled and coupled simulations – in particular equatorial zonal wind anomalies
  • response over Indian Ocean and west Pacific strongly affected by ocean-atmosphere coupling

AGCM response in JJA to warm North Atlantic

surface winds

Note: major changes in Pacific mean state develop in first decade of hosing – strong evidence that atmospheric teleconnections dominate

precipitation

change in annual cycle on equator

Warming of SST in boreal autumn and winter – reduced upwelling?

Westerly anomalies in boreal summer & autumn (consistent with AGCM)

Change in Annual Cycle on Equator

Precip anomalies suggest ocean-atmos coupling

Thermocl-ine depth responds to winds

Weakening of annual cycle in SST on Equator

changes in enso variance
Changes in ENSO variance

Weaker THC => enhanced ENSO variance & enhanced skewness

Peak change 0.6oC

composite sst anomalies 1 5 s
Composite SST anomalies (+/-1.5s)

Control

1.0 Sv experiment

understanding the changes
Understanding the changes
  • Mean warming of central Pacific SST in boreal summer and autumn implies an eastward extension of west Pacific warm pool in the seasons when El Nino events grow to largest amplitude
  • Likely cause of eastward displacement of westerly wind anomalies during El Nino events
  • Also favours larger amplitude El Nino events:
    • Higher absolute SST => higher rates of evaporation, precipitation and latent heating => stronger zonal wind anomalies
    • Upward slope of mean thermocline to the east implies that eastward displacement of wind anomalies will enhance importance of “upwelling feedback” (impact of anomalous upwelling on SST).
  • Anomalies in thermocline depth (shallower in March-June) could also favour growth of larger SST anomalies (“thermocline feedback”)

Mechanism based on ideas from Wang and An (2002); Codron et al (2001); Wu and Hsieh (2003); Kang and Kug (2002)

slide30

Rossby wave propagation

Mean westerly anomalies in boreal summer and autumn

Shoaling signal propagates into central/east Pacific in boreal spring/summer

relevance to multidecadal variability
Relevance to Multidecadal Variability?
  • No evidence of THC shutdown in twentieth century, but – as we have seen – there is evidence of THC variability.
  • Could this variability have affected ENSO?
  • B. Dong, R. Sutton, A. Scaife, Modulation of ENSO variance by Atlantic Sea Surface Temperatures, GRL 2006.
slide32

The amplitude of ENSO varies on interdecadal timescales

Possible association with AMO:

AMO- phase (1930-60):Nino 3 st dev: 0.63oC

AMO+ phase (1965-95):

Nino 3 st dev: 0.81oC

Coincidence or causality?

Nino3 amplitude based on standard deviation in a 13 year running window

slide33
Testing the hypothesis that the Atlantic could drive variations in ENSO amplitude: regional coupling experiments
  • 2 x 150 year experiments with HadCM3 coupled model (Atm: 2.5oX3.75o with 19 levels. Ocean: 1.25oX1.25o with 20 levels)
  • Initial state from a 1700y control simulation
  • +/- AMO: Atlantic SSTs relaxed to seasonally varying climatology from the coupled model

+/- 3 x EOF pattern in Atlantic.

  • Relaxation timescale = 2.5 days
slide34

Impact on annual mean climate

  • Opposite sign to hosing experiments but patterns very consistent
  • Seasonal evolution also very consistent
impact on enso
Impact on ENSO

Relative to the AMO- run, Nino 3 standard deviation decreases by 23% in the AMO+ run.

Taking into account the amplitude of forcing this suggests Atlantic might account for about half observed change (assuming perfect model)

slide36
Comparison between HadCM3 response to weakened THC and observed change in ENSO properties associated with 1976 “climate shift”
  • Wang and An (2002) suggested that the key decadal change in equatorial Pacific winds was a remote response to changes in the North Pacific.
  • Alternative hypothesis is that Atlantic changes were a key driver.
  • However, could be a generic response of the Pacific climate system to many potential triggers
conclusions 2
Conclusions 2
  • In the HadCM3 model changes in the THC, or related Atlantic SST anomalies, influence the amplitude of ENSO. Atmospheric teleconnections are responsible.
  • Weakened THC => enhanced ENSO variance and increased asymmetry: larger amplitude El Nino events.
  • Atlantic influence could help to explain interdecadal variability of ENSO characteristics in twentieth century and paleoclimate evidence linking North Atlantic cooling to more frequent or persistent El Nino conditions (e.g. Cobb et al, 2003).
  • Other coupled models appear to show similar ENSO response to THC weakening (Timmermann et al, 2006); extent of common mechanism to be determined / discussed.
  • Mechanism might also (help to) explain observed inverse association between annual cycle amplitude and ENSO amplitude (Fedorov and Philander, 2001; Guilyardi, 2006) – warming of central Pacific in boreal autumn is key to both.
some outstanding issues
Some Outstanding Issues
  • Model uncertainty in responses to AMO – e.g. Sahel rainfall, U.S. rainfall
  • More detailed understanding of mechanisms – e.g. impact of AMO on North and South American precipitation and – of course – on ENSO.
observational datasets all monthly mean gridded products
Observational Datasets(all monthly mean gridded products)
  • HadISST – 1x1 degree sea surface temperature and sea ice data (Rayner et al, 2003) 1871-2003
  • HadSLP1 – 5x5 degree SLP 1871-1998 (update of Basnett and Parker, 1997)
  • HadCRUT2 – 5x5 degree land surface temperatures 1856-2003 (Jones and Moberg, 2003)
  • CRU precipitation – 3.75x2.5 degree 1900-1998 (Hulme, 1992)
slide42

Timmermann et al, 2006

Obs fig by Buwen Dong, after Fedorov and Philander (2001)

GFDL CM2.1

HadCM3

MPI ECHAM5-OM1

CCSM3

CCSM2

slide43
Consistent with Gill-type response, vertical shear is reduced vertical shear in August-September-October

Vertical shear of zonal wind

See also: Vitart and Anderson (2001); Shapiro and Goldenberg (1998)

Shear anomaly in MDR ~ 2m/s cf 8m/s in control experiment

  • reduced shear favours hurricane formation & hence numbers (distinct from direct effect of SST on intensity)
jja anomalies 1951 60 1961 90
JJA Anomalies (1951-60)-(1961-90)

Obs

Response to global SST

Response to Atlantic SST

climate response to a basin scale warming cooling of the north atlantic ocean
Climate Response to a basin-scale warming/cooling of the North Atlantic ocean

Aims

  • Study the climate impacts of a warming or cooling of the N. Atlantic ocean in a carefully controlled way
  • Focus here:
    • Seasonal evolution of the climate response
    • [ paper also discusses other issues: Roles of tropical vs higher latitude SST; Nonlinearities with respect to the sign of SST anomalies ]

(R. Sutton & D. Hodson, J. Climate, 2006, in press)

the atlantic multidecadal oscillation or amo
The “Atlantic Multidecadal Oscillation” or AMO

North Atlantic SST Index (0-60N; 75-7.5W), annual mean, low pass filtered

Pattern derived by regression of annual mean SST on SST index

[Seasonal variation of the time series is very small; seasonal variation of the pattern is modest: 10-30%]

experimental design
Experimental Design
  • HadAM3 model (2.5 lat * 3.75 lon, 19 levels)
  • Control experiment (40 years)
  • Anomaly experiments (20 or 10 years, of which use last 19 or 9)
  • North Atlantic SST anomaly pattern derived from observations

Cautionary note: There is evidence of considerable inter-model variation in the response to Atlantic SST anomalies

sst anomalies
SST anomalies

Experiments: NA+/- TNA+/- XNA+/-

NA+ TNA+ XNA+

  • Anomalies set to zero north of 70N where large uncertainties
  • Applied anomalies are 4s but for comparison with observations results are scaled by ¼ to give a fair linear comparison
analysis
Analysis
  • Examine differences between time-means of pairs of experiments
  • Statistical significance based on t-test assuming years are independent
  • Signal/noise measure:

SST forced response

Internally generated variability

(Time mean anomaly between expts)

(interannual standard deviation within an experiment)

NB: S/N is independent of ensemble size

response to na na in surface temperature
Response to (NA+)-(NA-) in surface temperature

cooling

DJF MAM

JJA SON

Anomalies plotted are significant at 95% level

response to na na in sea level pressure
Response to (NA+)-(NA-) in sea level pressure

cooling

MAM

DJF

Contours: ensemble mean anomaly. Shading: signed signal/noise ratio

S/N:

> 1 tropics

< 1 extratrops

SON

JJA

S/N highest in the tropics, and in JJA and SON

response to na na in precipitation
Response to (NA+)-(NA-) in precipitation

MAM

DJF

SON

JJA

Shading: ensemble mean anomaly. Contours: signal/noise ratio

NB: Models vary considerably w.r.t. impact on Sahel rainfall

climatological sst
Climatological SST

MAM

DJF

SON

JJA

Mean SSTs are warmest in JJA and SON => larger precipitation and latent heating anomalies

contribution of internal variability to european slp change
Contribution of internal variability to European SLP change

SLP anomalies for western European region (30W-25E; 40-60N)

0

obs

slide57

SLP anomalies for western European region (30W-25E; 40-60N)

obs

0

SLP anomalies for North American region (130W-70W; 15-45N)

obs

0

causes of decadal variability in n atlantic sst
Causes of decadal variability in N. Atlantic SST

Comparison of observations with HadCM3 “all forcings” integrations; 20 year smoothing applied to all data

  • Warming 1910-1940 & Cooling 1950-1970 look unlikely given (small) ensemble => suggests a large internal fluctuation, most likely related to THC
  • Recent warming substantially forced but there may be a positive or negative THC contribution

North Atlantic SST

North Atlantic SST North of 40N

Figure from Peter Stott

slide63
Response in tropics very similar in NA and TNA experiments in all seasons
  • Very weak response (mostly insignificant) to XNA forcing alone
  • Higher latitude response seen in DJF and MAM in NA experiments not seen in either TNA or XNA experiments, implies nonlinear interactions
response to xna xna in slp
Response to (XNA+)-(XNA-) in slp

MAM

DJF

SON

JJA

“Heat low” (strongest in JJA)

response of atmospheric circulation
Response of atmospheric circulation
  • Response in tropics very similar in NA and TNA experiments in all seasons
  • Weak “heat low” response to XNA forcing alone
  • Higher latitude response seen in DJF and MAM in NA experiments partly a remote response to tropical SST
slide70
Met Office seasonal forecast of winter North Atlantic Oscillation based on preceding May Atlantic SST

Basis of “cold winter” forecast

Note prominent interdecadal variation