Why the Indian Ocean is Important for North Atlantic Climate Prashant. D. Sardeshmukh @ noaa.gov and Sang-Ik Shin Climate Diagnostics Center, CIRES, University of Colorado and Physical Sciences Division/ESRL/NOAA AGU Fall Meeting December 2008 - PowerPoint PPT Presentation

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Why the Indian Ocean is Important for North Atlantic Climate Prashant. D. Sardeshmukh @ noaa.gov and Sang-Ik Shin Climate Diagnostics Center, CIRES, University of Colorado and Physical Sciences Division/ESRL/NOAA AGU Fall Meeting December 2008

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... Pacific, coupling to a 50 mb slab mixed layer ocean ... Mean erroneous streamfunction tendency induced by erroneous daily vorticity flux convergences ... – PowerPoint PPT presentation

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Title: Why the Indian Ocean is Important for North Atlantic Climate Prashant. D. Sardeshmukh @ noaa.gov and Sang-Ik Shin Climate Diagnostics Center, CIRES, University of Colorado and Physical Sciences Division/ESRL/NOAA AGU Fall Meeting December 2008


1
Why the Indian Ocean is Important for North
Atlantic Climate Prashant. D. Sardeshmukh _at_
noaa.gov and Sang-Ik Shin Climate
Diagnostics Center, CIRES, University of
Coloradoand Physical Sciences
Division/ESRL/NOAAAGU Fall Meeting December
2008 San Francisco
  • Tropical SST trends, especially over the Indian
    ocean, have been proposed as important drivers
  • of NAO trends, from ensembles of atmospheric GCM
    simulations of the past half-century with
  • prescribed observed SSTs.
  • However, such uncoupled integrations account for
    only about 30 of the observed NAO trend.
  • Some proposed explanations include the neglect of
    air-sea coupling over the North Atlantic and
  • the neglect of direct radiative forcing changes.
  • Here, we propose an alternative explanation the
    neglect of air-sea coupling over the Indian
    Ocean.
  • We show that this neglect leads to a strong
    spurious transient-eddy feedback on the NAO.

2
From Hurrell et al (2004)
Evidence that Atmospheric GCMs with prescribed
observed SSTs severely underestimate the
magnitude of the observed NAO trend.
3
Determining the responses to Coupled vs.
Uncoupled Indian Ocean SSTs
NCAR/CCM3.10 atmospheric GCM integrations with
a prescribed constant 2 x EOF1 SST
anomaly in the eastern tropical Pacific, coupling
to a 50 mb slab mixed layer ocean model only over
the Indian ocean, and climatology elsewhere.
Experiments Indian Ocean SSTs Control
prescribed climatology Exp 1
Coupled Mixed Layer Model Exp 2 Uncoupled
prescribed from coupled run All experiments
were run for 105 years and the last 100 years
were used for the analysis. Coupled response to
Indian Ocean SSTs Exp. 1 - Control Uncoupled
Response to Indian Ocean SSTs Exp. 2 -
Control Error of Uncoupled response Exp. 2 -
Exp. 1
4
Mean 200 mb Height responses in DJF to Indian
Ocean SSTs (C.I. 5 m)
Coupled response Uncoupled response

Error of uncoupled response
Thick Lines Thin Lines - Gray Shading
95 significant.
5
100-yr time series of a winter-mean NAO index
In Coupled run Uncoupled minus Coupled
In Uncoupled run p.d.f of NAO index
in coupled and uncoupled runs
6
Mean T850 and Precipitation Responses in DJF to
Indian Ocean SSTs
Coupled response Error of uncoupled
response Uncoupled response Only
responses significant at the 95 level are shown
7
Mean 200 mb Vorticity responses in DJF to Indian
Ocean SSTs
Error of uncoupled response
Coupled response Uncoupled response

Shaded regions show location of 200 mb jet
8
Fractional Changes of Seasonal Variance forced
by Indian Ocean SSTs
Coupled response Error of uncoupled
response Uncoupled response Only changes
significant at the 90 level are shown
9
Errors in DJF 200 mb Streamfunction ?
Mean Error of uncoupled response Fractional
Variance Error of Uncoupled response Mean
erroneous streamfunction tendency induced by
erroneous daily vorticity flux convergences
10
Summary and Concluding Remarks
  • Tropical SST trends, especially over the Indian
    ocean, have been proposed as important drivers of
  • NAO trends, from ensembles of atmospheric GCM
    simulations of the past half-century with
    prescribed
  • observed SSTs.
  • However, such uncoupled integrations account for
    only about 30 of the observed NAO trend.
  • Proposed explanations of include the neglect of
    air-sea coupling over the North Atlantic and
  • the neglect of direct radiative forcing changes.
  • Here, we propose an alternative explanation the
    neglect of air-sea coupling over the Indian
    Ocean.
  • We show that this neglect leads to a strong
    spurious transient-eddy feedback on the NAO.
  • We suspect that this is the basic reason for the
    negative NAO trend bias seen in previous model
    studies.
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