A Link between Tropical Precipitation and the North Atlantic Oscillation

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A Link between Tropical Precipitation and the
North Atlantic Oscillation

• Matt Sapiano and Phil Arkin
• Earth Systems Science Interdisciplinary Center,
  University of Maryland
• Climate Diagnostics and Prediction Workshop, Penn
  State,
• Wednesday, October 6th, 2005

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Talk Outline

• Motivation
• Brief intro to datasets
• The link between Global Precipitation and the NAO
• A link between tropical precipitation and the
  NAO?
• Outstanding questions

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Motivation and Aims

• Hoerling et al. (Science, 2001) found that
  increasing trend in tropical precipitation from
  1950-1999 was related to similar trend in NAO
  (used NCAR CCM3)
• Aim to explore this link with observational data
  using a simple statistical model

Implication high NAO index associated with
greater Indian/Pacific Ocean tropical
precipitation?
Note blue is positive on this plot!
TOGA Precipitation
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Global precipitation

• Global Precipitation Climatology Project (GPCP
  Version 2) dataset DJF

GPCP precipitation dataset is too short to
compare directly with the results of Hoerling et
al. (2001), but maybe periods of high/low NAO
index are characterized by coherent anomaly
patterns away from the Atlantic Ocean Best
oceanic precip estimates available
Mean GPCP V2 Dec 1979 February 2005 , mm/day

• Global gridded merged gauge/satellite
  precipitation estimate
• 2.5 º by 2.5º spatial resolution 1979-present

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The North Atlantic Oscillation

-

• CPC NAO index
• From rotated EOF analysis of 700hPa geopotential
  heights
• 11 month moving average applied for plot above
  (Jan 1979- Dec 2004)
• Define ve (and ve) NAO as months that are
  within the upper (lower) quartile (calculated
  from DJF Dec 1979- Feb 2004).

Images courtesy Martin Visbeck
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What do we expect from the NAO
NAO positive North Atlantic close-up

• Positive conditions associated with
• Increased precip over N Europe and US East Coast
• Decreased precip over Mediterranean and Labrador
  Sea

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The link between Global Precipitation and the NAO

• NAO well correlated with precipitation in North
  Atlantic this is what we expect
• Quadripole extending from Tropics? Physical
  explanations audience participation.
• Tropical relationship?
• Correlation is linear technique

One point correlation map of DJF monthly GPCP
anomaly with DJF monthly NAO
?Scant evidence of linear tropical NAO link look
at NAO and NAO- separately
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A link between tropical precipitation and the NAO?

• Composites of precipitation based on DJF GPCP
  (V2) monthly anomaly for NAO positive and
  negative conditions
• Non-linear technique shows different patterns for
  positive and negative NAO

?Non-linear tropical effects which are not
evident from simple correlation map
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A simple parametric model

• Linear correlation does not highlight interesting
  relationships in the tropics.
• Non-linear composites show interesting
  connections, but make statistical hypothesis
  testing difficult.
• Need a simple model which is analogous to the
  composites.

Regression with NAO positive and NAO negative
explanatory variables
Response Precipitation
Explanatory variable NAO
Anomaly terms
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Is this link statistically significant? (1)
Intercept is December mean Plots below are
analogous to composite plots, but anomaly is
accounted for in the model
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Is this link statistically significant? (2)
?NAO/Indian Ocean link statistically significant
for monthly data
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Some (interim) conclusions

• The link between North Atlantic precipitation and
  the NAO is (unsurprisingly) statistically
  significant.
• There is a statistically significant link between
  precipitation over the Indian Ocean and the NAO
• BUT the response differs in location for
  positive and negative NAO phases
• There is also a statistically significant link
  between precipitation over North-East Brazil, but
  only during the negative phase
• Note no apparent ENSO link
• The Atlantic Quadripole is statistically
  significant

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Is there a timescale dependence?
Monthly GPCP composites
Pentad GPCP composites
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Outstanding questions

• A conclusion There is a statistically
  significant relationship between the NAO and
  Indian Ocean precipitation in DJF winter.
• Is the phenomenon an artifact of the dataset?
  Difficult to test this over the ocean.
• Why do the monthly and sub-monthly (pentad) maps
  disagree in certain areas?
• Indian Ocean Equatorial Pacific
• How strong is this link and how important is it
  for the climate system?
• What are the mechanisms behind this link?