A Link between Tropical Precipitation and the North

Atlantic Oscillation

Matt Sapiano and Phil ArkinEarth 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 precipitationGlobal Precipitation Climatology Project (GPCP Version 2) dataset DJF

•Global gridded merged gauge/satellite precipitation estimate

•2.5 º by 2.5º spatial resolution; 1979-present

Mean GPCP V2 Dec 1979 – February 2005 , mm/day

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

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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 NAONAO 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

techniqueOne 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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• 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

A link between tropical precipitation and the 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:

),0(~ NAONAOFebJan 2-43210 Ny

Response: Precipitation

Anomaly terms

Explanatory variable: NAO

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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…?