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A Link between Tropical Precipitation and the North Atlantic Oscillation

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 6 th , 2005. Talk Outline. Motivation

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A Link between Tropical Precipitation and the North Atlantic Oscillation

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  1. 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

  2. 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

  3. 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? TOGA Precipitation Note: blue is positive on this plot!

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. Is this link statistically significant? (2) →NAO/Indian Ocean link statistically significant for monthly data

  12. 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…

  13. Is there a timescale dependence? Monthly GPCP composites Pentad GPCP composites

  14. 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…?

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