Decadal Variability of the Indo-Pacific Warm Pool and Its Association with Atmospheric and Oceanic Variability in Winter

1. Decadal Variability of the Indo-Pacific Warm Pool and Its Association with Atmospheric and Oceanic Variability in Winter and Summer Vikram M. Mehta and Hui Wang The Center for Research on the Changing Earth System Columbia, Maryland  Importance of the Indo-Pacific Warm Pool in the Global Climate System  Decadal Variability of the IPWP Surface Temperature • Association Between Surface and Sub-surface Ocean Variability • Association Between IPWP, Atmospheric Circulation, and US Rainfall Variability • A Plausible Physical Hypothesis of IPWP Decadal Variability  Interactions Between Decadal and MJO Timescales  Summary Acknowledgement NASA – Ocean Physics Program

2. Importance of the Indo-Pacific Warm Pool in the Global Climate System Saturation vapor pressure non-linearly related to SSTdramatic increase in atmospheric moisture content and convection when SST ≥ ≈28.5°C Annual-average SST≥28°C from approximately 90°E-180°, 20°S-20°N; pronounced annual cycle THE major source of heat for the global atmosphere Largest pool of warmest surface ocean water on the Earth Numerous studies of possible mechanisms of maintenance of time-average state of the IPWP (Ramanathan and Collins, 1991; Wallace, 1992; Fu et al., 1992; Hartman and Michelsen, 1993; Waliser and Graham, 1993; Waliser, 1996; Sud et al., 1999) Natural variability has not received much attention; influences ENSO and marine ecosystems at interannual timescales (Delcroix et al., 2000; Picaut et al. 2000)

3. The Warm Pool Oscillation Climatology + low-pass filteredsea-surface temperatures from SODA: 1950-2001

4. Data Sets and Analysis Techniques • Sea-surface temperature: Smith and Reynolds (2004) version 2; 1948-2005; reduced to 1952-2001 after low-pass filtering (>7 years) • NCEP-NCAR Reanalysis: Kalnay et al. (1996); • Simple Ocean Data Analysis: Carton et al. (2000) • US precipitation: CPC US Unified Daily Precipitation Analysis; 1948-98 • Real-time US Daily Precipitation Analysis; 1999-2005; Higgins et al. (2000) Empirical Orthogonal Function analysis of boreal winter and summer low-pass filtered SST variability to decompose into dominant components Linear regression to composite oceanic and atmospheric fields, and US precipitation with respect to two dominant SST EOF patterns Significance of statistical results tested with the Monte Carlo technique

5. Average Indo-Pacific Warm Pool Sea-surface Temperature with and without Linear Trend IPWP Index: Average monthly SST  28ºC (20ºS-20ºN, 90ºE-180º) Linear Trend 0.1ºC/ decade Significant spectral peaks in detrended time series at 1.5, 2.1, 3.5, and 9.7 years

6. Regression and Principal Component Patterns of Monthly, Low-pass Filtered Sea-surface Temperatures Domain of EOF analysis Two std. dev. departure for PC 1 Normalized PC 1 45% Two std. dev. departure for PC 2 Normalized PC 2 18% ºC

7. EOF-Based Reconstruction of Low-pass Filtered Sea-surface Temperatures Annual average SSTs  2 std. dev. EOFs EOF1+ EOF2+ EOF2- EOF1- East-west and north-south expansion-contraction in EOF1 Changes in in situ warmth in EOF2

8. Equatorial Cross-section of Sub-surface Temperatures Associated with SST EOFs: Boreal Winter EOF1; DJF EOF2; DJF Upper 150m in C. & E. Pac. warm in +ve phase In situ changes along the thermo cline Depth (m) Lower 200m in W. Pac. warm in -ve phase Anom. T Total T

9. Equatorial Cross-section of Sub-surface Temperatures Associated with SST EOFs: Boreal Summer Differences between +ve and -ve phases generally similar to winter but weaker

10. Meridional Cross-section of Sub-surface Temperatures Along 150ºE Associated with SST EOFs: Boreal Winter WP shrinks in merid. dim. in +ve phase and expands in -ve phase; opposite to behavior in zonal direction. Depth (m)

11. Surface Temperature and Anomalous Current Patterns Associated with SST EOFs: Boreal Winter Zonal Expansion/Contraction and Warming/Cooling by Zonal Heat Advection?

12. Anomalous Meridional Currents At 150ºE Associated with Positive Phase of SST EOFs: Boreal Winter Stronger STCs in both Hemispheres can cool WP near the equator and expand WP in the tropics  Neg. feedback Weaker STCs in both Hemispheres can warm WP near the equator and contract WP in the tropics  Pos. feedback Characteristic advection time in STC ~ 10 years

13. Anomalous Hadley Circulation Between 60ºE and 160ºW Associated with Positive Phase of SST EOFs: Boreal Winter and Summer

14. Anomalous Walker Circulation Between 2.5ºS and 2.5ºN Associated with Positive Phase of SST EOFs: Boreal Winter and Summer

15. Composite 200 hPa Geopotential Height Anomalies Associated with Positive Phase of SST EOFs: Boreal Winter and Summer PNA-like pattern in Winter can modulate ENSO impacts Arctic Oscillation like pattern? Anticyclonic anomaly over central North America in summer

16. Rainfall, 850 hPa Wind, and 850 hPa Divergence Patterns Associated with SST EOFs

17. Summary • Two empirical orthogonal patterns of ~50 years’ SSTs “explain” more than 60% of decadal variability of the IPWP. • The decadal variability extends to at least 300m depth and involves zonal and meridional heat transport variability, suggesting that ocean dynamics are involved in this decadal variability. • STC variability consistent with negative feedback on IPWP SSTs. • Thermally-direct responses of Hadley and Walker Circulations to the IPWP variability. Significant upper level rotational flow anomalies in extratropics conducive to ENSO impacts modulation in winter and summer rainfall modulation in central North America. • Lower level wind and convergence-divergence anomalies over the US; winter precipitation anomalies in Florida, the Gulf Coast, southern Texas, Arizona, and along the West Coast; summer precipitation anomalies mainly in the Midwest and Southeast. • These long-persistent precipitation anomalies can potentially make a significant impact on US water resources and agriculture.

18. The Pacific Decadal OscillationCourtesy: Nathan Mantua, Stephen Hare Univ. of Washington • Major changes in northeast Pacific marine ecosystems have been correlated with phase changes in the PDO; warm eras have seen enhanced coastal ocean biological productivity in Alaska and inhibited productivity off the west coast of the contiguous United States, while cold PDO eras have seen the opposite north-south pattern of marine ecosystem productivity.