4th Annual CTB SAB Meeting ESSIC, UMD 11 September 2008

1. JISAO/UW and NOAA/PMEL NOAA/NCEP 4th Annual CTB SAB Meeting ESSIC, UMD 11 September 2008 Upper Tropical Pacific Variation during recent ENSO: An Evaluation of Global Ocean Data Assimilation System (GODAS) Dongxiao Zhang Yan Xue Michael J. McPhaden David Behringer

2. Like most other models, CFS had difficulty to predict the 2006 El Nino, even initialized in summer 2006. • CFS tends to overshoot in prediction of either warm (Dec. 2006) or cold (Jan. 2008) events. 2005-08 NINO3.4 Predictions

3. Approach Compare GODAS analyses with ocean temperature, salinity, and velocity observations, emphasizing the period 2005-08 Objective Evaluate the extent to which systematic errors in GODAS may limit skill in NOAA Climate Forecast System (CFS) forecasts

4. Data Products • GODAS Model: GFDL MOM v3, 75°S-65°N Horizontal resolution: 1° x 1° resolution (enhanced to 1/3° in latitude within 10 °of the equator) Vertical resolution: 40 levels with 10-m resolution in upper 200m Forcing: NCEP atmospheric reanalysis-2 wind stress; SST relaxed to weekly Reynolds analysis; SSS relaxed to annual mean climatology Data assimilation: Temperature profiles from TAO, TRITON, ARGO, and XBT; “synthetic” subsurface salinity based on mean T/S Available: 1980-present • Near real-time PMEL temperature/salinity analysis Data: Argo, TAO/TRITON (Temp & Salinity) Resolution: 10 days, 0.5° lat x 0.5° long Available: 2005-present Analysis on density surfaces

5. PMEL Argo-TAO/TRITON Analysis (Week Centered on 26 Dec 2007) SSS Potential Temp 115m Text Sal. 115m U, V at 115m (ref. 1000db) Arrows show Subtropical Cell pathways in the thermocline PMEL analysis

6. PMEL GODAS PMEL (seasonal cycle removed) GODAS (seasonal cycle removed) • Z20 anomalies associated with ENSO development are reasonably simulated in GODAS, with higher amplitude than in observation. • Z20 seasonal cycle amplitude in GODAS is however biased too high. 20°C isotherm standard deviation (2005-2008)

7. GODAS PMEL Z20 anomalies associated with ENSO development are reasonably reproduced, though GODAS anomalies tend to be larger. 20oC isotherm anomalies 5°S-5°N Average (2005-08)

8. PMEL GODAS PMEL (seasonal cycle removed) GODAS (seasonal cycle removed) SSS variability is underestimated in GODAS. SSS standard deviation (2005-2008)

9. GODAS PMEL GODAS-PMEL SSS SSS SSS Diff. MLD MLD MLD Diff. White contour shows the 29°C isotherm, indicating the edge of warm pool Large bias in MLD in western Pacific, presumably caused by salinity bias. Surface Salinity and Mixed Layer Depth, 5oS-5oN Average

10. Zonal Wind Stress PMEL 20°C isotherm OI sst • Slow eastward propagating of both downwelling Kelvin wave and SST anomalies during 2006 El Nino, after westerly anomalies in western Pacific, are characters of slow coupled waves. • No propagating signature is observed in Z20 or SST during 2007-08 La Nina. Persistent shallow thermocline anomaly indicates entrainment cooling. 5oS-5oN Averaged Anomalies

11. SSS at 165°E, 0°N vs. Nino3.4

12. Large dT/dt anomalies are forced by surface heat flux, which is however in response to damp anomalies from entrainment. • Zonal advection is large, and tends to follow large flux anomalies. • Dominance of zonal advection and surface heat flux during summer 2006 suggests a role of slow coupled waves (Hirst 1986) in El Nino development. • Persistent entrainment cooling is important during 2007-08 La Nina. Heat Budget in Nino3.4 Area

13. U, V at 115m (ref. 1000db) Meridional Structure Along 140°W Density surface sq=25 kg m-3 1950-1999 Temperature Shallow Overturning in the Subtropical Cells Salinity

14. Persistent Entrainment Cooling vs. Subtropical Cell (STC) PMEL U, V at 115m (ref. 1000db) Entrainment anomalies follow STC convergence transport, which advects subducted subtropical cold waters equatorward to feed equatorial upwelling. • Stronger STC since 2007 • Stronger equatorial upwelling • More entrainment cooling Interior STC transport convergence is estimated from equatorward thermocline transports, marked by arrows, at 9°S and 9°N.

15. Summary • PMEL has developed a near real-time gridded T, S, and geostrophic current product to validate GODAS over 2005-08. • All terms in mixed layer heat budget equation, except vdT/dy, are important in controlling SST tendency term. • The observed STC variability is well reproduced in GODAS, and contributes to the persistent entrainment cooling during 2007-08 La Nina. • The equatorial Pacific heat content, or Z20, associated with the development of ENSO cycle agree reasonably well in GODAS and PMEL analyses, though GODAS variability tends to be larger. • GODAS surface salinity variability is too weak and mixed layers are too deep in the western Pacific warm pool, with potential adverse affects on zonal currents, advection and ENSO SST development when used to initialize CFS.

16. Questions • Why does the annual cycle variability in GODAS upper ocean temperatures tend to be higher than observed? Does it contribute to the overshoot of CFS prediction during either warm or cold events? • What accounts for the systematic error in warm pool mixed layer currents and how do those errors affect temperature advection and ENSO SST evolution? • GODAS will benefit from assimilation of ocean salinity measurements. What is the optimal method for reducing systematic errors in analyzed fields?

17. 0°, 165°E 0°, 170°W Surface current at 170°W is better simulated in GODAS than in western Pacific at 165°E, where a systematic bias is apparent. Zonal velocity (cm/s) at 10m in TAO and GODAS