The 2007/08 La Niña Cycle: Evolution and Prediction

1. Yan Xue, Boyin Huang, Wanqui Wang, Arun Kumar, Philip Pegion and Michelle L’Heureux Climate Prediction Center The 2007/08 La Niña Cycle: Evolution and Prediction 33rd CDPW and CLIVAR Drought Workshop, Lincoln, Nebraska, Oct. 21-24 2008.

2. Outline • Characteristics of the 07/08 La Nina • Comparison with historical La Nina Events • SSTA propagation and spring warming in far E. Pac. • Equatorial heat content relationship with NINO3.4 • Surface zonal current relationship with NINO3.4 • Impacts of MJO-related winds on the ocean • Heat budget analysis in the surface mixed layer • Prediction of the 07/08 La Nina by NCEP’s Climate Forecast System (CFS)

3. Data Sources and References • Optimal Interpolation SST (OI SST) version 2 (Reynolds et al. 2002) • SST 1971-2000 base period means (Xue et al. 2003) • NCEP CDAS 850 mb winds (Kalnay et al. 1996) • NCEP CDAS2 wind stress (Kanamitsu et al. 2002) • NESDIS Outgoing Long-wave Radiation (OLR) • NCEP’s Global Ocean Data Assimilation System temperature, heat content, currents (Behringer and Xue 2004) • Ocean Surface Current Analyses – Realtime (OSCAR) (Bonjean and Lagerloef, 2002)

4. Nino 4 Nino 3.4 Nino 1+2 • The definition of ENSO is keyed on the NINO3.4 Index calculated with the OI SST. • La Nina is referred to as NINO3.4 =< -0.5C. • The 07/08 La Nina started in August 07 and ended in June 08, with a duration of 10 months. • The 07/08 La Nina had a prominent westward propagation of SSTA, and early spring warming in far E. Pac.. Evolution of Pacific NINO SST Indices Nino 3 start end

5. Comparison with Historical La Nina Events • - According to CPC’s ENSO definition based on ERSST (NINO3.4 =< -0.5C for five consecutive overlapping 3-month seasons), there were seven La Nina winters during 1982-2008 (84/85, 88/89, 95/96, 98/99, 99/00, 00/01 and 07/08). • The 2007/08 La Nina has a similar strength as that of the 1999/00 La Nina. • The moderate-to-strong 2007/08 La Nina was noteworthy since ENSO variability has been relatively weak during 2001-2006.

6. Evolution of Equatorial Pacific SST, Heat Content, TAUX, and Surface Zonal Current Anomaly • Negative SSTA propagated westward and positive SSTA appeared in far E. Pac. in early spring 08. • Negative heat content (upper 300m temperature average) anomaly presented in central-eastern Pacific in January 07, and persisted until early spring 08 with little propagation. • Easterly (westerly) wind stress anomaly in the central (eastern) Pacific propagated westward during August 07 – April 08. • Westward surface zonal current anomaly (ZCA) switched to eastward ZCA in February 08.

7. Central and Eastern Pacific SSTA for Seven La Nina Winters The onset of the 07/08 La Nina was similar to that of the 95/96 La Nina during which negative SSTA in the far E. Pac. led negative SSTA in the C. Pac by 4-6 months. onset • During the decay phases of La Nina, negative SSTA in the far E. Pac. tend to return to near-normal in early spring of the following year and then cool down to a second cold phase in next fall. • Compared to all other early spring warmings in the far E. Pac., the 08 spring warming was stronger and persisted much longer. decay

8. Warm Water Volume and NINO 3.4 Index in 1982-2008 • WWV_East and WWV_West are defined as average of depth of 20ºC in [155ºW-80ºW, 5ºS-5ºN] and [120ºE-155ºW, 5ºS-5ºN], and WWV is the sum of WWV_East and WWV_West (Meinen and McPhaden, 2000). • WWV is intimately linked to ENSO variability since WWV usually leads NINO3.4 by 6-9 months (Wyrtki 1985; Jin 1997). • WWV_East is largely in phase with NINO3.4. • The amplitude of NINO3.4 is well correlated with that of WWV_East. • Both WWV_West and WWV lead NINO3.4 by 6-12 months, suggesting a slow eastward propagation of heat content anomaly. • But, during the 07/08 La Nina, there was no eastward propagation of heat content anomaly.

9. NINO3.4, Surface ZCA of OSCAR and GODAS for Seven La Nina Winters • Zonal current anomaly (ZCA) of GODAS agrees well with that of OSCAR in the central-eastern equatorial Pacific except during the 97/98 El Nino. However, GODAS has substantial mean biases, positive in the central-eastern Pacific and negative in the W. and far E. Pacific (not shown). • ZCA tends to lead SSTA by a few months. • ZCA usually switches from negative to positive during the peak phases of La Nina in late winter.

10. Impacts of MJO-related Winds on Heat Content Anomaly • MJO-related easterly wind anomaly during November-December 06 contributed strongly to the sudden switch of heat content anomaly from positive to negative in January-February 2007. • A pair of westerly and easterly wind bursts during May-June 07 contributed to a positive heat content anomaly in the central-eastern Pacific. • A pair of easterly and westerly wind bursts during December 07 – January 08 and a westerly wind event in the far E. Pacific in February-March 08 had contributed to the sudden switch of heat content anomaly from negative to positive in the central-eastern Pacific in April 08.

11. Heat Budget Anomaly & SSTA for (170W-120W, Eq) Vertical entrainment at the bottom of the mixed layer contributed to negative SSTA tendency during Feb-Apr and Sep 07 – Mar 08. Zonal advection followed the SSTA tendency well, suggesting a dominant role of zonal advection in controlling the evolution of the La Nina. Net heat flux is a damping term to SSTA.

12. Onset Phase

13. Development Phase

14. Mature Phase

15. Decay Phase

16. 2007 CFS Nino34 forecast with mean bias correction

17. 2007 CFS Nino34 forecast with PDF correction

18. Conclusions (1) • Evolution of the 07/08 La Nina • Before the onset phase, negative heat content anomaly presented in the C.-E. Pac. due to the sudden demise of the 06/07 El Nino in Jan 07. • During the onset phase in Mar-Jul 07, SST in the far E. Pac. was below-normal while SST in the C.-E. Pac. was near-normal due to a balance between enhanced solar radiation warming and vertical entrainment cooling. During May-Jun 07, a pair of westerly and easterly wind bursts led to a net increase of heat content, helping maintain the near-normal NINO3.4. • During the development phase in Aug-Nov 07, vertical entrainment and zonal advection cooling were both strong. • During the mature phase in Dec 07 – Feb 08, vertical entrainment cooling was strong, but largely compensated by net heat flux warming. • During the decay phase in Mar-May 08, net heat flux warming dominated the heat budget.

19. Conclusions (2) • Comparison with historical La Nina Events • The 07/08 La Nina did not have a slow eastward propagation of heat content anomaly as it was seen in most La Nina events. • The moderate strength of the 07/08 La Nina was largely determined by the moderate amplitude of negative heat content anomaly in the C.-E. Pac.. • The westward propagation of SSTA was similar to that during the 1995/96 La Nina. • The decay phase of the 07/08 La Nina was similar to that of other La Nina events, characterized by a switch of zonal current anomaly near Eq. in late winter and SST warming in far E. Pac. in early spring. • Prediction of the 07/08 La Nina by CFS • CFS forecasted the onset of the La Nina too early and overshot in the prediction of the mature and decay phases. • Statistical PDF correction was effective in correcting the forecast errors listed above, suggesting that those errors were largely caused by model’s systematic biases and the impacts of MJO on model’s ensemble predictions were secondary.

20. NINO3.4, WWV_West and WWV_East for Seven La Nina Events • During the onset phases of La Nina, negative WWV_West anomaly usually leads negative WWV_East anomaly, indicating a slow eastward propagation of negative heat content anomaly. • But, during the 07/08 La Nina, near-zero WWV_West anomaly led negative WWV_East anomaly, suggesting no eastward propagation of heat content anomaly. • The onset phase of the 07/08 La Nina was similar to that of the 99/00 La Nina during which WWV_West was near-normal. onset • During the decay phases of La Nina, negative WWV_East and positive WWV_West anomaly both weakened quickly in early spring of the following year. decay

21. Impacts of MJO-related Winds on Surface ZCA • MJO-related easterly wind anomaly during November-December 06 forced westward zonal current anomaly (ZCA). • A pair of westerly and easterly wind bursts during May-June 07 forced eastward and westward ZCA in the western-central Pacific. • A pair of westerly and easterly wind bursts during November - December 07 had contributed to the switch of ZCA from westward to eastward in the central-eastern Pacific in January-February 08.