Linkage between Summer Indian Ocean SST and the Decay of ENSO events

1. Linkage between Summer Indian Ocean SST and the Decay of ENSO events Ren Rongcai(任荣彩) (rrc@lasg.iap.ac.cn) (LASG, Institute of the Atmospheric physics, Chinese Academy of Sciences, Beijing, China) Collaborators:SY Sun, Q Li, Y. Yang MCai (FSU, USA); GXWu

2. Outline • Background • Questions to address • Main results • Summary

3. A. Background Niño3-regressed winter SST Indian Ocean Basin (IOB) Tropical Eastern Pacific ENSO • What have been known?

4. 1 The intimate lead/lag relationship between ENSO and IOB SST at the inter-annual timescale A- Background • SST variation in India Ocean always lags ENSO anomalies for several months and peaks in spring(Chiu and Newell, 1983; Cadet, 1985; Nigam and Shen, 1993; Du et al., 2009；Weare，1979); ENSO (Curve) and IOB SST (shd) anom. Since 1870 • Indian Ocean is considered as a capacitor to mediate the ENSO’s influences on the Indo-Western Pacific climate, after ENSO has largely dissipated in spring. (Xie et al. 2009) ENSO-IOB Lead/lag Corr • SST response in India Ocean to ENSO is basin wide ! (Fieux and Stommel，1976；Brown and Havans，1981；Cadet and Diehl，1984；Shukla，1987；Rao and Goswami，1988） Li et al，2012

5. A- Background 2 ENSO and IOB are remotely connected through the ENSO-induced “Atmospheric Bridge” processes（Klein et al.，1999；Alexander et al.，2002） Anomalous Walker Circulation Cloud Warm SLP H • Cloud radiation; • Wind Evaporation Mechanism (WEM) • Oceanic Dynamics: Rossby waves

6. A- Background 3 Persistence of ENSO-relatedprocesses may affect the summerIOB • Besides that in spring, there exists a second warming peak of the North Indian Ocean SST in summer of El Niño decay years (Du et al. 2009); • The longer persistence of the WEM process over the IOB may have contributed to a summer warming of North Indian Ocean since 1970s. • (Du et al. 2009; Xie, 2009; 2010).

7. A- Background 4 But the ENSO-related processes over the summer IOB are largely undetermined. Schematics of the summer warming in the tropical IO (Xie et al. 2010) • ENSO => SIO Rossby waves • => SIO warming • => C-type asymmetric wind • => WEM process • => summer NIO warming • (Du et al. 2009; Wu et al. 2008) • Summer NIO warming • => anticyclonic circulation over Indo-Pacific • => North easterly surface wind over the NIO (Xie et al. 2009)

8. A- Background 5. Significant long-term warming in IOB but not in ENSO • Significant long-term warming trend in tropical IOB ; • (Lau and Weng, 1999; Terray and Dominiak, 2005; Ihara et al., 2008) • No companion warming trend in tropical eastern Pacific; (Lau and Weng, 1999; Kumar et al., 2010) Monthly Niño3 and IOB SST anomalies Since 1870 IOB SST Niño3 Li et al，2012 • The IOB warming has commonly been attributed to the anthropogenic effect associated with global warming （Barnett et al., 2005；Pierce et al., 2006）

9. A- Background 6. Decadal changes of ENSO may have effects on the IOB • ENSO frequency becomes lower (Mantua et al.，1997; An and Wang，2000）and ENSOamplitude becomes largersince 1970s （Terray and Dominiak, 2005; Ihara et al., 2008) ; • Therelationship between ENSO and Indian Ocean SST is found stronger since 1970s. (Xie et al.,2010) . • There exist epochal changes in the persistency of the El Niño-related warmer SST over the North Indian Ocean (Chowdary et al.，2012); • It is also not fully clear that “what have the decadal changes of ENSO particularly the delay of ENSO-decay resulted in for the IOB SST ?”

10. B. Questions to address

11. B- Questions to address • How is the delay of ENSO-decay related to the IOB SST, in the Interannual and in decadal timescale? • How about the changes of the “atmospheric- bridge” processes over the IOB due to the delay of ENSO-decay? • And their contributions to the SST changes over the summer IOB?

12. C. Data and method

13. C- Data and method • Data • Monthly SST：HadISST1 data set (Rayner et al., 2003); COBE SST(Ishii et al 2005) • SOT and SSH: GODAS dataset (NOAA/OAR/ESRL, PSD, USA) • ENSO index: the Niño3 index • IOB SST Index (IOBI): area mean SSTA in (40-110°E, 30°S-30°N)(Li et al.,2010) • Circulation fields: JRA55 (Kobayashi et al 2015) Niño3-regressed winter SST

14. C- Data and method • Method • Removing long-term warming trend before regression and composition. • Categorizing ENSO cases since 1950s: • Normal-decay/Later-decay; • An energy-balance-based decomposition method CFRAM (Coupled surface-atmosphere climate Feedback-Response Analysis Method，LuandCai, 2009；CaiandLu,2009)

15. D. Niño3-regressed winter SST Main results ENSO-decay & IOB SST Interannual timescale

16. D- ENSO-decay & IOB, interannual Identifying the contrasting Normal/Later ENSO-decay Evolutions of ENSOs in ENSO-decay years • ENSO events: • Definition: NDJ Niño3 > 0.7Std; • Normal-decay: Apr |Niño3| < its mean • Later-decay: Apr |Niño3| > its mean ENSO • Normal-decay ENSOreverses sign in April; • Later-decay ENSOlasts much longer; • Auto-corr. of Niño3 significant until June for Later-decay ENSO. Normal Later

17. D- ENSO-decay & IOB, interannual Summer IOB SSTa significant only for Later-decay ENSO Evolutions of IOB SSTa in ENSO-decay years IOB SST Index • Contrasting evolutions of the IOB SST following normal-decay/later-decay ENSO: • IOB SSTa weaker/stronger in normal-decay/later-decay ENSO years; • The contrast is symmetric between El Niño and La Niña; • Strongest contrastis in summer. Normal Later

18. D- ENSO-decay & IOB, interannual Summer IOB SSTa significant only for Later-decay ENSO Warm-minus-cold composite IOB SST anomalies • Normal-decay • Weaker IOB • SSTa diminished • in summer • Later-decay • Basin-wide SSTa • still significant • in summer • Later-decay ENSO may extend its effect on IOB into summer!

19. D- ENSO-decay & IOB, inter-decadal Niño3-regressed winter SST ENSO-decay & IOB SST Inter-decadal timescale

20. D- ENSO-decay & IOB, inter-decadal There exists significant inter-decadal warming over IOB IOBI 1870-2009 after the long-term trend removed 1970s 2000s Statistic-Z (Line) 1930s IOBI (Bar) 1900s 1940s • IOBI has significant inter-decadal changes, on top of its long-term warming; • Persistent warming in two period: 1900s-1930s and 1940s-2000s; • The warming during 1940s-2000s is much stronger (0.0687 °C/decade ) with longer duration than the warming in 1900s-1930s (0.0420 °C/decade).

21. D- ENSO-decay & IOB, inter-decadal Inter-decadal IOB warming in 1940s-2000s peaks in summer Inter-decadal summer IOB warming since 1970s Seasonal distribution of linear trend of IOBI 99% 95% 90% • 1900s-1930s warming: significant in winter-to-early-spring; • 1940s-2000s warming: significant throughout the year, peaks in summer months.

22. D- ENSO-decay & IOB, inter-decadal Effect of ENSO is extended to summer due to Inter-decadal delay of ENSO-decay phase 21-year running lagged correlation of the NDJ Niño3 with (a) Niño3 and (b) IOBI in the following months of the ENSO decay years • Since 1970s, abrupt increase in auto-correlation of Niño3 in spring-summer; • Indicating a delay of the seasonal timing of the ENSO decay phase. a) Niño3-Niño3 Calendar month of ENSO decay year b) Niño3-IOBI • Since 1970s, also an abrupt increase in the lagged correlation between winter Niño3 and the IOBI in the following summer. • Indicating an extension of the delayed effect of ENSO on IOB from spring to summer.

23. D- ENSO-decay & IOB, inter-decadal Warming/Cooling on IOB Warm/cold ENSO Niño3-regressed winter SST How to understand the inter-decadal Warming over the summer IOB?

24. D- ENSO-decay & IOB, inter-decadal Only the warming effect of ElNiño has been extended Lagged correlation respectively before 1976 (asterisk) and after 1976 (dotted) All Years ElNiñoyears LaNiñayears Niño3-Niño3 Calendar month of ENSO decay year  * * Before 1976 After 1976  Niño3-IOBI • Delay of the seasonal timing of decay phase for ElNiñois much more prominent than for La Niña; • Only the warming effect of ElNiñois delayed into summer, but not for the cooling effect of La Niña. • Therefore, resulting in a warming over the summer IOB.

25. D- ENSO-decay & IOB, inter-decadal Asymmetric inter-decadal changes of ENSO events List of Later-decay and Normal-decay ENSO cases since 1950s • It is the five emerging delayed-decay El Niño events after 1970s that have resulted in the asymmetric changes of ENSOand the interdecadal summer IOBwarming !

26. D- ENSO-decay & IOB, inter-decadal The 5 delayed-decay El Niño events are crucial Lagged correlation before 1976 (asterisk) and after 1976 (dotted) All ENSO events Without the 5 later-decay events.

27. D- ENSO-decay & IOB SST • Interannual: • ENSO effect on IOB always extends to summer in Later-decay ENSO years; • It is symmetric between El Niño and La Niña • Inter-decadal: • Interdecadal change of ENSO-decay since 1970s has resulted in an inter-decadal warming over the summer IOB. • It is mainly due to the 5 emerging Later-decay El Niño events.

28. D- ENSO-decay & IOB SST Niño3-regressed winter SST ENSO-decay & IOB SST The “atmospheric-bridge”processes

29. D- ENSO-decay & IOB, Walker circulation Anomalous Walker circulation persists to summer for Later-decay ENSO Warm-minus-cold composite zonal wind anomalies (10S-10N) Climatology Normal-decay ENSO Later-decay ENSO

30. D- ENSO-decay & IOB, the Indo-Pacific SLP High Warm-minus-cold composite SLP anomalies H H Normal decay Later decay H H H • The anomalous SLP High persists into summer in Later-decay ENSO years.

31. D- ENSO-decay & IOB, the cloud-radiation processe Warm-minus-cold composite Cloud amount and net radiation • Mainly over North Indian Ocean; • Less cooling effect in summer Normal-decay ENSO Later-decay ENSO • Slightly less cloud in summer of later-decay ENSO years; • Slightly less cooling (or warming) effect on the North Indian Ocean.

32. D- ENSO-decay & IOB, Oceanic dynamic processes Normal decay Delayed decay • The anomalous Westward propagating Oceanic Rossby waves stronger, and can affect summer IOB SST in Later-decay ENSO years. SSH SOT Summer SOT Warm-minus-cold composite SSH&SOT anomalies (10S-10N)

33. D- ENSO-decay & IOB, surface heat-flux processes WEM no longer effects on LH in summer of later-decay ENSO year! Warm-minus-cold composite zonal-mean 10m-wind and LH anomalies over IOB (40-110E) WEM: Wind Evaporation Mechanism • North Indian Ocean: climatological wind reverses around May. Climatology • The C-type wind patterns are clear; • WEM mechanism (weakening-wind& positive LH) effects on LH through summer. Normal-decay ENSO • In summer of delayed-decay ENSO years: weakening-wind <=> negative-LH Intensifying-wind <=>positive-LH • WEM seems no longer effects in summer ! Later-decay ENSO

34. D- ENSO-decay & IOB, surface heat-flux processes WEM no longer effects on LH in summer of later-decay ENSO year! Warmer/colder SST => LH cooling/warming in summer Warm-minus-cold composite 10m-wind, SST, LH and SH anomalies in summer IOB WEM => LH WEM ≠ LH SST => LH Normal-decay ENSO Later-decay ENSO • SST feedbacks replace WEM to dominate the LH process when SST is highly warmer in summer of later-decay ENSO years! [See arguments in Xie et al (2001, 1996)]

35. D- ENSO-decay & IOB SST Niño3-regressed winter SST ENSO-decay & IOB SST Quantifying the contributions of individual processes to the summer IOB SST changes due to the delayed decay of ENSO

36. D- ENSO-decay & IOB SST, quantitative contributions of each processes CFRAM （Coupled surface-atmosphere climate Feedback-Response Analysis Method）(LuandCai,CaiandLu,2009)

37. D- ENSO-decay & IOB SST, quantitative contributions of each processes Partialcontributions of radiative and non-radiative processes to the summer IOB SST warming Non-Radiative Radiative • Generally the radiative processes (cloud, water vapor, ozone) mainly contribute; non-radiative processes cool the IOB; • Individual non-radiative contributions much stronger, compensate for one another (Oceanic dynamics & LH+SH);

38. D- ENSO-decay & IOB SST, quantitative contributions of each processes Area-weighted mean partialcontributions to the summer IOB SST warming • IOB: • Radiative processes dominate; • Non-Radiative compensate each other • North Indian Ocean: • Cloud radiation dominates. • South Indian Ocean: • Oceanic Dynamics much stronger, but compensated for by LH; • Water and Ozone changes co-contribute.

39. E. Niño3-regressed winter SST Summary ENSO-decay & IOB SST

40. E- Summary • Later-decay of both El Niño and La Niña can extend their effects on IOB into summer; • The asymmetric changes of ENSO, or the 5 emerging Later-decay El Niño events after 1970s have resulted in the inter-decadal warming over the summer IOB since 1970s; • Most of the “atmospheric bridge” processes can extend to summer in Later-decay ENSO years, but WEM no longer contributes to LH because of the highly warmer SST then; • The extended radiative processes generally dominate the IOB SST changes due to Later-decay of ENSO, though the effect of the oceanic dynamic processes is the strongest which is however compensated for by surface heat-fluxes.

41. (rrc@lasg.iap.ac.cn) • Ren* R.-C., SY Sun, Y. Yang and Q. Li, 2016: Summer SST anomalies in the Indian Ocean and the Seasonal timing of ENSO decay phase, Climate Dynamics, DOI: 10.1007/s00382-015-2935-0. • Li, Qian, R.-C. Ren*, M. Cai and G. X. Wu, 2012: Attribution of the summer warming since 1970s in Indian Ocean Basin to the inter-decadal change in the seasonal timing of El Niño decay phase, Geophys. Res. Lett., VOL. 39, L12702, 5 PP., 2012，doi:10.1029/2012GL052150.