Drivers of Inter-decadal Variability of the Asian Summer Monsoon H. Annamalai IPRC/SOEST, University of Hawaii Contributions from: K. Lakshmi and V. Krishnamurthy (COLA) M. Rajeevan (MoES, India) R. Krishnan (IITM, Pune, India) Talk Outline • Interannual – decadal variability (analogy to intraseasonal interannual variability) • Observations (current and past), and models (current) • Decadal ENSO-monsoon association (observations and models) • AMO/PDO linkages from observations (land-sea thermal contrast ?) • Is there a breakdown in this decadal-multidecadal variability? • Issues Summer (JJAS) all-India rainfall anomalies (mm) and Nino3.4 SST anomalies (C) “decadal epochs” – alternating decadal clustering of strong and weak monsoons – economic conditions – green revolution (since 1960s) – education in 1950s “relationship between interannual and decadal variability?” – analogy to ISV - IAV “intensity of ENSO – no direct relationship with intensity of monsoon rainfall” – other factors? Decadal-multidecadal variations in monsoon rainfall over India Decadal variability and trend? 10% change in last 50-60 years! amplitude – observational constraints (Turner and Annamalai 2012) Proxy indications of multi-decadal monsoon rainfall variations M. Berkelhammer et al. / Earth and Planetary Science Letters 290 (2010) 166–172 Decadal-multidecadal rainfall variations since 600 AD M. Berkelhammer et al. / Earth and Planetary Science Letters 290 (2010) 166–172 Observed very heavy rainfall events (R> 150 mm/day) over central India (red line) and its smoothed variation (black line) for the period 1901–2004. 40 Rajeevan et al. (2009) - 30 20 0 1901 1905 1909 1913 1917 1921 1925 1929 1933 1937 1941 1945 1949 1953 1957 1961 1965 1969 1973 1977 1981 1985 1989 1993 1997 2001 10 All-India rainfall INDIAN OCEAN SST ANOMALIES AND HEAVY RAINFALL FREQUENCY OVER CENTRAL INDIA 0,8 0,6 30 IO SST ANOMALY RAINFALL FREQUENCY 25 20 0,2 0,0 15 -0,2 10 -0,4 5 -0,6 0 1901 1904 1907 1910 1913 1916 1919 1922 1925 1928 1931 1934 1937 1940 1943 1946 1949 1952 1955 1958 1961 1964 1967 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 -0,8 RAINFALL FRQ SST ANOMALIES 0,4 YEAR Smoothed variation of frequency of very heavy rainfall events over central India and SST anomalies over the Equatorial Indian Ocean. The smoothing has been done to remove the subdecadal fluctuations using a 13-point filter [IPCC, 2007]. Decadal variability of the Indian monsoon •Low-frequency (11-yr running mean) IMR and Nino-3 vary coherently with negative correlation and follow a common interdecadal variation •The regressions of SST on low-frequency IMR and Nino-3 show similar ENSO-like patterns •Hypothesized to be a low-frequency tropical ocean-atmosphere mode (Krishnamurthy & Goswami 2000) (a) MRI “need for ensemble” (b) CCSM4 “decadal modulation tied to decadal ENSO variance” Decadal Variability of Indian Monsoon Rainfall Field: Rainfall (5-year running mean) Domain: Indian monsoon region Data: IMD Period: 1901-2004 RC (1,2) - EOF RC (5,6) - EOF Result from MSSA: 3 statistical oscillatory modes RC (7,8) - EOF (All MSSA modes are significant at 95% confidence level based on MSSA Monte Carlo test) South of the mean monsoon trough K. Lakshmi (COLA) Relation between decadal modes in IMR and SST Regression of SST on rainfall RC index RC(1,2) RC(5,6) Mode Period Relation to SST RC (1,2) 52 AMO RC (5,6) 21 PDO RC (7,8) 13 Tripole “statistical oscillatory mode” RC(7,8) Dotted regions - 90% confidence level Shifts in PDO index – not seen in monsoon rainfall – Observations GFDL_CM2.1 Zhang and Delworth (2006) “time-scale” In-phase “dominated by Sahel rainfall” AMO has better relationship to monsoon rainfall variations at multi-decadal time scales? Does the relationship break down in recent decades? 10% change in last 50-60 years! Is there a break down in decadal monsoon variability? JJAS – Precipitation and SST Climatology I II III • Multiple regional heat sources • EIO and SPCZ – still experience high precipitation (thermal equator at 20oN) • Central India rainfall – dynamical effects; Rain-shadow regions • absolute ascent over a large domain CM2.1 Diagnostics Rainfall over tropical west Pacific in CM2.1 •Salinity observations since 1955 support this increase in rainfall (Cravatte et al. 2009) • Decadal-multidecadal SST variability over west Pacific (Guan and Nigam 2008) JJAS SST – Observations (trend /50 year) AMO…? PDO…? No significant warming over the equatorial Pacific (PDO? No AMO signature?) SLP and 850 hPa Vertical velocity 400 hPa Day 3 Day 6 Day 9 Figure 7: LBM solutions of perturbation SLP and wind at 850 hPa (left) and vertical velocity at 400 hPa (right) to SST anomalies over tropical west Pacific for Day 3 (top), Day 6 (middle), and Day 9 (bottom) Summary…. Existing ideas on the role of AMO/PDO on the monsoon – land/sea contrast What about the variability over tropical western Pacific? Thermodynamics – moist static energy principle – (i) Pre-industrial control runs with CMIP3/5 models (ii) Coordinated coupled model experiments by AAMP? Summer (JJAS) all-India rainfall anomalies (mm) and Nino3.4 SST anomalies (C) “decadal – multi-decadal epochs” – alternating decadal clustering of strong and weak monsoons “data manipulation or a natural mode of variability?” Coupled ocean-atmosphere-land (also by fixed orography) • Dynamics and thermodynamics – very different from simple land-breeze • Thermodynamics forcing during summer – rainfall poleward of Equator Yet, sufficient rainfall occurs along EIO “did not review the decadal variability” Turner and Annamalai (2012, Nature CC) “decadal modulation tied to decadal ENSO variance” CCSM4 CM2.1 diagnostics What governs the decadal-multidecadal modulation of ENSO ? Wittenberg (2006)- SLP and wind at 850hPa “how to choose sliding window” Moisture Budget (AM2.1 – SST trend over warm pool) Precipitation Moisture advection Annamalai et al. (2012) Moisture convergence Evaporation