NERC Centre for Global Atmospheric Modelling Department of Meteorology, University of Reading The role of the land surface in the climate and variability of the Asian Summer Monsoon Julia Slingo 1, Bernd Becker 2, Laura Ferranti 3 and Franco Molteni 4 1 CGAM, University of Reading 2 Met Office, Bracknell 3 ECMWF, Reading 4 ICTP, Trieste

Outline of Talk Focus on the impact of springtime Eurasian snow amounts on the Asian Summer monsoon Variability of spring Eurasian snow amounts and association with ENSO Ensemble seasonal forecasts designed to isolate the effects of Eurasian snow on monsoon variability Relative importance of land surface vs. SST forcing Suggest a possible mechanism to explain the association between Eurasian snow and monsoon variability

HYPOTHESIS: That anomalous springtime snow amounts over Eurasia may provide long term memory to the climate system by affecting the land surface energy and moisture budgets, and thereby influencing monsoon variability

MODEL ECMWF Integrated Forecast System (IFS) model Resolution: T63 (~ ), 31 vertical levels. Model version: Cy16r2, operational in the latter half of 1997

EXPERIMENTAL DESIGN 9 member ensembles for 7 years (1982/83, 83/84, 84/85, 86/87, 87/88, 88/89, 91/92) PRISM-W: 1-10 November to 1-10 April; observed SSTs; initial conditions from ECMWF Reanalyses PRISM-C: 1-10 April to 1-10 November; climatological SSTs; initial conditions from PRISM-W ensemble. PRISM-O: 1-10 April to 1-10 November; observed SSTs; initial conditions from PRISM-W ensemble.

Rationale(1) PRISM-W provides a range of possible scenarios for Eurasian snow amounts that might occur for the particular SST forcing during the preceding winter. PRISM-W enables the question of the relationship between Eurasian land surface conditions and SST forcing during winter to be addressed. PRISM-W provides a set of initial states at the end of March in which the land surface is in balance with the atmosphere.

Rationale (2) PRISM-W provides a set of springtime land surface conditions for initializing the summer ensembles, PRISM-C and PRISM-O PRISM-C, in which SST interannual variability is suppressed, will enable the influence of land surface conditions to be detected in isolation from the effects of SST forcing. PRISM-O will then show how observed SSTs influence monsoon variability relative to the effects of land surface conditions.

Composite differences (January – March) for high vs. low western Eurasian snow Surface Temperature 500hPa Height (m)

Composite differences in surface temperature (June – Sept.) for high vs. low western Eurasian snow PRISM-O PRISM-C

Composite differences in 500hPa height (June – Sept.) for high vs. low western Eurasian snow (metres) PRISM-C PRISM-O

CONCLUSIONS (1) High snow amounts over western Eurasia are linked to La Niña, suggesting that the El Niño/Southern Oscillation (ENSO) has an influence on the wintertime climate of Eurasia. Signature of these snow depth anomalies is carried through to the summer in terms of changes in soil wetness and surface temperatures. Land surface conditions can have a significant impact on the large scale monsoon circulation and Indian Summer Monsoon rainfall.

CONCLUSIONS (2) Mechanisms involved in the relationship between the monsoon and Eurasian land surface conditions may involve the response of the mid-latitude flow and its interactions with the monsoon circulation If interactions with the extratropics hold the key to understanding the link between Eurasian land surface conditions and monsoon variability, then predictability of this relationship is likely to be limited.