Transcript ENSO changes due to heat flux adjustment in current and

The role of the basic state in the
ENSO-monsoon relationship and
implications for predictability
Andrew Turner, Pete Inness, Julia Slingo
Motivation
• Asian summer monsoon affects more than 2
billion people in India, China and the rest of
Southeast Asia.
• Regional agriculture reliant on the timing,
duration and intensity of the ASM – GCMs
increasingly used to predict these details.
• State of equatorial Pacific SSTs long regarded as
an important predictor of the monsoon (e.g.
Charney and Shukla, 1981).
• Coupled GCMs generating mean climate closer
to observations are more likely to correctly
simulate the interannual variability of tropical
precipitation (Sperber and Palmer, 1996).
The model & datasets
• HadCM3 3.75lon x 2.5lat (~T42). 100 year
integration. (atmosphere: Pope et al., 2000,
ocean: Gordon et al., 2000).
• L30 used in this configuration which produces
more realistic representation of intraseasonal
tropical convection – MJO – than L19 (Inness et
al., 2001).
• ERA-40 Reanalysis (1958-1997).
• CMAP for tropical precipitation 1979-1997; Xie
and Arkin, 1997.
• All –India Rainfall (AIR) gauge-based dataset;
Parthasarathy et al., 1994.
What’s wrong with the model?
Summer DMI lag-correlated with Nino-3 SSTs
Mean summer surface temperature
HadCM3 mean summer (JJAS)
differences with ERA-40
Mean summer (JJAS) 850mb winds
HadCM3
differences with ERA-40
Mean summer (JJAS) precipitation
HadCM3
differences with CMAP
Heat flux adjustments
• Seasonally varying flux adjustments used in older
models (eg HadCM2) to prevent climate drift;
HadCM3 does not have this problem.
• Heat flux adjustments used here to study the effect
of mean state error on the monsoon-ENSO system.
• Devised by Inness et al. (2003) to investigate the
role of systematic low-level zonal wind and SST
errors on the MJO.
• Coupled model run for 20 years, Indian and Pacific
SSTs within 10S-10N relaxed back to climatology.
• Anomalous heat fluxes saved to generate a mean
annual cycle, then applied to a new 100 year
integration (HadCM3FA).
Heat flux adjustments
Annual Mean
• Large fluxes (up to
186Wm-2 at 120W)
into the cold tongue.
• Much smaller
(~30W.m-2) over
Maritime Continent
and Indian Ocean.
Standard deviation of cycle
• Small annual cycle
apart from upwelling
region off African coast.
Improvements to the mean state
HadCM3FA mean summer (JJAS) surface temperature
differences with HadCM3
HadCM3 differences with ERA-40
Improvements to the mean state
HadCM3FA mean summer (JJAS) 850hPa winds
differences with HadCM3
HadCM3 differences with ERA-40
Improvements to the mean state
HadCM3FA mean summer (JJAS) precipitation
differences with HadCM3
HadCM3 differences with CMAP
The monsoon-ENSO teleconnection
Lag-correlation of summer (JJAS) DMI with Nino-3 SSTs
• High summer Nino3 anomalies
associated with weak
Asian summer
monsoon
(dynamically).
• HadCM3 fails to
reproduce timing of
the teleconnection.
• Stronger and better timed teleconnection
with flux adjustments.
The monsoon-ENSO teleconnection
Lag-correlation of summer (JJAS) Indian rainfall with Nino-3 SSTs
• Indian rainfall shares
similar teleconnection
pattern.
• ERA-40 has poor
representation when
compared to gauge
dataset.
• Stronger and better
timed teleconnection with
flux adjustments.
• Monsoons feed back on Pacific system to further intensify ENSO.
The monsoon-ENSO teleconnection
Composite evolution of equatorial
Pacific total SSTs during El Nino
• 10 warm events composited
from each model integration.
• Warmest waters (absolute SSTs)
are further east, past the dateline.
• Convection and hence the rising
branch of the Walker circulation is
repositioned.
• Warmer mean state means that
even weak El Ninos in HadCM3FA
may drive the teleconnection.
• See Turner et al. (2005)
HadCM3
HadCM3FA
The effect of climate change
summer (JJAS) 850hPa wind differences: 2xCO2-1xCO2
HadCM3
HadCM3FA
The effect of climate change
summer (JJAS) precipitation differences: 2xCO2-1xCO2
HadCM3
HadCM3FA
The effect of climate change
summer (JJAS) surface temperature differences: 2xCO2-1xCO2
HadCM3
HadCM3FA
The teleconnection
Lag-correlation of summer (JJAS) Indian rainfall with Nino-3 SSTs
Future ENSO?
irregular period
biennial period
Summary
Current climate:
• Flux adjustments, whilst having some drawbacks, can
help correct mean state and have beneficial effect on
monsoon predictability.
• Stronger teleconnection (and greater coupling); more
realistic Walker circulation & El Nino development.
• Flux adjustments highlight the danger in assuming a
linear system, anomaly forecasting etc.
Future climate:
• Tendency to stronger monsoons in future climate
scenario, irrespective of flux correction.
• The sign and timing of the monsoon-ENSO
teleconnection may not be robust.
• Flux adjustment raises questions relating to the nature of
ENSO in future climate.