Land Surface Processes in NAMS

Dennis P. Lettenmaier

Department of Civil and Environmental EngineeringUniversity of Washington

for presentation at

Eighth Annual Meeting of WCRP/CLIVAR/VAMOS Panel (VPM8)

Mexico City

March 7, 2005

Motivation for talk

• Draws from our own work (esp. Zhu et al, 2005)

• Attempts to generalize hypotheses, and identify ways in which ongoing and future work can test them

Hypothesis for land surface role in NAMS

• Anomalously wet (dry) monsoon years tend to be preceded by anomalously dry (wet) antecedent winter precipitation (e.g. Higgins et al, 1998; 2000), and/or

• Anomalously wet (dry) monsoon years tend to be preceded by anomalously low (high) snowpack in the previous winter (e.g. Gutzler and Preston, 1997)

• Hypothesized mechanisms are different, but both lead to enhanced (suppressed) land-ocean contrast, hence strong (weak) NAMS

• Reality is, of course, more complicated – questions of lag/lead, geographic location of signal, stability over time

Study Domain

Monsoon regions are defined as in Comrie & Glenn (1998)

Monsoon West

Monsoon South

Monsoon North

Monsoon East

15-year Moving Average Correlation of PI versus MW JJAS rainfall

JFM Precipitation Index (PI)

0

40

80

120

160

1950 1960 1970 1980 1990

Year

Pre

cipi

tatio

n (m

m)

JFM PI

Winter Precipitation –

Monsoon Relationship

April Snow Index (SWE) 15-year Moving Average Correlation of SWE versus MW JJAS rainfall

Snow – Monsoon Relationship

Winter Precipitation - Monsoon Rainfall

feedback hypothesis

Higher (lower) winter precipitation & spring snowpack

More (less) spring & early summer soil moisture

Weak (strong) monsoon Lower (higher) spring & early summer surface temperature

JFM Precipitation in extreme monsoon

years

DRY WET

DRY WET

Apr-May Soil Moisture in extreme monsoon years

Dry MonsoonWet Monsoon

Soil moisture anomalies persist from spring until June

WetDry

Correlation of June Sm & JFM PI (1965-1999)

What is the feedback to the atmosphere ?

Correlation:April SWE & May-June Ts (Negative relationship )

Correlation:June Sm & June Ts (No significant relationship in MW )

SW Sm has no significant relationship with Ts

WetDry

WarmCold

June Ts in extreme monsoon years

June Sm in extreme monsoon years

× ×

Winter Precipitation - Monsoon Rainfall feedback hypothesis

Higher (lower) winter precipitation & spring snowpack

More (less) spring & early summer soil moisture

Weak (strong) monsoon Lower (higher) spring & early summer surface temperature

×?

June 30Wet years Dry years

SW desert daily precipitation in extreme years

Correlation:June Ts & July MW precipitation

Negative correlation between SW desert pre-monsoon Ts & July precipitation ?

North American Monsoon Conceptual Basis

• The combination of seasonally warm land surfaces in lowlands and elevated areas together with atmospheric moisture supplied by nearby maritime sources is conductive to the formation of a monsoon like system. ( Adams D. K and A. C. Comrie, 1997: The North American Monsoon. Bull. Amer. Meter. Soc.,2197-2213. )

• The inverse spring snow – summer rainfall relationship proposes the hypothesis intuitively: excessive spring snow means cold continental temperature, thus inhibiting the summer time land surface heating that drives the monsoon. (Gutzler D. S. and J. W. Preston, 1997: Evidence for a relationship between spring snow cover in North America and summer rainfall in New Mexico. Geophys. Res. Let., 24, 2207-2210.)

Higher (lower) June Ts Stronger (weaker)

monsoon

Z500 (m) anomalies in wet years

Wet Monsoon

Low

June

High

June Ts anomalies in wet years

Dry Monsoon

High

June

Low

Z500 (m) anomalies in dry years

June Ts anomalies in dry years

The strong positive relationship between June Ts and Z500 anomalies, suggesting that pre-monsoon Ts are not modulated by the local land-surface conditions.

Summary

● Southwest winter precipitation is a potential predictor for MW summer monsoon, even though this relationship varies with time

● Spring land surface conditions in the SW U.S. are strongly determined by the previous winter’s precipitation, and this land memory can persist through April and May into June. However, this memory appears to contribute little to the magnitude of NAM precipitation.

● June positive Z500 anomalies in dry years induce an increase in surface temperature in AZ and NV, and vice versa for wet year, which suggesting that the controlling factor for the pre-monsoon Ts anomalies may not be local

Comments• Question as to whether (lack of) persistence in

soil moisture from previous winter-spring to onset of monsoon is model artifact

• Coupled model experiments (possibly along lines of Koster-Suarez land-ocean-atmosphere coupling design) are probably needed to make further progress

• Such experiments have been hampered in the past by model-specific behavior, and/or lack of consistency between off-line and coupled land schemes.

• Latter can be resolved by multi-model ensemble framework for off-line runs, and consistent LDAS forcing data for NAMS region (which have now been assembled)