Conclusions:● The spring land condition in the SW U.S. has a memory of winter precipitation anomalies, and the spring land memory in this area seems to negatively affect the pre-monsoon seasonal surface thermal condition, which in turn is negatively correlated with monsoon onset in NW Mexico. This contrasts with previous findings for the Monsoon West (SW U.S.), where the persistence of land conditions from the previous winter was insufficient to induce enough of a surface temperature anomaly in late spring and early summer to affect monsoon onset. Thus, the land –sea thermal contrast concept for monsoon initiation is confirmed for MSa, unlike for MW. ● Upper-tropospheric circulation, which appears to be the major cause of monsoon onset anomalies in MW, also strongly influences the surface thermal condition in MSa, thus modulating the summer monsoon circulation. In fact, in MSa, large-scale circulation may play a more important role than land surface conditions in controlling monsoon onset.

Role of Antecedent Land Surface Conditions in Warm Season Precipitation over Northwestern Mexico

Chunmei Zhua, and Tereza Cavazosb , Dennis P. Lettenmaier a

aDepartment of Civil & Environmental Engineering, Box 352700, University of Washington, Seattle, WA 98195bDepartment of Physical Oceanography, Centro de Investigacion Cientifica de Educacion, Superior de Ensenada, Ensenada, Mexico

IntroductionWe explore the role of antecedent land surface conditions including precipitation (P), surface skin temperature (Ts), soil

moisture (Sm) and snow water equivalent (SWE) anomalies on the onset and intensity of monsoon precipitation during the

1950-2000 period in the core of the North American monsoon system (NAMS) region in northwestern Mexico, termed

MSa here. We use surface data from Maurer et al (2002) for the continental U.S. portion of the domain, and our newly

developed meteorological and hydrological dataset for Mexico (Zhu et al 2005a). We find a statistically significant positive

relationship between monsoon onset in MSa and previous winter precipitation in the Southwestern U.S. (SW) and

northwestern (NW) Mexico, and winter snow water equivalent (SWE) in the southern Rocky Mountains. The linkages are

strong during the 1960s-1980s and weak otherwise, which is a much shorter period than we found previously (Zhu et al,

2005b) for a southwestern U.S. target area termed monsoon west (MW). In the MW study, we proposed a land surface

feedback hypothesis: more winter P and SWE leads to more spring Sm, hence lower spring and early summer Ts, which

induces a weaker onset of the NAMS. We test this same hypothesis here for MSa by examining three links. First, we find

that in May not only the total column, but also the near-surface soil moisture in both the Southwest U.S. and Northwestern

Mexico have memory from the previous winter precipitation. This spring land memory negatively affects the pre-monsoon

seasonal surface thermal condition over most of the continental U.S. and Mexico except for the desert region of the

Southwest US and Northwest Mexico. The monsoon onset is negatively correlated with May surface temperature over an

area roughly consisting of New Mexico and some adjacent areas, suggesting that the antecedent land surface conditions

may influence the pre-monsoon surface thermal condition, which then affects monsoon onset. We also confirmed the

monsoon driving force concept that states that the strength of the monsoon should be related to pre-monsoon land - sea

surface temperature contrasts. We find in confirmation of this concept that late monsoon years are associated with colder

land and warmer adjacent ocean than early monsoon years. Besides the land surface conditions, we found a strong positive

relationship between May Ts anomalies and the large-scale mid-tropospheric circulation (Z500) anomalies, which suggests

that large-scale circulation may play a more important role in modulating the monsoon onset than the pre-monsoon land

surface conditions. (Note: this work is reported in full in a recently submitted paper (Zhu et al, 2005c))

References:Comrie A.C. and E.C. Glenn, 1998: Principal components-based regionalization of precipitation regimes across the southwest United States and northern

Mexico, with an application to monsoon precipitation variability. Clim. Res., 10, 201-215. Englehart P. J. and A. V. Douglas, 2001: The role of eastern Pacific tropical storms in the rainfall climatology of western Mexico. Intl. J. Climatology, 21, 1357-1370.Maurer E.P., A.W. Wood, J.C. Adam, D.P. Lettenmaier, and B. Nijssen, 2002: A long-term hydrologically-based data set of land surface fluxes and states for the conterminous United States. J. Climate, Vol. 15, 3237–3251.

Zhu C. M., D. P. Lettenmaier, 2005a: Long-term climate and derived surface hydrology and energy flux data for Mexico, 1925 – 2004. J. Climate,

submitted.Zhu C. M., D. P. Lettenmaier, and Tereza Cavazos, 2005b: Role of Antecedent Land Surface Conditions on North American Monsoon Rainfall Variability. J. Climate, 18, 2824-2841.Zhu C. M., Tereza Cavazos, and D. P. Lettenmaier, 2005c: Role of Antecedent Land Surface Conditions in Warm Season Precipitation over Northwestern Mexico. J. Climate, submitted.

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Land – sea thermal contrast

The early monsoon years are characterized by warmer land over the Southwest US and NW Mexico, and a weak positive SST signal over the Gulf of California (GOC) and the subtropical eastern Pacific

The late monsoon years show a reverse differential thermal contrast, with a stronger warming over the eastern Pacific and cooling over Northern Mexico and the Southwest US.

8

Monsoon regions are defined as in Comrie & Glenn (1998) based on the seasonality and variability of JJAS monsoon precipitation from 1961-1990. The Monsoon South domain is divided into 2 sub-regions MSa and MSb because MSb is much more influenced by tropical storm system (Englehart and Douglas, 2001). In this paper we will explore the land surface feedback mechanism on MSa.

Monsoon South

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Classification of Extreme Monsoon Years

Winter Precipitation-monsoon onset feedback hypothesis

Higher (lower) winter Precipitation and snowpack

More (less) spring or early summer soil moisture

lower (higher) spring and early summer surface temperature

Late (early) monsoon

4 Winter Precipitation, Snow – Monsoon Onset

Figure 4a: MSa winter P related region.Figure 4b: 15-year moving mean correlation of

monsoon onset with winter P index

Figure 4c: MSa winter SWE related region. Figure 4b: 15-year moving mean correlation of monsoon onset with winter SWE index

● There exists a winter precipitation positively correlated region in SW and NW Mexico (Fig.4a). A wet winter tends to be followed by late onset of the monsoon, and a dry winter by an earlier start. But this relationship is only robust from 1965 to 1980.● A snow index equal to JFM SWE in the southern Rock Mountain (blue area in Figure 4c) and MSa monsoon onset shows a positive correlation, which is significant during 1960 to 1980.

Winter precipitation – spring soil moisture link

Late Early

Late Early

Spring land – surface thermal condition – monsoon onset

Late Early

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May Sm anomaly composite

May Ts anomaly composite

Correlation of May first layer Sm vs. May Ts

Correlation of May Ts vs. monsoon onset

May Ts exhibits a strong inverse relationship with May Sm. in late monsoon years, May Ts is colder than normal in large areas of the

Southwest U.S. and Northwestern Mexico; the reverse pattern in the Southwest US is true for early monsoon years, which is consistent

with the thermal contrast concept for driving the onset of the monsoon

LateEarly

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Atmospheric Circulation effect?

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May Ts anomaly composite

May Z500 anomaly composite

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Correlation of May Ts vs. May Z500

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JFM precipitation relative anomaly composite

May soil moisture anomaly composite Correlation of winter P vs. May Sm

May soil moisture has a memory of the winter precipitation anomaly in Southwest U.S. and Northwestern Mexico, with late monsoon years showing anomalously wet spring soil and vice versa for early monsoon years .

Spring soil wetness condition has a negative feedback to the surface thermal condition over the key area in New Mexico and Colorado, thus influencing the monsoon onset.

The May Z500 anomaly maps exhibit a positive correlation with May Ts anomalies with low Z500 (high Z500) anomalies over the Southwest associated with negative (positive) Ts anomalies during late (early) monsoons, suggesting that the large-scale circulation may play a important role in the pre-monsoon land-sea thermal contrast, which in turn affects the monsoon onset.