Conclusions:

1. Late Early Late Early Late Early a b Late Early Monsoon South Monsoon North Monsoon West Monsoon East 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 98195 bDepartment ofPhysical Oceanography, Centro de Investigacion Cientifica de Educacion, Superior de Ensenada, Ensenada, Mexico 7 2 Introduction Studying the role of land surface conditions in the Mexican portion of the North American monsoon (NAMS) region has been a challenge due to the paucity of long-term observations. We produced a long-term gridded observation-based climate data set suitable for forcing land surface models, and model-derived land surface states and fluxes for a domain consisting of all of Mexico (Zhu et al 2006a). The data sets span the period of 1925 – Oct. 2004 at 1/8◦ spatial resolution at a subdaily (3h) time step. The simulated runoff matches the observations plausibly over most of 14 small river basins spanning all of the Mexico, which suggests that long term mean evapotranspiration is realistically reproduced. On this basis, and given the physically based model parameterizations of soil moisture and energy fluxes, the other surface fluxes and state variables such as soil moisture should be represented reasonably. In general, the derived surface fluxes in VIC (Variable Infiltration Capacity hydrological model) show compatible spatial patterns with North American Regional Reanalysis (NARR) data on a seasonal mean basis, but with some difference in the magnitude especially for downward shortwave radiation and net radiation. These validation results provide confidence that the data set can be used for exploring the land surface – atmosphere feedback mechanism over the core of the North American monsoon system (NAMS) region in northwestern Mexico, termed MSa here. We evaluated 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 the monsoon during the 1950-1999 period in MSa. We find a statistically significant positive relationship between monsoon onset date in MSa and previous winter precipitation in the Southwestern U.S. (SW) and northwestern (NW) Mexico, and winter 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 for a SW target area termed monsoon west (MW). In the MW study (Zhu et al, 2005), 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. This hypothesis broke down in MW due to the small contribution of land surface memory to surface thermal condition, and hence to monsoon strength. We test the same hypothesis here for MSa by examining three links (Zhu et al, 2006b). First, we find that in May not only the total column, but also the near-surface Sm in both SW and NW Mexico have memory from the previous winter precipitation. The spring Sm anomalies correlate negatively with Ts anomalies over most of the continental U.S. and Mexico except for the desert region of SW and NW Mexico. The monsoon onset is negatively correlated with May Ts over an area roughly consisting of New Mexico and some adjacent areas, suggesting that 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 apparent land surface feedback, 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 strong (possibly more important than land feedback) role in modulating the monsoon onset. 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, 2006a: Long-term climate and derived surface hydrology and energy flux data for Mexico, 1925 – 2004. J. Climate, accepted. Zhu C. M., D. P. Lettenmaier, and Tereza Cavazos, 2005: 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, 2006b: Role of Antecedent Land Surface Conditions in Warm Season Precipitation over Northwestern Mexico. J. Climate, accepted. Comparison of simulated and observed streamflow Spring land – surface thermal condition – monsoon onset Monthly calibrated routed simulated runoff (solid lines) versus observed streamflows (dashed lines)in m3/s. Correlation of May first layer Sm vs. May Ts May Sm anomaly composite The general match between simulation and observation shows that water balance components such as soil moisture and evapotranspiration is well presented at least in an acceptable quality, which support our analysis study of land surface feedback mechanism shown in later sections. Correlation of May Ts vs. monsoon onset May Ts anomaly composite 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 3 Spring soil wetness condition has a negative feedback to the surface thermal condition over the key area in New Mexico andColorado, thus influencing the monsoon onset. Study domain 8 Land – sea thermal contrast 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 the later sections, we will explore the land surface feedback mechanism on MSa. 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 Late - early 4 Winter Precipitation-monsoon onset feedback hypothesis 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. More (less) spring or early summer soil moisture Higher (lower) winter Precipitation and snowpack Late Early Late (early) monsoon lower (higher) spring and early summer surface temperature 5 Winter Precipitation, Snow – Monsoon Onset 9 Atmospheric Circulation effect? Fig 4b: 15-year moving mean correlation of monsoon onset with winter P index Fig 4a: MSa winter P related region. 1 Hydrological Model May Ts anomaly composite 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. VIC Model Features: ● Multiple vegetation classes in each cell ● Sub-grid elevation band definition (for snow) ● 3 soil layers used ● Energy and water budget closure at each time step ● Subgrid infiltration/runoff variability ● Non-linear baseflow generation Fig 4d: 15-year moving mean correlation of monsoon onset with winter SWE index Fig 4c: MSa winter SWE related region. ●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. May Z500 anomaly composite Late Early Model Development • Surface forcing data:Daily precipitation, maximum and minimum temperatures SMN daily meteorological data(-2003)recently updated and improved long-term surface station data, obtained courtesy of Ing. Alejandro Gonzalez Serratos Northwestern Mexico North American Monsoon Experiment Event Raingage Network (NERN)daily precipitation data (2002 - 2005)provided courtesy of David Gochis of NCAR for 86 stations that cross the Sierra Madre SMN daily historical precipitation data (1995 – near realtime) provided courtesy of Miguel Cortez Vázquez of SMN, around 1,000 stations. 6 Winter precipitation – spring soil moisture link • Soil parameters:derived from FAO global soil map. • Vegetation coveragefrom the University of Maryland 1-km Global Land Cover product (derived from AVHRR) JFM precipitation relative anomaly composite May soil moisture has a memory of 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 . Conclusions: ● We produced a long-term observation-derived and hydrological model-derived dataset of land surface fluxes and states with a period of 1925-2004, at 1/8 degree resolution for the whole Mexico. Because the simulated runoff is shown to match with observations well over some small river basins and the derived surface fluxes in VIC show compatible spatial patterns with NARR data on a seasonal mean basis (not shown in this poster), it is argued that the other energy fluxes and water balance components such as soil moisture and evapotranspiration are well presented at least in an acceptable quality, which provides confidence that the data set can be used for the evaluation the role of land surface – atmosphere interactions in the NAMS region. ● 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. Guage Station Distribution (SMN) Correlation of winter P vs. May Sm May soil moisture anomaly composite Late Early 2001 Near-real NERN