Introduction

1. Relationship between Antecedent Land Surface Conditions and Precipitation in the North American Monsoon Region Chunmei Zhua, Dennis P. Lettenmaier a, and Tereza Cavazosb aDepartment of Civil & Environmental Engineering, Box 352700, University of Washington, Seattle, WA 98195 bDepartment of Physical Oceanography, Centro de Investigacion Cientifica de Educacion, Superior de Ensenada, Ensenada, Mexico 5 2 Winter Precipitation - JJAS Monsoon West Rainfall Winter precipitation – spring soil moisture link Introduction We explore possible links between North American Monsoon System (NAMS) seasonal (Jun-Jul-Aug-Sep) precipitation and pre-monsoon (previous autumn, winter, and spring) land surface conditions, including precipitation, temperature, soil moisture and snow cover anomalies. We hypothesize land and sea surface feedback mechanisms associated with NAMS precipitation, and we propose an approach for determining their dynamical links. Following previous investigators, we partitioned the NAMS region into four sub-regions (Monsoon West, South, North and East) based on the seasonality and variability of JJAS monsoon precipitation from 1961-1990, and evaluated the possible effects of previous land surface conditions in various subcontinental “predictor regions” on Monsoon West (MW) monsoon precipitation. Data for the study were monthly aggregates from the retrospective Land Data Assimilation System (LDAS) archive for the period 1950 to 1999 (Maurer et al, 2002). The retrospective LDAS archive includes gridded precipitation (P), mean surface air temperature (SAT), and Variable Infiltration Capacity (VIC) land surface model-derived soil moisture (Sm), and snow water equivalent (SWE). We outline future work that will construct an exploratory seasonal monsoon precipitation predictive model based on antecedent conditions. 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. Guzler D.S., 2000: Co variability of spring snowpack and summer rainfall across the southwest United States. J. Climate, 13, 4018-1027. Gutzler D.S. and J.W. Preston, 1997: Evidence for a relationship between spring snow cover in North America and summer precipitation in New Mexico. Geophys. Res. Lett., 24, 2207-2210. Higgins R.W. and W.Shi , 2000: Dominant factors responsible for interannual variability of the summer monsoon in the Southwestern United States. J. Climate, 13, 759-776. Hu Q. and F. Song, 2002: Interannual rainfall variations in the North American Summer Monsoon Region: 1900-98. J. Climate, 15, 1189-1202. Liang, X., D. P. Lettenmaier, E. F. Wood, and S. J. Burges, 1994: A Simple hydrologically Based Model of Land Surface Water and Energy Fluxes for GSMs, J. Geophys. Res., 99(D7), 14,415-14,428. Lo F. and M.P. Clark, 2002: Relationships between spring snow mass and summer precipitation in the Southwestern United States associated with North American monsoon system. J. Climate, 15, 1378-1385. Matsui T, V. Lakshml and B. Small, 2003: Links between snow cover, surface skin temperature, and rainfall variability in the North American Monsoon system. J. Climate, 16, 1821-1829. 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. Figure 2b: 15-year moving average correlation of JJAS MW precipitation with winter precipitation in predictor region The figures show apparent relationships between strong and weak MW monsoon precipitation and soil moisture in the preceding spring. The left figure shows the strong (weak) monsoons are associated with dry (wet) antecedent soil moisture. Note that the left figure appears similar to Figure 2c, and indicates that spring soil moisture in the Southwest is a reflection of winter precipitation. The right figure is is for June, and confirms that in much of the Southwest, soil moisture anomalies persist from winter through the following spring (immediately prior to the monsoon). Note that the Great Plains and Southwest show reverse signals. Figure 2a: Monsoon West winter predictor region. ● The statistically significant negatively related region includes southern California, Nevada, Utah, Arizona, western Colorado and New Mexico, which is the potential winter predictor region for Monsoon West monsoon rainfall (figure 2a). ● This negative relationship varies in strength. It is strong during the 1965-1990 period, but weak otherwise. ●This negative signal is especially strong during extreme years (Figure 2c). Soil moisture – surface temperature link The figure at left shows that April soil moisture has a negative, but not very strong correlation with May and June surface air temperature. Figure 2c: Monsoon West JFM relative precipitation anomaly composite for wet and dry years. Period: 1965-1999. Snow – surface temperature link April SWE (in the region shown in Figure 3a) shows a strong negative correlation with May and June surface air temperature in the Four Corners region. 3 Snow-monsoon Relationship Pre-monsoon SAT – monsoon precipitation 1 Study Domain Antecedent June surface air temperature (SAT) in Northern AZ and in the Southern Rockies is positively correlated with July MW precipitation. It seems that in the core of the monsoon the relationship is negative, possibly because rainfall there comes earlier. The land –surface mechanisms associated with SAT and precipitation remain unclear, and need further investigation Figure 3b: 15-year moving average correlation of Monsoon West snow index versus JJAS monsoon rainfall Figure 3a: Monsoon West snow index area Conclusions: ● For the Monsoon West region (here referring to eastern AZ and western NM), land surface-monsoon relationships are not stable in time. The strength of land surface teleconnections in MW seems to be strongest for 1965-1990. JJAS (monsoon) precipitation is MW appears to be negatively related to the previous winter’s precipitation in the U.S. Southwest, and to spring snow accumulation in the mountainous part of the Southwest. ● The antecedent land surface link that we propose related to SWE, soil moisture, and air temperature) is stronger in the Southwest mountainous area and the Four Corners source areas. This is a mountainous area, where snow plays a strong role in land surface processes, however it is not yet clear that the apparent land memory mechanism is related to snow per se. Furthermore, the surface temperature – monsoon links remain unclear, and need further investigation. Monsoon West ● A snow index equal to April SWE in the mountainous part of the U.s. Southwest (blue area in Figure 3a) and JJAS MW precipitation shows a negative correlation. ● This negative relationship is especially strong during 1965 – end of 1980s. Monsoon South Monsoon North 4 Monsoon East Winter Precipitation-monsoon rainfall feedback hypothesis Higher (lower) winter precipitation and spring snowpack More (less) spring or early summer soil moisture Monsoon regions are defined as in Comrie & Glenn paper (1998) based on the seasonality and variability of JJAS monsoon precipitation from 1961-1990. In the following section we evaluate the possible effects of previous land surface conditions in various subcontinental “predictor regions” on Monsoon West (MW) monsoon precipitation. lower (higher) spring and early summer surface temperature Weak (strong) monsoon