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 4 2 Winter Precipitation - JJAS Monsoon West Rainfall Winter Precipitation-monsoon rainfall feedback hypothesis 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. The retrospective LDAS archive includes gridded precipitation (P), mean surface air temperature (Ts), 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. 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. 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. Higher or lower winter precipitation & winter , spring snow More or less spring or early summer soil moisture lower or higher spring and early summer surface temperature Weak or strong monsoon 5 Winter precipitation – spring soil moisture link Figure 2b: 15-year moving correlation of JJAS rainfall versus related winter precipitation predictor indices 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 potential winter predictor region for Monsoon West monsoon rainfall (figure 2a). ● This negative relationship is not robust always. It is strong during the 1965-1990 period, but weak in other periods from 1950-1999. ●This negative signal is very strong during extreme years (Figure 2c). The land has a memory effect from winter precipitation anomaly. During extreme years, the soil moisture anomaly signal persists through April. May and June. Great Plain and Southwest show reverse signal. Soil moisture – surface temperature link April soil moisture has a negative , but not very strong correlation with May , June surface temperature. Figure 3c: Monsoon West JFM relative precipitation anomaly composite for wet and dry years. Period: 1965-1999. Snow – surface temperature link 1 Study Domain 3 April snow index shows a strong negative correlation with May & June surface temperature in the Four Corners region. Snow-monsoon Relationship Pre-monsoon surface temperature – monsoon Antecedent June Ts in Northern AZ and in the Southern Rockies is positively correlated with July monsoon West rainfall. 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 surface temperature and rainfall needs further investigation Monsoon East Monsoon South Monsoon North Conclusions: ● For North American Monsoon region (here referring to western AZ and eastern NM), land surface-monsoon relationships are not stable in time. 1965-1990 is the significant period over 1950-1999. Southwest US winter precipitation, Four Corners regions and mountainous area of Nevada and Utah spring snow have negative correlation with JJAS monsoon. ● Antecedent land surface link that we propose (SWE, Sm, and Ts) is stronger in the Utah and Nevada mountain and Four Corners source areas where SWE may play a significant role in underpinning the land surface memory effect into the atmosphere. However the surface temperature – monsoon link need further investigation Monsoon East Figure 3b: 15-year moving average correlation of Monsoon West snow index versus JJAS monsoon rainfall Figure 3a: Monsoon West snow index area ● JFM & April SWE in the mountainous portions of Utah and Nevada and the Four Corners region show a strong negative correlation with monsoon rainfall. ● This negative relationship is strong during 1965 – end of 1980s. Monsoon regions are defined as Comrie & Glenn paper (Climate Research, 1998, Vol10 : 201-215) based on the seasonality and variability of JJAS monsoon precipitation from 1961-1990, and in the following section we evaluated the possible effects of previous land surface conditions in various subcontinental “predictor regions” on Monsoon West (MW) monsoon precipitation.