1. Winter precipitation – spring soil moisture link
Relationship between Antecedent Land Surface Conditions and Warm Season Precipitation in the North American Monsoon Region
Chunmei Zhua, Dennis P. Lettenmaier a, and Tereza Cavazosb
aDepartment of Civil & Environmental Engineering, Box , University of Washington, Seattle, WA 98195
bDepartment of Physical Oceanography, Centro de Investigacion Cientifica de Educacion, Superior de Ensenada, Ensenada, Mexico 2 5
Summary
We explore possible links between North American Monsoon System (NAMS) seasonal (Jun-Jul-Aug-Sep) precipitation and pre-monsoon seasonal land surface conditions including precipitation (P), temperature (T), soil moisture (Sm) and snow water equivalent (SWE) anomalies. We find statistically significant inverse relationships between the strength of the monsoon in an area of Arizona and western New Mexico defined as Monsoon West and the previous winter’s precipitation and snowpack in in the southwestern U.S. (SW) and the mountainous parts of UT and CO respectively. These linkages are strong from s and weak otherwise, as has also been suggested by previous studies. We propose a land surface feedback hypothesis: winter P leads to more winter and early spring SWE in the predictor area, hence more spring and early summer Sm, and lower spring and early summer Ts, which induces a weaker onset of the NAMS and vice versa. We tested all of the 3 links in this hypothesis and confirmed the existence of the land memory of winter precipitation and snow anomaly. This land memory can even persist from April and May into June. However, our results show that this land memory contributes little to the magnitude of NAM precipitation. It is interesting that the pre-monsoon (June) surface temperature over the U.S. Southwest desert shows am inverse relationship with monsoon precipitation, which is the reverse of what we expect based on the monsoon driving force concept of land-sea temperature contrasts. The apparent reason is the June upper-troposphere atmospheric circulation pattern: the June 500 mb positive anomalies in dry years induces an increase in surface temperature in the U.S. Southwest, and vice versa.
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, Guzler D.S., 2000: Co variability of spring snowpack and summer rainfall across the southwest United States. J. Climate, 13,
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,
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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.
Winter Precipitation, Snow - JJAS MW Rainfall
Soil moisture – surface temperature link
Correlation of June soil moisture vs. June SAT ×
Figure 2b: 15-year moving average correlation of JJAS MW rainfall with winter precipitation index
Figure 2a: Monsoon West winter predictor region.
Unexpectedly, the June surface air temperature anomaly map in extreme years (lower left) in the Southwest doesn’t show an inverse relationship with June soil moisture (upper left), as is implied by the correlation map of June soil moisture and surface temperature (upper right). In the Southwest, there is no significant inverse relationship between soil moisture and surface air temperature in June (pre-monsoon).
Figure 2d: 15-year moving average correlation of
MW snow index versus JJAS monsoon rainfall
Figure 2c: Monsoon West snow index area 6
● The region with a statistically significant inverse relationship between winter precipitation and strength of the subsequent monsoon includes southern California, Nevada, Utah, Arizona, western Colorado and New Mexico. This area is considered to be the potential winter predictor region for MW monsoon rainfall (Figure 2a).
● A snow index equal to JFM SWE in the mountainous part of the U.S. Southwest (blue area in Figure 2c) and JJAS MW precipitation shows a negative correlation with the subsequent monsoon’s strength.
● The inverse relationship varies in strength. It is strong during the period, but weak otherwise (Figure 2b,2d).
Pre-monsoon SAT – monsoon precipitation
Antecedent June surface air temperature (SAT) in Northern AZ and in the Southern Rockies is positively correlated with July MW precipitation. However in the core of the monsoon (SW lower desert) the relationship is negative. Daily area (red circle in left figure) mean precipitation shows this inverse relationship is not related with the earlier arrival of monsoon rainfall there (lower figure).
SW desert daily precipitation in wet years (red) and dry years (green) from 1 June to 30 July. Period: 3
Winter Precipitation-monsoon rainfall feedback hypothesis 1
Study Domain
More (less) spring or
early summer soil moisture
Higher (lower) winter precipitation
and spring snowpack 7
Monsoon West
Monsoon South
Monsoon North
Monsoon East
Weak (strong) monsoon
lower (higher) spring and early
summer surface temperature
500mb Geopotential height (Z500) – surface air temperature 4
Winter precipitation – spring soil moisture link
MW JFM relative precipitation anomaly composite for extreme years
The figures show apparent relationships between strong and weak MW monsoon precipitation and soil moisture in the preceding spring. The lower left figure shows that strong (weak) monsoons are associated with dry (wet) antecedent soil moisture. Note that the lower left figure appears similar to the upper left figure, 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 inverse signals.
Monsoon regions are defined as in Comrie & Glenn (1998) based on the seasonality and variability of JJAS monsoon precipitation from 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.
Conclusions:
● Southwest winter precipitation is a potential predictor for MW summer monsoon, even though this relationship varies with time (strong from s and weak otherwise)
● Spring land surface conditions in the southwestern 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 years. The atmospheric circulation pattern appears to be responsible for the inverse relationship between monsoon precipitation and June Ts in this SW desert region.
Significant correlation of Southwest desert SAT vs. Z500 over the whole domain Period:
June Z500 anomaly maps in extreme years (upper figures) show similar patterns as the June SAT anomaly maps (lower figure in section 5 ), suggesting a relationship between upper-tropospheric circulation pattern with surface air temperature. The Southwest desert SAT correlation map with Z500 (lower left figure) also shows that Z500 could have impact on surface air temperature. The June Z500 higher anomaly in dry years induces warmer surface temperature and vice versa for wet years.