Download Relationship between Antecendent Land

Relationship between Antecedent Land Surface Conditions and Warm Season Precipitation in the North American
Monsoon Region
a
a
Chunmei Zhu , Dennis P. Lettenmaier , and Tereza Cavazos
b
a
Department of Civil & Environmental Engineering, Box 352700, University of Washington, Seattle, WA 98195
b
Department of Physical Oceanography, Centro de Investigacion Cientifica de Educacion, Superior de Ensenada, Ensenada, Mexico
2 Winter Precipitation, Snow - JJAS MW Rainfall
Introduction
5
Soil moisture – surface temperature link
We explore possible links between North American Monsoon System (NAMS) seasonal (Jun-JulAug-Sep) precipitation and pre-monsoon seasonal land surface conditions including precipitation (P),
Correlation of June soil moisture vs. June SAT
temperature (T), soil moisture (Sm) and snow water equivalent (SWE) anomalies. We found
statistically negatively significant winter precipitation and snow related regions in SW and the
mountainous region in UT and CO respectively. Their linkage with Monsoon West (Arizona and
western New Mexico) monsoon rainfall is strong from 1965-1990s and weak otherwise, as has
suggested by previous studies. We proposed 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.
Figure 2a: Monsoon West winter predictor region.
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, 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 a negative 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.
Beyond what we expect, June surface air temperature
anomaly map in extreme years (lower left) in Southwest
doesn’t show the opposite pattern of June soil moisture
(upper left), which is consistent with the correlation map
of June soil moisture and surface temperature (upper
right). In Southwest, there is no significant negative
relationship between soil moisture and surface air
temperature in pre-monsoon season June.
Why?
Figure 2c: Monsoon West snow index area
Figure 2d: 15-year moving average correlation of
MW snow index versus JJAS monsoon rainfall
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.
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.
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.
● 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.
● The negative relationship varies in strength. It is strong during the 1965-1990 period,
but weak otherwise (Figure 2b,2d).
3
Study Domain
6
Winter Precipitation-monsoon rainfall feedback hypothesis
Higher (lower) winter precipitation
and spring snowpack
More (less) spring or
early summer soil moisture
Monsoon West
Weak (strong) monsoon
Monsoon South
Monsoon North
Monsoon East
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.
Conclusions:
● Southwest winter precipitation could be a potential predictor for MW summer monsoon, even
though this relationship varies with time, strong from 1965-1990s and weak otherwise
One reason may be the small evapotranspiration
signal in this semiarid area (right Fig.), the sparse
vegetation in the SW doesn’t favor much extraction
of soil water from the deep soils where most of the
moisture is stored. The long-term mean June
evaporation spatial distribution (right Fig.) exhibits
somewhat a consistent pattern with June Sm and
June Ts correlation map (upper right in this box)
● The statistically significant negatively related winter precipitation region includes
southern California, Nevada, Utah, Arizona, western Colorado and New Mexico, which is
the potential winter predictor region for MW monsoon rainfall (Figure 2a).
References:
1
×
Figure 2b: 15-year moving average correlation of
JJAS MW rainfall with winter precipitation index
4
lower (higher) spring and early
summer surface temperature
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 negative 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: 1965-1999.
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 the strong
(weak) monsoons are associated with dry (wet) antecedent
soil moisture. Note that the left figure appears similar to
higher 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
reverse signals.
7
500mb Geopotential height (Z500) – surface air temperature
● SW USA has land memory of winter precipitation, and this land memory can even persist through
April, May into June. However, it contributes little to the magnitude of NAM precipitation.
● June positive Z500 anomalies in dry years induce an increase in surface temperature in AZ/NV,
vice versa for wet years. The atmospheric circulation pattern causes the negative relationship between
monsoon precipitation and June Ts in SW desert region.
June Z500 anomaly maps in extreme years (upper figures) show similar pattern with June SAT anomaly
maps (lower figure in section 5 ), suggesting the correlation between upper-tropospheric circulation
pattern with surface air temperature. 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.