Conclusions:

1
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 of Physical 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, 32373251. 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 and Colorado, 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
lower (higher) spring and early summer surface
temperature
Late (early) monsoon
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 coverage from 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
NERN
2001 Near-real