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Chunmei Zhu and Dennis P. Lettenmaier

Abstract

Studying the role of land surface conditions in the Mexican portion of the North American monsoon system (NAMS) region has been a challenge due to the paucity of long-term observations. A long-term gridded observation-based climate dataset suitable for forcing land surface models, as well as model-derived land surface states and fluxes for a domain consisting of all of Mexico, is described. The datasets span the period of January 1925–October 2004 at 1/8° spatial resolution at a subdaily (3 h) time step. The simulated runoff matches the observations plausibly over most of the 14 small river basins spanning all of 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 addition, a comparison of the surface fluxes from this study is performed with North American Regional Reanalysis (NARR) data on a seasonal mean basis. The results indicate that downward shortwave radiation is generally smaller than in the NARR data, especially in summer. Net radiation, on the other hand, is somewhat larger in the Variable Infiltration Capacity (VIC) hydrological model than in the NARR data for much of the year over much of the domain. The differences in radiative and turbulent fluxes are attributed to (i) the parameterization used in the VIC forcings for solar and downward longwave radiation, which links them to the daily temperature and temperature range, and (ii) differences in the land surface parameterizations used in VIC and the NCEP–Oregon State University–U.S. Air Force–NWS/Hydrologic Research Lab (Noah) land scheme used in NARR.

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Chunmei Zhu, Tereza Cavazos, and Dennis P. Lettenmaier

Abstract

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 1950–99 period in the core of the North American monsoon system (NAMS) region in northwestern Mexico (termed MSa here) is explored. A statistically significant positive relationship is found between monsoon onset date in MSa and previous winter precipitation in the southwestern United States (SW) and northwestern (NW) Mexico, and winter SWE in the southern Rocky Mountains. The linkages are strong during the 1960s–80s and weak otherwise, which is a much shorter period than had been found previously for an SW target area termed monsoon west (MW). In the MW study, the following land surface feedback hypothesis was proposed: more winter P and SWE lead to more spring Sm, hence lower spring and early summer Ts, which induce 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. The same hypothesis is in this work for MSa by examining three links. First, it is found 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 United States 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 premonsoon surface thermal condition, which then affects monsoon onset. The monsoon-driving force concept that states that the strength of the monsoon should be related to premonsoon land–sea surface temperature contrasts is also confirmed. The confirmation of this concept shows that late monsoon years are associated with colder land and warmer adjacent ocean than early monsoon years. In addition to the apparent land surface feedback, a strong positive relationship between May Ts anomalies and the large-scale midtropospheric circulation (Z500) anomalies is found, which suggests that large-scale circulation may play a strong (possibly more important than land feedback) role in modulating the monsoon onset.

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Chunmei Zhu, Dennis P. Lettenmaier, and Tereza Cavazos

Abstract

Possible links between North American Monsoon System (NAMS) seasonal [June–July–August–September (JJAS)] precipitation and premonsoon seasonal land surface conditions including precipitation (P), surface air temperature (Ts), soil moisture (Sm), and snow water equivalent (SWE) anomalies are explored during the 1950–2000 period. A statistically significant inverse relationship is found between monsoon precipitation in an area defined as the Monsoon West (Arizona and western New Mexico) and antecedent winter precipitation in the southwestern (SW) United States and the mountainous region in Utah and Colorado (the predictor area). This linkage is strong during 1965–90 and weak otherwise, as has been suggested by previous studies. A land surface feedback hypothesis is proposed to explain this relationship: more 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 (and less precipitation) of the NAMS and vice versa. All three links in this hypothesis were tested and the existence of a land memory associated with winter precipitation and snow, which can persist until June, was confirmed. However, the results show that this land memory contributes little to the magnitude of NAM precipitation. Winter snow is negatively correlated to late spring Ts in the SW mountainous region, but not in extreme years. In fact, the premonsoon (June) Ts over the U.S. southwest is inversely related to monsoon precipitation, which is the reverse of what is expected based on the hypothesis. The lack of a significant Sm–Ts–P relationship in most of the SW suggests, based on the constructed Sm dataset, that local premonsoon soil wetness conditions play a minor role in the strength of the monsoon. A strong positive relationship between June Ts anomalies and the large-scale midtropospheric circulation before the onset of the monsoon was found, suggesting that the controlling factor for the premonsoon Ts anomalies may not be local (i.e., not from the land surface). The results suggest that further research is needed to elucidate the nature of land–sea–atmosphere interactions as related to the onset of the monsoon.

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