INTRODUCTION:
The summer time Mexican Monsoon is one of the largest seasonal climate signals in North America. During the months of July, August, and September, the Southwest United States and Northwest Mexico experience a dramatic directional shift in the lower atmospheric wind fields; this shift is coincident with a seasonal peak in precipitation rates. During these months northwest Mexico receives up to 80% of its annual rainfall. Nearly half of Arizona's precipitation is also received during this time.

PROJECT GOALS:
Current global models are unable to adequately simulate and predict variations in the Monsoon. Possible reasons for this global model failure include their inability to incorporate the warm SSTs over the Gulf of California as well as their inability to reproduce the correct low-level winds in this region. Our goal is to develop a regional model that incorporates the warm Gulf of California and which can adequately simulate and predict the summertime monsoon wind fields and hydrologic cycle on daily to monthly time scales.

METHODOLOGY:
We are using a global to regional modeling system developed at the National Centers for Environmental Prediction (NCEP) for these regional simulations. The domain of the model is shown in Fig. 1. Individual case studies as well as continuous simulations are being developed. In particular, we are now making daily to monthly forecasts for the U.S. Southwest. The modeling system nests the 25km resolution RSM within daily 7-day forecasts from NCEP's global spectral model (GSM); these GSM forecasts are initialized using NCEP's operational analysis data. A unique aspect of this modeling system is that it performs a continuous simulation in which the initial conditions for the RSM forecasts are supplied by the RSM 24Z forecast from the previous day. This allows perturbations resulting from the enhanced orography, land surface and resolution of the RSM to be carried forward from one simulation to the next, thus reducing spin-up effects.

RESULTS AND ACCOMPLISHMENTS:
Our modeling system has already been shown to be capable of simulating the hydrologic cycle for California (Chen et al. 1999) and the US (Roads et al. 2000). As a start to simulating the hydrologic cycle of the Mexican Monsoon, we have attempted to simulate the Gulf of California monsoon wind-fields and summertime surge events (Anderson et al. 1998, 2000a,b,c). Fig. 2 shows the third sigma-level (~450 m above ground level) RSM2 forecast wind vectors and meridional wind speed contours for the 30-day average beginning July 5, 1990 and for the 30-day average beginning July 30, 1995. Also included are the 435m AGL (above ground level) winds taken from the SWAMP-90 campaign, indicated on the figure by a circle centered on the station location. At night, RSM2-simulated winds for July 1990 show coherent southeasterly flow over the foothills of the Sierra Madre Occidental and eastern Gulf, in agreement with observations in this region. This low-level southerly flow extends into southwestern Arizona and southern Nevada with northwesterly flow over the eastern Pacific and southeastern portions of Arizona.

Vertical profiles of the 24-hour running mean July 1990 RSM2 and observed meridional wind components over Yuma are shown in Fig.3. A similar plot for the 1995 RSM2 and observed meridional wind components over Puerto Penasco, Mexico is presented in FIG. 4. As before, the model qualitatively reproduces the observed profiles for this period. In general, surges are well reproduced with proper timing of onset and southerly flow through approximately 2000m.

FIG.5 5 shows the diurnal cycle of the third sigma-level RSM2 forecast wind vectors and meridional wind-speed contours for the 1990 and 1995 surge period and non-surge period averages. Also shown are the SWAMP-90 450m AGL observations, similarly separated into surge and non-surge periods. During the surge periods there is coherent southeasterly flow throughout the Gulf of California which is comparable in magnitude to the northwesterlies found to the west of Baja (approximately 7-10m/s). This southerly flow extends through southeastern Arizona and into southern Nevada. Daytime wind patterns (24Z) resemble the 30-day averages however they show much stronger flow through the Gulf. Daytime observed winds show increased up-slope flow over the Sierra Madre Occidental and continued southerly flow over Yuma, in agreement with model output. At night (12Z) maximum wind speeds shift towards the foothills of the Sierra Madre Occidental but still show strong southeasterly flow over the entire Gulf; this shift results in a nocturnal intensification of the RSM2 southerly winds over the northern Gulf and southwestern Arizona. Simulations of the nighttime winds over the Sierra Madre stations do an excellent job reproducing both the magnitude and direction of the SWAMP-90 observations. As before, winds over Yuma are significantly under-predicted, possibly due to the positioning of the simulated jet core.

Non-surge low-level winds show markedly different characteristics from their surge-period counterparts. Most striking is the nighttime (12Z) wind field. Mean winds tend to be northwesterly over most of the Gulf with southerly winds located only over the northeastern Gulf and small portions of southwestern Arizona. In contrast to 30-day means and surge-event winds, flow over the foothills of the Sierra Madre Occidental is weak and variable. SWAMP-90 observed winds for these periods show similar features, including significantly weakened nocturnal winds at Yuma and over the Sierra Madre Occidental. During the day (00Z), the RSM2 indicates that southerly winds are still present over both the northern and central Gulf, however the flow through the Gulf is not continuous and is disrupted by westerlies over the central Gulf region; this discontinuity is again in contrast to the 30-day mean wind fields which seemed to suggest coherent southerly flow originating from the southern Gulf and extending through to northwestern Mexico. Over the foothills of the Sierra Madre Occidental, observed winds at Altar, Hermosillo and Obregon are all predominantly westerly, similar to that seen in the model output. At Altar, observations even show a northerly component, again in agreement with the model output.

By using the fine-scale model data to identify and characterize the low level wind fields, it has been possible to differentiate between the surge and non-surge variations which monthly to seasonal climatologies fail to capture. To show the strong intraseasonal influence surge events can have upon the dynamic and hydrologic fields in this region, a Hoevmoeller diagram of the low-level (~450m AGL) meridional winds and specific humidity over a north-south transect of the Gulf of California is plotted for the 30-day 1990 simulation (Fig.6); the vertical lines represent the starting and ending times (inclusive) of the surge periods according to the selection criteria as described in Anderson et al. (1999a). As can be seen, there are large variations in both the momentum and moisture fields associated with the onset and persistence of surge events. During surge periods, there is a dramatic increase in the low-level southerly winds throughout the Gulf. Associated with this increase in southerly winds is an increase in the low-level specific humidity extending from the mouth of the Gulf into northwestern Mexico.

In the model simulations, similar synoptic-scale variability is also found in the precipitation fields. Fig. 7 shows the vertically integrated RSM2 moisture flux vectors and precipitation values for the 1990 and 1995 surge periods, along with those for the non-surge periods. Surge events show dramatic increases in integrated moisture flux over the entire Gulf of California and into southeastern Arizona and southern Arizona. During these periods, simulated precipitation is found over most of the Sierra Madre Occidental as well as southwestern Arizona with rainfall over the Gulf occurring predominantly at night and over the Mogollon rim during the day. In contrast, the non-surge period low-level moisture flux vectors show markedly different characteristics from their surge-period counterparts. Over most of the Gulf and Sierra Madre Occidental, moisture fluxes have a northeasterly component, with streamlines originating from the Sierra Madre Occidental. Over the northeastern Gulf and southern California there is a southerly component to the moisture flux field but it is much weaker than that seen during surge periods. During these non-surge periods there is also a dramatic decrease in precipitation rates over the entire domain.

FUTURE WORK:
We are now in the process of extending our modeling system to investigate the intraseasonal and interannual variability of the atmospheric hydrologic cycle over the US southwest and northwest Mexico. The modeling system will initially be run for the summer of 1999 (May through October) to analyze climatological characteristics of extreme events associated with monsoon surges. The system will then be used to simulate the hydrologic cycle for the period Sept. 1997 through Sept. 2001, allowing us to investigate inter-annual variability associated with extremes in the ENSO cycle. As we develop these continuous simulations for this PACs projects, experimental daily to monthly forecasts using these continuous simulations as initial states will be displayed on the web on the Scripps Experimental Climate prediction home page (http://ecpc.ucsd.edu)

PUBLICATIONS RESULTING FROM THIS RESEARCH
Anderson, B. T., 1998: Investigation of Summertime Low-level Winds over the Gulf of California and the Southwestern United States* Ph.D. Thesis. Scripps Institution of Oceanography. UCSD, 177 pp.

Anderson, B.T., S. -C. Chen, J. O. Roads, H-M. H. Juang, 1999a: Model dynamics of low-level JETS over northwest Mexico. JGR (in press).

Anderson, B.T., S.-C. Chen, J.O. Roads, 1999B: Large-scale Forcing of Summertime Monsoon Surges over the Gulf of California and Southwest United States, JGR (submitted).

Anderson, B.T., S. -C. Chen, J. O. Roads, H-M. H. Juang, 1999C: Regional simulation of the low-level monsoon winds over the Gulf of California and southwest United States. JGR (submitted)

Chen, S., J. Roads, H. Juang, M. Kanamitsu, 1999: Global to regional simulations of California wintertime precipitation. J. Geophys. Res. (in press)

Roads, J., S. Chen, 2000: Surface Water and Energy Budgets in the NCEP Regional Spectral Model. JGR (submitted)

CONTACTS:

Principal Investigator:
John Roads
E-mail: jroads@ucsd.edu
Phone: (858) 534-2099
Facsimile: (858) 534 8561

Co-Principal Investigator:
Shyh Chen
E-mail: schen@ucsd.edu
Phone: (858) 534-0618
Facsimile: (858) 534 8561

Co-Investigator:
Bruce Anderson
E-mail: anderson@soest.hawaii.edu
Phone: (808) 956-3636

INSTITUTION:
SCRIPPS Institution of Oceanography

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