# Presentation: 200129

## PREDICTING THE ONSET OF THE NORTH AMERICAN MONSOON AND

- PROGRESS  TOWARD  A  MECHANISTIC  UNDERSTANDING
- David L. Mitchell1, Dorothea Ivanova1, Beth Hall1, Miguel F. Lavn2, and
- Affonso S. Mascarenhas, Jr.3
- Desert Research Institute, Reno, Nevada, USA
- 2. Centro de Investigacin Cientfica y de Educacin Superior de Ensenada (CICESE),
- Ensenada, Baja California, Mexico
- 3. Instituto de Investigaciones Oceanologicas, Universidad Autonoma de Baja California,
- Ensenada, Baja California, Mexico

## Annual Rainfall

- Contributions of the
- North American Monsoon:
- NW Mexico: 60-80%
- Arizona: 35%
- New Mexico: 45%

## Slide 3

## AZNM region cumulative normalized rainfall

- for periods having N. GC SSTs ≤ indicated
- SST.  Time is implicit with increasing SSTs.

- Mean rainfall rates for the AZNM region
- for N. GC SST intervals of 0.5oC based on
- five June-August seasons.

## 7 July 2002

- at 00 UTC,
- Total Precip.
- Water (TPW)

- 9 July 2002
- at 00 UTC,
- TPW

- 11 July 2002
- at 00 UTC,
- TPW

- 2.4oC increase in N. GC SST over 2 days,
- reaching 29.8oC.

## 11 July 2002

- at 00 UTC.
- 24 hour rainfall
- accumulation

- 15 July 2002
- at 00 UTC.
- 24 hour rainfall
- accumulation

## Sensitivity of CAPE (Convective Available Potential Energy) to GC

- N. GC 26oC; 28oC otherwise

- N. GC 29oC; 30oC otherwise

- N. GC. 30oC; 30oC otherwise

- sea surface temperatures.  Simulation time = 54 h; 11 pm LST.
- Yellow ≈ 1300 J/kg  (CAPE)
- Orange ≈ 2100 J/kg
- Red      ≈ 2900 J/kg
- Brown  ≈ 3700 J/kg

## Modeled surface circulation of the GC by Beier (1997; J. Phys. Oceanography).

- Islands in red; sea surface elevation in yellow.

## March-April

- Sea surface height images courtesy of Dr. Ted Strub, Cooperative Institute of Oceanography, Oregon State University

- SSTs on16 May 1999
- Red ≈ 29-30oC,
- Light orange ≈ 25oC

- 9-year
- climatology

- May-June
- climatology

- July-August
- climatology

- Sea surface heights

- Sea surface heights

- Sea surface heights

## Dynamic height and geostrophic current velocities (blue arrows) for 15 May 2003.

- Black line is 50 cm contour.  From NOAA/AOML website, developed by Joaquin
- Trinanes.

## Dynamic height and geostrophic current velocities (blue arrows) for 25 May 2003.

- Black line is 50 cm contour.  From NOAA/AOML website, developed by Joaquin
- Trinanes.

## Evolution of the 26oC and 28.5oC isotherms for the warm pool, based on SST 

- Evolution of the 26oC and 28.5oC isotherms for the warm pool, based on SST
- climatology (1983-2000).  Images are 5-day means centered on June 3rd and June 28th.
- Courtesy of Miguel Lavn at CICESE.

## Sea height anomaly (SHA) index based on anomalies in 3 coastal and 1 open ocean region.

- Coastal: GC entrance, between the “Cape” and GC entrance, and southern Mexico
- Open Ocean: Off southern Mexico (the “dome”)
- 1 rating point = | 3 cm sea surface height |
- Negative coastal ratings: SHA Index = |coastal rating – 1.5| + off-shore rating
- Positive coastal ratings:  SHA Index = off-shore rating – coastal rating

- Based on 10 seasons of
- TOPEX/POSEIDON
- satellite altimeter data.

- Preliminary analysis relating the May SHA Index to the onset of the 29.5oC N. GC SST.

## Slide 14

## Statewide AZ June-August rainfall amounts, 1983-2002, vs. N. GC 29.5oC

- Statewide AZ June-August rainfall amounts, 1983-2002, vs. N. GC 29.5oC
- onset date, using 18 or 14 km weekly SSTs from JPL or NOAA.  Zero and
- negative values are prior to July 1st.  Horizontal line = median rainfall value.

## Deviation from normal for Jul-Sep precipitation, based on the 9 wettest AZ seasons for the

- period 1950-99, relative to the long-term mean 1950-95.  Color legend gives std. deviations.

## Poleward Migration of the North American Anticyclone:

- Working Hypothesis

- As warm SSTs ( > 26oC) move up the Mexican coast, deep convection is triggered
- as shown previously and in earlier work.
- 2.	The poleward advance of deep convection over the Sierra Madre Occidental is
- accompanied by a branch of descending air aloft to the north of the deep convection.
- This may position the center of the anticyclone.
- 3.	The observed SST-rainfall relation implies the anticyclone’s position depends on
- the poleward extent of the warm SSTs.
- The mid-level monsoon moisture supply is largely governed by this anticyclone,
- with moist tropical air often to the south and west of the anticyclone center.  The summer
- circulation over North America is dominated by this anticyclone.

## Evolution of NA Monsoon Onset

- From Higgins et al. 1999, J. Climate
- Note timing of onset coincides with
- the arrival of SSTs > 26oC north
- of Cabo Corrientes, as shown in
- Mitchell et al. 2002 (J. Climate).

## Evolution of the 27.5oC isotherm and the 500 mb anticyclone

- Evolution of the 27.5oC isotherm and the 500 mb anticyclone

## Evolution of the 27.5oC isotherm, the 500 mb anticyclone center, and the 250-260 W/m2

- Evolution of the 27.5oC isotherm, the 500 mb anticyclone center, and the 250-260 W/m2
- OLR gradient (transition between regions of rising and descending air).

## MM5 simulations of the anticyclone at 600 mb 

- MM5 simulations of the anticyclone at 600 mb
- for July, using 3 convection schemes: (a) Betts-
- Miller-Janjic, (b) Kain-Fritsch and (c) Grell.
- Taken from Gochis et al. 2002, *Sensitivity of the *
*Modeled North American Monsoon Regional *
*Climate to Convective Parameterization*, Mon.
- Wea. Rev., Vol. 130, 1282-1298.
- Simulations based on Reynolds-Smith SST data
- (optimum interpolation method), which under-
- estimate the GC SSTs by 2-6oC during July,
- especially in the northern GC.  The position of the
- anticyclone below US-Mexico border is consistent
- with hypothesis that its position depends on the
- latitude of the warmest coastal SSTs.

## Reynolds-Smith climatological SSTs (1974-1993) for July with OLR fields

- (numbers and solid curves).  Dark orange = 29oC, with 1oC change per color
- change.  Northern GC is about 24oC.

## Mean 500 mb circulation showing anticyclone center (in yellow) and mixing

- ratios.  Based on MM5 simulation for 10 July – 10 August, where GC SST =
- 29.5oC.  Taken from Stensrud (1995; J. Climate).

## SUMMARY

- Previous work based on satellite observations and modeling reveal the existence of a
- threshold SST of about 29.5oC for the northern GC that triggers widespread and relatively
- heavy rainfall in Arizona.  Moreover, the incremental advance of SSTs > 26oC up the
- Mexican coast appears necessary for the northward advance of the monsoon.
- The extremely warm water in the GC appears primarily due to advection of warm
- pool water into the GC during May-July, resulting in a dramatic reconfiguration of the
- warm pool 28.5oC isotherm (based also on earlier studies from UABC and CICESE).
- More research is needed to confirm this.
- The onset of relatively heavy rainfall in AZ may be forecast 1-2 months in advance,
- based on a relationship linking sea surface topography with the arrival of N. GC SSTs 
- 29.5oC.  The timing of this onset indicates whether AZ ( the Midwest) is likely to be
- average-to-wet (average-to-dry) or whether AZ is likely to be average-to-dry during the
- period June-August.  A more rigorous analysis is needed to confirm these results.
- 4.	Satellite SST and OLR data, and reanalysis data, suggest that the poleward
- migration of the North American 500 mb anticyclone during spring/summer may
- depend on the poleward advance SSTs > 26oC up the Mexican coast.  This corroborates
- well with MM5 modeling results, that show the mean July position of the anticyclone
- is much farther south when unrealistically cool GC SSTs are used.  This anticyclone
- dominates the summer mid-level circulation over North America.