Wayne Higgins, Wei Shi and Christopher Hain Climate Prediction Center / NCEP

1. Relationships Between Gulf of California Moisture Surges and Precipitation in the Southwestern United States Wayne Higgins, Wei Shi and Christopher Hain Climate Prediction Center / NCEP Climate Diagnostics and Prediction Workshop Oct. 21, 2003

2. Outline • Objectives • Background • Data Set Selection • Identification of Surges • Features Associated with Surges • Summary

3. Objectives • To examine relationships between Gulf of California moisture surges and precipitation in the southwestern United States using standard surface observations and observed precipitation. • To examine potential mechanisms relating surges to precipitation in order to determine why some surges are wetter than others.

4. Background • Previous diagnostic and modeling studies of GOC surges (e.g. Fuller and Stensrud 2000; Stensrud et al. 1997) have not emphasized relationships between the surges and the associated precipitation pattern. • Improved understanding and more realistic simulation of these relationships are required for improved warm season precipitation prediction. • Improved warm season precipitation prediction is the guiding goal of NAME.

5. Data Set Selection • Hourly Surface observations • Td, surface wind speed, and surface wind direction • Stations: Yuma, AZ and Tucson, AZ (sensitivity) • Precipitation • US_Mexico daily precipitation analysis (Higgins et al. 2000) • Resolution (lon, lat)=(1.0°x1.0°) • Atmospheric circulation • 700-hPa and 200-hPa winds from NCEP/NCAR Reanalysis • Period of Analysis: July-August 1977-2001* • * except 1992

6. Yuma Tucson

7. Identification of Surge Events • Application of method given in Fuller and Stensrud (2000) • Onset Characteristics: • Rapid increase in surface Td > mean* (PRIMARY) • Surface winds > mean* (SECONDARY) • Surface wind direction ~ southerly (SECONDARY) • 25-hr running means are used to avoid misleading aspects of the diurnal cycle in the desert (little impact on surge ID) . * climatological mean: July-August 1977-2001

8. Identification of Surge Events • Definestrong, weak, wet, dry surges • Strong: Td > climatological mean for at least 3 days • Weak: Td > climatological mean for less than 3 days • Wet: positive precipitation anomalies in AZNM* for the 5 day period (day 0 to day +4) after onset • Dry: negative precipitation anomalies in AZNM* for the 5 day period (day 0 to day +4) after onset * AZNM:Arizona/New Mexico (112.5°-107.5°W; 32°-36°N)

9. 25-hr Running Mean Values of Td (˚C), Wind Direction (˚) and Wind Speed (m s-1) at Yuma, AZ for July-August 1986 W S S W S S S S * Note: Fuller and Stensrud (2000) identified the same set of events.

10. Number (%) of Surges by Category 65% of Yuma surges were also identified at Tucson (strict) 82% of Yuma surges were associated with an upward trend in Td at Tucson.

11. Composite Evolution of Td (˚C) Keyed to Onset of Surges at Yuma • Strong surges have higher Td values for a much longer period after onset than weak surges. • For both weak and strong surges, Td values remain elevated above those observed prior to onset throughout the period examined. • Both before and after onset: • Diurnal maximum Td: 16Z • Diurnal minimum Td: 00Z • Average Diurnal Range: 3.0°C (5.4°F) • Agrees well with climatology

12. 25-hr Running Mean Values of Td (˚C) at Yuma and Daily Precipitation Anomaly (mm) in AZNM* for July-August 1986 (W)eak or (S)trong W S S W S S S S Precip Td (W)et or (D)ry D W W D D W D W • Some surges are associated with wetter-than-normal conditions in AZNM while others are associated with drier-than-normal conditions . • This is somewhat independent of surge strength. *AZNM:Arizona/New Mexico (112.5°-107.5°W; 32°-36°N)

13. Number (%) of Surges by Category 46% (37%) of the surges identified at Yuma (Tucson) are dry.

14. Composite Evolution of Precipitation Anomalies (mm) for Moisture Surges Keyed to Yuma • All surges: SENW progression of +ve anomalies along the west coast • The evolution tends to be enhanced for strong surges and suppressed for • the weak ones.

15. Composite Evolution of Precipitation Anomalies (mm) for Moisture Surges Keyed to Yuma • The evolution for wet surges is similar to that for strong surges, except +ve anomalies are larger and more widespread. The evolution for dry surges is dominated by -ve anomalies. • The composites show strong evidence of westward propagation, suggesting a relationship between the surges and: • TEW’s; • tropical storms moving northward; • middle / upper level inverted troughs / cyclones moving westward; • westward shifts in the boundary separating dry east Pacific / continental air from moist subtropical air.

16. Mechanisms Relating Surges to Rainfall • Scientific Question: Why are some GOC surges wetter than others? • Previous Studies: Stensrud et al. (1997) and Fuller and Stensrud (2000) suggested that when the passage of a westerly wave trough across the western United States (110°W) precedes the passage of an easterly wave trough across western Mexico (110°W) by several days , the resulting Gulf Surge is strong. But they did not relate this to the precipitation pattern. • Strategy: Examine how the different categories of surge events (i.e. weak, strong, wet, dry) are related to TEW’s and MWW’s in an effort to explain the associated precipitation patterns. TEW’S: 700-hPa V (20°-25°N) MWW’s: 200-hPa V (37.5°-42.5°N)

17. Longitude-Time Section of 700-hPa V Anomalies (m s-1) (20-25N) During July-August 1986 easterly wave trough axis • As the TEW’s shift westward across 110 W, • the wave troughs delineate a coherent transition from northerly (blue) to southerly (red) winds. • If a surge begins at Yuma within 3 days after the passage of an easterly wave trough across 110°W, then the two events are defined as being related. • (FS00 criterion).

18. Fraction (in %) of Surge Events at Yuma Related to TEW’s Crossing 110°W by Surge Category. Results Based on July-August 1977-2001 • Strong and wet surges are most often associated with TEW’s.

19. Longitude-Time Sections of Composite Mean 700-hPa V Anomalies (m s-1) (20-25N) Keyed to Yuma Surges • Both wet and dry surges are associated with easterly wave troughs across 110°W, but the • southerly regime that follows the trough passage is stronger for the wet surges.

20. Longitude-Time Section of 700-hPa V (m s-1) Anomalies (20-25N) and 200-hPa V Anomalies (m s-1) (37.5-42.5N) During July-August 1986 TEW’s MWW’s easterly wave trough westerly wave trough • As MWW’s shift eastward across 110 W, there is a coherent transition from southerly (red) to northerly (blue) winds. • The MWW trough axis (highlighted by the diagonal line) appears to cross 110°W a few days prior to an easterly wave trough axis in a few cases (consistent with FS00).

21. Longitude-Time Sections of Composite Mean 200-hPa V Anomalies (m s-1) (37.5-42.5N) Keyed to Yuma Surges • The composites do not show a strong relationship to eastward propagating MWW’s. • The composites do show a strong relationship to the strength and location of • the upper tropospheric monsoon anticyclone. • Wet: ridge axis to east Dry: ridge axis to west

22. Composite Evolution of 700-hPa Streamfunction Anomalies (m2 s-1 x106) and Vector Wind Anomalies (m s-1) for Moisture Surges Keyed to Yuma Wet Dry • Wet surges: • Ridge axis to the east. • The flow around the monsoon anticyclone phases with the flow around the tropical wave allowing a deep layer of tropical moisture to be advected into the region from the south and east. • Dry surges: • Ridge axis to the west. • The flow around the monsoon anticyclone “caps” the atmosphere inhibiting convective development, even when a shallow, moist southerly flow is present below 700-hPa. H H L L

23. Schematic of the 700-hPa Circulation (Heights and Winds) for Wet and Dry Moisture Surges Keyed to Yuma, AZ Wet Dry

24. Summary • Relationships between gulf surges and precipitation were examined. • Emphasis was placed on the relative differences in precipitation and atmospheric circulation patterns for several surge categories (weak, strong, wet, dry). • A significant fraction of the events in all categories are related to the passage of tropical easterly waves across western Mexico, but the explicit role of other types of tropical disturbances (e.g. tropical storms, upper-level inverted troughs and cyclones) was not considered. • The relative location of the upper-level anticyclone in midlatitudes at the time of the gulf surge affects the response to the surge in the southwestern United States.Ridge axis to the east  wetter-than-normal conditions in AZNM. • Ridge axis to the west drier-than-normal conditions in AZNM.

25. Summary, cont. • Wet surges:The flow around the monsoon anticyclone phases with the flow • around the tropical wave allowing a deep layer of tropical moisture to be • advected into the region from the south and east. • Dry surges:The northerly flow on the east side of the monsoon anticyclone • “caps” the atmosphere inhibiting convective development, even when a shallow, • moist southerly flow is present below 700-hPa. • Given the persistent orographic forcing for vertical motion always present in the • core monsoon region, it is likely that an important driver for precipitation is how • surges affect the intensity and areal extent of boundary layer CAPE, though this • was not investigated.