Relationships Between Gulf of California Moisture Surges and Tropical Cyclones

in the Eastern Pacific Basin

Wayne Higgins and Wei ShiClimate Prediction Center / NCEP

29th Climate Diagnostics and Prediction WorkshopMadison, WI

18-22 October, 2004

Objectives

• To examine relationships between Gulf of California moisture surges and tropical cyclones using standard surface observations, the Regional Reanalysis circulation and moisture fields, tropical cyclone track data, and observed precipitation.

• To distinguish the precipitation and atmospheric circulation patterns for several categories of surge events, including those that are directly related, indirectly related and not related to TC’s.

Data Set Selection

• Hourly Surface observations at Yuma, AZ• Td, surface wind speed, and surface wind direction

• Precipitation • US_Mexico daily precipitation analysis (Higgins et al. 2000)• Resolution (lon, lat)=(1.0°x1.0°)

• Tropical Cyclone (HURDAT) track data (NHC)

• Atmospheric Circulation and Moisture• NCEP Regional Reanalysis (Mesenger et al. 2004)

• Period of Analysis: July-August 1977-2001*

* except 1992 in surface observations and 1977-78 in RR

Identification of Surge Events

Application of method given in Fuller and Stensrud (2000)

Onset Characteristics:

Rapid increase in surface Td > mean*

Surface winds > mean*

Surface wind direction ~ southerly

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

Td (˚C), Wind Direction (˚) and Wind Speed (m s-1) at Yuma, AZ July-August 1986

WW WW SSSS SS SS SS

* Note: Fuller and Stensrud (2000) identified the same set of events.

SS

Classification of Surge Events

Surges at Yuma were further further classified based on their relationships to eastern Pacific TC’s.

TC-related: A TC crosses 110W within 3 days of a surge at Yuma, AZ.

Direct Influence: If the TC center of circulation moves within 3° of Baja California, GOC, or Mexican mainland (poleward of 22 ° N) within 3 days of onset.

Indirect Influence: If the direct influence criterion is not satisfied.

Not TC-related: No TC crosses 110W within 3 days of a surge at Yuma, AZ.

TROPICAL CYCLONE TRACKS IN THE REGIONAL REANALYSIS

• The RR has realistic TC tracks that compare well to observations, so it is appropriate to use the circulation and moisture fields from RR to link the TC’s and surges at Yuma, AZ.

direct

indirect

mean direct (RR) mean indirect (RR)

mean direct (Obs.)

mean indirect (Obs.)

Number (%) of Yuma Surges by Category

Surge Category Number of Events

All 142

TC-Related 65

(46%)

Not TC-Related 77

(54%)

Direct 38

(58%)

Indirect 27

(42%)

Classification of Surge Events(Higgins et al. 2004)

Define strong, 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)

25-hr Running Mean Values of Td (˚C) at Yuma and Daily Precipitation Anomaly (mm) in AZNM* for July-August 1986

WW WW SSSS SS SS SS SS

DD WW WW WW WWDD DD DD

(W)eak or (S)trong(W)eak or (S)trong

Td

Precip

(W)et or (D)ry(W)et or (D)ry

* AZNM:Arizona/New Mexico (112.5°-107.5°W; 32°-36°N)

• 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.

Number (%) of Yuma Surges by Category

• 45% of the TC-related surges are strong and wet, while only 27% of the surges not related to TC’s are strong and wet • 50% of surges with a direct relationship to TC’s are strong and wet and 82% are strong.

Surge Category TC-Related

Not TC-Related

Direct Indirect

Strong and Wet 29 (45%) 21 (27%) 19 (50%) 10 (37%)

Strong and Dry 17 (26%) 14 (18%) 12 (32%) 5 (19%)

Weak and Wet 9 (14%) 18 (24%) 4 (10%) 5 (19%)

Weak and Dry 10 (15%) 24 (31%) 3 (8%) 7 (25%)

Composite Evolution of Precipitation Anomalies (mm) for Moisture Surges Keyed to Yuma

• All surges: SENW progression of +ve anomalies along the west coast • The positive precipitation anomalies are much larger and more widespread for the TC-related surges than for surges not related to TC’s.

Composite Evolution of Precipitation Anomalies (mm) for Moisture Surges Keyed to Yuma

• Surges that are directly related to TC’s have much larger +ve anomalies over most of

Mexico than those that are indirectly related.

Fraction (%) of Total Precipitation and Accumulated Precipitation Anomalies (mm)

July-August 1977-2001 base period

• The fraction of surge-related precipitation during TC-related surges is larger than that during surges not related to TC’s, yet TC-related surges account for less than half of the total surge events (65 of 142 events).

• Most of rainfall during TC-related surges occurs when the TC’s are directly related to Yuma surges, yet these cases are relatively rare (38 events over 25 years).

Fraction (%) of Total Precipitation and Accumulated Precipitation Anomalies (mm)

The low-level southerly flow is stronger and there is more low level moisture present in the GOC region for the TC-related surges than for surges not related to TC’s

Mean 925-hPa Vector Wind Anomalies (m s-1) and Specific Humidity Anomalies (g kg-1)

Mean 925-hPa Vector Wind Anomalies (m s-1) and Specific Humidity Anomalies (g kg-1)

Differences between direct and indirect cases are pronounced with moisture transport from the southeast along the entire GOC in the direct cases

• Surges that are directly related to TC’s have dramatically stronger low-level southerly flow and the most significant increases in moisture along the GOC. Specific humidity anomalies are as large as 25% of the mean value at Puerto Penasco.

Vertical Profiles of Wind Direction and Wind Speed Anomalies (tenths of knots) and Specific Humidity Anomalies (g kg-1)

• Surges with direct TC influences show a much stronger cyclonic signature in the wind field and larger moisture anomalies than those with indirect TC influences.

Vertical Profiles of Wind Direction and Wind Speed Anomalies (tenths of knots) and Specific Humidity Anomalies (g kg-1) along 110oW

850-hPa Mean Vector Wind (m s-1) and Specific Humidity (g kg-1)(July – August 1979-2001)

Summary

• Relationships between gulf surges and eastern Pacific Tropical Cyclones (TCs) were examined.

• Emphasis was placed on the relative differences in precipitation and atmospheric circulation patterns for several surge categories (TC-related, not TC-related, direct and indirect).

• TC-related surges account for slightly less than half of all surges.

• Surges that are directly related to TC’s have dramatically stronger low-level southerly flow and the most significant increases in moisture along the GOC. They occur on average about once per year, and are the most prolific rain producers.