Relationships Between Gulf of California Moisture Surges and Precipitation in the Southwestern Unite

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
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Outline

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

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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.

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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.

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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)(1.0x1.0)
• Atmospheric circulation
• 700-hPa and 200-hPa winds from NCEP/NCAR
  Reanalysis
• Period of Analysis July-August 1977-2001
• except 1992

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Yuma
Tucson
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Identification of Surge Events

• Application of method given in Fuller and
  Stensrud (2000)
• Onset Characteristics
• Rapid increase in surface Td gt mean (PRIMARY)
• Surface winds gt 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

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Identification of Surge Events

• Define strong, weak, wet, dry surges
• Strong Td gt climatological mean for at least 3
  days
• Weak Td gt 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
• AZNMArizona/New Mexico (112.5-107.5W
  32-36N)

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25-hr Running Mean Values of Td (C), Wind
Direction () and Wind Speed (m s-1) at Yuma, AZ
for July-August 1986
W
W
S
S
S
S
S
S
Note Fuller and Stensrud (2000) identified
the same set of events.
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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.
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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 3.0C (5.4F)
• Agrees well with climatology

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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
W
S
S
S
S
S
S
Precip
Td
(W)et or (D)ry
D
W
W
W
W
D
D
D

• 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.

• AZNMArizona/New Mexico (112.5-107.5W
  32-36N)

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Number () of Surges by Category

46 (37) of the surges identified at Yuma
(Tucson) are dry.
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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.

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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
• TEWs
• 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.

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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 (110W) precedes the passage of an
  easterly wave trough across western Mexico
  (110W) 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 TEWs and MWWs in an effort to explain the
  associated precipitation patterns.
• TEWS 700-hPa V (20-25N) MWWs 200-hPa V
  (37.5-42.5N)

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Longitude-Time Section of 700-hPa V Anomalies (m
s-1) (20-25N) During July-August 1986
easterly wave trough axis

• As the TEWs 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
  110W, then the two events are defined as being
  related.
• (FS00 criterion).

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Fraction (in ) of Surge Events at Yuma Related
to TEWs Crossing 110W by Surge Category.
Results Based on July-August 1977-2001

• Strong and wet surges are most often associated
  with TEWs.

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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 110W, but the
• southerly regime that follows the trough
  passage is stronger for the wet surges.

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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
TEWs
MWWs
easterly wave trough
westerly wave trough

• As MWWs 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 110W a few days prior to
  an easterly wave trough axis in a few cases
  (consistent with FS00).

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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 MWWs.
• 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

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

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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.