An Updated Climatology of Tropical Cyclone Impacts on …ritchie/hurrlab/pdf/WoodRitchie2013.pdf · An Updated Climatology of Tropical Cyclone Impacts on the Southwestern United States

An Updated Climatology of Tropical Cyclone Impacts on the Southwestern United States

KIMBERLY M. WOOD AND ELIZABETH A. RITCHIE

Department of Atmospheric Sciences, The University of Arizona, Tucson, Arizona

(Manuscript received 7 March 2013, in final form 7 August 2013)

ABSTRACT

A dataset of 167 eastern North Pacific tropical cyclones (TCs) is investigated for potential impacts in the

southwestern United States over the period 1989–2009 and evaluated in the context of a 30-yr climatology.

The statistically significant patterns from empirical orthogonal function (EOF) analysis demonstrate the

prevalence of amidlatitude trough pattern when TC-related rainfall occurs in the southwesternUnited States.

Conversely, the presence of a strong subtropical ridge tends to prevent such events from occurring and limits

TC-related rainfall toMexico. These statistically significant patterns correspondwell with previous work. The

El Ni~no–Southern Oscillation phenomenon is shown to have some effect on eastern North Pacific TC impacts

on the southwestern United States, as shifts in the general circulation can subsequently influence which re-

gions receive rainfall fromTCs or their remnants. The Pacific decadal oscillationmay have a greater influence

during the period of study as evidenced by EOF analysis of sea surface temperature anomalies.

1. Introduction

The eastern North Pacific is the densest region of

tropical cyclogenesis in time and space (Molinari et al.

2000). Most tropical cyclones (TCs) in this basin form

near the coast and are eventually steered westward by

a subtropical ridge extending from North America over

the eastern North Pacific for much of the season (Fig. 1),

where they generally dissipate over cooler sea surface

temperatures (SSTs). However, many of these systems

impact the coast before moving away from land, bring-

ing winds and heavy rain even as the center remains over

the ocean. When synoptic conditions are favorable,

eastern North Pacific TCs make landfall, and the large-

scale flow can bring these systems or their remnant mois-

ture far inland even though the TC itself has weakened

(e.g., Ritchie et al. 2011, hereafter R11).

Upon interaction with the complex topography of

Mexico and the southwestern United States, this advec-

ted moisture can result in locally heavy precipitation

during the summertime rainy season associated with the

North American monsoon (NAM; Adams and Comrie

1997; Higgins et al. 1997).Midlatitude trough interactions

(R11; Corbosiero et al. 2009) or the influence of a strong

subtropical ridge (R11; Wood and Ritchie 2012) are

generally responsible for eastern North Pacific TCs or

their remnants entering the southwestern United States.

However, it is difficult to predict the pattern and in-

tensity of rainfall that will result from a TC remnant, as

many factors impact the nature of the remnant’s in-

teraction with the midlatitude flow. Examples include

the strength of theTCwhen it interacts with amidlatitude

trough, the location of the TC with respect to the trough,

the strength and meridional extent of the trough, and the

strength of the subtropical ridge over southwesternNorth

America.

This study briefly examines a 30-yr climatology of the

large-scale flow in the eastern North Pacific over the

period 1981–2010 in order to establish climate-scale

context. It also builds on the climatology of TCs af-

fecting the southwesternUnited States described byR11

through the use of a single global reanalysis dataset and

a period of study expanded from1992–2005 to 1989–2009.

Over this 21-yr period, the number of tracked cases in-

creases from 43 to 167 TCs, producing a robust sample

for statistical analysis. Objective analysis is applied to

determine the statistically significant patterns that lead

to TC-related impacts on North America. Section 2

describes the data and methods used in the study, sec-

tion 3 provides a background climatology of the eastern

North Pacific hurricane season over 30 yr and NAM

Corresponding author address:Kimberly M.Wood, Department

of Atmospheric Sciences, The University of Arizona, P.O. Box

210081, Tucson, AZ 85721-0081.

E-mail: [email protected]

4322 MONTHLY WEATHER REV IEW VOLUME 141

DOI: 10.1175/MWR-D-13-00078.1

� 2013 American Meteorological Society

mailto:[email protected]

precipitation over 21 yr, section 4 evaluates large-scale

patterns associated with TC-related impacts and exam-

ines them in light of the seasonal climatology, section 5

discusses the rainfall patterns associated with these sys-

tems, and section 6 presents a summary and conclusions.

2. Data and methodology

a. Data

The global, 6-hourly, T255 (nominally 0.78) resolutionInterim European Centre for Medium-Range Weather

Forecasts (ECMWF) Re-Analysis (ERA-Interim; Dee

et al. 2011) is used to investigate atmospheric fields as-

sociated with eastern North Pacific TC remnants. The

ERA-Interim data are obtained from theResearchData

Archive (RDA) maintained by the Computational and

Information Systems Laboratory at the National Center

for Atmospheric Research (NCAR; http://rda.ucar.edu).

Monthly mean data derived from the ERA-Interim are

also obtained from the NCARRDA in order to examine

seasonal circulation patterns.

Daily, 0.258 global gridded SST data (Reynolds et al.

2007) for the period 1982–2010 are obtained from the

Earth System Research Laboratory (ESRL) Physical

Sciences Division hosted by the National Oceanic and

Atmospheric Administration (NOAA; http://www.esrl.

noaa.gov/psd/). Monthly, 28 SST data (Smith and

Reynolds 2004) for the period 1981–2010 are also ob-

tained from NOAA/ESRL. Climate Prediction Center

(CPC) unified U.S.–Mexico daily gridded precipitation

data at 18 spatial resolution are used to examine sea-

sonal precipitation totals and precipitation associated

with each TC remnant. Geostationary water vapor (WV)

imagery, obtained from theGlobal International Satellite

Cloud Climatology Project B1 Browse System (http://

www.ncdc.noaa.gov/gibbs/) at 3-h intervals, are used to

track each remnant.

All TCs that moved within 550 km of the Mexican or

Californian coastlines (Englehart and Douglas 2001) in

the National Hurricane Center best track (Davis et al.

1984) from 1989 to 2009 are examined to evaluate their

impacts on North America. In addition, all TCs during

this period that did not come within 550 km of the coast

but reached 258N are tracked in WV satellite imagery in

order to assess whether or not any remnantmoisture was

advected over land. This produced 167 cases over the 21-yr

period. Every TC in this database is tracked to determine

FIG. 1. ERA-Interim monthly average 500-hPa geopotential heights (m) over the period 1981–2010 for (a) June,

(b) July, (c) September, and (d) October. Note that the pattern in August (not shown) greatly resembles the pattern

in July.

DECEMBER 2013 WOOD AND R I TCH IE 4323

http://rda.ucar.edu

http://www.esrl.noaa.gov/psd/

http://www.esrl.noaa.gov/psd/

http://www.ncdc.noaa.gov/gibbs/

http://www.ncdc.noaa.gov/gibbs/

the time and location of the storm at which it interacted

with the large-scale flow. This point is commonly found

when the TC recurved (the direction of motion changed

from northwestward to northeastward) or otherwise

changed its direction of motion and is thus denoted the

‘‘turning point.’’ The 43 cases examined in R11 are in-

cluded in this database.

Following a similar methodology to R11, the moisture

associated with each case is tracked inWV imagery until

the clouds associated with the TC or TC remnant dissi-

pated or could no longer be distinguished from other

features. The resulting track is used to extract an associ-

ated swath of rainfall from the unified U.S.–Mexico CPC

precipitation dataset. Precipitation patterns are then ex-

plored using these swaths. As rainfall contributions from

three pairs of TCs could not be attributed to a single

system, this dataset comprises 164 swaths.

b. Statistical analysis

Patterns in the eastern North Pacific are explored using

empirical orthogonal function (EOF) analysis, a method

that objectively determines those patterns that explain

the maximum amount of variance in a two-dimensional

dataset (e.g., Hannachi et al. 2007). The full basin is

investigated for seasonal patterns, and storm-centered

fields spanning 408 latitude and 508 longitude are ex-

tracted from the ERA-Interim using the turning point for

each case in order to evaluate the large-scale flow asso-

ciated with each remnant that impacted North America.

To obtain the EOF patterns, singular value decom-

position (SVD; Bretherton et al. 1992; Hannachi et al.

2007) is applied to the anomalies of a given field, such

as 500-hPa geopotential height. In this study, the mean

500-hPa geopotential height field from the storm-

centered dataset is used to compute these anomalies.

The North test (North et al. 1982) is used to assess the

95% confidence error of the resulting EOF patterns. A

physical representation of the anomalies denoted by

each EOF pattern is obtained by regressing the nor-

malized principal components (PCs) onto the original

data. The normalized PCs can also be regressed onto

other fields, such as temperature and wind, to examine

their related patterns.

Unlike 500-hPa geopotential height patterns, standard

EOF analysis of precipitation fields tends to produce

highly localized, bull’s-eye-type features due to the het-

erogeneous nature of rainfall distributions in space. To

minimize these unphysical results, EOFs can be rotated;

that is, the orthogonality constraint is relaxed in order to

better represent fields with large spatial variability and

zero value regions (Hannachi et al. 2007). In the current

study, the varimax criterion (Kaiser 1958; Sherin 1966),

given by

V51

4�m

j51

" �k

i51

a4ij

!2

1

k

�k

i51

a2ij

!2#, (1)

is applied to the input matrixA in order to create rotated

EOFs (REOFs). In Eq. (1), V is the resultant rotated

matrix, A comprises the EOFs of the rainfall swath data

found by performing SVD, a refers to components of A,

and j and k refer to rows and columns of A, respectively.

Maximum covariance analysis (MCA; Bretherton

et al. 1992; Wallace et al. 1992) is also used in this study

to explore relationships between the storm-centered

500-hPa geopotential height patterns and the associated

rainfall swaths.MCAdecomposes the covariancematrix

between two datasets via SVD in order to objectively

find structures in each dataset that correspond with

structures in the other dataset. The resulting patterns

thus explain the maximum amount of covariance be-

tween the two sets of data. The 500-hPa geopotential

height fields at the turning point time are used in this

analysis in order to assess the impact of the interaction

on the subsequent rainfall pattern.

3. Seasonal climatology

a. Tropical cyclone activity

Figure 1 shows the dominance of the subtropical ridge

overMexico and the southern United States for much of

the hurricane season. As a result, most eastern North

Pacific TCs gain a strong westward component to their

motion, with over 80% of the 463 named systems from

1981 to 2010 never moving north of 258N (e.g., Fig. 2).

However, as summer transitions into autumn, this ridge

weakens and retreats southward (Figs. 1c,d), increasing

the chance for a TC to interact with the midlatitude

flow. The likelihood for recurvature is much greater in

September and October, as these two months account

for just 37% of all TC activity but 58% of all recurving

TCs in the 1981–2010 period.

The seasonal activity peak occurs inAugust, and nearly

70% of the TCs during the 1981–2010 period developed

during July, August, and September (Fig. 3). However,

there is considerable year-to-year variability in TC ac-

tivity, with one of the least active years on record, 2010,

developing no TCs during the month of July. The low

activity in 2010may have been related to a developing La

Ni~na event (Stewart and Cangialosi 2012).

Because of the influence of the El Ni~no–Southern Os-

cillation (ENSO; Trenberth 1997) on SSTs in the eastern

North Pacific, warm and cool events are chosen for com-

parison by setting a Ni~no-3 (58S–58N, 1508–908W) July–

September (JAS) threshold anomaly value of greater than

0.58C or less than 20.58C, respectively. This produced six


El Ni~no events (1982, 1987, 1991, 1997, 2002, and 2009)

and seven La Ni~na events (1981, 1985, 1988, 1995, 1999,

2007, and 2010).

An average of 16.2 TCs ($34 kt; 17m s21) developed

per year during El Ni~no events compared with an av-

erage of 12.4 TCs during La Ni~na events. This is not

a statistically significant difference at the 95% signifi-

cance level. The 17 ENSO-neutral years averaged 16.4

TCs per year. This higher average compared with the La

Ni~na events is partially explained by a low-to-high ac-

tivity shift in 1982 followed by a shift back to lower ac-

tivity in 1999 (Zhao and Chu 2006). The low activity

periods thus include four of the seven La Ni~na events

from 1981 to 2010. In addition, it corroborates previous

work that found little difference in TC frequency between

ENSO and ENSO-neutral years (e.g., Whitney and

Hobgood 1997).

More TCs are able to track farther westward and

northward during El Ni~no years (Fig. 2b) versus LaNi~na

years (Fig. 2c). Note that more tracks are clustered near

the Mexican coastline during the La Ni~na hurricane

seasons (Fig. 2c). This distribution corresponds well with

previous work on ENSO and eastern North Pacific TC

tracks (e.g., Irwin and Davis 1999; Chu 2004; Camargo

et al. 2008). Monthly genesis frequency is shifted to

earlier in the season during La Ni~na years and slightly

later in the season during El Ni~no years (Fig. 3).

Anomalies in July–September 300–700-hPa geopoten-

tial thickness fields (not shown) support these differ-

ences. Greater thicknesses in the genesis region tend to

steer TCs farther westward during El Ni~no years, and

the midlatitude troughs that extend farther south have

a higher chance of eventually interacting with a TC.

Conversely, lower thicknesses in the genesis region and

a weakness in the ridge during La Ni~na years can lead to

more TCs approaching the Mexican coast. This cor-

roborates previous work by Gutzler et al. (2013) who

found that near-coastal cyclones occurred with greater

frequency during the early months (May–July) of La

Ni~na years. In addition, a weakness in the subtropical

ridge during the month of September can allow more

TCs to track northward (not shown), such as Hurricanes

Ismael (1995) and Henriette (2007).

FIG. 2. (a) All 463 TC tracks from 1981 to 2010. (b) Average El

Ni~no minus average 1981–2010 gridded tracks. (c) Average La

Ni~na minus average 1981–2010 gridded tracks.

FIG. 3. Percentage of annual genesis events by month for TCs

that reached wind speeds of 34 kt (17m s21) or greater over the

period 1981–2010. Black represents all years, dark gray represents

El Ni~no years, and light gray represents La Ni~na years.


b. Precipitation in southwestern North America

The bulk of TC-related rainfall during the NAM

season (15 June–30 September) occurs along the west-

ern coast of Mexico, with secondary maxima in Arizona,

New Mexico, and Texas (not shown). However, these

states receive an average of 10% or less of their NAM

precipitation from TCs each year, while northwestern

Mexico and regions of California derive a much larger

portion from these systems (Fig. 4a). Though the aver-

age annual contribution is low, the amount of NAM

rainfall derived from TCmoisture can vary greatly from

year to year. For example, some parts of Arizona re-

ceived half of their monsoon precipitation from a series

of TCs in 1993 (Fig. 4b). Locally heavy rainfall enhanced

by orography is common with these events.

Tropical cyclones can also contribute moisture after

the official end of the NAM season. During the rela-

tively dry October period, these systems can produce

significant amounts of rainfall. In some years an in-

dividual TC remnant is the source of much, if not all, of

the measured precipitation for the entire month. One

such example is Hurricane Raymond in 1989 (Fig. 4c),

which contributed over 80% of October’s rainfall to

some parts of Mexico, Arizona, and New Mexico.

Conversely, the rainfall associated with Hurricane

Kenna in 2002 occurred in Texas and east-central

Mexico even though this was also an October event

(Fig. 4d).

Though there is some uncertainty because of the

limited number of years in each composite, the regions

impacted by TC-related rainfall appear to shift slightly

over the 1989–2009 period with respect to ENSO events

(Fig. 5). During El Ni~no years, the average TC produces

less precipitation in northern Mexico and the south-

western United States but more precipitation in eastern

Mexico and parts of the central and eastern United

States (Fig. 5b). In La Ni~na years, average rainfall in-

creases in northern Mexico and parts of the southwest-

ern United States but decreases along theMexican coast

south of Baja California (Fig. 5c). This precipitation

shift may be related to the average location and strength

of the subtropical high during warm and cool ENSOphases

(section 3a).

FIG. 4. Percentage of 15 Jun–30 Sep precipitation from TCs (a) over the period 1989–2009 and (b) only in 1993.

Percentage of October precipitation from TCs in (c) 1989 and (d) 2002.


4. Large-scale patterns associated with TCremnants

a. EOF analysis of geopotential heights

Table 1 provides the variance explained by each of

the first five EOFs found via SVD analysis of the

storm-centered 500-hPa geopotential height fields. The

first and fourth EOFpatterns were significant at the 95%

confidence level according to the North test and are

shown in Fig. 6. Note that the geography is centered on

the average turning point location of the 167 TCs in-

cluded in the analysis.

EOF1, which explains 41.8% of the variance in the

dataset, shows the TC approaching a midlatitude trough

through a break in the subtropical ridge in the positive

mode (Fig. 6a). To quantify regions of enhanced vertical

motion associated with a midlatitude trough that may

lead to increased thunderstorm activity when additional

moisture is advected into the region from a TC, Fig. 6e

shows the 400-hPaQ vectors associated with the positive

mode of EOF1. TheseQ vectors are calculated from the

EOFs of 400-hPa geopotential height and the regression

of those PCs onto the associated 400-hPa ERA-Interim

temperature fields. Because the convergence ofQ vectors

as calculated from the quasigeostrophic omega equation

represent forcing for ascent, they are useful for quanti-

fying regions of vertical motion (e.g., Holton 1992). The

400-hPa level is used to reduce the effects of topography

on the Q vector fields.

Conversely, the negative mode of EOF1 depicts a

strong subtropical ridge centered over northern Mexico

and the southwestern United States deflecting the TC

away from land (Fig. 6b). The associatedQ vector pattern

(Fig. 6f) shows a corresponding lack of support for ascent

in the upper troposphere due to the strong ridge.

EOF4, the other statistically significant pattern from

the 500-hPa geopotential height analysis, explains 9.0%

of the variance. The positive mode (Fig. 6c) resembles

the positive mode of EOF1, but the ridge is stronger and

the trough farther north in comparison. Again, there is

some Q vector support for upward motion at 400hPa in

the vicinity of the trough (Fig. 6g), but it is weaker than

that found in the positive EOF1mode. The negativemode

(Fig. 6d) shows a ridge centered over northern Mexico

and the southwestern United States and a midlatitude

FIG. 5. (a) Mean accumulated precipitation (mm) per TC for all

164 swaths. (b) Average deviation from the mean (mm) per TC

during four El Ni~no years (1991, 1997, 2002, and 2009; 32 TCs).

(c) Average deviation from themean (mm) per TC during three La

Ni~na years (1995, 1999, and 2007; 18 TCs).

TABLE 1. Percent variance explained by the five leading EOF

modes from analysis of storm-centered 500-hPa geopotential

heights 24 h before the turning point, at the turning point, and 24 h

after the turning point, as well as extracted rainfall swaths and SST

anomalies spanning the North Pacific Ocean.

500-hPa geopotential height

Rainfall

SST

anomalies224 h 0 h 124 h

1 40.0% 41.8% 47.7% 27.8% 13.2%

2 17.4% 17.0% 14.9% 24.9% 7.3%

3 15.7% 14.5% 11.5% 10.7% 5.5%

4 8.4% 9.0% 9.8% 9.3% 4.8%

5 3.7% 3.4% 2.8% 8.8% 4.3%

Total 85.2% 85.8% 86.7% 81.6% 35.1%


FIG. 6. 500-hPa geopotential height (m) for (a) EOF11, (b) EOF12, (c) EOF41, and (d)

EOF42. 400-hPa Q vectors (1 3 1025 Pam21 s21; barbs) and Q-vector convergence (1 310211 Pam22 s21; contours; divergence not shown for simplicity) computed by regression of the

geopotential height PCs for (e) EOF11, (f) EOF12, (g) EOF41, and (h) EOF42. Black dots in

each panel represent the location of the TC.


trough approaching from the west. Upward motion oc-

curs ahead of this trough (Fig. 6h).

EOF analysis was also performed on storm-centered

geopotential height fields from 24h before the turning

point (165 TCs) and 24h after the turning point (161

TCs). The full 167 TC dataset is not represented in each

because the circulation had yet to develop 24h before the

turning point for two systems or had dissipated 24h after

the turning point for five systems. The leadingmode from

each analysis is shown in Fig. 7, and the EOF1 patterns

from0h (Fig. 6) are repeated to show temporal evolution.

In the positive EOF1 pattern, a break in the ridge de-

velops 24h before the turning point, and the TC moves

largely northwestward (Fig. 7a). Most of the uplift asso-

ciated with the midlatitude trough is just beginning to

move over land at this time (Fig. 7d). The break in the

ridge intensifies at the turning point time (Fig. 7b), and

24h later the TC has turned to the northeast. Associated

upward motion occurs in a large swath that extends from

the TC, now merging with the midlatitude trough, across

much of the southwestern United States (Fig. 7f). Con-

versely, the negative EOF1 pattern exhibits a strong ridge

situated over southwestern North America 24h before

the turning point (Fig. 7g), which subsequently deflects

the TCwestward (Fig. 7h). The subtropical ridge remains

in place through 24h after the turning point time (Fig. 7i).

Little to no upward motion is observed during this se-

quence of events (Figs. 7j–l).

The statistically significant EOF patterns also com-

pare favorably with the subjectively identified patterns

found in R11 (Fig. 8). The positivemode of EOF1 shares

characteristics with groups 1 and 2 fromR11 (Figs. 8a,b).

Both groups were composed of TCs that recurved,

where group 1 TCs recurved farther north earlier in the

season and group 2 TCs farther south later in the season.

The negativemode of EOF4 resembles group 3 fromR11

(Fig. 8c), with a similar ridge and ridge weakness pattern

and a shallow troughwell northwest of the storm.None of

the statistically significant patterns provided a good

match to group 4 fromR11; instead, the negativemode of

EOF3 most closely resembled this subjective grouping

(Fig. 8d). This emphasizes the point that nonstatistically

significant EOF patterns may still have physical signifi-

cance. Though a ridge blocks the TC from affecting land

much like the negative EOF1 pattern, there is a trough to

the northwest of the TC in both the negative EOF3 and

group 4 patterns. Note that EOF3 explains 14.5% of the

variance in the dataset.

b. Comparison with climatological patterns

Figure 9 shows a time series of the TCs in the dataset

with an absolute normalized PC value of at least 1.0 for

the two significant EOF modes from the turning point

time, a subset comprising 85 systems (see Table 2 for

a detailed list of TCs).Most cases that correspond with the

negative EOF1 pattern (Figs. 6b,f) occur during the first

half of the season when the subtropical ridge is strongest

(Fig. 1). Conversely, the frequency of the other three

patterns is shifted to later in the season when the ridge

weakens and midlatitude troughs are able to protrude

farther south. The strongest trough interaction pattern

(the positive mode of EOF1) occurs most often during

September and October, which are the two months with

the highest overall frequency of recurvature.

An average of 4.0 TCs that met or exceeded the nor-

malized PC threshold of 1.0 occurred each year over the

21-yr period, ranging from 0.9 to 1.2 per pattern. How-

ever, there was a great deal of variability from year to

year, with 2004 and 2009 each having seven cases and

1999 having none (Fig. 9b). Compared with the 1981–

2010 average of 15.4 named storms per year, below-

normal seasonal activity occurred in 1999 (9 TCs) and

2004 (12 TCs). Conversely, above-normal activity oc-

curred in 2009 (17 TCs).

Four El Ni~no years (1991, 1997, 2002, and 2009) and

three La Ni~na years (1995, 1999, and 2007) discussed in

section 3 overlap with the 1989–2009 period. Though the

sample size is limited, some observations can be made

using these data. From the 85-case subset shown in Fig. 9,

except for 2002, all of the El Ni~no years had zero ridge

deflection events (e.g., Figs. 6b,f), and 2002 had only one

such case (Fig. 9b). Three total cases occurred per year

during each El Ni~no event save for 2009, which had seven

cases. Four total cases occurred during each of the La

Ni~na events of 1995 and 2007; both years had at least one

ridge deflection case. The sole year with no cases, 1999,

was also a La Ni~na year. None of the La Ni~na years pro-

duced cases from the positive EOF1 mode (Figs. 6a,e),

while the El Ni~no events of 2002 and 2009 did (one and

three TCs, respectively). Note that 1999 did have four

TCs that influenced North America, but none of them fit

the criteria for the 85-TC subset (Table 3). These results

imply that ENSOmay have some influence on the nature

of TC impacts in southwestern North America.

c. SST analysis and comparison with climatology

Though ENSO-related SST anomalies asmeasured by

the Ni~no-3 index do not appear to have a significant

impact on eastern North Pacific TC activity, the poten-

tial influence of SST anomalies elsewhere in the basin

should also be considered. For example, anomalously

cool SSTs northwest of the main genesis region may

have contributed to below-normal activity in 2004

(Avila et al. 2006).

EOF analysis is performed on the daily SST anomalies

derived from the 0.258 global gridded SST dataset using


subset fields that cover 108S–508N, 1208E–908W.Anomalies

are computed using the base period 1982–2011. The

analysis data are chosen based on the turning point day of

each of the 167 TCs. The first two EOFs are statistically

significant, and they explain 13.2% and 7.3% of the var-

iance in the dataset, respectively (Table 1).

Figure 10 shows the positive and negative modes of

these two EOFs. The positive mode of EOF1 (Fig. 10a)

FIG. 7. EOF11 (a)–(c) 500-hPa geopotential height (m) and (d)–(f) regressed 400-hPa Q vectors (1 3 1025 Pam21 s21; barbs) and

Q-vector convergence (13 10211 Pam22 s21; contours; divergence not shown for simplicity). EOF12 (g)–(i) 500-hPa geopotential height

(m) and ( j)–(l) regressed 400-hPa Q vectors and Q-vector convergence. At (left) 224, (middle) 0, and (right) 124 h. Black dots in each

panel represent the location of the TC.


FIG. 8. 500-hPa geopotential height (m) EOF patterns (shaded) and composite fields (contours)

for (a) EOF11 and group 1, (b) EOF11 and group 2, (c) EOF42 and group 3, and (d) EOF32 and

group 4. The dots represent the location of the composite storm. Composite rainfall swaths (mm)

for (e) group 1, (f) group 2, (g) group 3, and (h) group 4. Group composites are from R11.


is characterized by warm SST anomalies along much of

the equator and the western coast of North America,

though the waters off southern Mexico and Central

America are near normal. Cool anomalies cover much

of the northwestern Pacific above 308N, and the overall

pattern resembles that of the Pacific decadal oscillation

(PDO;Mantua et al. 1997). The negative mode of EOF1

(Fig. 10b) is characterized by near-normal conditions

across most of the eastern North Pacific, with one small

cool anomaly near Mexico, and extensive warm SST

anomalies in the western North Pacific, particularly

north of 308N. A distinct El Ni~no–like signature is ap-

parent along the equator in the positive mode of EOF2

(Fig. 10c). The negative mode of EOF2 depicts cool

anomalies confined to the eastern North Pacific genesis

region south and west of Mexico (Fig. 10d).

Further insight can be gained by examining the

subset with absolute normalized PC values of at least

1.0 (Table 4). All of the cases in 1997 and all but one case

in 2009 correspond with the positive mode of EOF2,

which were both strong El Ni~no years. The positivemode

ofEOF1also includes a number of cases from theElNi~no

years of 1991, 1997, and 2002. Conversely, the negative

modes of each EOF include many cases from La Ni~na

years. No El Ni~no cases correspond with the negative

EOF modes, and no La Ni~na cases correspond with the

positive EOF modes.

However, there are a number of included seasons,

notably the years with at least 10 cases (1989, 1990, 1992,

2000, and 2008), which are ENSO-neutral years. This

implies that ENSO may not exert the most significant

influence on the frequency of TCs affecting the south-

western United States. As briefly mentioned earlier, the

EOF1 patterns (Figs. 10a,b) do resemble the PDO, and

Table 4 presents a general shift from the positive EOF1

pattern in the 1990s to the negative EOF1 pattern in the

mid-2000s. Though there is some fluctuation in the in-

termediate years, 2005–08 dominate the number of cases

that correspond with the negative mode. However, the

1989–2009 period of this study does not cover a full os-

cillation of the PDO. Further work is necessary to ex-

amine the connection between TC impacts on North

America and decadal to multidecadal oscillations such

as the PDO, as links have been previously found be-

tween the PDO and the NAM (e.g., Castro et al. 2001).

5. Rainfall patterns associated with TC remnants

The first four REOFs computed for the 164 rainfall

swaths in the dataset are shown in Fig. 11. Though var-

iations exist between each REOF, there is a dominant

pattern of rainfall along the coast of Mexico with di-

verging amounts of rain along a southwest-to-northeast

FIG. 9. Data from the 85 TC subset of cases with absolute PC

values of at least 1.0. (a) Turning point occurrences by month for

each mode of the two statistically significant EOFs. Black repre-

sents EOF1 and gray represents EOF4. Positive modes are solid,

negative modes dashed. (b) Number of TCs in the dataset sepa-

rated by year: total cases (black), cases with a trough present (dark

gray), and cases with only a ridge present (EOF12; light gray).

TABLE 2. Individual TCs with absolute normalized PC values of

at least 1.0 for each statistically significant 500-hPa geopotential

height EOF mode. This table includes a total of 85 cases.

EOF11 EOF12 EOF41 EOF42

1989 Octave 1989 Barbara 1989 Raymond 1989 Priscilla

1992 Virgil 1989 Flossie 1990 Kenna 1990 Douglas

1992 Winifred 1989 Narda 1990 Rachel 1990 Lowell

1992 Zeke 1990 Boris 1991 Hilda 1990 Vance

1994 Rosa 1993 Irwin 1991 Nora 1991 Linda

1996 Fausto 1995 Gil 1992 Lester 1994 Ileana

2000 Norman 1996 Boris 1993 Calvin 1995 Henriette

2002 Kenna 1996 Douglas 1993 Lidia 1995 Juliette

2003 Kevin 1998 Blas 1995 Ismael 1997 Blanca

2004 Agatha 1998 Celia 1998 Madeline 1997 Nora

2004 Javier 1998 Frank 2000 Bud 1997 Rick

2004 Lester 2000 Hector 2002 Iselle 1998 Lester

2005 Otis 2000 Ileana 2004 Blas 2000 Miriam

2006 Miriam 2002 Genevieve 2004 Frank 2001 Adolph

2006 Norman 2003 Ignacio 2005 Hilary 2001 Manuel

2008 Norbert 2004 Georgette 2007 Ivo 2004 Howard

2008 Odile 2005 Calvin 2007 Kiko 2006 Paul

2009 Olaf 2005 Dora 2009 Blanca 2008 Douglas

2009 Patricia 2005 Eugene 2008 Julio

2009 Rick 2005 Irwin 2009 Andres

2006 Carlotta 2009 Ignacio

2006 Gilma 2009 Jimena

2006 Ileana

2007 Gil

2007 Henriette


track extending into the United States. Only REOF1 is

statistically significant at the 95% confidence level, and

it explains less than 28% of the variability in the dataset.

Also, none of the patterns appear to correspond with the

presence of the ridge in the negative EOF1 mode (Fig.

6b).

In contrast to the composite rainfall swaths presented

in R11 (Figs. 8e–h), where group members were de-

termined in part by the nature of each case’s corre-

sponding rainfall pattern, the analyses of large-scale

circulation patterns and rainfall swaths in this study are

accomplished independently through objective statisti-

cal EOF and REOF analysis. The simplest way to make

a physical connection between a particular large-scale

pattern and a given rainfall swath is to examine the in-

dividual cases for correlations. An alternative, objec-

tive method is to use the MCA methodology described

in section 2 to evaluate the covariance between the

500-hPa geopotential height patterns and the rainfall

swaths. Though the geopotential height fields are taken

from a single time, unlike the multiple-day accumulated

rainfall swaths, this comparison can provide information

regarding the subsequent precipitation resulting from

a particular type of interaction between the TC and the

large-scale flow.

The first MCA pattern (Fig. 12) accounts for 79.4% of

the covariance between the 500-hPa geopotential height

patterns and the rainfall swaths. The positive mode of

the geopotential height field (Fig. 12a) features a weak-

ness in the subtropical ridge and a southward-protruding

midlatitude trough, much like the positive modes of

EOF1 and EOF4 from the geopotential height EOF

analysis (Figs. 6a,c). The corresponding rainfall pattern

(Fig. 12c) resembles the average rainfall swath computed

from all group 1 and 2 cases in R11 (not shown). Regions

of greater precipitation are associated with mountainous

terrain and with additional moisture advection from the

Gulf of Mexico east of the Rocky Mountains. The Q

vectors (not shown) depict support for upward motion in

the vicinity of the trough.

The geopotential height pattern given by the negative

mode of MCA1 (Fig. 12b) includes a subtropical ridge

centered over northernMexico and the southern United

States. Though it is weaker than the ridge found in the

TABLE 3. Breakdown of all 167 TCs in the 1989–2009 dataset by

year as well as the subset of 85 TCs listed in Table 2. Boldface text

denotes El Ni~no years and italicized text denotes La Ni~na years.

Total EOF11 EOF12 EOF41 EOF42

1989 12 1 3 1 1

1990 11 0 1 2 3

1991 8 0 0 2 11992 11 3 0 1 0

1993 7 0 1 2 0

1994 4 1 0 0 1

1995 7 0 1 1 2

1996 7 1 2 0 0

1997 8 0 0 0 3

1998 7 0 3 1 1

1999 4 0 0 0 0

2000 10 1 2 1 1

2001 7 0 0 0 2

2002 8 1 1 1 02003 8 1 1 0 0

2004 7 3 1 2 1

2005 7 1 4 1 0

2006 9 2 3 0 1

2007 7 0 2 2 0

2008 10 2 0 0 2

2009 8 3 0 1 3

All 167 20 25 18 22

FIG. 10. The two statistically significant patterns from the EOF

analysis of daily SST anomalies (8C): (a) EOF11, (b) EOF12,

(c) EOF21, and (d) EOF22.


negative mode of EOF1 (Fig. 6b), the ridge in the MCA

pattern also extends far westward over the Pacific

Ocean. It does bear similarity to the group 4 pattern

(Fig. 8d) and thus to the negative mode of EOF3. The

corresponding precipitation pattern (Fig. 12d) restricts

most of the rainfall to the western coast of Mexico with

limited precipitation occurring in theUnited States. This

implies that most cases involving a strong subtropical

ridge primarily produce rain in Mexico, but that mois-

ture advection by the subtropical ridge can result in

precipitation in the southwestern United States.

6. Summary and conclusions

This paper presents an expanded 21-yr climatology of

eastern North Pacific TC impacts on the southwestern

United States. Confirming results found in the refereed

literature, ENSO events are not observed to have a sig-

nificant impact on the frequency of TC genesis in the

eastern North Pacific, though the subsequent tracks of

theTCs that do formmaybe altered by shifts in the general

circulation. La Ni~na tracks tend to cluster near the North

American coast, particularly earlier in the season, while El

Ni~no tracks often move farther westward and may turn

TABLE 4. Number of cases per year that correspond with nor-

malized PC values at or greater than 1.0 or at or less than21.0 for

the first twoEOFs from the SST anomaly analysis. Years not shown

had no cases that fit the criteria. Boldface text denotes El Ni~noyears and italicized text denotes La Ni~na years.

EOF11 EOF12 EOF21 EOF22

Year

No. of

cases Year

No. of

cases Year

No. of

cases Year

No. of

cases

1991 4 1998 5 1997 8 1989 5

1992 5 1999 4 2003 2 1992 10

1993 5 2000 5 2005 2 1993 6

1997 8 2001 3 2006 7 1995 2

2002 3 2005 1 2008 8 1999 4

2004 3 2007 4 2009 7 2000 2

2008 9 2007 1

FIG. 11. Rainfall swath patterns (mm) for (a) REOF1 (27.8%), (b) REOF2 (24.9%), (c) REOF3 (10.7%), and

(d) REOF4 (9.3%).


northward farther from land. These track shifts may also

affect which regions are impacted by TC-related rainfall

during ENSO events.

The statistically significant geopotential height EOF

patterns highlight the importance of midlatitude trough

interactions on subsequent TC-related impacts in the

southwestern United States. These interactions are

more frequent later in the season when the subtropical

ridge weakens and troughs protrude southward. Con-

versely, cases that involve a strong subtropical ridge often

approach the Mexican coast without producing rainfall

in the United States. TCs are deflected by the ridge more

often earlier in the season when this feature is strongest.

TheQ vector fields are indicative of upward forcing in the

vicinity of the trough; upward motion is suppressed for

cases deflected by the ridge. The objectively deter-

mined EOF patterns also support the findings presented

in R11.

Trough interaction events appear to be somewhat

more frequent and ridge deflection events less frequent

during El Ni~no years. Conversely, ridge deflection events

occur with greater frequency and trough interaction

events with less frequency during La Ni~na years. The

statistically significant SST anomaly patterns related to

TC impacts highlight anENSO signal in the secondEOF

mode, but the PDOmay have a greater influence as shown

by the first EOFpattern of SST anomalies.A longer period

of study is necessary to properly examine the relationship

between TC-related impacts and the PDO.

REOF analysis of individual rainfall swaths do not

clearly distinguish between different patterns. Instead,

that analysis highlights an overall trend of rainfall along

the Mexican coast with lower accumulations in the

United States. The influence of a midlatitude trough or

the subtropical ridge on the subsequent rainfall pattern

is clearer in the MCA analysis results. More pre-

cipitation occurs in northern Mexico and the United

States when a trough is present. Ridge deflection cases

produce less precipitation in the same region, and rain-

fall that does occur north of Mexico during these events

FIG. 12. First MCA pattern of 500-hPa geopotential heights (m) for (a) the positive mode and (b) the negative

mode. The dots represent the location of the composite storm. FirstMCApattern of rainfall (mm) for (c) the positive

mode and (d) the negative mode.


tends to be a result of moisture advection around the

periphery of the subtropical ridge.

Future work will investigate the variability of large-

scale patterns in the eastern North Pacific on interannual

and longer time scales in order to better characterize the

influence of longer period oscillations, particularly the

PDO, on eastern North Pacific TC impacts on North

America.

Acknowledgments. The research presented in this

manuscript is based on work done as part of the first au-

thor’s dissertation. The authors would like to acknowledge

Drs. Katherine Hirschboeck and Yolande Serra for their

constructive comments during the course of this research.

Insightful feedback from Dr. Ron McTaggart-Cowan and

two anonymous reviewers helped improve this manu-

script. This work was supported by a grant from the

National Science Foundation (Grant AGS-1132131).

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