Tropical Cyclones of Thl Eastern North Pacific and Their Effects on the Climate of the Tern Unit Ed States

Tropical Cyclones of the Eastern North Pacific and Their Effects on the Climate of the Western United States: A Study of Circulation Features That May Be Recorded by Tree Rings, Final Report

Item Type text; Report

Authors Douglas, Arthur V.; Fritts, Harold C.

Publisher Laboratory of Tree-Ring Research, University of Arizona (Tucson, AZ)

Download date 30/09/2021 07:41:12

Link to Item http://hdl.handle.net/10150/302705 TROPICAL CYCLONES OF THL EASTERN NORTH PACIFIC AND THEIR EFFECTS ON THE CLIMATE OF THE TERN UNIT ED STATES

A Study of Circulation Features That May Be Recorded by Tree Rings

Final Report

NOAA Contract 1-35241

Second Year

Prepared by

Arthur V. Douglas and. Harold C. Fritts

Laboratory of Tree-Ring Research University of Arizona Tucson) Arizona 85721

Prepared for: Environmental Data Service National Oceanic and Atmospheric Administration United States Department of Commerce

Disclaimer:

Publication of this Technical Report does not constitute official Government approval of the report's findings or conclusions. Its contents reflect the views of the Con- tractor who is responsible for the facts and accuracy of the views presented herein, and do not necessarily reflect the views or policy of the Government. PREFACE

by H. C. Fritts

Dendroclimatology, the use of tree rings to reconstruct variations in past climate, is the only discipline that is presently capable ofproviding

quantitative information on the yearly and decadal variations in prehistoric

climate. Information about climate is contained in rings because physiological

processes which control ring growth and wood properties areaffected by climatic

factors. Also, rigorous procedures can be used to assure precise dating so

that the exact year each ring was formed can be established with certainty even

for very old trees. Such accuracy in dating allows the averaging of annual

growth from many trees so that the similarities in yearly growth responses

among trees are reinforced and thedifferences (usually due to non -climatic

factors) are minimized in the averaging process.

Such well dated and replicated proxy series of climate can be calibrated

by use of multivariate techniques with the variations in certainparameters of

historical climate. Transfer functions can be obtained by this procedure which

may be applied to the growth amountsduring years prior to the historical rec-

ords to calculate and reconstruct the past variations in the parametersof

climate.

However, reconstruction of a climate parameter from tree rings is of

limited value if we have little understanding of the climatologicalconditions

that cause the parameter to vary from one year to the next. We wish to identify

not only past periods of high or low moisture but also the featuresof atmospher-

ic circulation that caused the moisture to fall.

This report is the first of several studies aimed at understanding the

climatology affecting the trees of the North American west.Later papers will

deal with other features of climate and will describe applications where varia-

tions in ring widths of trees are used to reconstruct anomaliesof past climate. We hope to include estimates of anomalies in the general circulation as well as the specific climatic conditions affectingthe trees. In short, we feel confident that we have a good understanding of the prefix "dendro" and are now attempting to develop in a similar fashion anunderstanding of "climatology"

We recognize that there are certain details we have overlooked, but we hope our efforts will enhance the interpretation of our tree -ring analyses in terms mean- ingful to students of climate. INTRODUCTION

In an earlier paper by Douglas (1972) the summer climatology of tropical

storm development is reviewed with reference to Sea Surface Temperature (SST) distribution and upper- and lower -level winds.An apparent increase in yearly

storm totals recorded since 1965 is believed to be the direct result of satellite detection of small, well off -shore storms. However, monthly variations

in storm totals appear to be caused by anomalous SST either off Baja California

or along the equator west of South America. During the tropical storm season the region of greatest storm formation is found to shift towards the northwest

and then southeast. This regional variation in storm development may be caused by changes in SST and upper troposphere shearing off Baja California and in the movement of the Inter- Tropical Convergence Zone (ITCZ) off mainland Mexico.

Data presented by Douglas (1972) indicate that tropical storm formation

is most common during the months of July, August and September. During the

latter part of August through the first part of October, tropical storms can

enter the southwestern United States from either a track up the Gulf of

California or up the Pacific Coast of Baja California. This report will re-

view some additional circulation features associated with tropical storm

activity in the eastern North Pacific. The major emphasis will be directed

towards the effects of these storms upon the climatological conditions of the

southwestern United States.

MOVEMENT OF STORMS

The percentage frequency distribution of the direction of motion for all

eastern North Pacific tropical cyclones was determined for the 8 compass points

and for each 5° square of latitude and longitude during the period 19+7 -1971

(figs. 1 and 2). The direction and length of the lines represent the move-

-1- ,0. 147

11\ 00' IJ0 0 I3o

Figure 1. -- Percentage frequency distribution of the direction of motion of tropical cyclones by 5° squares for May, June, July and August. The number in the lower right hand corner represents the number of storms observed in each square. The length of each vector gives the percentage frequency of storms moving in each 45° sector centered at that direction. Dashed lines indicate that there are 2 or less observations per square. so.

10.

i / t /

;

1 1 1 1 16

. 6.-....1. ,.....11 8 17' 7 15 1 v 6 7-7 12 I 7 3 MO% 2j U-LAJ 50

Figure 2.--Same as figure 1 except for the months of September, October and November. ments of storms in each 5° square. Those represented by two or less storms are shown in dashed lines and the number of storms analyzed isindicated in the corner of each square. At the beginning and end of the tropical storm season, a greater variance in direction of movement of storms isindicated.

During the months of July and August, there is less variance in the direction of storm movement because there is more persistent steering of the stormsby the western extension of the Bermuda high. As a result of this persistent steering current and the positioning of the major source regions of storm development (Douglas, 1972, fig. 1), the storms move in a westerly or northwesterly direction. During these same months the points of recurvature to the north or northeast generally are not encountered south of 30 °N.Earlier in the season and at the end of the season the Bermuda high islocated farther south and therefore recurvature occurs at lower latitudes.

May. Very few storms have formed in May and the movement of thesestorms

is not persistent in any one direction, though west -to -northmovements do appear to be common (fig. 1).

June. A large number of storms have occurred during this month which marks the beginning of the period of major tropical storm activity in the

eastern North Pacific. East of 120 °W most of the storms move toward the

northwest, parallel to the coast (fig. I). In this same region, however,

some storms do move westward. The northwest direction of movement is associ-

ated with the flow on the west side of the upper-level Bermuda high. A due -

west direction of movement may occur when the Bermuda high is connected with

the North Pacific high and there is a resultant easterly flow in the atmos-

pheric circulation over the North American tropics. A small number of storms

pass west of 110 °W and north of 20 °N where cool SSTwould preclude storm forma-

tion as well as foster rapid storm deterioration.The 300 mb, 200 mb and 100 mb charts for the eastern North Pacific (Sadler, 1972) also suggest that upper tropospheric shearing is encountered in June west of 115 °W. Therefore both cool SST west of 110° and upper tropospheric shearing west of 115° appear to be responsible for the scarcity of storms west of 115 °W. Storm activity in

June appears to affect only those land areas south of Mazatlan (23 °N). No storms are believed to have entered southwestern United States during this

month. -

July. A larger number of storms are observed in July throughout the study region (Douglas, 1972, fig. 2). A westward penetration of storms south of 25 °N is most evident. This increase in westward movement is associated with the seasonal warming of the ocean to the south and west of Baja California.

Renner (1963) has shown that SST increases by as much as 6° in this region from Jurie to July, and farther south, near 17 °N, SST increases by 1° or 2 °F.

As a result of higher SST during July, a greater area of the eastern North

Pacific becomes favorable for the development and maintenance of tropical storms. In addition to these favorable surface changes in July, Sadler (1972) has shown that the upper -tropospheric westerlies shift westward of 120 °W, thus resulting in favorable conditions (non -shearing) for tropical storms in the region south of Baja California.

The movement of coastal storms in July north of 15 °N is predominantly northwesterly. Landfall of storms can occur in western Mexico, but no storms enter the western United States due to very cool SST north of 24 °N and tropospheric shearing over northern Baja California.

August. The concentration of storms during August, in comparison with

July, shows a decrease south of 20 °N and an almost twofold increase in storms travelling north of 20 °N (fig. 1). This increase in storms is associated with a slight northward movement of the !TCZ due to SST warming of3° to 4 °F between 20 °N and 30 °N east of 118 °W. In addition upper tropospheric shearing of storms probably does not occur until they pass north of 30 °N as indicated in Sadler's charts (1972).

The general movement of August storms is northwesterly, commonly along a line from 20 °N, 130 °W to 15 °N, 110 °W. A larger number of storms found west of

110 °W and north of 20 °N suggested higher SST and less upper tropospheric shearing in this region. The upper -level Bermuda high may also have a great influence in the eastern North Pacific at this time with a resultant steering of storms towards the northwest. Near the end of the month a greater number of northwest -moving storms could also be related to the southward digging of the temperate eastern Pacific troughs at the end of summer.These troughs result in the movement of air from the eastern North Pacific equatorial regions towards the north and thereby storms are steered in a more northerly direction than would be expected if the upper -level Bermuda high were controlling the steering. As will be shown later, this late season circulation feature results in the movement of tropical storms or their remains into the western United

States.

September. Movement of storms in September is more variable than those in August. North of 15 °N movements are towards the west to north (fig. 2).

The movement of storms parallel to Baja California is also more common than in

August. The failure of storms to dissipate farther south, as in August and

July, is probably associated with slightly warmer SST in September than in any of the previous (and subsequent) months. Sadler's (1972) upper troposphere charts would indicate, however, a strong shearing at most longitudes in the region north of 25 °N. Thus, while September SST's are at an optimum state to maintain a tropical storm off Baja California, tropospheric shearing across northern Baja California would tend to preclude the movement of intense tropical storms into the area. It should be noted, however, that periods with strong upper -level southwesterlies alternate with periods of strong westerlies, and under the former circulation, a tropical storm could "ride" the current into the Southwest.The stores need not be intense for even a decayed tropical stormor depression could be caught in such a southwesterly flow, and the attendant cloudiness and rain associated with the disturbance could bring significant precipitation to western United States.

October. There is considerable recurvature of storms towards the northeast during October and many storms enter western Mexico (fig. 2). There appears to be greater variation in the direction of movement of October storms even at quite low latitudes. This condition is a reflection of the breakdown and southward movement of the subtropical highs of both the North Pacific and the North Atlantic.

As noted by Douglas (1972) the sudden decrease in October storm activity is a result of midlatitude westerlies spreading south across the region of summer storm development. Sadler's (1972) charts for October indicate that upper -level westerlies and associated vertical shear are almost nonexistent south of 20 °N and east of 110 °W. North and west of this region, storms either recurve toward the east or they dissipate due to their upper -tropospheric shearing. The frequency of storms in each 5° square and their movement in areas north of 20 °N does indicate that considerable curvature of tracks occurs and dissipation is rapid. Storms off Mexico move more northerly and northeasterly and many make landfall in western Mexico.

November. As in May storms are rare in November and any persistent tendencies in their movement are difficult to ascertain. It appears from the charts (fig. 2) that all storms have moved in a westerly to northerly or northeasterly direction. These storms either dissipated upon landfall in Mexico, or over the ocean, due to strong tropospheric wind shearing, a feature common to the mean upper tropospheric charts of November.

STORM FREQUENCY

The frequency of tropical storms varies greatly from year to year by a factor approaching 2. In most years this variability is often the result of one or two months in which tropical storm activity is anomalous. Variations

in frequency of storms appear to be associated with the strength and areal distribution of the subtropical highs at the 700 mb level. The average monthly

700 mb circulations associated with maximum and minimum development of storms falling within given monthly classes have been constructed for the period

1963 -1971. A detailed analysis of each month is presented below.

June. The average number of storms developing during this month is two

(1963 -1971 average). Years in which the June total was > 3 storms were classed as years of above -normal development, while those with storm totals < 1 were classed as below -normal development. The average 700 nab pressure field of all years of above -normal development is shown in figure 3. Development of storms

is associated with below -normal pressure over Mexico and the northeastern tropical Pacific and a southeasterly flow of moist air south of 25 °N and east of 110 °W. In contrast, the Junes with below -normal storm development are

characterized by higher pressures aloft over Mexico and the eastern tropical

Pacific. This chart suggests possible moderate westerly flow could occur in

Mexico during these years (fig. 3). Higher- than -normal pressure over western

Mexico and the adjacent Pacific Ocean may be unfavorable for stormdevelopment because of tropospheric shearing aloft. The difference in the circulation for months of June classified as having maximum and minimum sto.m development

show more difference in circulation than those charts for any othermonth of the year. ó b b ó ti a N ---' £ / OS/F x OO/F .0 \0 OSO E o *O0-.Z1Ico / Ts- OO£ .' 00 02 X 000£ vra

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x 00 ..,.;43. ,h0 0 0 O x k '') ,/ bx

o Ó Ó N Ó July. An average of 3 storms occurs in July. Years with July storm totals of > 4 storms constituted above- normal activity, while those with < 1

storm constituted below -normal activity. The Julies experiencing above - normal activity are characterized by an above - normal belt of pressure at the 700 mb

level lying along 30 °N (fig. 3), This high pressure would produce a strong

southeasterly flow in the tropical eastern North Pacific - -a situation bringing moisture for storm development. Below- normal development of storms in July

is associated with a weakened subtropical high (fig. 3) and a possible trough

extending into the tropics near the 90 °W parallel.

August. An average of 3 storms occurs in August. If 2 or less storms

occurred, the month was considered to have experienced below- noLmal activity;

if 4 or more storms occurred, it was classified as a month with above- normal

activity. The average 700 mb chart for Augusts with above - normal activity is

characterized by a strong 700 mb subtropical high pressure ridge from the

Atlantic which terminates in the Southwest (fig. 4). The flow in the north-

eastern Pacific would, under this regime, be from the southeast, and thus

favorable for the formation of tropical storms. The average 700 mb chart

for Augusts with above -normal activity (fig. 4) does not show a strong ridge into

the Southwest from the Atlantic. Instead, a separate 700 mb high center is

found in west Texas with a trough to the east extending from the Mississippi

Valley into the Gulf of Mexico. Under this regime dry air from the Plains

(as described by Namias, 1938) would occasionally flow into eastern Mexico and

then across the pacific near 23 °N.This relatively warm dry flow could also

result in vertical shearing at various levels from the surface to 300 mb.

Storm formation under this regime would, therefore, be precludeddue to both

dry upper air and vertical shearing. An analysis of August rainfall at Mazatlan,

Mexico (near 23 °N), indicates that, under the drier northeasterly flow,the monthly precipitation averages about 2.00 in. less than under the more moist southeasterly flow. This same phenomenon could occasionally occur in July but was not sufficiently pronounced to show in thedata compiled for figure +.

September. An average of 3 storms occur in September. Years in which

September storm totals were < 2 were considered to have had below- normal activity, while those with 4 or more stor.us were considered to have hadabove - normal activity. The frequency of storm development in September appears to be associated with variations in the breakdown and southward movement ofthe summer subtropical highs. Months with above- normal activity are characterized by a continuance of the summer circulation from August(fig. 4). As in the other months with above -normal storm activity, the chart forabove - normal storm frequency in September indicates southeasterly winds in theeastern tropical North Pacific. Below- normal storm activity, however, is associated with a very broad west coast trough (fig.4). This trough may be responsible for vertical shearing west of 105 °W, a condition whichimpedes organization of disturbances into deep circulations. Normally, this is a region in which storm formation is common unless shearing is present.

UPPER-LEVEL WINDS AT MAZATLAN

An analysis of upper -level data for Mazatlan (1965 -1971) seemsto affirm the hypothetical circulation in eastern North Pacific asdiscussed in the last

section. During the June with no storm activity (1969), considerablevertical

shearing was evident from the 850 mb level to the 300mb level (fig. 5).

During the June with greatest storm activity(1970 - -3 storms), shearing was

not evident below the 500 mb level, and instead, a ratherdeep and uniform

flow was present.

The July with maximum storm frequency (1971 --7storms) was characterized

by moderate east -southeasterly winds above Mazatlan atthe 700 mb, 500 mb, and MAZATLAN - MAXIMUM STORM FREQUENCY 300-

500-

700

850

1010mb JUN, JUL. AUG. SEPT. 1970 1971 1968 1966

MAZATLAN- MINIMUM STORM FREQUENCY

500

700

850-

1010mb M- JUN. JUL. AUG. SEPT, 1969 1966 1969 1970

Figure 5.--Selected monthly upper-level winds for Mazatlan, Mexico, during periods of maximum and minimum tropical storm frequency 1963-1971. 300 mb levels. In contrast, in July, 1966, no storms were reported and the upper -level winds were from a more easterly direction (possibly drier) and at low speeds.

Only 2 storms occurred in August, 1969. The Mazatlan upper -level data for this month shows (fig. 5) that a strong flow from the northeast occurred, thus suggesting the transport of warm, dry air into the eastern tropical Pacific from the Mexican highlands and plains states of the United States. In addition to having been drier, the air mass itself may have been descending. In the

August with greatest storm activity (1968 - -8 storms), the upper -level flow was southeasterly at the 700 mb and 500 mb levels. Storms forming south of Mazatlan would, in this case, be favored by a fairly deep, moist flow with little or no shearing.

The September upper -level data for Mazatlan do not reflect conditions expected from the 700 mb charts just presented (fig. 5). Unexpectedly, the year with little storm activity in September (1970) shows a strong deep southeasterly flow at Mazatlan. This flow would supposedly be favorable for storm development. However, an examination of the SST chart for September, 1970, shows that negative departures covered a large area of the eastern tropical

Pacific. Presumably these low temperatures were a result of intense mixing of the surface waters by tropical storms and hurricanes during the previous months

(3 storms in June, 6 in July and 4 in August). Many of these storms traversed a section of the eastern North Pacific in which there is commonly ridging of the thermocline (Cromwell, 1958, fig. 1 -C).The thermocline in this ridge is very shallow and if it becomes subjected to wave induced vertical mixing, much cooler subsurface water can be brought to the surface resulting in SST which are below the critical value (80 °F) for tropical storm development. As a plausible result of the cooling of the ocean north of 10 °N, the ITCZ would probably shift southwards (or become less organized) over warmer waters lying equatorward of the region of previous storm activity. The cooler portion of the ocean would gradually undergo a period of warming and once it became warmer than 80 °F, tropical storm development could again become possible. In addition, the TTCZ would again shift northwards (if it had been farther south) or become more organized over the warmer region. Periods of tropical storm inactivity during periods when the upper -level flow is condensed for storm formation appears to be related to the development of other unfavorable conditions such as cool SST in a region which is normally very warm.

The upper -level data for September, 1966, indicate that during this period of maximum storm development, winds were quite light at all levels (fig. 5). In this case the subtropical high was centered very close to Mazatlan and thus the upper -level winds at this station were less representative of the tropical circulation to the south where southeasterly winds may have predominated. During this month storms formed and moved westward at more southerly latitudes than normal, a condition probably associated with the southward displacement of the upper -level high to a position near Mazatlan.

RECONSTRUCTING STORM FREQUENCY, 1921 -1940

The argument has arisen that since recent satellite coverage of storms has resulted in an almost twofold increase in named storms, actual numbers of storms reported by surface observations in the past are not representative of the total number. This seems to be a relatively valid argument considering that in 1970, of the 18 named storms, 9 were totally undetected by conventional surfacedata.

The perplexing problem arises, however, when the year1968 (the year of greatest storm activity) is considered. Of the 19 tropical storms during this year, every single storm was noted in a ship report at least onceduring the storm's existence. During the 6 -year period, 1966 -1971, of the 95 named storms,83% were detected by ships. As a result of this finding, it was believed that ship gale data were a fairly good indication of storm activity in the North

Pacific.

During the 1920's and 1930's, gale data were published in the Monthly

Weather Review. In analyzing the gale data a number of criteria were developed in order to classify gale reports as possible indicators of tropical storm activity. These criteria are presented in the appendix. In table 1 the data for 1921 -1940 are presented. This table indicates great year -to -year and month -to -month variability in storm frequency during the 1921 -1940 period, a condition also prevalent in the data from 1947 -1971 (table 2). Yearly storm activity was found to range from only 3 storms in 1924 to 19 storms in 1938.

Storm totals in the recent record ranged from 1 in 1948 to 19 in 1968. It can be argued that perhaps many of the newly detected (previously unrecognized) storms for the 1921 -1940 period were merely tropical depressions and, therefore, the monthly and yearly storm totals are possibly misleading.With the recent advent of satellite data, however, it should be pointed out that in 1969, when only 10 storms formed, there was not an overabundance of tropical depressions.

That is to say "inactivity" in the northeastern tropical Pacific may be the result of decreases in both tropical depression formation, as well as tropical storm formation. The presented storm analysis from 1921 -1940 may, therefore, be a good approximation of "activity" in the eastern North. Pacific.

At the beginning of the tropical storm season of 1939 (as well as 1953,

1957, 1966, and 1972) below- normal activity may have been the result of a warm

SST along the equator due to "El Niño" conditions a few months previous.Warm

SST along the equator would act as a modifier of cross -equatorial flows(Fujita et al., 1969) and as a result, few intense "bursts" would occur along the ITCZ.

Assuming that some storms form along this burst, their numbers would decrease Table 1.-- Tropical storms and gales during 1921 -1940

May June July August Sept. Oct. Nov. H H H H H H r-{ cüam c Total in u) cSU) úrh vm j H rn toH Tropical m + < á áá Year ; ó Ñ ó +°' ó . ó ó .° 0 + Ho -Storms ;AM r M d M 3 c7 E+ C7 É-+ cH É+ [

1921 0 0 0 0 00 22 11 1 1 1 0 4

1922 0 0 10 11 11 22 22 0 0 6

1923 1 1 22 3 3 11 11 42 4 0 10

1924 00 1 1 o 0 0 o 2 2 0 0 2 0 3

1925 o0 2 2 2 2 1 1 54 3 3 3 1 13

1926 3 0 1 1 3 3 2 2 2 2 3 1 4 0 9

1927 0 0 22 2 2 1 1 3 3 11 10 9

1928 2 2 2 2 2 2 1 1 5 5 2 2 1 0 14

1929 1 1 1 1 0 0 3 3 3 3 2 0 0 0 8

1930 o 0 1 1 2 2 2 2 1 1 4 4 1 0 10

1931 0 0 1 1 4 4 2 2 4 4 1 0 2 0 11

1932 3 o 1 1 o o 5 5 1 1 2 0 5 0 7

1933 1 1 1 1 3 3 1 1 2 2 1 0 3 0 8

1934 1 o 11 2 2 1 1 3 3 2 1 2 0 8

1935 o o 0 0 1 1 2 2 11 3 o 2 0 4

1936 o c 3 3 0 0 3 3 1 1 4 3 2 0 10

1937 1 1 2 2 1 1 1 1 3 3 3 3 2 0 11

1938 1 1 44 2 2 3 3 4 3 6 6 2 0 19

1939 o 0 2 2 3 2 1 1 3 3 3 1 5 o 9

1940 o o 11 5 5 2 2 3 3 3 2 2 1 14

o U\ in o o Ll\ l!\L!\ LnO OO oo Lr Mean t- rc) Cl} C- C-L- --1- \.O CU r-f re) r-i4 44 i--t r-i N CV N r-I al O\ Table 2.--Tropical storms, 1917 -1972

Year Number recorded per month Total

May June July August Sept. Oct. Nov.

1947 0 0 0 0 2 0 0 2

1948 0 0 0 0 0 1 0 1

1949 0 2 0 0 4 0 0 6

1950 0 1 2 0 0 1 0 4

1951 1 2 1 2 2 0 1 9

1952 1 1 2 0 2 1 0 7

1953 0 0 0 1 2 1 0 4

1954 0 1 3 0 4 2 0 10

1955 0 2 1 0 1 2 0 6

1956 2 2 2 1 3 1 0 11

1957 0 0 1 2 3 3 0 9

1958 0 2 3 2 3 2 0 12

1959 0 2 3 3 2 2 0 12

1960 0 2 1 2 1 2 0 8

1961 0 1 4 1 1 2 2 11

1962 0 1 1 2 3 1 0 8

1963 0 1 2 0 4 1 0 8

1964 0 0 3 2 1 O 0 6

1965 0 4 0 3 3 0 0 10

1966 0 1 0 4 6 2 0 13

1967 0 3 4 4 14. 3 0 18

1968 o 1 4 8 3 3 0 19

1969 0 0 3 2 4 1- 0 10

1970 1 3 6 4 1 18

1971 1 1 7 4 2 2 1 18

1972 1 0 1 6 2 1 1 12 with a decrease in bursts. A broader area of warm SST could also result in the

ITCZ lying closer to the equator, as well as being less organized along a single band. Presumably, this condition would tend not to be favorable for storm

development.

Minimal storm development during the middle of the season, e.g., September,

1923, 1930, 1932, 1935, and 1936, may have been the result of abnormally cool

SST off tropical Mexico or the development of shearing tropospheric westerlies

in the region north of 20 0N and west of 105 °W, as described in an earlier

section. Minimal storm development during these years could also have been the result of the advection of upper -level dry air into the region accompanied by vertical shearing from the 850 mb to 300 mb level and a general descending

(stabilizing) air column. Years with continued maximum monthly storm development (e.g., 1925, 1928, 1938, and 19 0) may have been characterized by storms which occurred over a broader region of the eastern North Pacific (rather than along a narrow tract) whereby SST cooling would tend not to be very pronounced along the prominent thermocline ridge off southwestern Mexico. Tree -ring

chronologies may be used to estimate sea surface temperatures, to reconstruct upper air conditions and to test the chronology of storm frequencies proposed here.

TROPICAL STORM PRECIPITATION IN THE SOUTH=

Cry (1967) has evaluated the importance of tropical storm precipitation in the eastern and southern United States as it is related to periods of both drought and normal precipitation. A similar analysis for the southwestern

United States was desired in this report to assess the importance of eastern

North Pacific tropical storms to the climate of southwestern United States.

The synoptic conditions in the Southwest are often more difficult to ana-

lyze than those in eastern United States due to lack of adequate data over a vast region to the west and south of the United States. Therefore, the actual

sources of moisture which result in precipitation are often unclear. Also, there may be several sources of lloisture for a given storm in the Southwest.

An analysis of the last five years of satellite data has suggested that

some of this precipitation is the result of surges of moisture up the Gulf, a

condition described in a later portion of this report. In addition to this unique type of synoptic condition, the satellite data have indicated that a

large percentage of the tropical storm precipitation in the Southwest has been derived from the interaction of tropical storms with cold cored disturbances to the north of the storm (either over Arizona or to the west).Satellite data have shown that upon interacting with a cold trough, a tropical storm will begin to expand principally in its northern and eastern quadrants. This large area of cloudiness is then entrained into the larger trough at which time the region of interaction is characterized by baroclinic instability and heavy

precipitation (e.g., September 3 -7, 1970). In order to carry out a long -term analysis of tropical storm precipitation in the Southwest, it was decided to

combine all of the synoptic conditions in which a tropical storm was believed to have supplied some or all of the measured precipitation during a given time

period.

The effects of these storms can vary throughout the region and the vari-

ance of precipitation intensities tends to depend upon the time of day the moisture arrives as well as the topography of the surrounding region. In order to assess these differences, 5 Arizona stations of different topographic con-

figurations were selected: Yuma, a lowland desert; Phoenix, central moderate

elevation desert; Holbrook, northeast plateau;Tucson, southeast upland

desert; Prescott, Mogollon Rim. In California 10 stations were chosen as

follows: Indio, Brawley, Blythe, and Needles --all in a lowland desert;Death Valley, a lowland desert to the north; Cuymaca, a station in the peninsular mountain range; San Bernardino and Yorba Linda, intermediate and coastal valley localities respectively; and San Diego and Los Angeles, metropolitan coastal areas.

All storms which passed north of 21 °N and east of 135 °W were examined for the two periods, 1921 -1940 and 1947 -1971, to ascertain possible influences on the synoptic conditions of the western United States. Historical weather maps were used in the evaluation of the large storms, though not too infrequently it was found that these maps did not always indicate the positions of storms once they passed into a region lacking ship reports. From 192.1 -1940 and 1947-

1971, 89 storms were found to have been associated with precipitation in

Southern California and Arizona. In a large number of cases the arrival of moisture associated with the storm occurred after a period in which little or no precipitation had been reported in the region. These rainless preceding days were helpful in distinguishing the tropical storm rainfall from other nontropical storm sources. In some cases convective showers developed soon after the storm entered the mouth of the Gulf, a condition which results from the northward surge of maritime tropical air into the lower Colorado River valley. In order to confirm this hypothesized synoptic condition of a surge up the Gulf, the 24 -hour temperature and wind data for Yuma were analyzed for the apparent advection of tropical air into this region (as indicated by cooler daytime temperatures, sometimes higher nighttime temperatures and higher wind velocities). When the given tropical storm did not approach the Gulf of

California and instead moved northwestward offshore, the first precipitation associated with the storm often did not begin until the storm had passed north of 24 °N. In these cases the precipitation was in theform of scattered thunder- showers which became more numerous and of greater intensity as the storm approached the Southwest. Widespread rains did not begin until the largest stores (in the Pacific or in the Gulf of California) moved north of at least 28 °N, while

the smaller sized and less intense storms brought precipitation to the United

States once they were north of 30 °N.

For the two periods, 1921 -1940 and 1947 -1971, the number of storms each

month which have affected either Arizona or California are summarized in table

3 and 4. In both periods an average of 2 storms per year have contributed to

the precipitation of the region. From 1921 -1940 (table 3) the storms seem to

have been much more frequent duringthe month of September, while during the

period 1967 -1971 (table 4-) storms were slightly more frequent during the other

months of the season except for September.

The average annual precipitation due to tropical storms, as mapped in

figure 6, indicates that the center of the area experiencing tropical stoma

precipitation differed between the two periods. Throughout most of Southern

California and the lower Colorado River valley the annual amount of precipitation

due to tropical storms decreased by about one -half during the later period, while

in Arizona (excluding Yuma) the annual amount has increased or remained about

the same. The percentage of annual precipitation due to tropical storms has

not decreased as drastically in Southern California as has the actual amount.

In Arizona the percentage of annual precipitation due to tropical storms appears

to have increased slightly (fig. 7). The continued importance of tropical storm

precipitation in all areas except coastal California is the result of a gradual

decrease in winter precipitation which has not been matched by the same magni-

tude of decrease in precipitation from tropical storms.

An analysis of the total annual precipitation and annual storm precipi-

tation at Yuma from 1921 -1940 and 1947 -1972 is given infigure 8. At first

glance it is apparent that annual precipitation was greater at the station Table 3.-- Tropical storms affecting the Southwest, 1921 -194 +0

Year June July August September October Total

1921 0 0 1 1 0 2

1922 0 0 0 0 1 1

1923 0 0 0 o 1 1

1924 0 0 O 1 0 1

1925 0 0 0 0 1 1

1926 o 1 o 3 o 4

1927 o 1 o 1 0 2

1928 0 0 0 0 0 0

1929 0 0 1 2 0 3

1930 o o o o 1 1

1931 o o o 3 o 3

1932 0 o o 0 o o

1933 o o o o o o

1934 o 0 o 1 1 2

1935 0 o 2 0 0 2

1936 0 0 2 0 1 3

1937 0 0 1 3 1 5

1938 o o 1 1 o 2

1939 0 o o 3 o 3

1940 0 0 0 1 0 1

Total O 2 8 20 7 37

Mean .1 »4- 1.0 .3 1.9 Table 4. -- Tropical storms affecting the Southwest, 1947 -1971

Year June July AugustSeptember October Total

1947 0 0 0 0 0 0

1948 o 0 0 o 0 0

1949 0 0 0 3 o 3

1950 1 1 0 o 1 3

1951 O o 1 0 o 1

1952 O 0 0 1 0 1

1953 o O 1 0 o 1

1954 o 1 O 1 0 2

1955 o 0 0 o 1 1

1956 0 0 0 0 0 0

1957 o 0 1 o 2 3

1958 0 1 0 1 1 3

1959 1 0 1 1 0 3

1960 o o 1 1 1 3

1961 o o o 1 o 1

1962 0 0 o 1 1 2

1963 o o o 1 1 2

1964 o 1 1 1 0 3

1965 o o 1 i 0 2

1966 0 0 1 2 0 3

1967 o o 1 o o 1

1968 0 0 1 1 2 4

1969 o o 0 1 O 1

1970 0 o 1 1 0 2

1971 o 0 2 1 0 3

Total 2 13 19 l0 48

Mean .2 .5 .7 .4 1.9 Figure 6.-- Average annual precipitation due to tropical storms(inches)

for the two periods 1921 -1940 and 1947 -1971. Stations

indicated are: 1) Holbrook, 2) Prescott, 3) Phoenix,

4) Yuma, 5) Tucson, 6) Greenland Ranch /Death Valley,

7) Needles, 8) San Bernardino, 9) Los Angeles, 10) Yorba

Linda, 11) Indio, 12) Blythe, 13) Cuymaca, 14) San Diego,

and 15) Brawley. TROPICAL STORM PRECIPITATION 1921-1940

.25 '.50 .50 .75 AVERAGE ANNUAL 1.00 1.00 TROPICAL STORM .75,50 PRECIPITATION 1947 -1971

Figure 6. D/O 50

30- 30 PRESCOTT p to u_ 50

70 -70 YUMA

50 50

30- -30

10- -10

1921 1925 1935 1940 1947 1950 1955 1960 1965 1970 YEAR

Figure 7. -- Percentage of annual precipitation due to tropical storms for 5 stations in Arizona. during 1921 -1940. In addition tropical storm precipitation appears to have been more frequent during this earlier period.The contribution of tropical storm precipitation to the annual :rec. :'itati on total for the period 1947- 19'72, is quite high in 8 of the 26 years, though the actual precipitation amounts are not much higher than those observed in the earlier period. As noted above, this difference is the direct result of decreasing winter precipitation throughout much of the Southwest since the early 1900's. In a recent examination of thunderstorm activity in the mountains of Southern California, Tubbs (1972) has found an apparent increase in tropical storm related thunderstorm activity for the period 1958 -1968 compared to the period 1947 -1957. Our data would tend to indicate that in terms of precipitation from tropical storms the period 1958-

1968 was the wettest. In addition the increase appears to have continued during the past three years (1970 -1972).

The tropical storm precipitation data for individual stations in Arizona have suggested that tropical storms can bring excessive precipitation to the

Southwest. Likewise, the yearly data for these stations (see Yuma, fig. 8 and

9) tends to indicate a cluster of years with numerous tropical storms, while other clusters of years indicate few tropical storms. The summer monsoon of

Arizona may, therefore, be composed of two quite different synoptic regimes.

The first summer regime is the flow of warm, moist air into the Southwest from the Gulf of Mexico. At other times of the monsoon season (particularly near the season's end) precipitation may move into the Southwest as tropical storms from the eastern North pacific move inland or dissipate off Baja

California.

Since excessive precipitation in the summer may be associated with tropical storms, it was decided to analyze station data in terms of periods of monthly excesses or deficiencies. A recent re ^ort by the Atmospheric Physics YUMA

ANNUAL TROPICAL STORM

:41:"*".. r r r 1921 1925 1930 1935

Figure 8.-=Total annual precipitation from all storms and total annual precipitation due to tropical storms for Yuma during the period 1921-1940.

YUMA

[1] ANNUAL TROPICAL STORM

-T T 1 1947 1950 1955 1960 1965 1970

Figure 9.--Same as figure 8 but forperiod'1947-1972. Department of the University of Arizona provides a usefulbasis for judging

precipitation anomalies at 69 stations in Arizona. This technical report

contains "...emperical probabilities that the amount ofprecipitation (inches)

during each month of the year will be less than a computed amount"(Kangieser

and Green, 1965). The numbers of stations reporting precipitation values

either in the upper 25% class (wettest) or the lower25% class (driest) were

tabulated for the months of July, August and September for theperiod 1934-

1963. Stations were segregated into regional categories and ineach category

the percentage of stations reporting excessive or deficientprecipitation in

each given month was calculated.

The data revealed that for the period of study there wereconsecutive

years with repeated dry or wet conditionseither in July or in September

(fig. 10). Even more surprising was the relationship between theperiods of

deficiencies or excesses. From the mid -1930's to the mid- 1940's, most stations

reported very dry conditions in July which were commonly followedby near nor-

mal precipitation in August and excessive precipitation in September. By the

1950's, precipitation in July was high while precipitation forSeptember was

decidedly low. Considering the number of stations used in this analysis(69

in Arizona and 19 in northwestern Mexico) and the criteria forselection of

wet and dry years (upper 25% wettest and lower25% driest), it seems quite

clear that there is a definite alternation in the type of monsoonexperienced

in the Southwest. That is to say that for many years themonsoon is primarily

an early summer phenomenon, which isthe result of the arrival of upper -air

moisture from the Gulf of Mexico. During other clusters of years the monsoon

appears to be a later arriving phenomenonwhich is in part composed of precipi-

tation associated with the movement of tropical airinto the Southwest from the

Gulf of Mexico (during August) followed by the movementof tropical storms into O °Q \ \ \IDSlA l G 1 -t r---TT--7 I r f 1935 1940 1945 1950 1955 1960 YEAR NUMEROUS TROPICAL STORMS --->I TRANSITION FEW TROPICAL STORMS - iTRANSITION I'

Figure 10.- -The percentage of stations (out of 17) in southeastern Arizona reporting dry conditions (the driest 25%) or wet conditions (the wettest 25%) during July, August and September for the period 1934 -1963. the area during late August and September. The areal extent and duration of tropical storm rainfall in Arizona. appears to be a function of either the intensity of the tropical storm as it approaches the United States or the interaction of the tropical storm with a "digging" cold trough along the west coast.

The 500 mb level maps for 1950 -1971 were analyzed for possible circulation features associated with given anomalous precipitation in Arizona. A summary of the average 500 mb circulations associated with abnormalities in precipitation is given below.

July. Extreme dryness during July (1960, 1963, 1971) was associated with an upper level 500 mb high pressure cell centered over New Mexico (fig. 11).

At the surface quite warm and dry atmospheric conditions are conducive to the formation of these upper -level continental highs. Over Texas and New Mexico extremely dry air may flow southward along the east side of the high pressure cell (Namias, 1938). This drier air from the continent may mix with more moist air from the Gulf of Mexico, but even with this type of mixing, the air may still be dry as it begins to recurve westward and then northward. into Arizona.

As a result of a possibly drier air mass and an upper -level high associated with descending air, thunderstorm activity would not be favored in Arizona. However, above normal rainfall for southern Sonora and. northern Sinaloa during July, 1960 and 1963 suggests that the flow to the south was more moist - -a condition not surprising since this region lies far south of the high pressure cell with its drier, mixed air. Heavy precipitation in July (1950, 1953, 1954, 1955) is not characterized by an upper level high pressure cell in the Southwest. Instead, the main circulation feature is an extremely strong upper -level high pressure ridge extending from the Atlantic into central New Mexico. This circulation feature would preclude the flow of warm, dry air into northern Mexico from the

Great plains. The extended ridge would foster the development of a broad, moist = 150° 30° , ~ ` ` = `` ' /y ' . A. °~

o ' 9' ~ '' `h 00 ' ^ .` :9-o ' .. _ . .,., ~^ * 5900 _ ' ~/ , ``__H 20°

^ ~

~ , . ~ JULY- WET JULY -- DRY 1950, 1955' \0° 0° r// 1950, 1963. 1971 500mb 1954, |955

. / . _,._ 'O° s, =

170° 150° 90° 30° o , .` o 1 q' ,0 = '/]

/ vz6 , . ^

` . ,, '

^ ' «9o"- , ~ 0° ^ `0 `` AUGUST --DRY ~ } AUGUST-WET 1950, 1953' \. |955' 959'|96J ~/ v' 1856'|952 500mb 1968'1371 500mb ' ' \ / .0° °~ '1° 170° 30° ' 30° ^ '' m ' ',If ."- o | 1- A.0 ^ -- 40 ,k5f2t, u` / ^ ,^0° tt '0 0. . ^ r . ' . * 00 , . `. o" ^ . . ~//ôJ `~ ~~ q. ,, _-_--- --~~ ~ o ` / ^ ., /4 ~ 5850 SEPTEMBER-DRY 1953' 1955' 1956' /SEPTEMBER-WET / \7( = 1957,' 1968 5OOmb /1958,1966,1970 500mb ^- °^ , 90° '"" 93.

Figure 11.--Average 500 mb charts associated with dry and wet periods in Arizona for the period 1950-1971. flow into the Southwest from the Gulf of Mexico. Since there is not a strong

New Mexico high pressure cell suggested for a wet July, it is also postulated that the air would not have the tendency to descend as in a dry July. As a result of these conditions, thunderstorm activity would be favored for longer periods of time in the Southwest.

August. The average 500 mb chart for the dry Augusts (1950, 1953, 1956,

1962) is like that for July (fig.11). A warm upper -level high is centered over west Texas with a ridge into New Mexico andeastern Arizona. As noted above, this configuration should preclude frequent thunderstorm activity in

Arizona due to generally drier descending air. In addition to these more stable conditions, the placement of the high farther south than in July would suggest the tendency for occasional southwesterly flows to occur across Arizona.

Reitan (1957) has found that these periods of southwesterly flows during the summer are generally characterized by decreased thunderstormactivity. The supplementary data from southern Sonora and northern Sinaloa indicate that, un- like July, the variation in August precipitation in this region is generally

similar in sign to the variation in Arizona values.This similarity may be a result of a greater influence in Sonora of the southward displaced high pressure cell and its attendant descending dry air. The extensive dryness in this Mex-

ican region could also be due to a less persistent warm, moist southeasterly flow (a condition definitely suggested for dry Septembers).The average 500 mb

chart for the wet Augusts (1955, 1959, 1963, 1966, 1971) does not show an upper-

level high pressure cell over the Southwest though a very weak high pressure

center is indicated over east Texas. This configuration would not be associated with descending air in the Southwest or northwestern Mexico. Though not evident

from the 500 mb charts, the upper -level winds during the wetAugusts were gen-

erally southeasterly from south Texas to Arizona. This would indicate a flow of warm, moist air from the Gulf of Mexico into the study region.As noted above, this type of circulation should foster the development ofthunderstorm activity in the monsoon region of the Southwest.

September. The average 500 mb charts for wet and dry Septembers are quite different from those previously described for July and August (fig. 11). A thermally derived high pressure cell is not found to be maintained over the continent due to normal seasonal cooling in September. For both the wettest and the driest Septembers, the average 500 mb charts are characterizedby a westerly flow north of 35 °N. South of 35 °N along the western margin of North

America, two types of flow regimes are suggested. During the dry September the pressure gradients to the southof 35 °N are quite weak, suggesting either nearly calm conditions or a weak westerly flow aloft.As noted by Reitan (1957), this dry westerly flow alternating with calms would preclude thedevelopment of thunderstorms in Arizona. Since tropical storms most frequently enter the

Southwest during September, it should be noted that a weak flow,especially a weak westerly flow, would not tend to steer a storm towardsthe Southwest.

Instead, the storms would either decay over relatively cool waters off Baja

California (under a weak flow) or they would move inland in anortheasterly direction (under a westerly or southwesterly flow). The possibility of precipitation would be low during a September with a weak or westerly flow,since neither Gulf of Mexico air masses nor tropical storms would be ableto pene- trate into the region. The average 500 mb chart for the wet Septembers(1958,

1966, 1970) indicates a prominent west coast upper -level trough. The flow on the east side of this trough would steer tropical storms intothe Southwest.

The wettest Septembers for the 1934 -1972 period were as follows: 1939, 1940,

1946, 1958, 1966, 1970, 1972.The synoptic conditions associated with the heavy rains were as follows: a well- developed upper -level west coasttrough w to warm, moist sou t to southwesterly winds aloft over Arizona occurring in

1939, 1940, 1946, 1958, 1966, 1970, 1972.The presence of a tropical storm along the coast of Baja California occurred in 1939, 1958, 1966, 1970 and

1972. Thus, in almost every case of heavy September precipitation, it was found that an upper -level trough was either stationary or entering the region from the west and a tropical storm occurred to the south along the coast of Baja

California. Present satellite data analyses suggest that under such conditions considerable moisture can be entrained into the troughs from the region of tropical cloudiness associated with the tropical storms. Since the upper -level troughs are cold cored, the movement of tropical air into the cooler region of the trough results in extremely heavy precipitation due to baroclinic instability. As a result of this interaction, the heaviest precipitation amounts are found along the eastern edge of the cold trough where the tropical air is being entrained. Less precipitation may be found west of this line near the trough itself or to the south near the tropical storm. Locally severe flooding occurred near this region of interaction in 1939, 1958, 1966 and 1970.Though no tropical storms were detected during the days of heavy rains in September, 1940 and 1946, this need not indicate there were no storms present.With the use of satellite coverage of the eastern North Pacific, it has been found that some storms (occasionally up to 40% of the yearly total) are not indicated by surface reports- -land station reports or ship reports. It could be hypothesized, therefore, that either a tropical depression or a tropical storm had contributed to the heavy rains of 1940 and 1946.

CONCLUSIONS

Variations in monthly tropical storm totals and storm movement are believed to be caused in part by the distribution of the upper -level high pressure cells over North America. Few storms form when either dry shearing upper -level westerlies or dry upper -level northeasterlies enter the tropical eastern North

Pacific. In the southwest summer drought conditions occur during either of

these regimes associated with minimal storm development. Maximum storm development occurs when there is an upper -level high ridge extending westward from the

Atlantic with an associated southeasterly flow in the eastern North Pacific.

Under this regime storms may enter the southwestern United States. Storms most

commonly enter the Southwest during the month of September when steering of the

storms is from a southerly direction. In Arizona and Baja California periods

of drought in July versus wetness in Septemberor wetness in July and dryness

in September are shown to last for periods as long as 10 years. Namias (1971)

has shown that for the period with high precipitation in July the eastern North

Pacific was characterized by cool SST from 1947 to 1957. By 1958 these SST

anomalies reversed themselves. In the earlier paper by Douglas (1972), it

was shown that warm SST off Baja California, as in the period examined by

Namias (1958 to 1969) would be favorable for the movement of tropical storms

into the Southwest, a condition borne out by the data presented here. Assuming

the SST patterns described by Namias and the summer upper -level patterns as

described in this paper last for about 10 years, it is possible to expect that

within the next few years a possible cooling of the SST along the coast of

Baja California could occur as well as a decrease in September precipitation

in the Southwest. REFERENCES

Cromwell, Tmmsend, "ThenrJocline Topography, Horizontal Currents and I Rid[§ing I

in the Eastern Tropical Pacific ", Inter-American Tropical Tuna Comr-"ission

Bulletin, Vol. 3, No.8, 1958, pp. 135-165.

Cry, George W., "Effects of Tropical Cyclone Rainfall on the Distribution of

precipitation over the Eastern and Southern United States ", ESSA

Professional paper I, U. S. Department of Commerce, 1967, pp. 1-67.

Douglas, Arthur, ''The Climatology of Northeastern Pacific Tropical Storms and

Hurricanes", Interim Report, NOAA Contract 1-35241, Tree-Ring Laboratory,

University of Arizona, Tucson, Arizona, 1972, pp. 1-28.

Fujita, Tetsuya T., Kazuo vlatanabe, and Tatsuo Izmm, "FOF..nation and structure

of Equatorial Anticyclones Caused by Large-scale Cross Equatorial FIO'Hs

Determined by ATS-l Photographs ", Department of GeOI)hysical Sciences,

The University of Chicago SMHP Research Paper No. 78, January, 1969,

pp. 1-37.

Kangieser, Paul C. and Christine R. Green, "ProbabiUties of Precipitation at

Selected points in Arizona", The University of Arizona, Institute of

Atmospheric Physics, Technical Report No. 16, Tucson, Arizona, 1965,

pp. 1-46.

Namias, Jerome, ''Th1.ll1derstonn Forecasting with the Aid of Isentropic Charts",

Bulletir: of the American t'1eteorological Society:, Vol. 19, 1938, pp. 1-14.

Namias, Jerome, "Large-scale and Long-term Fluctuations in Some Atmospheric and

Oceanic Variables Presented at the Nobel Symposhun, Gottenburg, Svrcden,

August, 1971, pp. 16-20. Renner, James, "Sea Surface Temperature Monthly Average and Anomaly Charts,

Eastern Tropical Pacific Ocean, 191+7- 1958 "., United States Fish and

Wildlife Service Special Scientific Report -- Fisheries No. 11+2, 1963,

pp. 1 -57.

Reitan, Clayton H., 'The Role of Precipitable Water Vapor in Arizona's Summer

Rains ", The University of Arizona, Institute of Atmospheric Physics,

Technical Report No. 2, Tucson, Arizona, 1957, pp. 1 -19.

Sadler, James C.,The Mean Winds of the Upper Troposphere over the Central

and Eastern Pacific", Environmental Prediction Research, Naval Post-

graduate School, Technical Paper No. 8 -72, Monterey, California, 1972,

pp. 1 -29.

Tubbs, Anthony M., "Summer Thunderstorms over Southern California ", Unpublished

manuscript submitted for publication in Monthly Weather Review, Scripps

Institution of Oceanography, La Jolla, California, 1972, pp. 1 -25. APPFMIX

Analysis of Gale Data for the Determination of Tropical Storms

1. Gale data were examined for the period May through November - -the known

tropical storm season in the northeastern Pacific.

2. All gales were recorded for the area bounded by 0 °N to 35 °N and from

80 °W to 140 °W (see fig. 12).

3. The following entries were made for each observation:

a. date of gale

b. latitude and longitude

c. barometric pressure at time of peak wind

d. direction of peak wind

e. value of peak wind

4-. Suspected tropical storms:

Since many of the gale reports were associated with a single storm,

the data were segregated into sets of data that were believed to rep-

resent an individual storm which was either stationary or westward

through northward moving.

a. Average mature storm size was taken at 5° latitude, though much

smaller storms are common. Gales on a given day that were more than

5° apart were analyzed for the possibility of 2 storms being present.

b. Storms were assumed to move from 0° to 8° per day (the latter figure

the greatest movement of any storm in the period 1956 -1971.

c. From a and b it was assumed that gale data within a 13° latitude

could represent a single storm during a given 24 -hour period (8° of

movement plus 5° of diameter).

d. Analysis of all data within a given 13° latitude square:

1. Wind directions were examined to locate the center of the ex- STO M 1 ,'

SH1r C Ì ------ISHIP SYORMI2 REGION "TEHEPS"

Figure 12. -- Sample gale datamap. See text for explanation. petted storm-e.g., a northwest wind would indicate that the

storm was to the east and south.

2. Strength of the gale and barometric pressure were examined to

indicate the proximity of the storm- -e.g., strongest gales and

lowest barometric pressure indicating a closer position of the

storm compared to other ship reports (see Ship A, fig. 12).

3. With approximate location of the center being determined, all

outlying data within 10° latitude or longitude of the gale were

examined to see if they were associated with the expected storm

at the estimated location. Assuming increasing pressure and

decreasing wind speeds from the storm center, the wind direction

was the best estimate of a gale report being associated with the

given storm. For example, if a ship to the east of an expected

storm reported a southerly or southeasterly gale, it could be

assumed that this report indicated a definite relationship to the

circulation to the west (see figure for s hi p B and storm 1) .

If the ship (C) to the east of the storm reported a northeasterly

or northerly wind, it was assumed that another storm had formed

to the east of both the first storm and the ship (C). This

second storm is indicated in figure 12.

5. Nontropical storms:

a. Tehuantepecers: During the month of May and the months of

September through November, localized gales can occur in the Gulf

of Tehuantepec as a result of extremely high pressure to the north

and a resultant southward flow of air. Relatively high pressure

and consistency of northerly winds characterize these gales which

may last from 1 to 4 days. Evaluation of the anticyclone tracks for North America can associate these winds with high pressure to the

north, and thus, these storms can be easily separated into a non-

tropical set of gale data (see fig. 12). b. Northerly trades off Baja. California and Southern California:

During the early part of the summer (May through early July) and

again in the late summer (mid - August through October) an abnormally

strong high pressure cell may ridge northward along the coast of

North America. This high pressure off the coast, plus lower pres-

sure in the deserts of the Southwest and northwest Mexico, can re-

sult in a very strong northwesterly to northerly flow off the coast.

These conditions are easily segregated from tropical storm data

since these winds are accompanied by high pressure, and they are

restricted to the region generally north of 25 °N. c. Strong easterly trades: Occasionally strong trades accompanied by

relatively high barometric pressure were reported off southern

Mexico. Possible wind shifts during the gale (as noted in the

published gale data) and the depression of the barometer at the time

of the wind shift were taken as indication of a disturbance. In

this case the data were believed to indicate the existence of a

tropical storm. All other cases were then examined to see if they

propagated westward or northward, and if so, they were again assumed

to indicate the existence of a tropical storm. d. Cross -equatorial flows:A few cases of strong southerly or south-

easterly winds near the equator were reported. These winds were

assumed to indicate a cross- equatorial flow since tropical storms

have not been reported along the equator in this sector ofthe Pacific

(in fact they never occur near the equator).