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) Rights Copyright © Arizona Board of Regents. The University of Arizona. 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 sat- ellite 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 ,o 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° sec- tor centered at that direction. Dashed lines indicate that there are 2 or less observations per square. so. 10. i / t / I ; 1 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 north- westerly 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 tropo- spheric shearing over northern Baja California.