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An Investigation of Possible Consequences of Recurving Western Pacific on the General Circulation

GregoryJ. Hakim Department ofAtmospheric Science, State University of New York at Albany, 1400 Washington Ave., Albany, New York 12222

December 1991 ABSTRACT

Palmen (1958) demonstrates that a tropical which transforms into an can generate enormous amounts of kinetic energy and available potential energy. Given that these systems occur fairly regularly each autumn over the western Pacific, an attempt is made to assess the impact of recurving typhoons on the general circulation for both short and long time scale. To do so, average kinetic energy time series for 30-60° N were generated for the period 1 August to 31 December 1965-1988 at 250mb. These data indicate years which have above normal kinetic energy in September also had above normal eddy kinetic energy the following 3 months. An examination of recurving activity for 1965-1988 revealed that September was also the month with the greatest frequency of events. These separate results may suggest that recurving typhoons could have significance in short term climate anomalies.

Results of a seven case composite of individual events indicates that recurving typhoons are associated with a downstream positive anomaly over western North America which is stable on short time scales (5 days). In addition, a composite of six Septembers with above normal typhoon activity also indicates that these years were also associated with a westward propagating positive anomaly over the eastern North Pacific which is speculated to be a low frequency Rossby wave excited by extratropical typhoon activity.

The best correlation of monthly typhoon frequency and deviant kinetic energy at 250mb was found for September (.53) and the five month average (.53). Given the natural variability of baroclinic eddies, these relatively modest correlations may indicate that although recurving typhoons represent only a slight fraction of all eddies, their impact on the the general circulation may be important. Based on the results obtained in this study, some speculation of specific consequences is put forth. 1.Introduction

Tropical represent a very large local concentration of kinetic and latent potential energy in the atmosphere. This energy is realized primarily through surface fluxes from the ocean surface over which they develop (Emanuel, 1986). Thus, it is not surprising that the regions of maximum frequency are located over the warmest sea surface waters; namely the tropical Atlantic, west Pacific, and Indian oceans. Frequently, these cyclones will form, mature, and dissipate completely in the realm of the tropical atmosphere. However, a certain fraction of the time over the Atlantic and western Pacific these cyclones migrate northward intact, and interact with midlatitude baroclinic disturbances. These interactions often result in a transformation of the tropical cyclone into an extratropical cyclone. Details of such transformations are certainly case dependent, however even general aspects of such events have not been extensively explored.

In one of the few studies to address this issue, Palmen (1958) examines the transformation of hurricane Hazel into an extratropical over eastern North America on 15 October 1954. Palmen shows that the transformation process was characterized on the large scale by local production of kinetic energy (hereafter KE) by a thermally direct circulation, with sinking cold air within and west of the baroclinic wave, and rising warm air near the cyclone. This process can be viewed as a transformation of eddy available potential energy into eddy kinetic energy (Lorenz, 1955). Palmen goes on to show that while the transformation depletes the available potential energy, there was actually a net increase in this quantity due to the extraordinary amount of latent heat release in the warm air mass.

When viewed in terms of the classic four box energetics diagram (Fig. 1), recurving tropical cyclones which evolve into extra tropical cyclones differ from "ordinary" cyclones in that they contribute markedly to P' as well as K'. This additional P' furnished to the atmosphere can further be realized to K' by other disturbances. Eventually, through momentum flux convergence, this energy maintains the westerlies against dissipation. Using representative dissipation values, Palmen estimates that only 2 to 3.5 disturbances of the kind similar to Hazel would be necessary to provide the atmosphere north of 30° N with sufficient kinetic energy to offset frictional dissipation on an annual basis.

During the autumn, the increase of the midlatitude westerlies is due almost exclusively to the aforementioned eddy momentum flux convergence, since the time mean meridional circulation in this region is indirect. Since this conversion process essentially originates with P' (Fig. 1), it seems reasonable to conclude that system which furnishes a generation of P' through diabatic processes will play an important role in the autumnal increase of the westerlies. With the further assumption that the Hazel is somewhat representative of a recurving tropical cyclone which becomes extratropical, then the work of Palmen also suggests that these systems may be important to this problem, since they primarily occur during the time of the autumnal increase of the westerlies.

The work of Erickson and Winston (1972) illustrates one way in which tropical cyclones can effect midlatitude weather. It was noticed on satellite imagery that occasionally a band of high clouds would emmenate from a typhoon in the western Pacific and extend east/northeast across the Pacific ocean. By studying a number of such cases, Erickson and Winston show that these bands are associated with warming on the anticyclonic side of the jet stream, which increases the horizontal pressure gradient, and hence the jet stream velocity. They also observed an increase in zonal KE, eddy KE, and total KE in the 850-200mb layer several days after the events. In addition there was a surge in eddy available potential energy several days later.

The present study is conducted to address some questions regarding recurving tropical cyclones (hereafter recurving is meant to imply those systems which transform and become extra tropical cyclones):

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- do recurving tropical cyclones contribute significantly to the autumnal spinup of the Northern Hemisphere westerlies?

-are there short (5 day) and long range (30-90 days) effects on the general circulation in years with anomalous recurving tropical cyclone activity?

In our attempt to address these questions, we focus attention on the tropical cyclones of the western Pacific. This region was chosen since it averages a large number of tropical cyclones in close proximity to the Asiatic jet stream. The analysis is carried out primarily through an subjective analysis of recurving western Pacific tropical cyclone activity (typhoons) and kinetic energy at 250mb for the period 1965-1988. A statistical approach is used to explore possible relations between the two. The methodology to this study is outlined in section 2, with the results of KE calculations presented in section 3. Results of the recurving typhoon study appear in section 4. A statistical analysis of possible connections between deviant KE at 250mb and anomalous typhoon activity is discussed in section 5. Section 6 provides some speculation on potential implications of the presented material as well as avenues of future research. 2. Methodology

Effects on the jet stream were explored through computing KE parameters for 250mb, using the National Meteorological Center's (NMC) octagonal gridded wind data for the Northern Hemisphere poleward of 20° N for 1 August through 31 December 1965-1988. In this study, KE refers to KE per unit mass:

(1)

This quantity was averaged over the latitude band 30-60° N. where the midlatitude jet stream is typically located. For most of the work presented in this study no correction was made for the fact that the amount of atmospheric mass per zonal strip of earth decreases poleward. For cases in which the jet speed remains constant with an equatorward displacement this method will

4 result in no observed increase in KE, despite the fact that due to the increased mass involved there is a substantial increase in the total mass weighted KE. From the ..x..mination of several years data for which the effect was accounted for by a cosine normalization, there is an approximately 15% underestimate of the amount of actual spinup of the westerlies. In addition, since we are only using a 30° latitude band we are undoubtedly neglecting important influences such as the subtropical jet stream.

The total KE was decomposed at each grid point into contributions from a zonal average and perturbation components:

TKE =ZKE + EKE (2) where:

TKE =.5 IV 12is the total kinetic energy per unit mass ZKE =.52 is the zonal kinetic energy per unit mass EKE = .5(u'2+v2) is the eddy kinetic energy per unit mass

Brackets denote an instantaneous zonal average, and it was assumed that the meridional velocity component was an eddy contribution only. These quantities were computed by two methods to ensure proper computations. One method involved computing EKE from TKE-ZKE while the second involved

, direct evaluation of the eddy term, with agreement within machine roundoff error. The gridded results were then averaged in latitude bands every 2.5°. The computations were performed at 00 UTC and 12 UTC daily from 1 August to 31 December. The two times were averaged to provide a representative field of KE for each day. An average of the data from 30-60N produced the final KE data on which most of this study is based.

It should be noted that with the aforementioned caveats with regard to the use of KE/mass and the fact that only one level was chosen, these results represent a conservative estimate of temporal trends in KE. Thus, these results

5 should be viewed with the understanding that this simplistic approach potentially underestimates the KE response, to such events as recurving typhoons. It was deemed prudent to address the problem from a conservative approach so that any signals which do appear would have a greater likelihood of being genuine.

Statistics for recurving typhoons in the western Pacific were derived from analyses of the microfeisch archive of the NMC surface analysis. The area of interest is defined as 125E-180E during the period 1 August to 31 December 1965-1988. The 12 UTC analysis were used as representative of the activity for a given day, although intermediate analyses were used for tracking disturbances. The criteria used to define a transformation ev~nt is a system of at least storm intensity (>17ms-l) which is eventually analyzed as an extratropical cyclone with frontal symbols. The 12 UTC central pressure of extratropical typhoons was recorded, frequently using the inner most isobar when a central pressure was not given. It must be noted that there was some subjective interpretation required at times, and that the analyses may not actually provide accurate representation of the storm. It may be presumed that due to the relative sparsity of data over the ocean that the analyses may systematically underestimate these , although perhaps not preferentially with respect to other oceanic storms. In any event, these factors will be ignored and left unaccounted for in this study.

Possible effects of typhoons on the jet level KE are explored from a statistical approach, relYing mainly on the use of correlation coefficients and t-test's of composite analyses to asses significance. 3. KE Results

A 24 year daily mean time series ofTKE, ZKE, and EKE is shown in Fig. 2. The autumnal buildup of TKE is characterized by a quasi-linear increase from 1 August values near 250 J/kg to 650 J/kg by 1 December. ZKE and EKE also

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increase during the period, being of comparable magnitude until mid October, at which point the ZKE continues to linearly increase while the EKE gradually levels off. Viewed in terms of the box diagram shown in the section 1, this suggests that the eddies become more efficient at increasing ZKE later in the autumn. Note also that the ZKE trace is considerably smoother than that for the EKE.

Analyses similar to that of Fig. 2 were produced for each individual year, and reveal a wealth of variability. For each year, daily anomalies from the 24 year daily mean values were computed, and normalized by the appropriate daily standard deviation. A mean of these normalized anomalies was found for each year and is graphed in Fig. 3. For example, in 1965 the TKE averaged greater than 1 standard deviation above normal for the entire five month period; a significant long term anomaly. In general, all three KE quantities decline from maxima in the later half of the 1960's to relative minima in the early 70's with a general increase during the rest of the period. It is not known to what extent changes in data analysis contribute to the variability in this figure. In any event, there are some years where EKE and ZKE deviations are in phase, and others when they are out of phase. The out of phase situation may be an indication of primarily barotropic conversions between ZKE and EKE (i.e. zonal flow vs. blocking).

As an example of a trace for an individual year Fig. 4 contains the results for 1965, which as pointed out was well above normal for the season. when compared with Fig. 2, the eddy activity appears slightly above normal in August, and considerably above normal in September, while ZKE was near normal. EKE remained above normal in October and subsided in November, while ZKE increased markedly. From late November to the end of the year there is a cyclic behavior in both the ZKE and EKE, which are completely out of phase. This may reflect a regime which vaccilates from high-amplitude blocking patterns to zonally dominant patterns.

7 As an example of a considerable below normal year, the data for 1975 (Fig. 5) indicates near normal EKE and ZKE in August gave way to below normal eddy activity and normal ZKE in September. This situation persisted through October, and November when the EKE increased to normal levels, while ZKE remained essentially constant until the end of the year. Thus, the westerlies never achieved the mean values for December and resulted in TKE well below normal (see Fig. 2).

Comparing 1965 and 1975, substantial differences fIrst appear in September and October, when EKE was much larger in 1965. After 1 November the EKE traces are similar, while the ZKE during 1965 continued to increase when it was constant in 1975. Based on these two years there is an indication that excessive eddy activity early in autumn results in above normal TKE which persists for the remainder of the season and is reflected by increased ZKE even after the eddy activity subsides to normal levels. Note also that EKE increases in the active year occurred without a compensating decrease in ZKE.

Another example of a yearly trace of KE for a relatively "normal" year, 1974, illustrates some interesting activity (Fig. 6). In mid October a major surge in ZKE is followed several days later by a major surge in EKE. In early November another surge in ZKE is followed by a surge in EKE and decline in ZKE indicative of a blocking type of weather" regime.

A spectral analysis was performed on the TKE time series (adjusted for latitude effects by a cosine normali~ation) for 1965 by a Fast Fourier Transform (FFT). The power spectrum of the transform (Fig. 7) indicates peak activity at periods of 51, 17, and 6 days. These periods are quite similar to the major periods in global angular momentum (Rosen and Salstein, 1983). It is perhaps surprising that the signal was so robust for a single level near the northern hemisphere jet stream. 4. Typhoon Results

8 The results of the recurving typhoon study are summarized in Table 1, with some of these results graphically illustrated in Figs. 8 ad 9. Of the 171 cases of typhoons which become extratropical during the 24 year period, 62 occurred during September, while only 2 occurred in December (Fig. 8). August and October were closely matched with 44 and 46 respectively. October had the greatest number of strong «981mb) and very strong «961mb) events with 31 and 12 respectively. Although August had a similar number of total events, only 11 were strong, and 0 were in the very strong category. This is graphically illustrated by plotting the strong and very strong events as a percentage of the total number per month. A linear increase for the strong events from 25% in August to 100% in December, and similar increase for very strong storms suggests several interesting possibilities. Although there are a greater number of storms early in the period, later storms are more likely to be intense. This is probably a reflection of greater baroclinicity later in the autumn, which is accompanied by stronger disturbances in the westerlies with which typhoons interact with. This effect is also indicated by the deepest storm (former typhoons) observed during each month for the 24 year period. The deepest storm in August out of a total of 44 events was 968mb while December, with only 2 events total produced the deepest observed (934mb). The fact that October, November, and December's deepest storms had comparable magnitude indicates that the background baroclinicity is large enough as early as October such that given given the proper initial disturbance a cyclone of noteworthy proportions is possible.

As a simple means of evaluating which month may be the most important In effecting the general circulation, a "power ranking" was developed which equally weights the total number and intensity «981mb). With 1.0 being a perfect score (a month which had the greatest total number and greatest number <981mb), September ranks highest with .94, followed by October (.87) , and August (.53).

9 Further inspection of Table 1 indicates that 47% of the storms which recurved at one point reached 980~b or lower. Recall that only the daily 12 UTC central pressure was used, so this this percentage is in fact a conservative estimate. While it is unknown what fraction of all baroclinic disturbances at this time of the year reach this intensity it is probably not that large.

While there is an average of 7.13 storms per year, there is also significant annual variability (Fig. 9). A maximum of 12 events occurred in both 1967 and 1968, and only a single event in 1973. Since September represents a large fraction of this total, it is not surprising that the curve for September follows quite closely.

Based on 88 years of typhoon data (Central Weather Bureau, Taiwan) the average number of typhoons per month decreases with time from 4.6 in August to 1.1 in December (Table 1). Use of our 24 years of data (not a subset of the Taiwan dataset) indicates that the greatest percentage of recurvature events occurs in September and October (60%), with only 8% recurving in December. One possible explanation of these data is that although there are more typhoons early in the season the jet stream does not extend far enough equatorward to initiate recurvature and transformation. To the contrary, in November and December the jet is much farther south although the ocean waters in in lower latitudes over which the typhoon must traverse ar~ colder, weakening the system before it can interact. It is also possible that increased shear in lower latitudes would be more likely to destroy the tropical cyclone at this time of the year. 5. Test of Hypothesis

Although a recurving typhoon is a single local event, one may speculate as to whether it has a short term signature in average jet stream KE. For the active year 1968, a running five day average of TKE normalized deviations indicates that most of the recurving typhoons were associated with above normal

10 activity at their time of OCCUlTenceor shortly thereafter (Fig. 10). This is a situation for which the lack of accounting for the latitude effect on KE would underestimate the anomalies. Of course, another inherent problem with such an analysis is that these are not the only events taking place which effect the KE. As previously stated, one of the possible effects of recurving typhoons is that they not only increase K' but P' as well. An open question is on what timescale this energy is transformed by the atmosphere. Does it happen immediately, or at a continuous rate, or at some later time?

From a more general perspective we wish to explore what relation may exist between the computed KE parameters. As a start, correlation coefficient were computed for normalized deviations averaged over monthly periods (Table 2). Deviations in TKE and ZKE have the highest correlation in August at .86 and gradually decline with time to .58 by November when eddy activity is usually increased. EKE also correlates with TKE quite well on a monthly basis, reaching a peak in September (.92). Although these results could have been anticipated since TKE is a sum of the elements it is being correlated with, the temporal changes in the correlation coefficient provide meaningful information. For example, the only 2 months when EKE and ZKE have any significant correlation are August and September, when the eddies are acting to increase the jet.

As for long time correlation, TKE deviation in November correlate fairly well with eddy KE deviations in August, September, and October (Table 2). For December there is a good correlation with eddy activity in September and October, but surprisingly none with November. On the scale of an entire season, deviations in TKE and ZKE (EKE) have a correlation coefficient of .80 (.88) (Table 2). EKE and ZKE deviations are much less correlated on this time scale (.43).

11 Since September has the highest correlation for most of the comparisons considered, EKE deviations for the month were correlated with the next several months (Table 3). Reasonably large correlations exist for October (.55) and December (.55) and weaker for November (.40). When the monthly values for October through December are averaged, the correlation with September is a robust.72. This indicates that if the eddies are more active than normal in September they will remain above normal for the next three months, on average. Physically, this result corroborates with the discussion in Section 1, where the significance of eddy contributions of potential and kinetic energy were discussed, and supported by the correlations in Table 2.

The correlation coefficients in Table 4 address the fIrst question posed in the introduction: the relationship between typhoon activity and normalized KE deviations. For the TKE there appears to be little to no correlation except possibly in September and October. For the entire five month season, the correlation is .39. The ZKE shows even weaker correlations, which do not appear to suggest any significance. The EKE however, has a fairly substantial correlation of .53 with September and the entire season. Although most of the correlations in this table are small and suggest limited correlation, the temporal trends may be significant. For instance, the EKE deviations correlate best in September and October which were shown to be the significant months for recurving typhoons ("power ranking" in Table 1). Other months, with lower power rankings also have smaller correlations.

As a further exploration of the shorter term effects of recurving typhoons in September and early October, 7 cases of storms in the strong and very strong categories in the region 45-60° N, 160-180° W were chosen for compositing (Table 5). The composite surface pressure field. (Fig lla) indicates a large cyclonic circulation centered over the Aleutian islands, which is an 18mb anomaly from climatological values (Fig. lIb). The Icelandic low is also present,

12 and deeper than normal. At-test of the 7 case anomaly field (Fig. l1c) indicates that the mean extratropical typhoon position is confident at 99%, as is the negative anomaly northwest of the Himalyas. The 8mb positive anomaly upstream of the mean cyclone position is also significant at 95% confidence.

At 500mb (Fig. 12), the mean height field indicates a large closed cyclone near the mean position of the composite surface cyclone, indicating an equivalent barotropic structure. This 22dam anomaly is also significant at the. 99th percentile. Other significant structures are the positive anomalies over the west coast of North America and the central and western Atlantic. In general, the anomaly field displays a wavenumber 5 signature, which is the approximate scale of the extratropical typhoon circulation. Five.days after this time (Fig. 13) there is no evidence of the closed low over the North Pacific. However, a positive (negative) anomaly over western (eastern) North America is very similar to that observed at time O. The ridge and a small portion of the trough are significant at 99% confidence. Another significant anomaly extends from China to the central Pacific where there is a positive anomaly. These results suggest that even though the effects are locally transient on short time scales, recurving typhoons which take a path similar to the events in this sample can produce large amplitude circulation anomalies downstream, which are stable in the short term.

As a means of exploring possible longer range influences of these systems, a composite of the six Septembers which had four or more recurving typhoons was constructed (Fig. 14). The most prominent anomaly, located over the Gulf of Alaska, has twice the magnitude of any other anomaly. Portions of this anomaly are significant at both the 95% and 99% confidence levels, as are the positive anomalies over the eastern United States and Siberia.

Composite anomalies for the Octobers which follow (Fig. 15) indicate a positive anomaly just south of the Aleutian islands. This anomaly, significant at

13 the 95% level, is located 20° longitude west of the September positive anomaly, as is the positive anomaly over the eastern Soviet Union. A similar anomaly field is also present for the following November (Fig. 16), with a large positive anomaly over the north Pacific, negative anomalies on either side, and positive anomalies over eastern North America and the eastern Soviet Union. However, the t-test for this data indicate only a small portion of the Pacific positive anomaly is significant at the 95% level.

One possible interpretation of these results is that the recurving typhoons produce a stable low frequency Rossby wave over western North America which retrogrades in time to the north Pacific. One means of testing this hypothesis would be to infer the wavelength (and hence the wavenumber) from the mean propagation speed of the anomaly, and cross compare these anomaly fields with the anomaly height field for that wavenumber in each month from a spatial Fourier transform. However, it must be reiterated that this was only a six case sample, which does not allow us to draw general conclusions. 6. Speculative Conjectures

The following is a summary list of some of the outcomes of this research:

-KE time series reveal that in the mean, the eddy energy at jet level grows concurrently with the ZKE until mid October, when the trend in EKE levels off .' . as ZKE continues to grow until the end of the year.

- Individual years exhibit a wealth of behavior among which are apparent regimes which involve interchanges of ZKE and EKE.

- Years with above normal EKE in September had above normal EKE the following 3 months. This is supported by a robust. 72 correlation of EKE deviations in September with average EKE deviations for October, November, and December.

14 - Monthly deviations in TKE and EKE correlate very well, with a peak of .92 in September. The only months which had significant correlations between EKE and ZKE deviations are August and September, when the eddies are acting to increase the jet.

- The greatest number of recurving typhoons occurs in September with the greatest number of strong «981mb) and very strong «961mb) storms in October. The percentage of strong and very strong storms per month was found to increase linearly from August to December, reflecting the increase in background baroclinicity.

- Climatological typhoon data revealed that 60% of all typhoons recurve in September and October, while only 8% do in December. This reflects the favorable combination of warm ocean waters and active jet stream which characterize the early autumn in this part of the world.

- The number of recurving typhoons correlates weakly with deviations in KE parameters on a monthly basis. The best correlations (.53) occurred for EKE in September and EKE for the 5 month season. Temporal behavior of the correlation coefficients are similar to the trend in the "power ranking".

- A seven case composite revealed that recurving typhoons are associated with a significant positive anomaly downstream over western North America which stabilizes on short time scales (5 days). A six year composite of Septembers with above normal recurving typhoons indicates that these years were associated with a westward propagating positive anomaly over the eastern Pacific which may be a low frequency Rossby wave excited by the typhoon activity.

Keeping in mind the inadequacies of the dataset (which would underestimate the effect which was sought) and the small sample on which the statistics and composites were based, there are nonetheless coherent results

15 which may permit some further speculation. It must also be reiterated that there is a wealth of variability inherent to the dataset which was not accounted for, and which could have been expected to overwhelm any signal which exists for recurving typhoons. For example, variability of ordinary baroclinic wave activity accounts for most of the KE variability. However, the data appear to suggest that:

- Recurving typhoons are associated with stable short term weather regimes which may have effects which last for at least 2 months.

- Years with above normal recurving typhoons are associated with above normal EKE at jet level.

Whether or not recurving typhoons play an active role in the annual spinup of the westerlies remains an unanswered question.

Based on these suggestions one may speculate as to the effects of these systems on weather regimes, and even those weather regimes which favor typhoons to recurve. This line of reasoning may be extended to long range weather prediction for which some usefulness may be made of the statistical data presented here.

Another item of spec.ulation is the possible connection between sea surface temperature (SST) anomalies in the tropical Pacific and recurving typhoons. Above normal SSTs over the western Pacific favors above normal typhoon activity, which increases the likelihood of more recurving storms. Such conditions seem plausible at the end of a La Nina cycle, and would provide a means of connecting tropical SST anomalies with circulation changes in the extratropics on short time scales.

A comparison of recurving typhoons during September 1991 with the 24 year data composite indicates it was an abnormal month. Although the 4 storms

16 observed is comparable with other years, 3 of these storms were in the strong category. The central pressure of 2 of these storms, Luke and Mirielle, rank 1st and 3rd respectively out of the 62 September storms in this study. These systems accompanied a regime breakdown, with the establishment of a large amplitude ridge along the west coast of North America, very similar to that in the composite. Also similar to the composite, this feature was stable for a long period of time, reflected by the fact that it did not rain in Seattle, Washington for a record 44 days. There is a suggestion that this positive anomaly propagated westward in October (NMC Climate Diagnostics Bulletin).

The results of this research seem to indicate that the effects of recurving tropical cyclones deserves further research. Inadequacies of this study could be overcome by using data throughout the troposphere and generating a larger sample size. In addition, it would be desirable to have a better understanding of what happens during the transformation process in these storms. Individual case studies would be of better benefit than the simple composite approach presented here. With these steps it may be possible to assess with greater authority the effect of recurving tropical cyclones on the general circulation. 7. Acknowledgements. During the course of this research, the author has benefited from numerous discussions with Gary Lackmann which resulted in a better understanding of the results. Thanks are also extended to Tiros Lee for providing the Typhoon summary produced by the Central Weather Bureau in Taiwan, as well as my advisors Lance Bosart and Dan Keyser for permitting me to stray from thesis research to explore this exciting topic.

17 References

Emanuel, KA., 1986: An air-sea interaction theory for tropical cyclones. Part I: Steady-state maintenance. J. Atmos. Sci., 43, 433-471. Erickson, C.O. and J.S. Winston, 1972: Tropical storm, mid-latitude cloud-band connections and the autumnal buildup of the planetary circulation. J. Appl. Meteor., 11, 23-36. Lorenz, E.N., 1955: Available potential energy and the maintenance of the general circulation. Tellus, 7, 157-167. Palmen, E., 1958: Vertical circulation and release of kinetic energy during the development of hurricane Hazel into an extra tropical storm. Tellus, 10, 1-23. Rosen, R.D., and D.A. Salstein, 1983: Variations in atmospheric angular momentum on global and regional scales and length of day. J. Geosphys. Res., 88, 5451-5470.

18 FIGURE CAPTIONS

Figure 1. Four box diagrams for (a) "ordinary" cyclones and (b) tropical cyclones which have become extra tropical. P' (P) represents eddy(zonal) potential energy, and K' (K) eddy (zonal) kinetic energy. Black (red) boxes indicate the relative magnitude of the energy variables before (after) the event. Figure 2. Twenty-four year mean trends in total kinetic energy (solid), zonal kinetic energy (dotted), and eddy kinetic energy (dashed).

Figure 3. Yearly (August-December) averaged daily normalized deviations for TKE (solid), ZKE (dashed), and EKE (dotted).

Figure 4. Daily time series of TKE (solid), ZKE (dotted), and EKE (dashed) for 1965. Day one is 1 August and day 153 is 31 December.

Figure 5. As in fig. 4 except for 1975.

Figure 6. As in fig. 4 except for 1974.

Figure 7. Power spectrum of the 1965 TKE time series with wave period on the abscissa, and amplitude on the ordinate.

Figure 8. Total number of typhoons which recurved per month in the 24 year period 1965-1988. Curve A is for all cases, curve B for strong «981mb) storms, and curve C for very strong storms «961mb). Dotted lines represent the percentage of strong (top) and very strong (bottom) storms.

Figure 9. Number of recurved typhoons per year (dotted) and September (solid) for 1965-1988.

Figure 10. Running five day mean daily deviations of TKE in 1968, normalized by the standard deviation. Shaded areas represent times of recurving typhoons.

Figure 11. Seven case composite of recurving typhoons (see Table 5) (a) mean sea level pressure (mb), (b) anomalies from 24 year mean (mb), and anomaly t-test (dashed is signicant at 95% level; solid is significant at 99% level). Markers indicate the positions of the individual storms.

Figure 12. Seven case composite of recurving typhoons (a) mean 500mb heights (m), (b) anomalies from 24 year mean (m), and (c) anomaly t-test as in Fig. 11. Markers indicate positions of the individ~al storms.

Figure 13.As in Fig. 12, except for five days after time zero.

Figure 14. Composite of 6 Septembers with above normal recurving typhoons (a) mean 500mb heights (m), (b) 500mb height anomalies (m), and (c) anomaly t-test as in Fig. 12.

Figure 15. As in Fig. 14 except for the following Octobers.

Figure 16. As in Fig. 14 except for the following Novembers. TYPHOON STATISTICS

Aug. Sep. Oct. Nov. Dec. Year 65 1 4 3 1 0 9 66 3 7 1 1 0 12 67 5 3 3 1 0 12 68 2 2 "3 2 0 9 69 2 0 4 2 0 8 70 2 4 0 0 0 6 71 2 4 0 1 0 7 72 2 2 3 0 0 7 73 1 0 0 0 0 1 74 1 2 1 0 0 4 75 2 3 1 3 0 9 76 1 3 0 1 0 5 77 0 2 3 0 0 5 78 1 3 2 1 1 8 79 2 1 3 0 0 6 80 3 1 3 1 0 8 81 4 3 2 0 0 9 82 3 4 2 0 0 9 83 0 1 1 1 0 3 84 2 1 4 1 1 9 85 1 4 1 0 0 6 86 2 2 2 1 0 7 87 2 2 2 0 0 6 88 0 4 2 0 0 6

44 62 46 17 2 171 Totals 25% 36% 27% 10% 1%

1.8 2.6 1.9 .7 .1 7.1 Average #

11 27 31 12 2 81 <981mb 25% 44% 67% 71% 100% 47%

0 4 12 6 1 23 <961 0% 6% 26% 35% 50% 13% " . 968mb 948mb" 936mb 938mb 934mb Deepest .53 .94 .87 .33 .05 Power Ranking 4.6 4.2 3.2 2.2 1.1 15.3 Avg. # Typhoons 40% 61.% 60% 32% 8% 47% % recurvers

'Table 1. Statistical summary of recurving typhoons. Numbers in the table refer to transformation events. See text for details. Aug. Sep. Oct. Nov. Dec. Year T,Z .86 .79 .66 .58 .61 .80 T,E .84 .92 .85 .60 .68 .88 Z,E .45 .50 .17 .30 .15 .43 TDN .49 .41 .47 .60 TDD .28 .56 .49 .04

rable 2.Correlation coeficients between deviations in the KE components. r(Z,E) represents monthly averaged total (zonal, eddy) kinetic energy deviations, ~nd TDN(TDD) represents the average total KE deviation in November (December) .

Oct. Nov. Dec. TKEDOND EKEDOND EKES .55 .40 .55 .62 .72

~able 3.Correlation coefficients between eddy KE deviations in September (EKES) ~nd eddy KE deviations in October, November, December, and total (eddy) KE ieviations averaged over October, November, and December (TKEDOND, EKEDOND).

NUMBER OF TYPHOONS Aug. Sep. Oct. Nov. Dec. Year TKE .08 .30 .37 .19 .05 .39 ZKE .04 .15 .25 .08 .14 .07 EKE .09 .53 .33 .13 .12 .53 i lable 4.Correlation coefficients between number of typhoons per month and Kinetic Energy normalized deviations averaged over the same months.

18 September 1966 948 52,160 October 1980 972 48,162 October 1980 968 46,178 5 September 1965 960 55,172 October 1966 972 52,168 ~ September 1968 968 55,160 October 1979 960 46,173

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