E Advq hPa diff

Figure 10. (Continued )

termed the interactive Rodwell-Hoskin (IRH) mechanism, while the advection associated with the cross-continental winds is termed ventilation. Thus, the local effects of evaporation and Fnet initiate the convection and the effects of ventilation, and the IRH mechanism modifies the summer monsoon rainfall pattern.

In this study, we first analyzed the local effects, including evaporation and the dynamical feedback of Fnet. The evaporation over land is associated with soil moisture. Soil moisture does affect the summer monsoon rainfall and extend the rain zone farther northward, but its impact is not as dominant as the IRH mechanism and ventilation. We then examined the ventilation and the IRH mechanism. Both mechanisms not only determine the poleward extent of the summer monsoon rain zone but also induce the east-west asymmetry of the rain zone. In ventilation, the cross-continental flow transports low moist static-energy air from ocean regions to the western part of continent and disfavors convection over the region. In the IRH mechanism, the Rossby wave subsidence induced by the monsoon convective heating and the interaction with the monsoon circulation suppress convection over the western part of the continent and enhance convection over the eastern part of the continent. In the IRH mechanism, the horizontal transport of temperature and moisture is a feedback via the monsoon circulation, whose strength is determined by land-ocean heating contrast. In other words, land-ocean heating contrast also affects the poleward extent of the summer monsoon rain zone through the IRH mechanism. Both land and ocean conditions affect land-ocean heating contrast. The land condition can be influenced by topography and surface type associated with soil moisture and surface albedo. The ocean condition is affected by ocean heat transport and ocean heat storage.

We further examined impacts of those mechanisms on the Asian summer monsoon. The increase of soil moisture enhances the Asian summer monsoon rainfall and extends its rain zone northward. However, ventilation and the IRH mechanism have more substantial impacts on the Asian summer monsoon rainfall. For these mechanisms, the effect associated with the moisture transport is particularly important to the Asian summer monsoon. When suppressing ventilation and the IRH mechanism, the pattern of the Asian summer monsoon becomes similar to the Fnet pattern even with unsaturated soil moisture. In land-ocean heating contrast, the elevated heating source of the Tibetan Plateau enhances the meridional gradient of tropos-pheric temperature and strengthens the Asian summer monsoon circulation. Land surface albedo associated with snow cover and ocean heat transport also modify land-ocean heating contrast and change the Asian summer monsoon circulation and the northward extent of the monsoon rain zone. Local SST can sometimes be important to subregional monsoon systems when the variation of land-ocean heating contrast is weak, and thus it can also affect the northward extent of the Asian summer monsoon rain zone.


This work was supported under National Science Council grant 93-2111-M-001-001. It has been a pleasure to work with Prof. J. D. Neelin, Dr. H. Su, Prof. J.-Y. Yu and others.

[Received 29 December 2005; Revised 5 September 2007; Accepted 17 September 2007.]


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Coupling of the Intraseasonal Oscillation with the Tropical Cyclone in the Western North Pacific during the 2004 Typhoon Season

Huang-Hsiung Hsu, An-Kai Lo, Ching-Hui Hung, Wen-Shung Kau and Chun-Chieh Wu Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

[email protected]. ntu. edu. tw

Yun-Lan Chen

Weather Forecast Center, Central Weather Bureau, Taipei,, Taiwan

A strong in-phase relationship between the intraseasonal oscillation (ISO) and the tropical cyclone (TC) was observed in the tropical western North Pacific from June through October 2004. The ISO, which is characterized by the fluctuations in the East Asian monsoon trough and the Pacific subtropical anticyclone, modulated the TC activity and led to the spatial and temporal clustering of TCs during its cyclonic phase. This clustering of strong TC vortices contributed significant positive vorticity during the cyclonic phase of the ISO and therefore enlarged the intraseasonal variance of 850 hPa vorticity. This result indicates that a significant percentage (larger than 50%) of observed intraseasonal variance along the clustered TC tracks in the tropical western North Pacific came from TCs. Numerical simulation confirmed that the presence and enhancement of TCs in the models enlarged the simulated intraseasonal variance. This implies that the contribution of TCs has to be taken into account to correctly estimate and interpret the intraseasonal variability in the tropical western North Pacific.

1. Introduction

The tropical western North Pacific is an active region for the tropical cyclone (TC) and the intraseasonal oscillation (ISO) in the northern summer (e.g. Lau and Chan, 1986; Wang and Rui, 1990; Elsberry, 2004; Hsu, 2005). Many studies have revealed the modulation effect of the ISO on the TC activity in this region (e.g. Nakazawa, 1986; Heta, 1990; Liebmann, et al., 1994; Maloney and Dickinson, 2003). According to these studies, TCs tend to cluster in the westerly and positive vorticity phase of the ISO in the lower troposphere. Under the circumstances, one would wonder whether the clustering of strong TC vortices may in turn increase the overall amplitude of positive vorticity in this phase, and therefore increase the intraseasonal variance (ISV) of vorticity. If this effect is in action, the ISV may not result entirely from the ISO itself. Instead, part of the variance may come from the clustered TC activity. This possibility has been explored and confirmed by Hsu et al. (2008), using the European Centre for Medium-range Weather Forecast reanalysis (ERA40, 1958-2002). This new finding suggests that the TC contribution must be considered to obtain a better understanding of the intrasea-sonal variability in the tropical western North Pacific.

The typhoon season (defined as June-October, JJASO) of 2004 was a unique season in the western North Pacific, in terms of the strong TC and ISO activity, and the in-phase relationship between the two. The most significant phenomenon was the record-breaking number (10) of typhoon landfalls in Japan. Another interesting feature was the temporal clustering, on the intraseasonal time scale, of the tropical cyclone genesis during the summer and early autumn. It was this strong in-phase relationship that led to the record-breaking typhoon landfalls in Japan, because the clustering effect of the ISO on TC resulted in the tendency for the typhoons to move along similar tracks (Nakazawa, 2006). In view of this close relationship between the TC and the ISO, this unique season enables an excellent case study of the coupling of the ISO and the TC. It is the goal of this study to explore the relationship between the TC and the ISO in this particular season, and to evaluate the contribution of the TC to the ISV.

In this study, an unconventional approach applied by Hsu et al. (2008) was employed in removing TCs from the global analysis. The potential contribution of TCs was estimated, which was defined as the variance difference between the original and TC-removed vorticity fields at 850 hPa. Numerical experiments based on a regional model and a general circulation model (GCM), with/without TCs and with enhanced TCs, were also carried out. Since similar results were obtained in GCM and regional model simulations, only the GCM results will be shown here for the sake of brevity. The approaches adopted in this study were designed to shed light on the TC contribution to the ISV along the TC tracks. Our results indicate that the differences in variance (i.e. the effect of TCs on the ISV) are large enough to be of concern on the intraseasonal time scale.

The arrangement of this article is as follows. Section 2 describes the data and methodology. ISO modulation on TCs is presented in Sec. 3, and the TCs' potential contribution to the ISV is reported in Sec. 4. Simulation results and conclusions are presented in Secs. 5 and 6, respectively.

2. Data and Methodology

The wind and mean sea level pressure (MSLP) data used in this study were retrieved from National Centers for Environmental Prediction (NCEP) Reanalysis I on a 2.5°-by-2.5° grid, while the TC statistics and best-track data were obtained from the Japanese Meteorological Agency (JMA) and the Joint Typhoon Warning Center (JTWC), respectively. A 32-76-day Butterworth band-pass filter (Kaylor, 1977) was applied to NCEP Reanalysis I to extract the intraseasonal fluctuations from the daily mean vorticity. The reason for the choice of the 32-76-day band will be presented in a later section. To reduce the end effect of the Butterworth filter, only data from 16 June to 15 October were used to calculate the ISV.

In reality, due to the possible nonlinear TC-climate interaction, it is impossible to exactly quantify the TC contribution. This essentially holds good for all studies involving multiple temporal and spatial scales. Despite this concern, temporal and spatial filtering has often been used to decompose a total field into perturbation and mean flow, or even into several subfields of distinct temporal or spatial scales. This linearthinking approach has proved useful in providing important insights into climate processes, such as the wave-mean flow interaction. In order to evaluate the possible TC contribution to the ISV in the western North Pacific during the 2004 typhoon season, a spatial filtering approach was taken to remove the TC vortices from the 850 hPa vorticity. This filtering procedure is similar to the decomposition of the total field into perturbation and background flow, but is performed in a more sophisticated manner. The potential contribution of TCs to the ISV was estimated by calculating the difference between the ISV of the original and the TC-removed vor-ticity at 850 hPa.

Removal of TCs from the analysis data has been a common practice in typhoon and hurricane simulation and forecasting. The procedure, which has been used in the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane prediction system (Kurihara et al., 1993, 1995) and in typhoon simulations (Wu et al., 2002) for enhancing the representation of the environmental field in the initial condition, has proved effective in improving the overall TC track forecast. Following the procedure proposed by Kurihara et al. (1993, 1995), the four-time daily 850 hPa winds associated with each TC, based on the JTWC best track, were subtracted from the 850 hPa wind field during the typhoon season.

The basic procedure, demonstrated in Fig. 1, is briefly described as follows. [See Kurihara et al. (1993, 1995) for details.] The zonal and meridional winds were individually separated into basic and disturbance fields using a smoothing operator. The winds associated with a TC were isolated in the filter domain, which

Figure l. Flow chart of the TC removal procedure.

defines the extent of a TC in the global analysis, and subtracted from the disturbance field to create a non-TC component. In the procedure, 1200 km is specified as the radius of the filter domain. This does not mean that everything in the domain is identified as the TC component. It is simply the longest distance for the procedure to automatically search the effective radius of a TC in 24 directions (for every 15°) surrounding the TC center. When the radius of a TC, e.g. 500 km, is identified in a particular direction, the searching stops, and only the TC winds within the radius are subtracted. The reason for choosing a larger domain is to avoid missing the TC circulation in a TC (or typhoon) that has a large radius, and which would lead to underestimation of the TC circulation. After removing the TC component from the disturbance field, the non-TC component was added to the basic field to form the environmental flow, which is the TC-removed wind field in this study. The environmental flow outside the filter domain is identical to the original global analysis. The original and TC-removed vorticity fields were calculated based on the corresponding wind fields.

Hsu et al. (2008) demonstrated that the TC-removing procedure is able to correctly separate the TC and environmental components. One example is shown in Fig. 2 — three typhoons appeared simultaneously in the western North Pacific on 28 August 2004. They appear as three isolated vortices (i.e. the TC component) in Fig. 2(b), while the environmental flow (background flow plus non-TC component) is clearly characterized by the zonally elongated monsoon trough [Fig. 2(c)] across the South China Sea and the Philippine Sea. This result, as in many other cases, indicates that the TC removal procedure removes mainly the TC vortex and accurately retains the large-scale circulation, along with the climate variability contributed by the large-scale fluctuations.

Figure 2. 850 hPa vorticity fields on 26 August 2004, when three typhoons were observed in the tropical western North Pacific: (a) total field, (b) TC and (c) environmental component. The unit for the contour is 10_6 s~1.

One potential problem in removing TCs from the global analysis, such as NCEP Reana-lysis I, is the possible mismatch between the JTWC TC track, which defines the center of a TC, and the position of the TC-corresponding vortex in NCEP Reanalysis I. This is partially attributed to the coarse resolution of the data set commonly used in climate study. The JTWC best track and NCEP Reanalysis I were examined, and a general consistency, although not an exact match, was found. The main goal of this study is to statistically assess the gross contribution of TCs to the climate variability in a large domain covering the entire tropical western North Pacific. A mismatch of a few degrees in latitude and longitude would not seriously affect the overall results. As will be seen later, the removed vorticity is collocated nicely with the TC tracks. The possible mismatch does not seem to cause problems in this study. TCs may be presented differently in different global analyses or in different resolutions. Hsu et al. (2008), who compared the results derived from the European Centre for Medium-range Weather Forecast reanalysis and the NCEP reanalysis, and also between different spatial resolutions, demonstrated the general consistency between the analyses and the effectiveness of the TC-removing procedure adopted in this study. Their results suggest that the overall results presented here are not affected by different analyses and spatial resolutions, although certain quantitative differences may be found.

Another concern is the underrepresentation of the wind speed and vorticity of TCs in the global analysis. This is an existing problem, which cannot be solved in this study. The study's goal is to demonstrate how TCs contribute significantly to the ISV, based on presently available global analysis. The results should be viewed as the TC effect represented in currently available global analysis, which has been widely used to estimate the ISV. If the exact location and strength of TCs are represented in the global analysis, the actual contribution of TCs will likely be larger than what was found in this study.

Based on the JMA statistics, there were 29 named TCs (including tropical storms and typhoons) in the western North Pacific during the 2004 typhoon season, slightly more than the climatological mean of 26.7. Most of the TCs in this period tended to appear in clusters quasi-periodically, as noted in previous studies

(e.g. Gray, 1979; Nakazawa, 2006). This clustering phenomenon is closely associated with the fluctuations of the monsoon trough and subtropical anticyclone (e.g. McBride, 1995; Harr and Elsberry, 1998; Elsberry, 2004). The tendency of TC occurrence in or near the East Asian (EA) monsoon trough was particularly evident in 2004. Climatologically, the movement of TCs can be roughly classified into two types: straight-moving track and recurving track. The former type of TC moves across the Philippine Sea in the northwest direction toward southern China and the Indochina peninsula, while the latter type recurves northeastward following a certain period of northwestward movement over the Philippine Sea. During the June-October period, the ratio between the straight-moving and the recurving track is about 1:1 for those TCs formed south of 20° N (Harr and Elsberry, 1998). However, most of the TCs in JJASO 2004 moved northwestward over the Philippine Sea, recurving northeastward when approaching the subtropics. This clustering phenomenon can be seen clearly in Fig. 3, which presents the

Figure 3. Monthly mean 850 hPa streamline and named TC tracks from (a) June to (e) October 2004, and (f) the difference between the June/August/October and July/September mean circulation at 850 hPa.

monthly-mean 850 hPa streamlines and the TC tracks for each month from June to October. Note that the recurving occurred most evidently in June, August, and October, when TCs were more active.

The EA monsoon trough extended southeastward from the Indochina peninsula to the Philippine Sea, and contracted westward intermittently from June to October. There were more-than-average numbers of tropical storms in June (5), August (8), and October (4), compared to the 1971-2000 climatological mean numbers of 1.7, 5.5, and 2.8, when the EA monsoon trough was active and extended further southeastward than normal. Fewer tropical storms occurred in July (2) and September (3), compared to the climatological mean numbers of 4.1 and 5.1, respectively, when the EA monsoon trough was weak and contracted westward. It is known that TCs tend to occur in or near the EA monsoon trough (e.g. Elsberry, 2004; Chen et al, 2004). This relationship seems particularly evident in JJASO 2004. Most of the TCs also seemed inclined to move in a clockwise direction along the southern and western peripheries of the Pacific anticyclone. This movement pattern is similar to the recurve-south track identified by Harr and Elsberry (1998). It appears that the fluctuations of the EA monsoon trough and the Pacific anticyclone strongly modulated the TC activity in this particular season.

The intermittent occurrence of the eastward extension and westward contraction of the EA monsoon trough in JJASO 2004 was associated with strong ISO activity. To identify this intraseasonal signal, an index was designed to represent the fluctuation in the EA monsoon trough. The index is defined as the MSLP averaged over the region (120°E-150°E, 10°N-20°N), where the extension and contraction of the EA monsoon trough is most evident (Fig. 3). Wavelet analysis (Torrence and Compo, 1998) on this daily index from April to December 2004 was performed to identify the dominant periodicity. As shown in Fig. 4(a), large fluctuations are evident in the 32-76-day and 1127-day bands throughout the period. The accumulated variance explained by the 32-76-day perturbations accounted for 54.5% of the total variance in JJASO 2004, while the 11-27-day period accounted for 29.3%. The 32-76-day ISO was apparently the major fluctuation to affect the EA monsoon trough. In view of its dominance in variance, the following discussion will focus on the 32-76-day ISO. To further reveal the uniqueness of the this ISO in 2004, wavelet analysis was performed on the EA monsoon trough index during the June-October season from 1951 to 2004 annually. The result shown in Fig. 4(b) reveals that the 32-76-day variance in 2004 was the largest from 1951 to 2004, indicating the strongest intraseasonal fluctuation of the EA monsoon trough in this 54-year period. This variance (almost 3hPa2) is much larger than the second-largest variance (about 2hPa2) occurring in the 1979 summer, which was known as a summer of strong ISO activity (Lorenc, 1984). The JJASO of 2004 was obviously a unique season for the 32-76-day ISO. The reason for this large ISV is not clear and will be explored in other studies.

Spatial distribution of the 32-76-day ISV of MSLP in JJASO 2004, shown in Fig. 5(a), exhibits two centers of maximum variance: one over the Philippine Sea, and in the extratropical North Pacific. A comparison between Fig. 3 and Fig. 5(a) indicates that the large variances over the Philippine Sea and the extratropical North Pacific were associated with the movement and fluctuation of the EA monsoon trough and the Pacific anticyclone during the season, respectively. The shading shown in Fig. 5(a) indicates the percentile of the 32-76-day variance in JJASO 2004 at every point, compared to all JJASO variances in the 54-year period. The area exceeding the 90th percentile covers most of East Asia and the western North Pacific, while small variance appears in the Indochina peninsula, the South China Sea, and the equatorial

Figure 4. (a) Wavelet coefficients for the EA monsoon trough index (see text), (b) the percentages of the index variance explained by the 11—27-day (blue bar) and 32—76-day (red bar) bands for the 54 years from 1951 to 2004. The vertical axis in (a) and (b) denotes the period in days and percentage, respectively, while the horizontal axis in (a) and (b) denotes dates from June to October 2004 and years from 1951 to 2004, respectively.

Figure 4. (a) Wavelet coefficients for the EA monsoon trough index (see text), (b) the percentages of the index variance explained by the 11—27-day (blue bar) and 32—76-day (red bar) bands for the 54 years from 1951 to 2004. The vertical axis in (a) and (b) denotes the period in days and percentage, respectively, while the horizontal axis in (a) and (b) denotes dates from June to October 2004 and years from 1951 to 2004, respectively.

western Pacific. The area chosen to construct the index is well inside the 95% region. The index designed to reflect the large fluctuation in the monsoon trough appears adequately chosen. The distribution shown in Fig. 5(a) indicates that the unusually active ISO in JJASO 2004 occurred not only in the EA monsoon trough but also in other EA summer monsoon regions (e.g. eastern China, Taiwan, Japan, and Korea).

The spatial distribution of the 32-76-day OLR variance is shown in Fig. 5(b) to reveal the ISV in convection. A large variance area exceeding the 90th percentile is observed in the tropical western North Pacific, located to the southeast of its MSLP counterpart. A large variance and high percentile area is also found near Japan, reflecting the large number of typhoons affecting Japan and possibly the clustering effect of the ISO on the typhoon activity. These results indicate that the anomalously active intraseasonal fluctuations existed in both the circulation and convection fields during JJASO 2004.

The strong ISV of the EA monsoon trough and the Pacific anticyclone apparently resulted in the spatial and temporal clustering of TCs. This close relationship is shown in Fig. 6, which presents the composite of MSLP and the TC tracks during three cyclonic phase months (June, August, and October — upper panel) and two anticyclonic phase months (July and September — lower panel) appearing intermittently as shown in Fig. 3, respectively. The majority of TCs during JJASO occurred in the cyclonic phase, as indicated by the negative MSLP anomaly, and tended to take a recurving track, while only a few appeared in the anticyclonic

Figure 5. Spatial distribution of the 32-76-day filtered (a) MSLP and (b) OLR variance in JJASO 2004. The contour intervals are 2 hPa2 and 200 (W/m2 )2 for MSLP and OLR, respectively. Shading indicates the percentile of the 2004 variance in the 54-year period from 1951 to 2004.

phase in an environment of the positive MSLP anomaly.

The close ISO-TC relationship is further demonstrated in Fig. 7, which presents the 32-76-day fluctuation in the MSLP averaged between 120°E and 150°E, and the latitudinal positions of TSs that happened to be situated in this longitudinal band. A sequence of the 3276-day ISO propagated northward from 10°N to 30°N regularly from June to October. Evidently, many more TSs appeared in the negative phase than in the positive phase of the ISO and moved northward in an anomalous low-pressure environment. As found in many previous studies, the genesis and track of the TCs in the tropical western North Pacific during JJASO 2004 were strongly modulated by the intermittent

Figure 6. Composites of MSLP for (upper) three cyclonic phases and (lower) two anticyclonic phases. The TC tracks during these two phases are also marked. Periods chosen for composites are shown at the top of each figure.

extension and retraction of the monsoon trough, which was in turn affected by the unusually strong ISO.

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