Long-term trends of winter monsoon synoptic circulations over the maritime continent:

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1 ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 11: (2010) Published online 6 April 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: /asl.272 Long-term trends of winter monsoon synoptic circulations over the maritime continent: Liew Juneng* and Fredolin T. Tangang Research Centre for Tropical Climate Change System, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM Bangi Selangor DE, Malaysia *Correspondence to: Liew Juneng, Research Centre for Tropical Climate Change System, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM Bangi Selangor DE, Malaysia. juneng@ukm.my This article was published online 6 April An error was subsequently identified. This notice is included to indicate that it has been corrected 15 April Abstract The present study examines the long-term trends of the winter monsoon northeasterly cold surge and Borneo vortex over the South China Sea. There has been a significant increase in the frequency of the Borneo vortex within the study period of The location of the center of the Borneo vortices has shifted slightly offshore; northward of western Borneo. This could result in less vortex land interaction and lengthen the life span of the vortices due to less frictional shear and hence increase the vortex days. These changes could be related to the strengthened easterly component of the cold surge winds. Copyright 2010 Royal Meteorological Society Keywords: Borneo vortex; northeasterly cold surge; linear trends; maritime continent; South China Sea; winter monsoon Received: 20 April 2009 Revised: 3 January 2010 Accepted: 15 February Introduction The maritime continent s climate is part of the large Asia Australia monsoon system. Surface circulations are dominated by two opposing monsoon regimes. During the winter (summer) monsoon, the surface winds over the maritime continent are predominantly northeasterlies (southwesterlies). Over the southern South China Sea (SCS), the winter monsoon is generally associated with the rainy season, while the region is generally dry during the summer monsoon. Apart from the annual cycle, there is a considerable interannual signature associated with the El Niño Southern Oscillation (ENSO) (Kripalani and Kulkarni, 1997; Aldrian and Susanto, 2003; Hendon, 2003; McBride et al., 2003; Chang et al., 2005; Juneng and Tangang, 2005) and the Indian Ocean Dipole (IOD) (Saji et al., 1999; Behera et al., 2008). At a shorter time scale of days, the surface climate may be influenced by the Madden-Julian Oscillation (MJO) (Chang et al., 2005; Tangang et al., 2008). During the winter monsoon, the weather is characterized by two synoptic systems the northeasterly cold surge and the low level cyclonic system known as the Borneo vortex (Chang et al., 2005). The cold surges originate from the cold air outbreak of the Siberian High, which then spreads toward the equator flanking the eastern edge of the anticyclone over eastern Asia (Chang et al., 2005). The low level northerly winds merge with the easterly trade winds over the SCS and are moistened by overwater trajectories (Johnson and Houze, 1987). The Borneo vortex appears as a quasi-stationary synoptic scale cyclonic disturbance located in the vicinity of northwestern Borneo Island. These disturbances may interact with the cold surge winds and intensify (Ramage, 1971; Chang et al., 2005). Occasionally, the intensified cyclonic system in the strength of a tropical depression or a tropical storm may propagate westward over the SCS and cause severe weather over the eastern coast of Peninsular Malaysia (Juneng et al., 2007). The monsoonal circulation is a component of the global hydrological cycle and is expected to be sensitive to recent global climate changes. Several studies have reported long-term and interdecadal changes in the large-scale winter monsoon circulations. Observational studies have reported a weakening of winter monsoon over the last decade (Gong and Ho, 2002, 2004; Xu et al., 2006; Wu et al., 2007; Yu and Zhou, 2007). Also, several global climate model (GCM) [Correction made here after initial online publication.] experiments have shown that under the global warming scenario, there will be a weakening of the Siberian High and a shrinking of the Aleutian Low. Generally, the studies propose a weaker East Asian winter monsoon in the coming decades (Hu et al., 2000; Hori and Ueda, 2006). In addition, despite the weakening winter monsoon, the relationship between Copyright 2010 Royal Meteorological Society

2 200 L. Juneng and F. T. Tangang Table I. The definition of indices characterizing the Borneo vortex and the cold surge winds during the winter monsoon. Index FREQ VORT/S VORT/L VLAT VLON USURGE VSURGE Definitions The frequency of vortex days in the 3-month period of December February. Through streamline analysis of the 925 hpa winds, any particular day with an identified closed circulation within the domain (Figure 1) and with a wind speed of 2 ms 1 wasidentifiedasavortexday. Spatially averaged seasonal vorticity within a small box (Figure 1). Spatially averaged seasonal vorticity within 105 E Eand2.5 S 5 N. Seasonal mean of the latitudinal positions of the identified vortices. Seasonal mean of the longitudinal positions of the identified vortices. Spatially averaged of the mean seasonal 950 hpa u-wind component within 110 E Eand 12.5 N 15 N. Spatially averaged of the mean seasonal 950 hpa v-wind component within 110 E Eand 12.5 N 15 N. the overall Asia Australian monsoon and ENSO has strengthened since the late 1970s (Wang et al., 2008). These changes are attributed to the increased magnitude and periodicity of ENSO. These large-scale changes are expected to modulate the synoptic flows downstream in the maritime continent, hence altering the characteristic of the maritime heat source. In the present study, we examine the historical longterm trends ( ) of the Borneo vortex and the northeasterly cold surge winds characteristic of the southern SCS during the winter monsoon. The next section provides a brief description of the data and methods used. The results and discussion are given in Section 3., while Section 4. concludes the study. 2. Data and methods Time series indices that represent the vortex frequencies, average vortex intensity, position of the vortex center and the northerly and easterly components of the cold surge winds were established for the trend analysis. The National Center for Environment Prediction (NCEP) reanalysis product (Kalnay et al., 1996) was used to derive these indices. The variables involved include the 6-h u- and v-components of 925 hpa winds. Only the data at 0000 UTC is used in the definition of the indices. The NCEP reanalysis product has been used to examine the East Asia winter monsoon and was found reasonable for the study (Zhang et al., 1997; Chase et al., 2003; Chang et al., 2005). The indices defined include vortex frequencies, seasonal average vorticity, the latitudinal and longitudinal positions of the vortex centers and the average speed of the u- and v-components of the surge wind. The definitions of the indices used are provided in Table I. Latitude (N ο ) Longitude (E ο ) Figure 1. The spatial distribution of the identified Borneo vortex centers. The dotted box is the area where the 925 hpa u- and v-wind components were averaged to define the USURGE and VSURGE indices, respectively. The solid large (small) box is the area from which the seasonal mean vorticity was averaged to form the index VORT/L (VORT/S). Refer to the text for a further description of these indices. The temporal trends of the indices were examined on the basis of seasonal values using a linear regression analysis (parametric). In addition, the sequential Mann Kendall (nonparametric) test was also used to ensure the robustness of the trends detected. Generally, the results from both the trend tests were identical. The discussion of the trends in the following sections of this article is based only on the linear regression method. 3. Results and discussion 3.1. Long-term changes of the Borneo vortex Figure 1 shows the geographical domain from which the centers of the Borneo vortices are identified. Climatologically, the vortices were concentrated at the western tip of Borneo Island. We first examined the long-term changes of the vortex frequencies (FREQ). Interestingly, the time series showed a considerable upward trend, significant at the 5% level (Figure 2(a)). Based on the trend estimated by the linear regression analysis, the vortex occurrence had increased by a rate of 7% per decade over the study period of An increase in vortex frequencies may have increased the regional background vorticity, which may have had an implication on the seasonal moisture budget in the southern SCS. The area s average vorticity within the smaller box (VORT/S; Figure 2(b)) showed a remarkable downward trend, whereas the vorticity averaged within the larger box (VORT/L; Figure 2(c)) shows an upward trend. The upward trend of the VORT/L is consistent with the increment of the FREQ. The opposing trends of the VORT/S and VORT/L may suggest a spatial shift of the vortex centers from the mean location.

3 Long-term trends of winter monsoon synoptic circulations 201 Figure 2. The time series and the linear regression lines for the defined indices: (a) the frequency of the vortex day, (b) the area average seasonal mean vorticity within the smaller box, (c) the area average seasonal mean vorticity within the larger box, (d) the average latitudinal positions of the identified Borneo vortices, (e) the average longitudinal positions of the identified Borneo vortices, (f) the average u-wind component of the cold surges and (g) the average v-wind component of the cold surges. To verify possible long-term changes of the vortex center positions, we examined the seasonal mean of the latitudinal (VLAT) and longitudinal (VLON) positions of the identified vortices (Figure 2(d) and (e)). Generally, the trend analysis showed a significant northward shift of the latitudinal position of the vortex center from 1.7 to2.3 N over western Borneo. The VLON also portrayed a slight westward shift but the trend did not pass the 5% level significance test. The results suggested that there was a slight northwest shift of the vortex centers over the 46-year study period. The shift is particularly remarkable after the 1980s. This implies more vortices over the marine area and less over the Borneo land mass. Figure 3 shows the differences of the climatology of the 925 hpa horizontal vorticity between the two epochs of and The map suggests a decreasing background vorticity over the western Borneo land mass, while the vorticity over the marine area of the southern SCS increased markedly. The result was in tandem with the described northwestward shift of the vortex centers. The shift may have modified the life cycle of the vortices as frequent positioning of the vortex center over the marine area means less interaction with the surface topography. This may therefore promote more vortex days Long-term changes of the northeasterly cold surge The Borneo vortex and the northeasterly cold surge interacting with each other and with the local topography coupled with the modulation by the MJO and the ENSO resulted in a complicated weather system over the region (Chang et al., 2005; Tangang et al., 2008). We further examined the u- and v-components of the northeasterly cold surge wind. The trend analysis suggests no significant long-term trend in the VSURGE (Figure 2(g)). However, the USURGE (Figure 2(f)) component showed a significant negative trend. This indicates that the easterly component of the surge wind was getting stronger but the northerly component did not change over the study period. The low level cold surge wind over the SCS

4 202 L. Juneng and F. T. Tangang system to remain for a longer period of time on the marine area, which thereafter lead to more vortex day counts (Figure 2(a)). As a result, the background vorticity over the southern SCS increased over the study period of (Figure 2(c)). 4. Summary and concluding remarks Figure 3. The seasonal vorticity climatology (unit: 10 5 s 1 ) computed from period minus that which was computed from period. Table II. The cross-correlation table for six of the defined indices. FREQ VORT/ L VLAT VLON USURGE VSURGE FREQ VORT/L VLAT VLON USURGE VSURGE 1.00 The correlation coefficients significant at 0.01 level are shaded. had become more easterly and intense, while the axis of the surge wind had titled northwestward. The result is consistent with Wang et al. (2009). To understand the relationships between the variations in the cold surge wind and that of the Borneo vortex, we computed the correlation coefficients between the defined indices. The correlation matrix is provided in Table II. The result shows that the average vorticity is not directly associated with the USURGE, but is negatively correlated to the VSURGE. This implies that a stronger northerly wind increased the background VORT/L. However the vortex occurrence, FREQ does not appear to be correlated to VSURGE, but strongly correlated to USURGE ( 0.63), VLAT (0.54) and VORT/L (0.79). This suggests that more vortex days are associated with the stronger zonal component of the cold surge wind when the center positions of the vortices are more north. Note that there is a significant correlation between the VLAT and the USURGE ( 0.46). Based on this result, we hypothesized that the changes in the cold surge axis (a stronger easterly surge wind, Figure 2(f)) resulted in less steering of the cyclonic vortex far south toward the Borneo land mass (Figure 2(d)). This enabled the In this study, we analyzed the possible long-term changes in the Borneo vortex and the northeasterly cold surge two synoptic scale systems that characterized the surface meteorology during the winter monsoon. The results suggest that both of these systems exhibited significant long-term trends during the 46- year period of The occurrence frequency of the cyclonic vortex increased markedly. We hypothesized that this was due to the strengthening of the cold surge easterly component, which tilted the surge axis and promoted less steering of the vortices toward the Borneo land mass. This allowed the cyclonic vortices to remain for a longer period of time over the marine environment, resulting in an increase in the average low level vorticity. An interesting question to ask is how the changes in this cold surge easterly component relate to the large-scale circulation changes over the study period. The Siberian High has weakened due to reduction of sea level pressure over most of the middle to high latitude Eurasia since 1970s (Gong and Ho, 2002). Together with changes in other teleconnection patterns, such as the Artic Oscillation and the Eurasian teleconnection pattern, the weakening of Siberian High was associated to the warming of the entire middle to high latitude Asia. This may have altered the land sea thermal contrast between the continent and adjacent SCS and resulted in stronger easterly surges during the winter monsoon. It is worth to mention that apart from the winter monsoon, there is also pronounced weakening trends of the summer monsoon since the late 1970s, which may be related to the warming of Indian Ocean and Western Pacific, the weakening of sensible heat source over the Tibetan Plateau, and the aerosol forcing, as well as internal variability (Zhou et al., 2009). The weakening tendency of winter and summer monsoons is also evident in the global land monsoon precipitation, which is actually a measure of local summer minus winter monsoon precipitation change (Zhou et al., 2008a, 2008b). Generally, the exact mechanisms that are responsible for the observed changes in the large-scale monsoon system remain elusive. On the other hand, changes in the tropical maritime latent heat source may also result in the modification of the regional circulation over the SCS. Hence, it is particularly unclear whether this change is part of the large-scale monsoon circulation changes or whether it is a response to modified regional air sea processes. Nevertheless, these synoptic scale changes are expected to be associated with the changes in

5 Long-term trends of winter monsoon synoptic circulations 203 rainfall distribution. Further studies will focus on linking these regional changes and the local hydrological cycles and monsoon rainfall characteristics. Also, further clarification is needed to understand the physical link between these observed changes and that of the large-scale circulation associated with global climate change. Acknowledgements This research is funded by the Malaysian Government Science fund Grants No SF413, SF0437, UKM-GUP-ASPL and the Universiti Kebangsaan Malaysia Research University Grant, UKM-OUP-PI / References Aldrian E, Susanto RD Identification of three dominant rainfall regions within Indonesia and their relationship to sea surface temperature. International Journal of Climatology 23(12): Behera SK, Luo JJ, Yamagata T Climatic impacts of Indian Ocean dipoles, El Niño-Southern oscillation, and their interaction with the monsoon systems in the Asia-Oceania region. 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