Effects of monsoon trough interannual variation on tropical cyclogenesis over the western North Pacific

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Key Points: The thermodynamic impact is comparable to the dynamic impact The strong year sets a more favorable condition Supporting Information: Readme Figures S1 S4 Text S1 Correspondence to: T. Li, timli@hawaii.edu Citation: Cao, X., T. Li, M. Peng, W. Chen, and G. Chen (2014), Effects of monsoon trough interannual variation on tropical cyclogenesis over the western North Pacific, Geophys. Res. Lett., 41, 4332 4339, doi:. Received 22 APR 2014 Accepted 22 MAY 2014 Accepted article online 27 MAY 2014 Published online 17 JUN 2014 Effects of monsoon trough interannual variation on tropical cyclogenesis over the western North Pacific Xi Cao 1,2,3, Tim Li 2, Melinda Peng 4, Wen Chen 1, and Guanghua Chen 1 1 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China, 2 International Pacific Research Center and School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA, 3 School of Earth Science, University of Chinese Academy of Sciences, Beijing, China, 4 Naval Research Laboratory, Monterey, California, USA Abstract The western North Pacific monsoon trough (MT) exhibits marked interannual variation (IAV) associated with El Niño Southern Oscillation forcing. The role of MT IAV in tropical cyclone (TC) development was investigated using the Advanced Research Weather Research and Forecasting model placed on a beta plane. It was found that MT IAV has a great influence on vortex development. In strong years, the MT provides more favorable environmental conditions primarily through enhanced low-level vorticity, convergence and midlevel moisture for TC formation and vice versa in weak years. Sensitivity experiments that separated the dynamic and thermodynamic (moisture) factors from strong MT IAV showed that the thermodynamic impact associated with MT IAV is comparable to the dynamic impact. 1. Introduction The western North Pacific (WNP) is globally the most active basin for tropical cyclogenesis. In boreal summer, the largest warm pool is situated over the WNP with sea surface temperatures (SSTs) typically higher than 29 C. The summer mean circulation over the WNP is characterized by a monsoon trough (MT), a convergence between the monsoon westerlies and the trade easterlies, and a meridional shear line in the lower troposphere over the region of (5 20 N, 120 150 E). Compared to the thermodynamic factors, such as SST, the dynamic factors, such as low-level vorticity and divergence, play a more important role in determining tropical cyclone (TC) formation in the WNP [Fu et al., 2012; Peng et al., 2012]. Feng et al. [2014] indicated using 20 year reanalysis data that monsoon shear, confluence, and a reverse-oriented MT account for approximately 80% of TC genesis over the WNP. TC genesis frequency in the WNP shows a pronounced intraseasonal-to-interannual fluctuation [see Li, 2012 for a review]. Previous observational studies indicate that the interannual variation (IAV) of TC genesis is closely associated with the El Niño Southern Oscillation (ENSO) phenomenon [e.g., Chan, 1985, 2005; Lander, 1994; Chen et al., 1998; Wang and Chan, 2002; Camargo et al., 2007; Chen and Huang,2008;Cao et al., 2014]. During El Niño summers, TCs appear more frequently in the southeast quadrant and less frequently in the northwest quadrant over the WNP [Wang and Chan, 2002]. Such an eastward extension coincides well with the underlying warmer SST, enhanced convection and low-level cyclonic vorticity, increased midlevel relative humidity, and reduced zonal vertical shear in the southeast quadrant over the WNP [Wu et al., 2012]. In spite of the large-scale nature of El Niño and its control, the correlation between the number of TCs generated over the WNP and Niño3.4 index from July to October is not statistically significant during the 32 year period, from 1979 to 2010 [Li, 2012]. Li [2012] indicated that the insignificant correlation is due to the fact that the number of WNP TCs increases slightly during El Niño developing summers when eastern Pacific SST anomalies (SSTAs) are significantly large, but decreases markedly during El Niño decaying summers when eastern Pacific SSTAs are near normal. This is because SSTA-induced westerly anomalies in the equatorial western central Pacific strengthen the cyclonic shear and shift the MT southeastward during El Niño developing summers, whereas a pronounced large-scale anomalous anticyclone suppresses TC formation in the WNP during El Niño decaying summers [Wang et al., 2003; Li, 2012]. This ENSO-phasedependent TC characteristic resembles that of atmospheric intraseasonal oscillation (ISO). Lin and Li [2008] CAO ET AL. 2014. American Geophysical Union. All Rights Reserved. 4332

found that there is stronger ISO activity over the WNP during El Niño developing summers and weaker activity during decaying summers. It has also been shown that ISO activity greatly influences TC formation in the WNP [e.g., Liebmann et al., 1994; Maloney and Hartmann, 2001; Hsu et al., 2011; Cao et al., 2012]. One recent modeling study showed that MT ISO exerts strong control over TC formation through both dynamic and thermodynamic (moisture) effects (see the supporting information). It was that study which motivated us to investigate to what extent MT IAV modulates TC formation, and what the major differences are between the effects of MT ISO and MT IAV. The structure of this paper is as follows. In section 2, we describe the large-scale anomalies of MT IAV and the design of our numerical experiment. In section 3, we examine the physical processes through which MT IAV affects vortex development, with particular focus on the relative role of the dynamic and moisture fields of strong MT IAV in affecting TC formation. In section 4, we conclude the paper with a summary and brief discussion of the major findings. 2. The Patterns of MT IAV and Experimental Design 2.1. Large-Scale Anomalies of MT IAV To examine how MT IAV may affect vortex development, the large-scale anomalies associated with MT IAV were first obtained from National Centers for Environmental Prediction (NCEP) reanalysis data [Kalnay et al., 1996]. The monthly outgoing longwave radiation (OLR) data set from the National Oceanic and Atmospheric Administration (NOAA) [Liebmann and Smith, 1996] was used to measure the strength of MT IAV. The period for all variables spanned from 1979 to 2008 for the TC season from June to August. The climatological annual cycle was removed from the original data set. According to the climatological mean distribution of OLR and 850 hpa wind, an area (5 20 N, 120 150 E), average OLR index was used to identify the intensity of MT IAV (see Figure S1 in the supporting information). Three strong (1985, 1999, and 2000) and three weak (1980, 1983, and 1998) years of MT IAV were chosen according to the normalized area-averaged OLR index (with its absolute amplitude larger than a standard deviation of 1.3). The composite technique was then applied to derive the large-scale anomalous fields during the strong and weak years of MT IAV. Figures 1a and 1b show the horizontal patterns of wind fields at 850 hpa. During the strong years of MT IAV, there is a cyclonic low-level circulation anomaly, and during the weak years, there is an anticyclonic low-level anomaly. The width of the low-level anticyclonic circulation is obviously greater than the cyclonic circulation anomaly. Figures 1c 1f show the vertical profiles of the area-averaged (15 35 N, 112 132 E) relative vorticity, temperature, divergence, and specific humidity over the anomalous cyclonic circulation center during the strong MT IAV years, as well as those area-averaged (12 32 N, 112 132 E) over the anomalous anticyclonic center during the weak MT IAV years. It can be seen that positive relative vorticity anomalies appear throughout the troposphere, with a maximum magnitude of 2.0 10 6 s 1 at 700 hpa in the strong years (Figure 1c, solid black line). This equivalent barotropic vertical structure is somewhat different from MT ISO, which has a typical baroclinic structure composed of low-level cyclonic vorticity and upper level anticyclonic vorticity (figure not shown). The negative temperature anomalies appear throughout the whole troposphere, with a minimum cold core of 0.5 K at 150 hpa (Figure 1d), which is almost opposite to that of MT ISO. The tropospheric cooling during the strong years may be associated with the spreading of equatorial waves in response to negative heating in the eastern equatorial Pacific. In fact, our composite maps show that the strong MT composite is associated with the persistence or decaying of a La Niña event, whereas the weak MT composite is associated with the decaying of an El Niño event. Besides, convergence prevails below 400 hpa and divergence dominates above 250 hpa (Figure 1e). Positive specific humidity anomalies appear throughout the troposphere, with a maximum magnitude of 0.17 g kg 1 appearing in the middle level (Figure 1f). The vertical profiles of MT IAV during the weak years are almost opposite to those for the strong years, with slightly different magnitudes, except for specific humidity (Figure 1f, solid gray line). It is shown that the generally positive specific humidity anomalies appear throughout the whole troposphere, but with less magnitude during the weak years. Therefore, the main differences between the strong and weak years reside in the relative vorticity and divergence fields. CAO ET AL. 2014. American Geophysical Union. All Rights Reserved. 4333

Figure 1. The 850 hpa wind fields (vectors) of the initial conditions in (a) the strong years and (b) the weak years. The blue box in Figures 1a and 1b represents the areas (15 35 N, 112 132 E) and (12 32 N, 112 132 E) for the vertical profiles. The vertical profiles of area-averaged 20 20 (c) relative vorticity (10 6 s 1 ), (d) temperature (K), (e) divergence (10 6 s 1 ), and (f) specific humidity (10 3 kg kg 1 ) during the strong (solid black line) and weak (solid gray line) years of MT IAV centered in the circulation anomalies. The dashed blue line indicates zero. 2.2. Experimental Design In this study, we used the nonhydrostatic Advanced Research Weather Research and Forecasting model (version 3.3) developed by the National Center for Atmospheric Research [Skamarock et al., 2008]. The modeling configuration and the initial vortex structure are described in detail in Text S1 in the supporting information. Three groups of experiments were performed on a beta plane (see Table 1). In the control experiment, a weak vortex with a maximum tangential wind of 8 m s 1 was specified in a resting environment (i.e., no MT IAV fields; hereafter CTL). In the second group of experiments, the strong IAV composite patterns were added into the resting mean background state with and without an initial vortex (named STV and ST, respectively). The difference between the results of two experiments reveals the pure vortex evolution. In the third group of experiments, the weak IAV composite patterns were specified with and without an initial vortex (named WKV and WK, respectively). In order to quantify the dynamic and thermodynamic (moisture) impacts of MT IAV during the strong years, two groups of sensitivity experiments were performed (see Table 1). In the first set of sensitivity experiments, the dynamic variables including zonal wind, meridional wind, surface pressure, geopotential height, and temperature anomalies (u, v, ps, hgt, and T) derived from the strong-year composite were specified, whereas the specific humidity (sh) anomaly associated with strong MT IAV was set to zero. The experiments with and without a vortex were named STV_NOSH and ST_NOSH, respectively. Similarly, in the second set of sensitivity experiments, the specific humidity anomaly associated with the strong IAV composite was specified, whereas the dynamic fields such as wind, pressure, and temperature associated with strong MT IAV were set to zero. The experiments with and without a vortex were identified as STV_SH and ST_SH, respectively. CAO ET AL. 2014. American Geophysical Union. All Rights Reserved. 4334

Table 1. Model Experiment Description a Experiment Name Description CTL Initial vortex with a maximum wind speed of 8 m s 1 in a resting environment (G1) STV Similar to CTL except with the strong IAV anomalies involved (G2) ST Strong IAV anomalies added into the resting mean background (G2) WKV Similar to CTL except with the weak IAV anomalies involved (G3) WK Weak IAV anomalies added into the resting mean background (G3) STV_NOSH Similar to STV except without the thermodynamic IAV field (S1) ST_NOSH Similar to ST except without the thermodynamic IAV field (S1) STV_SH Similar to STV except without the dynamic IAV fields (S2) ST_SH Similar to ST except without the dynamic IAV fields (S2) a G denotes the group and S denotes the sensitivity. Note that ST and WK in the experiment names denote the strong and weak years of MT IAV. SH denotes the sensitivity experiment with the IAV moisture field but no dynamic fields. NOSH denotes the case with the IAV dynamic fields but no moisture field. V denotes the case with a specified initial bogus vortex. 3. Results Figure 2 shows the time evolutions of the minimum sea level pressure (MSLP) and maximum azimuthal mean winds (MAMW) at the 10 m height in five cases: CTL, STV, WKV, STV_SH, and STV_NOSH. The vortex development and evolutions show very different features between the strong and weak years of MT IAV. The vortices seldom develop in the first 60 h for the CTL, STV, and WKV runs. After this, they are characterized by three obviously different evolution paths (Figure 2). A strong TC develops at t = 108 h in STV, while a TC cannot form in the whole 108 h integration period in WKV. At t = 96 h, the respective MSLPs of the three experiments CTL, STV, and WKV are 1004.4 hpa, 959.7 hpa, and 1011.1 hpa, while the respective MAMWs in the three runs are 11.9 m s 1, 40.3 m s 1, and 5.4 m s 1. These results show that the large-scale background conditions associated with the strong years of MT IAV create a more favorable environment for TC formation. This is consistent with the findings of Gray [1968] and Zehr [1992], who suggested that positive low-level vorticity, low-level convergence, and enhanced moisture are favorable conditions for TC formation. Figure 2. Time evolution of (a) MSLP (hpa) and (b) MAMW (m s 1 )at10min the five experiments CTL, STV, WKV, STV_SH, and STV_NOSH on a beta plane. The abscissa represents time (hours) and the ordinate corresponds to the value of intensity. Because both the vortex perturbation and large-scale IAV circulation are included in STV and WKV, to examine pure vortex evolution, we calculated the difference between STV and ST and the difference between WKV and WK. Due to the same vortex, such differences reflect the impact of the large-scale IAV fields on vortex evolution. CAO ET AL. 2014. American Geophysical Union. All Rights Reserved. 4335

Figure 3. Vertical-time cross section of perturbation diabatic heating (10 4 Ks 1 ) averaged over 420 km 420 km centered in the TC center in the first 10 h for (a) CTL, (c) STV relative to ST, and (e) WKV relative to WK. (b, d, f) The same as Figures 3a, 3c, and 3e except for perturbation vertical velocity (10 2 ms 1 ). Both the enhanced large-scale cyclonic vorticity and convergence fields associated with the strong IAV could strengthen friction-induced moisture convergence and diabatic heating, providing an energy source for the vortex growth. Figure 3 shows the evolutions of diabatic heating and vertical motion profiles in the first 10 h in CTL, STV relative to ST, and WKV relative to WK. It can be seen that the heating contour of 0.1 10 4 Ks 1 appears in the low level at t = 6 h in CTL (Figure 3a), whereas it appears at t = 4 h during the strong years and at t = 7 h during the weak years (Figures 3c and 3e). After t = 7 h, shallow heating could develop into the upper level in the strong years in comparison with CTL; while in the weak years, the heating is always confined below 800 hpa with less magnitude during the first 10 h. The structure and evolution features of the perturbation vertical velocity are consistent with the heating fields among the three runs (Figures 3b, 3d, and 3f). Next we turn our attention to discussing the specific process through which the background cyclonic circulation associated with MT IAV affects vortex development. A stronger inflow and convergence in the boundary layer develops due to the greater midlevel outflow induced by the vortex-mean flow interaction (figure not shown) and the low-level convergence forcing of the strong IAV, which promote the upward penetration of friction-induced ascending motion and diabatic heating. Conversely, the IAV flow in the weak years imposes an inward flow in the middle level, along with low-level divergence forcing, inducing anomalous subsidence below. Therefore, the vortex-ascending motion associated with frictional convergence is confined within the boundary layer and is unable to develop into the high level. During the later stage, CAO ET AL. 2014. American Geophysical Union. All Rights Reserved. 4336

Figure 4. The vertical-time cross section of perturbation vertical velocity (10 2 ms 1 ) averaged over 420 km 420 km centered in the TC center in the first 10 h for (a) STV_NOSH relative to ST_NOSH and (b) STV_SH relative to ST_SH. The differences (c) between Figures 4a and 3b and (d) between Figures 4b and 3b are shown. the positive contribution from the strong IAV induces the vortex to develop through positive feedback, whereas the negative contribution from the weak IAV always prevents the vortex from developing (Figure 2). To quantify the relative importance of dynamic (lower tropospheric cyclonic vorticity and convergence) and thermodynamic (moisture) impacts during the strong years of MT IAV, sensitivity experiments were performed that separated the dynamic and thermodynamic (moisture) factors of MT IAV (see Table 1 for details). The sensitivity simulation results show that the contribution of the thermodynamic (moisture) factor to TC development is essentially comparable to that of the dynamic factors. Both types of factor make positive contributions to TC formation compared to CTL (Figure 2). This result is consistent with the observational study of Camargo et al. [2007], who found that relative humidity and vorticity are important for the eastward shift of TC genesis location during El Niño years. More precisely, the relative role of the dynamic and thermodynamic (moisture) impacts may be estimated based on the averaged ratio of MSLP differences between STV_SH and CTL and between STV_NOSH and CTL, during the period from t =84htot = 108 h. It is found that the intensifying rate due to the thermodynamic (moisture) factor associated with MT IAV is approximately 80% that of the dynamic factors. This indicates that, while the thermodynamic factor is certainly important, the dynamic factors associated with MT IAV play a slightly more important role in affecting TC formation. This result is consistent with the cyclogenesis time, which is 3 h earlier in STV_NOSH than in STV_SH if the MAMW of 15 m s 1 is defined as the time of TC genesis. Figure 4 shows the evolutions of perturbation vertical motion profiles in the first 10 h in STV_NOSH relative to ST_NOSH (hereafter NOSH) and STV_SH relative to ST_SH (hereafter SH) and their differences with CTL. It is seen that the obvious difference of the vertical motion contour of 0.05 10 4 ms 1 appears at t =6hin NOSH (Figure 4c), while it is at t = 9 h in SH (Figure 4d). Meanwhile, the vertical motion difference in NOSH is greater than that in SH during the initial 10 h. This result is consistent with the intensification ratios induced by the dynamic and thermodynamic factors mentioned above. CAO ET AL. 2014. American Geophysical Union. All Rights Reserved. 4337

The results derived in the current study are somewhat different from the MT ISO simulation result (see Figure S3), in which the vortex intensification rate associated with the ISO thermodynamic (moisture) impact is nearly twice as large as that associated with its dynamic impact. One explanation for the difference is suggested as follows. It is noted that the ratio of the strength of the low-level relative vorticity between MT IAV and ISO is approximately 0.6:1, while the ratio of the low-level specific humidity anomaly is approximately 0.3:1 (see Figure S4). This implies that, compared to MT ISO, the decrease of the specific humidity anomaly associated with MT IAV is approximately twice as large as the relative vorticity decrease. This background vorticity and moisture change ratio is consistent with the modeling result that the thermodynamic (moisture) impact of MT IAV makes a similar contribution to TC formation as the dynamic impacts of MT IAV. Thus, in addition to the low-level circulation anomaly, the moisture anomaly is also an indispensable element that controls the IAV of TC genesis frequency over the WNP. 4. Summary and Discussion Previous studies have suggested that the IAV of TC genesis in the WNP is closely associated with MT activity, which is affected by remote and local SST anomalies. However, the relative importance of dynamic (circulation) and thermodynamic (moisture) factors of strong MT IAV in contributing to TC formation is not clear. In this study, the effects of MT IAV on TC formation were investigated through idealized model experiments on a beta plane that separated the circulation and moisture impacts of strong MT IAV. The numerical results showed that the large-scale conditions of MT IAV during strong years contribute more to TC formation and development compared to during weak years. The dynamic impacts of MT IAV on vortex development are similar to those of MT ISO. Nonlinear interaction between the vortex and the IAV cyclonic background state induces outflow in the middle level during strong MT IAV years and inflow during weak years. The midlevel outflow together with low-level convergence promotes the upward development of the vortex-ascending motion induced by friction-induced boundary layer inflow. Conversely, the midlevel inflow along with low-level divergence has an inhibitory effect on this process. The enhanced low-level moisture from strong IAV also strengthens diabatic heating. It is noted that the perturbation heating appears 3 h earlier during strong years than during weak years. The heating quickly develops into the upper troposphere in the strong year simulation, while it is confined in the low level in the weak year simulation. Sensitivity experiments showed that the dynamic (circulation) and thermodynamic (moisture) impacts associated with strong MT IAV are approximately the same. This differs from the active MT ISO case in which the thermodynamic (moisture) impact is approximately twice as large as the dynamic impact. Both the dynamic and thermodynamic factors associated with strong MT IAV make positive contributions to TC formation compared with CTL. Thus, it is concluded that both the low-level vorticity/convergence and specific humidity anomalies of strong MT IAV play an important role in TC formation over the WNP. Accordingly, the results from the current study may help establish an empirical statistical method to predict TC formation using both dynamic and thermodynamic factors. The ratio of intensifying rates associated with the thermodynamic and dynamic impacts of strong MT IAV is different from that of the active MT ISO. This ratio difference could be attributable to the relative change of intensity of MT large-scale fields on both time scales. It is seen that the ratio of relative vorticity change between MT IAV and ISO is nearly half the specific humidity change. As a result, the dynamic and thermodynamic modulations of the MT on TC development are consistent between the two time scales. It is worth mentioning that the moisture effect may depend on the cumulus convective scheme applied. It would be desirable to examine the sensitivity of the model results to various convective schemes in the model. It is also worth mentioning that the current study focused on the role of MT IAV in TC formation in a quiescent environment. It would be interesting to examine how the summer mean flow and its 3-D structure might affect TC formation, given that it may also interact with the vortex. Therefore, more realistic examinations with specified summer mean circulation over the WNP are desirable, and we intend to conduct these experiments in future work. From observational analysis, it is found that strong MT IAV has a barotropic structure with a cold core in the upper level, whereas the active MT ISO has a typical baroclinic structure with an upper level warm core. The cause of such a contrasting structure will also be examined in the future. CAO ET AL. 2014. American Geophysical Union. All Rights Reserved. 4338

Acknowledgments The author X.C. would like to thank Baoqiang Xiang for his discussion. This work was supported by the Office of Naval Research (grant N00014-1210450), the National Science Foundation (grant AGS-1106536), and the International Pacific Research Center (IPRC), which is sponsored by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). This is SOEST contribution number 9130 and IPRC contribution number 1062. W.C. and G.H.C. were supported by the National Natural Science Foundation of China (grant 41275001). The observation data are from NCEP and NOAA. The simulation data are obtained from the WRF model (version 3.3). The Editor thanks two anonymous reviewers for their assistance in evaluating this paper. References Camargo, S. J., K. A. Emanuel, and A. H. Sobel (2007), Use of a genesis potential index to diagnose ENSO effects on tropical cyclone genesis, J. Clim., 20, 4819 4834. Cao, X., P. Huang, G. H. Chen, and W. 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