2682 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 Interactions between Typhoon Megi (2010) and a Low-Frequency Monsoon Gyre* MINGYU BI International Laboratory on Climate and Environment Change, and Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China, and International Pacific Research Center, and Department of Atmospheric Sciences, University of Hawai‘i at Manoa, Honolulu, Hawaii TIM LI International Pacific Research Center, and Department of Atmospheric Sciences, University of Hawai‘i at Manoa, Honolulu, Hawaii MELINDA PENG Naval Research Laboratory, Monterey, California XINYONG SHEN International Laboratory on Climate and Environment Change, and Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China (Manuscript received 21 September 2014, in final form 3 March 2015) ABSTRACT The ARW Model is used to investigate the sharp northward turn of Super Typhoon Megi (2010) after it moved westward and crossed the Philippines. The NCEP analyzed fields during this period are separated into a slowly varying background-flow component, a 10–60-day low-frequency component representing the monsoon gyre, and a 10-day high-pass-filtered component representing Megi and other synoptic-scale motion. It appears that the low-frequency (10–60 day) monsoon gyre interacted with Megi and affected its track. To investigate the effect of the low-frequency mode on Megi, numerical experiments were designed. In the control experiment, the total fields of the analysis are retained in the initial and boundary conditions, and the model is able to simulate Megi’s sharp northward turn. In the second experiment, the 10–60-day monsoon gyre mode is removed from the initial and lateral boundary fields, and Megi moves westward and slightly northwestward without turning north. Tracks of the relative positions between the Megi and the monsoon gyre centers suggest that a Fujiwhara effect may exist between the monsoon gyre and Megi. The northward turning of both Megi and the monsoon gyre occurred when the two centers were close to each other and the beta drift was enhanced. A vorticity budget analysis was conducted. It is noted that the Megi moves toward the maximum wavenumber-1 vorticity tendency. The sharp change of the maximum vorticity tendency direction before and after the track turning point is primarily attributed to the change of the horizontal vorticity advection. A further diagnosis shows that the steering of the vertically integrated low-frequency flow is crucial for the change of the horizontal advection tendency. 1. Introduction * School of Ocean and Earth Science and Technology Contri- bution Number 9310, International Pacific Research Center Con- Tropical cyclones, to a large degree, move with the tribution Number 1110, and Earth System Modelling Center Contribution Number 50. environmental steering flow (Chan and Gray 1982), while the beta effect and tropical cyclone (TC) struc- tures also play a role (Fiorino and Elsberry 1989; Li and Corresponding author address: Tim Li, International Pacific Research Center, and Department of Meteorology, University of Zhu 1991). Despite the prevailing control of the large- Hawai‘i at Manoa, 1680 East-West Road, Honolulu, HI 96822. scale environmental flow and the great improvement E-mail: [email protected] made in track prediction, some cases of large track error DOI: 10.1175/JAS-D-14-0269.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 09/25/21 10:58 PM UTC JULY 2015 B I E T A L . 2683 still occur because of complex interactions of TCs with other scales of motion, such as the low-frequency mode (Carr and Elsberry 1990). It has been noticed that the largest error in the pre- diction of TC tracks is observed during TC recurvature and sudden turns. Previous studies have examined the rela- tionship between midlatitude waves and TC recurvature. For example, George and Gray (1977) found that, if the 2 upper-level westerlies are greater than 25 m s 1 within 208 poleward of a typhoon, the typhoon may recurve. Hodanish and Gray (1993) compared the sharply and gradually recurving cases and found that typhoons begin to turn when upper-tropospheric westerlies penetrate to within 68 from the typhoon’s center. Holland and Wang (1995) found that typhoons tend to recurve into the FIG. 1. The JTWC best track of Megi from 13 to 24 Oct 2010. midlatitudes when a synoptic-scale trough moves away from East Asia into the subtropical ocean. waves or disturbances (Zhou and Li 2010). On one Some TCs in the tropical western North Pacific (WNP) hand, the ISOs can influence the development of occasionally experienced a sudden northward track synoptic-scale disturbances through barotropic energy change. Megi (2010) is one example wherein most oper- conversion (Maloney and Hartmann 2000). On the ational numerical models failed to predict the sharp turn other hand, the synoptic-scale perturbations may feed at the right time (see a more detailed description of this back to the ISO through the nonlinear rectification of super typhoon in section 2). Using a barotropic model, surface latent heat flux, diabatic heating, and eddy Carr and Elsberry (1995) investigated the sudden north- momentum transport (Hsu and Li 2011; Hsu et al. ward turning of a vortex when it approached a large-scale 2011). In general, the ISO in the WNP exhibits two monsoon gyre. They suggested that Rossby wave energy spectral peaks at periods of 30–60 days and 10–20 days dispersion associated with the monsoon gyre is critical in (Chen and Chen 1993; Chen and Sui 2010; Mao and causing the sudden northward-turning track. Chan 2005). Harr and Elsberry (1991) found that TC TCs in the WNP are usually accompanied with multi- tracks alternate between westward and recurving scale waves, including intraseasonal (10–90 day) oscilla- clusters at the intraseasonal time scale. Kim et al. tions (ISOs) and synoptic-scale (3–10 day) disturbances (2008) revealed a close relationship between land- (Li and Wang 2005; Li 2012). A typical example of low- falling TCs in the WNP and the phase of the MJO. frequency systems in the WNP is the monsoon gyre Thus, it is likely that the ISO flows may affect not only (Lander 1994). Li et al. (2006) demonstrated the effect of TC formation but also TC tracks. the monsoon gyre in promoting TC genesis in a 3D The objective of the current study is to investigate how model. One of the important aspects of the ISOs in the and to what extent Typhoon Megi (2010) interacted with tropical WNP is their interaction with synoptic-scale low-frequency monsoon gyre flow and how such an 5 2 24 FIG. 2. The wavelet power spectrum (10 W m ) of OLR over the region 58–238N, 1138– 1308E from 1 Aug 2010 to 1 Jan 2011. The black contour is the significant level. The red line indicates Megi turning time. Unauthenticated | Downloaded 09/25/21 10:58 PM UTC 2684 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 25 21 FIG. 3. The evolution of 10–60-day bandpass-filtered wind (vectors) and vorticity (shaded; 10 s ) fields averaged between 850 and 300 hPa. The black dots and red typhoon marks denote the centers of the low-frequency monsoon gyre and the TC center, respectively. interaction may have led to its sudden northward-turning In section 3, we describe the experiment design and track. The rest of the paper is organized as follows. In simulation results. In section 4, we investigate mecha- section 2, we present an overview of the evolution for nisms responsible for the sudden northward-turning Typhoon Megi and the nearby low-frequency circulation. track. Diagnostics of vorticity tendency are given in Unauthenticated | Downloaded 09/25/21 10:58 PM UTC JULY 2015 B I E T A L . 2685 FIG. 5. The JTWC best track (black) and simulated Megi tracks in the control (red) and NO_MG (blue) experiments. The pink and purple ovals indicate the before-turning and after-turning periods, respectively. 2. Overview of Megi and associated low-frequency flow Megi can be traced back to a pregenesis tropical de- pression that emerged east of the Philippines (around 1408E) early on 13 October 2011 (Fig. 1). The low pressure system strengthened throughout the day and became a named tropical storm, Megi, by 1200 UTC 13 October, when its central pressure fell to 998 hPa. In the following 3 days, Megi continued to develop while moving north- westward. It was upgraded to typhoon category by the Joint Typhoon Warning Center (JTWC) at 1200 UTC 16 October. It then moved west-southwestward and con- tinued to strengthen, reaching its peak intensity of super typhoon by 1200 UTC 17 October, with a central minimum 2 pressure of 895 hPa and maximum wind speed of 72 m s 1. It caused 11 deaths, 16 injuries, and the evacuation of more than 200 000 people. After passing through the northern Philippines, Megi weakened a little and slowed down as it entered the South China Sea. Beyond 19 October, Megi moved in a north- west direction. By 0000 UTC 20 October, Megi’s track turned straight northward from its original westward and northwestward movement. The angle of pre- and post- turning tracks from before 0600 UTC 19 October to be- FIG. 4. The patterns of (a) the unfiltered initial wind field, yond 20 October was almost 908. Most operational TC (b) the 10-day high-pass-filtered wind field, and (c) the initial wind forecast models failed to predict such a rather sudden field in the NO_MG experiment averaged from 850 to 300 hPa at 0000 UTC 18 Oct 2010. The red sign indicates the typhoon track change, as most models predicted that Megi would location.