M

a/r1

Figure 9. Summary of the Rankine vortex binary interaction regimes as a function of the dimensionless gap A/Rl, and the vorticity strength ratio 7 = /C2 for the radius ratios r = Rl/R2 =1/2, 1/3, and 1/4. As indicated by the code at the lower right, the structures are categorized as follows: elastic interaction (EI), merger (M), straining-out (S), tripole (T), concentric (C). The concentric regime is shadowed. (Adapted from K04.)

5. Concluding Remarks

There are observations and model simulations of binary tropical cyclone interactions that resemble the theoretical work of DW92 (see e.g. Larson, 1975; Lander and Holland. 1993; Kuo et al., 2000; Khain et al., 2000; Prieto et al., 2003). We have enriched the DW92 dynamics with the tripole and concentric structures. We have used vorticity dynamics to explain the formation of the concentric eyewall structure. Implicitly assumed in our approach is that deep convection, such as that observed in Fig. 1, may be a signature of vorticity. It is reasonable to expect that the (f + Z)V • u term in the vorticity equation allows vorticity to be generated by the lower tropospheric convergence associated with convection in the neighborhood of the vortex core. There is evidence that convection and vorticity are highly correlated. For example, Simpson et al. (1997) reported an elastic interaction (mutual rotation) of two 100 km scale convective systems and Hendricks et al. (2004) reported a "vortical hot tower" of deep convection that had a scale of several tens of kilometers. Zhang et al. (2005) also found that enhanced vorticity is associated with convecti-ve bands in MM5 simulations of the concentric eyewalls of Typhoon Winnie.

Since tropical cyclones often weaken after the formation of a concentric eyewall, intensity forecasting would obviously benefit from an improvement of concentric eyewall prediction. From the work presented here, it may be important to observe and understand the spatial and temporal characteristics of the vorticity field outside the cyclone core, as well as the detailed core structure, in order to better predict the formation of concentric eyewalls. The convection in the cyclone environment has been treated as an existing asymmetric vorticity in our model. The mesoscale vorticity-generating processes that determine the scale and strength of vorticity in the cyclone environment require further exploration. In this regard, there are several studies describing vorticity/convection initiation processes that can occur in concert with the organizational dynamics discussed here. For example, Montgomery and Kallenbach (1997) have identified the mechanism of radial propagation of linear Rossby waves in the presence of a critical radius outside the radius of maximum wind, which may be important for the formation of concentric eyewalls. Nong and Emanuel (2003) discussed the formation of concentric eyewalls in their axisymmetric model via an upper level external forcing-triggered finite amplitude WISHE instability.

In closing, we note that the appearance of tripoles in our study adds to a long list of papers in which these structures are documented. For example, tripoles emerge from unstable initial states in laboratory experiments with both rotating fluids (Kloosterziel and van Heijst, 1991; van Heijst et al., 1991; Denoix et al., 1994) and pure electron plasmas (Driscoll and Fine, 1990). They also are found in two-dimensional turbulence simulations as coherent structures (Legras et al., 1988), as a result of collisions of two di-poles (Larichev and Reznik, 1983; Orlandi and van Heijst, 1992), as a result of finite amplitude quadrapolar (i.e., azimuthal wave number 2) distortions of a monopolar Gaussian vortici-ty distribution (Rossi et al., 1997), as a result of the barotropic instability across the annular region separating a strong core vortex from a weaker vorticity ring (Kossin et al., 2000), and as the end state of an initial vorticity distribution in which a low vorticity eye is uncentered within a region of high vorticity (Prieto et al., 2001). Given their robustness and multitude of production methods, it is possible that tripoles may actually play a role in tropical cyclone dynamics. The tripoles may be an indication of incomplete mixing in the tropical cyclone core. This follows from the statistical mechanics arguments, such that tripoles are a restricted statistical equilibrium far from the end state of strong mixing (Robert and Rosier, 1997; Chavanis and Sommeria, 1998). The idea of incomplete mixing appears to be in agreement with our results that the tripoles serve as a demarcation for the merger and the concentric eyewall regimes.

Acknowledgements

We thank Wayne Schubert, Terry Williams, C.-P. Chang, Yu-Ming Tsai, and Li-Huan Hsu for their helpful comments and suggestions. This research was supported by the National Research Council of Taiwan through the grants NSC 96-2111-M-002-002, NSC94-2745-P-002-002, NSC95-2745-P-002-004 and MOTC-CWB-97-2M-01 to the National Taiwan University.

[Received 14 February 2007; Revised 22 May 2007; Accepted 22 May 2007.]

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Rare Typhoon Development near the Equator

Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan and Department of Meteorology, Naval Postgraduate School, Monterey, California, USA

[email protected]

Teo Suan Wong

Meteorological Services Division, National Environment Agency, Singapore

The formation of Typhoon Vamei on 27 December 2001 in the southern South China Sea was the first-observed tropical cyclogenesis within 1.5 degrees of the equator. This rare event was first detected by observations of typhoon strength winds from a US navy ship, and the existence of an eye structure was confirmed by satellite and radar imageries. This paper reviews these observations, and discusses the dynamic theory that may explain the process suggested by Chang et al. (2003) in which a strong cold surge event interacting with the Borneo vortex led to the equatorial development. As pointed out by Chang et al., the most intriguing question is not how Vamei could form so close to the equator, but is why such a formation was not observed before then.

1. Introduction

One of the generally accepted conditions for tropical cyclone formation has been that the location is "away from the equator." This condition is based on the lack of Coriolis effect at the equator, and supported by observations over more than a century that show most tropical cyclogeneses to occur poleward of 5° latitude (Gray, 1968; McBride, 1995). The previous record was set by Typhoon Sarah in 1956 at 3.3°N (Fortner, 1958). Typhoon Vamei formed at 1.5°N at the southern tip of the South China Sea at 00 UTC 27 December 2001, a latitude that most textbooks (e.g. Anthes, 1982) ruled out for development. The cyclone was named by the Japan Meteorological Agency, which initially identified it as a tropical storm with estimated winds of 21 ms-1. It was upgraded to a typhoon by the Joint Typhoon Warning Center (JTWC) in Hawaii. Figure 1 shows the best track and intensity of Vamei, published by the JTWC. The storm made landfall over southeast Johor at the southern tip of Peninsular Malaysia, about 50 km northeast of Singapore, at 0830 UTC 27 December 2001. Upon making landfall, it weakened rapidly to a tropical depression. It continued in its west-northwest track across southern Johor, the Malacca Straits, and made landfall again in Sumatra. Upon entering the Bay of Bengal, the storm regenerated and continued in its northwest track before dissipating in the central Bay of Bengal on 31 December 2001. During the short period of 12 h as a typhoon and another 12 h as a tropical storm, Vamei caused damage to two US Navy ships, including a carrier, and flooding

Figure 1. Best track and intensity of Vamei from 1200 UTC 12 December 2001 to 00 UTC 1 January 2002. (Diagram courtesy of JTWC.)

and mudslides in southern Peninsular Malaysia's Johor and Pahang states. More than 17,000 people were evacuated and 5 lives were lost.

The upgrade of Vamei to the typhoon category by the JTWC was based mainly on the shipboard observations from several US Navy ships within the small eyewall, with reports of sustained winds of 39 ms-1 and gusts of up to 54 ms-1. Because of its equatorial latitude, there was considerable interest among tropical cyclone forecasters regarding the typhoon's structure and the process of its development. This article will review the relevant data used to observe the development of Typhoon Vamei and discuss some theoretical considerations regarding its possible formation mechanism.

2. Background Flow and the

Observed Development

Vamei developed in late December 2001, near the middle of the Asian winter monsoon season, which is characterized by strong baroclinicity in the middle latitudes and northeasterly winds at lower levels. Freshening of the northeasterly winds, or cold surges (Chan and Li, 2004; Chang et al., 2004, 2005), occur sporadically and spread equatorward. Although cold surge winds are typically dry, they are moistened by the over-water trajectory (Johnson and Houze, 1987) and have been associated with increased deep convection and enhanced upper-tropospheric outflow over the Maritime Continent, which is related to an enhanced East Asian local Hadley cell (Lau and Chang, 1987). The cold surge air can reach the equator in about two days (Chang et al., 1983). Conservation of potential vorticity causes the air to turn eastward after it crosses the equator. These Southern Hemisphere equatorial westerlies may enhance the Australian monsoon trough farther south, between 10°S and 20°S, where tropical cyclogenesis occurs frequently (e.g. Holland, 1984; McBride, 1995).

Synoptic-scale disturbances are also found to occur in the vicinity of the island of Borneo (Johnson and Houze, 1987; Chang et al., 2005).

Over this region, the low-level basic-state background vorticity is cyclonic, due to the mean northeasterly wind maximum over the South China Sea and the equatorial westerlies associated with the Asian winter monsoon. Therefore, perturbations in this basic state often amplify into synoptic-scale cyclonic circulations. These disturbances are often found southeast of the primary region of cold surge northeasterly winds. Often, the circulation is present as a quasi-stationary, low-level cyclonic circulation, which is a persistent feature of the boreal winter climatology (Johnson and Houze, 1987; Chang et al., 2005). Although the circulation may not be completely closed on the east side over the island, it has been referred to as the Borneo vortex (Chang et al, 2004, 2005). The mean location of the vortex along the northwest coast of Borneo may be seen in Fig. 2, which shows the 1999/2000-2001/2002.

December-February mean 850 hPa vorticity from the 1° x 1° Navy Operational Global Atmospheric Prediction System (NOGAPS) analysis, overlaid with the surface vorticity derived from the QuikSCAT satellite scatterometer winds. The Borneo vortex is often associated with deep convection and intense latent heat release, and upper-level divergence is often present. However, because most of the time a significant part of the vortex circulation is over land (Fig. 3), even when a vortex drifts to northern Borneo between 5°N and 7°N, which are latitudes considered more favorable for tropical cyclone development, it is very difficult for the vortex to develop into a tropical cyclone (Chang et al., 2003).

Chang et al. (2003) provided the following description of the synoptic events preceding the development of Vamei. Starting from 19 December 2001, a cold surge developed rapidly over the South China Sea while the center of the Borneo vortex was located near 3° N on the northwest coast (not shown). The 850 hPa NOGAPS wind analysis and vorticity in Fig. 4 depict the southwestward movement of

Figure 2. 1999/2000-2001/2002 boreal winter (DJF) mean of 850 hPa NOGAPS 1° x 1° wind and vorticity (contours: solid — positive; dashed — negative; interval 2 x 10_5 s_1), and surface vorticity based on 25 km resolution QuikSCAT winds (yellow — positive; green — negative). [Diagram from Chang et al. (2003) by permission of American Geophysical Union.]

Figure 2. 1999/2000-2001/2002 boreal winter (DJF) mean of 850 hPa NOGAPS 1° x 1° wind and vorticity (contours: solid — positive; dashed — negative; interval 2 x 10_5 s_1), and surface vorticity based on 25 km resolution QuikSCAT winds (yellow — positive; green — negative). [Diagram from Chang et al. (2003) by permission of American Geophysical Union.]

Figure 3. Analyzed Borneo vortex center locations based on streamlines of unfiltered 925 hPa winds. (NCEP/NCAR reanalysis winds at 925 hPa at 2.5° x 2.5° grids, for 21 boreal winters (December 1980-February 2001.) [Diagram from Chang et al. (2005) by permission of American Meteorological Society.]

Figure 4. NOGAPS 1° x 1° 850 hPa wind and vorticity (red — positive; green — negative) at 00 UTC 20-26 December 2001.
Figure 5. MODIS satellite image on 27 December 2001, showing Typhoon Vamei near Singapore. (Diagram courtesy of Professor Lim Hock, National University of Singapore.)

the vortex from along the Borneo coast toward the equator. By 21 December, the center of the vortex had moved off the coast over water, where the open sea region at the southern end of the South China Sea narrows to about 500 km, with Borneo to the east and the Malay Peninsula and Sumatra to the west. This over-water location continued for several days. While the vortex center remained in the narrow equatorial sea region, the strong northeasterly surge persisted, and was slightly deflected to the northwest of the vortex. This near "trapping" of the Borneo vortex by a sustained surge is unusual, because normally the vortex center would be pushed eastward by the strengthening surge that streaks southwestward in the middle of the South China Sea. Consequently, the cross-equatorial flow wrapped around the vortex and provided a background area of cyclonic relative vorticity with a magnitude of >1 x 10"5 s_1, which is comparable to that of the Coriolis parameter 5o or more away from the equator.

Figure 5 shows the MODIS satellite image on 27 December 2001. Vamei's circulation center can be estimated to be just north of 1oN, but an eye is not observable under the clouds. Even though the size of the typhoon is quite small, which is considered a special characteristic of low latitude TCs by some researchers

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