Abstract
On 6 August 2009, typhoon Morakot encountered a giant warm ocean eddy approximately 700 km by 500 km in the southern eddy rich zone of the western North Pacific Ocean. Soon after passing over the warm ocean eddy, Morakot reached its peak intensity at category 2. Results based on multiple satellite observations and numerical modelling suggest very favourable ocean conditions provided by the warm ocean eddy during this earlier developmental stage of Morakot. It is found that in the presence of the observed warm ocean eddy, the upper ocean heat content increased significantly by ~100%, from ~60 to 120 KJ cm-2. This very deep and warm subsurface temperature effectively reduced the negative feedback of typhoon-induced ocean cooling. As a result, the during-storm sea surface temperature remained high at ~29 - 30°C. This very warm duringtyphoon sea surface temperature (SST) provided an increase in air-sea enthalpy flux supply by ~200% (i.e., ~500 W m(superscript -2) under the warm eddy situation .vs. the ~170 W m(superscript -2) under the without eddy situation). Also, since the during-typhoon SST remained high, the moisture supply was increased to enhance convective activities. Numerical experiments using the Weather Research and Forecasting (WRF) model suggest that the presence of the warm ocean eddy does not change the overall structure orcharacteristics of Morakot. Rather, it contributes to a ~10% increase in Morakot’s precipitation. This research shows that in addition to the favourable atmospheric conditions such as the Intra-Seasonal Oscillation or the southwestern monsoon flow, there also exist favourable ocean conditions provided by the presence of a warm ocean eddy during the early developmental stage of Morakot. Further studies based on a full-physics typhoon-ocean coupled model are needed to quantify the role of the upper ocean features in affecting the evolution of Morakot, including its rainfall over Taiwan.
Highlights
1.1 MotivationThe rainfall associated with typhoons, while affected by the high mountains, is the most critical to research and forecast challenges in Taiwan (Wu and Kuo 1999; Wu 2001; Wu et al 2002; Alpers et al 2007, 2010)
It can be seen that Morakot was travelling on the southern eddy-rich zone (SERZ) and it encountered two warm ocean eddies of sea surface height anomalies (SSHA) of +10 to +40 cm (Fig. 1)
As reported by Lin et al (2005, 2008); warm ocean eddies are usually characterised by a SSHA of more than +6 cm and eddies of such high +SSHA of 10 - 40 cm are very prominent warm ocean eddies
Summary
The rainfall associated with typhoons, while affected by the high mountains, is the most critical to research and forecast challenges in Taiwan (Wu and Kuo 1999; Wu 2001; Wu et al 2002; Alpers et al 2007, 2010). In 2009, Typhoon Morakot passed over Taiwan and became one of the deadliest typhoons in Taiwan’s historical record. As the SERZ located amidst the frequent passages of typhoons, there are frequent interactions between eddies and typhoons (Lin et al 2005, 2008). Based on 13 years’ of observations and numerical modelling, it has been found that warm ocean eddies in the SERZ are critical to the Western North Pacific super-typhoons (typhoons of maximum sustained 1-minute surface wind speed exceeding 130 kts) (Lin et al 2005, 2008).
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