Chun-Chi Lien

“Category‐6” supertyphoon Haiyan in global warming hiatus: Contribution from subsurface ocean warming

With the extra-ordinary intensity of 170 kts, supertyphoon Haiyan devastated the Philippines in November 2013. This intensity is among the highest ever observed for tropical cyclones (TCs) globally, 35 kts well above the threshold (135kts) of the existing highest category of 5. Though there is speculation to associate global warming with such intensity, existing research indicate that we have been in a warming hiatus period, with the hiatus attributed to the La Nina-like multi-decadal phenomenon. It is thus intriguing to understand why Haiyan can occur during hiatus. It is suggested that as the western Pacific manifestation of the La Nina-like phenomenon is to pile up warm subsurface water to the west, the western North Pacific experienced evident subsurface warming and created a very favorable ocean pre-condition for Haiyan. Together with its fast traveling speed, the air-sea flux supply was 158% as compared to normal for intensification.

Tropical Cyclone–Induced Ocean Response: A Comparative Study of the South China Sea and Tropical Northwest Pacific*,+

AbstractThe thermocline shoals in the South China Sea (SCS) relative to the tropical northwest Pacific Ocean (NWP), as required by geostrophic balance with the Kuroshio. The present study examines the effect of this difference in ocean state on the response of sea surface temperature (SST) and chlorophyll concentration to tropical cyclones (TCs), using both satellite-derived measurements and three-dimensional numerical simulations. In both regions, TC-produced SST cooling strongly depends on TC characteristics (including intensity as measured by the maximum surface wind speed, translation speed, and size). When subject to identical TC forcing, the SST cooling in the SCS is more than 1.5 times that in the NWP, which may partially explain weaker TC intensity on average observed in the SCS. Both a shallower mixed layer and stronger subsurface thermal stratification in the SCS contribute to this regional difference in SST cooling. The mixed layer effect dominates when TCs are weak, fast-moving, and/or small; ...

Atmospheric Fronts along the East Coast of Taiwan Studied by ERS Synthetic Aperture Radar Images

The existence of quasi-stationary alongshore atmospheric fronts typically located 30-70 km off the east coast of Taiwan is demonstrated by analyzing synthetic aperture radar (SAR) images of the sea surface acquired by the European Remote Sensing Satellites ERS-1 and ERS-2. For the data interpretation, cloud images from the Japanese Geostationary Meteorological Satellite GMS-4 and the American Terra satellite, rain-rate maps from ground-based weather radars, sea surface wind data from the scatterometer on board the Quick Scatterometer (QuikSCAT) satellite, and meteorological data from weather maps and radiosonde ascents have also been used. It is shown that these atmospheric fronts are generated by the collisions of the two airflows from opposing directions: one is associated with a weak easterly synoptic-scale wind blowing against the high coastal mountain range at the east coast of Taiwan and the other with a local offshore wind. At the convergence zone where both airflows collide, air is forced to move upward, which often gives rise to the formation of coast-parallel cloud bands. There are two hypotheses about the origin of the offshore wind. The first one is that it is a thermally driven land breeze/katabatic wind, and the second one is that it is wind resulting from recirculated airflow from the synoptic-scale onshore wind. Air blocked by the mountain range at low Froude numbers is recirculated and flows at low levels back offshore. Arguments in favor of and against the two hypotheses are presented. It is argued that both the recirculation of airflow and land breeze/katabatic wind contribute to the formation of the offshore atmospheric front but that land breeze/katabatic wind is probably the main cause.

Air-sea fluxes for Hurricane Patricia (2015): Comparison with supertyphoon Haiyan (2013) and under different ENSO conditions

Hurricane Patricia formed on October 20th, 2015 in the Eastern Pacific and, in less than 3 days, rapidly intensified from a Tropical Storm to a record-breaking hurricane with maximum sustained winds measured around 185 knots. It is almost 15 knots higher than 2013s supertyphoon Haiyan (the previous strongest tropical cyclone (TC) ever observed). This research focuses on analyzing the air-sea enthalpy flux conditions that contributed to hurricane Patricias rapid intensification, and comparing them to supertyphoon Haiyans. Despite a stronger cooling effect, a higher enthalpy flux supply is found during Patricia, in particular due to warmer pre-TC sea surface temperature conditions. This resulted in larger temperature and humidity differences at the air-sea interface, contributing to larger air-sea enthalpy heat fluxes available for Patricias growth (24% more than for Haiyan). In addition, air-sea fluxes simulations were performed for hurricane Patricia under different climate conditions to assess specifically the impact of local and large-scale conditions on storm intensification associated with six different phases and types of El Nino Southern Oscillation (ENSO) and long-term climatological summer condition. We found that the Eastern Pacific El Nino developing and decaying summers, and the Central Pacific El Nino developing summer are the three most favorable ENSO conditions for storm intensification. This still represents a 37% smaller flux supply than in October 2015, suggesting that Patricia extraordinary growth is not achievable under any of these typical ENSO conditions but rather the result of the exceptional environmental conditions associated with the build-up of the strongest El Nino ever recorded.

Influence of the Size of Supertyphoon Megi (2010) on SST Cooling

AbstractSupertyphoon Megi (2010) left behind two very contrasting SST cold wake cooling patterns between the Philippine Sea (1.5°C) and the South China Sea (7 °C). Based on various radii of radial winds, we found that the size of Megi doubles over the South China Sea when it curving northwards. On average, the radius of maximum wind (RMW) increased from 18.8 km over the Philippine Sea to 43.1 km over the South China Sea; the radius of 64 kt typhoon-force wind (R64) increased from 52.6 km to 119.7 km; the radius of 50 kt damaging-force wind (R50) increased from 91.8 km to 210 km; and the radius of 34 kt gale-force wind (R34) increased from 162.3 km to 358.5 km. To investigate the typhoon size effect, we conduct a series of numerical experiments on Megi-induced SST cooling by keeping other factors unchanged, i.e., typhoon translation speed and ocean subsurface thermal structure. The results show that if it were not for Megi’s size increase over the South China Sea, the during-Megi SST cooling magnitude woul...