Vol. 404: 39–50, 2010 MARINE ECOLOGY PROGRESS SERIES Published April 8 doi: 10.3354/meps08477 Mar Ecol Prog Ser Strong enhancement of chlorophyll a concentration by a weak typhoon Liang Sun1, 2,*, Yuan-Jian Yang3, Tao Xian1, Zhu-min Lu4, Yun-Fei Fu1 1Laboratory of Atmospheric Observation and Climatological Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, PR China 2LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, PR China 3Anhui Institute of Meteorological Sciences, Hefei 230031, PR China 4Key Laboratory of Tropical Marine Environmental Dynamics, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, PR China ABSTRACT: Recent studies demonstrate that chlorophyll a (chl a) concentrations in ocean surface waters can be significantly enhanced due to typhoons. The present study investigated chl a concen- trations in the middle of the South China Sea (SCS) from 1997 to 2007. Only the Category 1 (minimal) Typhoon Hagibis (2007) had a notable effect on chl a concentrations. Typhoon Hagibis had a strong upwelling potential due to its location near the equator, and the forcing time of the typhoon (>82 h) was much longer than the geostrophic adjustment time (~63 h). The higher upwelling velocity and the longer forcing time increased the depth of the mixed-layer, which consequently induced a strong phytoplankton bloom that accounted for about 30% of the total annual chl a concentration in the middle of the SCS. Induction of significant upper ocean responses can be expected if the forcing time of a typhoon is long enough to establish strong upwelling. KEY WORDS: SCS · Forcing time · Upwelling Resale or republication not permitted without written consent of the publisher INTRODUCTION the typhoon-induced upwelling and the pre-existing eddies favor the enhancement of chl a (Walker et al. Investigations of the impacts of typhoons on the 2005, Shi & Wang 2007, Zheng et al. 2008, McClain upper water levels of the ocean have determined that 2009, Sun et al. 2009). The stronger the upwelling is, surface chlorophyll a (chl a) concentrations substan- the more nutrients are transported to the surface. tially increase after the passage of a typhoon (Subrah- However, whether typhoons have notable impacts on manyam et al. 2002, Lin et al. 2003, McClain et al. ocean primary production is a question still open to 2004, Zheng & Tang 2007, Gierach & Subrahmanyam discussion. 2008, McClain 2009), especially in oligotrophic waters On the one hand, single case studies, in which the (Babin et al. 2004). The physical mechanism primarily impacts of super typhoons have been considered, responsible for the increase in chl a concentration is showed that typhoons do have notable impacts on the typhoon’s strong wind, which induces mixing and regional ocean primary production. In the East China upwelling in the upper ocean (Price 1981) and brings Sea, Typhoon Meari induced a 3-fold increase in pri- both subsurface chl a to the surface and subsurface mary production, contributing 3.8% of the annual new nutrients into the euphotic zone (Subrahmanyam et al. production (Siswanto et al. 2008). For the South China 2002, Lin et al. 2003, Babin et al. 2004, Zheng & Tang Sea (SCS), it was also estimated that, on average, 30- 2007, Gierach & Subrahmanyam 2008). Although it is fold increases in surface chl a concentrations were trig- not clear whether the extra chl a is due to the gered by Typhoons Kai-Tak (2000) (Lin et al. 2003) and upwelling of nutrients or of chl a, we are sure that both Lingling (2001) (Shang et al. 2008). Typhoon Kai-Tak *Email: [email protected] © Inter-Research 2010 · www.int-res.com 40 Mar Ecol Prog Ser 404: 39–50, 2010 (2000) alone and typhoons occurring during the entire resolution of 1/4° × 1/4°, obtained from the daily year induced from about 2 to 4% and 20 to 30% of the QuikSCAT (Quick Scatterometer), provided by SCS’s annual new primary production, respectively Remote Sensing Systems (www.remss.com/). Wind → (Lin et al. 2003). stress τ was calculated with the bulk formula (Garratt On the other hand, the integrated impact of tropical 1977) such that: cyclones on sea surface chl a contradicts the above- → →→ τρ= CUU (1) mentioned observations (Hanshaw et al. 2008, Zhao et aD → ρ al. 2008). A comparative study of Typhoons Lingling where a and U are the air density and wind vector and × –3 (2001) and Kai-Tak (2005) indicated that most of the CD = (0.73 + 0.069U) 10 is the drag coefficient. typhoons in the SCS were relatively weak compared to According to recent studies (Powell et al. 2003, Jarosz Kai-Tak and Lingling and that typhoons accounted for et al. 2007), the above-mentioned drag coefficient is an 3.5% of the annual primary production in the oligo- overestimation for wind speeds >40 m s–1. The poten- trophic SCS (Zhao et al. 2008). A similar conclusion tial upwelling velocity Ve due to wind was calculated was drawn for the North Atlantic; there the chl a using the Ekman pumping velocity (EPV) formula concentration contributed to only 1.1% of the positive (Price et al. 1994): → chl a anomaly within the hurricane season, which ⎛ τ ⎞ = ⎜ ⎟ implies that an integrated impact of tropical cyclones Ve curl (2) ⎝⎜ ρƒ⎠⎟ may justifiably be ignored (Hanshaw et al. 2008). Such conclusions are not surprising if we recall the investi- where ρ = 1020 kg m–3 is the density of seawater and ƒ gation of the hurricane-induced phytoplankton blooms is the Coriolis parameter. The thermocline displace- in the Sargasso Sea (Babin et al. 2004); here it was ment (or isopycnal displacement) Δη due to a typhoon found that 13 hurricanes induced, on average, a 3-fold with translation speed UT was estimated according to (range from 1- to 9-fold) chl a increase for about 2 wk. Price et al. (1994) such that: Thus, each hurricane induced <1% of the chl a in- → τ crease above the annual mean. Considering that about Δ = (3) η ρ 3 to 4 hurricanes occur annually, the integrated impact ƒUT of tropical cyclones on the chl a increase is very small, The second type of wind data used was the ‘best- which is consistent with integrated estimations (Han- track dataset’ for the western North Pacific, obtained shaw et al. 2008, Zhao et al. 2008). from the Joint Typhoon Warning Center (JTWC). Each As the typhoon-induced chl a enhancement depends best-track file contains locations and intensities of trop- on the amount of subsurface nutrients transported into ical cyclone centers (i.e. the maximum 1 min mean sus- surface waters due to upwelling, the main question to tained 10 m wind speed), at 6 h intervals. Such maxi- be answered is whether a typhoon induces sufficiently mum wind speeds can be found in Table 1. These 1 min strong upwelling. Motivated by such investigations mean sustained wind speeds are relatively large com- and the above-mentioned arguments, the present pared with 10 min mean sustained wind speeds, which, study investigated the chl a concentrations in the SCS according to recent studies (Powell et al. 2003, Jarosz et from 1997 to 2007 and found that only the Category 1 al. 2007), leads to even greater uncertainty in the calcu- Typhoon Hagibis (2007) had a notable impact on the lation of wind stress. Thus, we used the MSW and a chl a concentration. more modern drag law, i.e. CD = (–2.229 + 0.2983U– 0.00468U 2) × 10–3 (Jarosz et al. 2007), to calculate wind → stress τ. In addition, wind with a fixed radius of 200 km MATERIALS AND METHODS and radii of specified winds (35, 50, 65, or 100 knots) for 4 quadrants were also considered; these data are useful The merged daily and monthly chl a concentration for wind stress curl calculations. Finally, both wind → data (Level 3), with a spatial resolution of 9 km from stress τ and wind diameter D were used to perform EPV 2 ocean color sensors (MODIS and SeaWiFS), were pro- calculations for comparison such that: → duced and distributed by the NASA Goddard Space τ Flight Center’s Ocean Data Processing System (ODPS). V = (4) e ρ Typhoon tracking data, taken every 6 h, including ƒD center location, central pressure and maximum 10 min For more details on the calculations of EPV and ther- mean sustained wind speeds (MSW), were obtained mocline displacement see the ‘Discussion’ section. from the Shanghai Typhoon Institute (STI) of the China On the other hand, in the present study, we also con- Meteorological Administration (CMA). In addition, sidered the forcing time of typhoons (Table 1), i.e. the 2 other types of wind data were used. One was the sea typhoons’ maximum wind blowing time Tb in the surface wind (SSW) vector and stress, with a spatial region (see Appendix 1 for details). The adjustment Sun et al.: Enhancement of chlorophyll a concentration by a typhoon 41 Table 1. Typhoons passing over the study area; data and calculations from the present study. MWS/BWS: maximum wind speed/ best-track wind speed; CT: category of typhoon; MS: translation speed; EPV: Ekman pumping velocity Ve; AT: adjustment time Ta; FT: forcing time Tb;, superscripts ‘a’ and ‘b’: the values calculated according to Eq. (2) and Eq. (4), respectively; NA: not available Typhoon Latitude MWS/BWS CT MS Max. EPV AT FT (name & date) (°N) (m s–1) (m s–1) (10–4 m s–1) (h) (h) Faith (Dec 1998) 10–12 30/46.2 2 4.3 NAa/NAb 63 NA Lingling (Nov 2001) 13 50/59 4 4.6 45.2a/21.6b 53 40 Nepartak (Nov 2003) 13–15 30/38.5 1 4.6 7.8a/17.8b 50 20 Muifa (Nov 2004) 9–12 30/46.2 2 3.1 10.7a/NAb 69 26 Chathu (Jun 2004) 12–14 33/38.5 1 6.6 12.9a/NAb 53 15 Kai-tak (Oct 2005) 12–15 40/43.6 2 2.2 15.7a/17.9b 53 52 Chanchu (May 2006) 14–15 50/64 4 2.0 21.5a/32.9b 50 44 Chebi (Nov 2006) 15 30/36 1 4.1 2.9a/16.8b 45 16 Durian (Dec 2006) 11–14 40/46.2 2 4.2 38.7a/19.6b 53 26 Utor (Dec 2006) 14–15 45/41 1 4.5 14.8a/26.1b 50 18 Hagibis (Nov 2007) 10–12 35/41 1 2.5 48.1a/34.6b 63 82 times were also calculated for the typhoons passing the 1/2, or Envisat) sea surface height anomaly (SSHA) study area (Table 1); these were estimated by assum- data, which are high resolutions of the 1/4° × 1/4° Mer- ing that the upwelling is weak at the beginning of cator grid, are available at www.aviso.oceanobs.com.