10) on sinking particle fluxes in the 10°N thermocline ridge area of the northeastern equatorial Pacific

Deep-Sea Research I 67 (2012) 111–120

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˜ o events (1997/98 and 2009/10) on sinking particle Impact of strong El Nin fluxes in the 101N thermocline ridge area of the northeastern equatorial Pacific Hyung Jeek Kim a, Kiseong Hyeong a,n, Chan Min Yoo a, Boo Keun Khim b, Kyeong Hong Kim a, Ju Won Son a, Jong Seong Kug c, Jong Yeon Park c, Dongseon Kim c a b c

Deep-Sea & Marine Georesources Research Department, KORDI, Ansan P.O. Box 29, 425-600, Republic of Korea Department of Oceanography, Pusan National University, Busan 609-735, Republic of Korea Climate Change & Coastal Disaster Research Department, KORDI, Ansan P.O. Box 29, 425-600, Republic of Korea

a r t i c l e i n f o

abstract

Article history: Received 27 December 2011 Received in revised form 9 May 2012 Accepted 20 May 2012 Available online 30 May 2012

˜ o events (1997/98 and 2009/10) were monitored at a Sinking particle fluxes during two strong El Nin station (10130N, 131120W) located in the 101N thermocline ridge area (91–131N, 1051–1401W) of the ˜ o event resulted in two- to fourfold higher organic northeastern equatorial Pacific. The 1997/98 El Nin carbon and biogenic silica fluxes than the non-ENSO levels. In combination, these findings suggest an increase in the productivity of surface waters. The strengthening of the North Equatorial Countercurrent, due to intensified westerlies and the resulting shoaling of the thermocline, likely induced upwelling of nutrient-enriched subsurface water and enhanced biological productivity. In contrast, the ˜ o event was only accompanied by a substantial increase in planktonic foraminifera, with 2009/10 El Nin minor increases in organic carbon and biogenic silica fluxes. The distinct differences in the particle ˜ o events are composition and the biological response of the marine ecosystem during the two El Nin attributed to different oceanographic and atmospheric conditions generated by two different types of El ˜ o: the cold-tongue (CT) type and the warm-pool (WP) type. Our results, together with the findings Nin ˜ o event, suggest that enhanced biological productivity of a previous study of the 1982/83 strong El Nin ˜ o conditions. Our study also provides occurs in the 101N thermocline ridge area under strong El Nin ˜ o conditions. insight into equatorial biological dynamics under the extreme CT- and WP-type El Nin ˜ o conditions are projected to increase in frequency in the This is important because the WP-type El Nin future. & 2012 Elsevier Ltd. All rights reserved.

Keywords: 101N thermocline ridge area Sinking particle fluxes ˜o Strong El Nin ˜o Cold-tongue and warm-pool type El Nin Biological productivity

1. Introduction ˜ o Southern Oscillation (ENSO) is one of the major The El Nin drivers of interannual global ocean–atmospheric climate variability. It has also been shown to affect biological productivity and sinking particle fluxes in the equatorial Pacific (e.g., Barber and Chavez, 1983; Chavez et al., 1984, 1999; Honjo et al., 1995; Kawahata and Gupta, 2003; McPhaden, 2004). For example, El ˜ o has been shown to reduce surface ocean productivity and to Nin decrease sinking particle fluxes in the eastern equatorial Pacific (Fig. 1a and b) (Chavez et al., 1999; Dymond and Collier, 1988;

n

Corresponding author. Tel.: þ82 31 400 6382; fax: þ82 31 418 8772. E-mail addresses: [email protected] (H.J. Kim), [email protected] (K. Hyeong), [email protected] (C.M. Yoo), [email protected] (B.K. Khim), [email protected] (K.H. Kim), [email protected] (J.W. Son), [email protected] (J.S. Kug), [email protected] (J.Y. Park), [email protected] (D. Kim). 0967-0637/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr.2012.05.008

Honjo et al., 1995; Karl et al., 1995; Barber et al., 1996; Kawahata and Gupta, 2004). Primary productivity and particle flux varia˜ o and tions had been monitored during 1992 moderate El Nin ˜ o in the eastern equatorial Pacific as a part 1982/83 strong El Nin of the United States Joint Global Ocean Flux Study (US-JGOFS) Equatorial Pacific (EqPac) Experiment and the Manganese Nodule Project (MANOP), respectively (Fig. 1). In the EqPac experiment, ˜ o event led to a decrease in sinking particle fluxes the 1992 El Nin between 51S and 51N along 1401W meridian (Fig. 1a and b) (Honjo et al., 1995). Exceptions were found in sites 4 (21N) and 5 (01), where sinking particle fluxes were similar to or somewhat higher ˜ o period (Honjo et al., 1995). In all than those in post-El Nin stations, however, reductions in primary productivity and export flux of particulate organic matters were clear at the base of the ˜o euphotic zone (0–120 m water depth) during the 1992 El Nin period (Bacon et al., 1996; Barber et al., 1996; Berelson et al., 1997; Luo et al., 1995). The suppression of primary production and decrease in sinking particle fluxes were also evident under

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Fig. 1. (a) Map showing the locations of the study sites and the major current systems in the equatorial Pacific. Contour lines show the thermocline depth (Wilson and Adamec, 2001). (b) Total mass fluxes, measured in previous and this studies, carried out in the eastern equatorial Pacific. The locations of the sites are shown in Fig. 1a. Bars represent the entire range of observed particle fluxes. The gray rectangle indicates the latitudinal range of the 101N thermocline ridge area. NEC, North Equatorial Current; NECC, North Equatorial Countercurrent; SEC, South Equatorial Current; EUC, Equatorial Undercurrent; NGCUC, New Guinea Coastal Undercurrent. Data for the moderate El ˜ o and post-El Nin ˜ o periods at 51S, 21S, 01, 21N, 51N and 91N are taken from Honjo et al. (1995). However, post-El Nin ˜ o flux data at 91N are those of the normal cold Nin ˜ o flux data by Honjo et al. (1995) was found to include the seasonal effect (Kim et al., 2011a). Data for site S and site C, 151N, season from Kim et al. (2011a) as the El Nin ALOHA station and KOMO are from Dymond and Collier (1988), Honjo et al. (1982), Karl et al. (1995) and Kim et al. (2011a), respectively.

˜ o conditions at site C (11N, 1391W; the 1982/83 strong El Nin Fig. 1a and b) (Dymond and Collier, 1988). In detail, biogenic particle fluxes were 2–4 times lower during the 1982/83 strong El ˜ o period than those during the post-El Nin ˜ o period. In Nin ˜ o has been shown to depress surface ocean addition, El Nin productivity and sinking particle fluxes in the off-equatorial Pacific region (101–151N, 1351E–1351W, site 1 in Fig. 1a and b) (Chavez et al., 1999; Kim et al., 2011a) and in the north Pacific subtropical gyre region (151–351N, 1351E–1351W, Aloha station in Fig. 1a and b) (Karl et al., 1995; Thunell, 1998; Wong et al., 2002). These findings suggest that the productivity of surface waters is ˜ o condition because the upwelling of reduced under El Nin nutrient-enriched subsurface water in the eastern equatorial Pacific is inhibited (Barber and Chavez, 1983; Chavez et al., 1999; Kang et al., 2008; Strutton and Chavez, 2000). An exception to this pattern was reported in the 101N thermocline ridge area (91–131N, 1101–1401W; the shallowest thermocline doming region in the eastern Pacific) during the ˜ o. Dymond and Collier (1988) reported a 1982/83 strong El Nin two- to fourfold increase in particle fluxes, compared with those ˜ o season, at site S (111N, 1401W; Fig. 1a and b). of the post-El Nin In contrast, site C (11N, 1391W; located in the core region of El ˜ o) experienced a significant reduction in particle fluxes during Nin ˜ o event as discussed earlier (Fig. 1a and b) the same El Nin (Dymond and Collier, 1988). The elevated particle flux at site S is remarkable because the 101N thermocline ridge area is located near the core region of the ENSO effect. In addition, the biological ˜ o conditions, in both the response to weak and moderate El Nin region of site S and the equatorial region, was consistent in both cases (Kim et al., 2011a). Dymond and Collier (1988) attributed the increase in biological productivity and sinking particle fluxes

at site S to intensification of the NECC associated with shoaling of the thermocline. However, the mechanism is not fully understood because of limited meteorological and oceanographic data covering the 1980s. Furthermore, these particle flux measurements are ˜ o event in the 101N thermocline the only data for a strong El Nin ridge area. Therefore, it is not possible to test the significance of these results without additional particle flux measurements for a similar meteorological event. ˜ o event took place in 2009/10. The most recent strong El Nin ˜ o, and This was classified as a warm-pool type (WP-type) El Nin was characterized by a large sea surface temperature (SST) ˜ o 4 region, anomaly confined to the central equatorial Pacific (Nin 51S–51N, 1601E–1501W) (Fig. 2b) (Kim et al., 2011b; Lee and McPhaden, 2010; Takahashi et al., 2011). Unlike the 2009/10 El ˜ o, the 1982/83 and 1997/98 strong El Nin ˜ o events were Nin characterized by a maximum SST anomaly restricted to the ˜ o 3 region, 51S–51N, 901–1501W) eastern equatorial Pacific (Nin (Fig. 2a) and were classified as an cold-tongue type (CT-type) El ˜ o (Lee and McPhaden, 2010; Ratnam et al., 2011; Ren and Jin, Nin 2011). Distinct spatial patterns of SST anomalies during the two ˜ o events result in different oceanic and meteortypes of El Nin ological conditions in the eastern equatorial Pacific (e.g., Ashok et al., 2007; Kim et al., 2009; Kim et al., 2011b; Mo, 2010; Weng et al., 2009). These conditions probably have contrasting impacts on marine ecosystems, and therefore on biological productivity and sinking particle fluxes (e.g., Turk et al., 2011). However, ˜ o event has been encountered because the strong WP-type El Nin only once (in 2009/10) since the start of ENSO monitoring in the late 19th century (e.g., Kug et al., 2009; Lee and McPhaden, 2010; Takahashi et al., 2011; Yeh et al., 2009), no data are available for comparing the biological response during the two types of strong

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Fig. 2. Sea surface temperature (SST) anomalies observed during the peak season ˜ o strong events. The anomalies are of the (a) 1997/98 and (b) 2009/10 El Nin averaged for three months from December to February (DJF). Shading indicates SST anomalies normalized for the monthly average of the last 30 years. The contours show the SST with a 11C interval, averaged for the last 30 years.

˜ o events in the equatorial Pacific and in the 101N thermoEl Nin cline ridge area. For this study, we operated a time-series sediment trap at a station (called ‘‘KOMO’’) in the 101N thermocline ridge area (Fig. 1a) for 8 years (i.e., July 1997 to May 1998, and July 2003 to June 2010), and were able to monitor the particle flux ˜ o events. variations during the 1997/98 and 2009/10 strong El Nin These data provide insight into the influence of these two strong ˜ o events on sinking particle fluxes in the 101N thermocline El Nin ridge area. The study site is located near site S (i.e., where the unexpected increase in particle fluxes was reported during the ˜ o event; Fig. 1a and b). These new data, 1982/83 strong El Nin together with those from site S, provide an overview of biological ˜ o events in the surface waters of the response to strong El Nin 101N thermocline ridge area. This study also provides the opportunity to explore the influence of two different types of strong El ˜ o (i.e., the 1997/98 CT-type and the 2009/10 WP-type events) Nin on surface water ecological processes and sinking particle fluxes in the region.

2. Study site The Korea Ocean Research and Development Institute (KORDI) has operated a monitoring station (KOMO Station at 10.51N, 131.31W; ca. 5000 m water depth) to understand the physical and biological characteristics of the 101N thermocline ridge area in the northeastern equatorial Pacific (Fig. 1a). This region is located in the high nutrient and low chlorophyll (HNLC) area, which extends to the north Pacific subtropical gyre (Karl, 1999; Murray et al., 1995). The 101N thermocline ridge area has the shallowest thermocline in the eastern equatorial Pacific ( o100 m), due to a divergence between the westward flowing NEC and the eastward flowing NECC (Dymond and Collier, 1988; Pennington et al., 2006; Wilson and Adamec, 2001). The New Guinea Coastal Undercurrent (NGCUC) flows northwest along the

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northern coast of New Guinea and feeds the eastward flowing NECC and the Equatorial Undercurrent (EUC) (Fig. 1a) (Christian et al., 2004; Messie´ and Radenac, 2006). The intensity and zonal location of these currents fluctuate both seasonally and with the ENSO cycle. In particular, the speed and transport of the NECC ˜ o conditions (Johnson et al., 2000; intensifies under strong El Nin Kessler and McPhaden, 1995; Meyers and Donguy, 1984; Qiu and Joyce, 1992). Properties of the surface waters at KOMO station (e.g., SST, sea surface salinity (SSS), and wind speed) are governed by seasonal variations of the inter-tropical convergence zone (ITCZ). Here, the northeast and southwest trade winds converge (Amador et al., 2006; Kim et al., 2011a; Romero-Centeno et al., 2007). In the eastern Pacific region, the ITCZ migrates to 111N in the summer–fall and to 41N in the winter–spring. This is accompanied by latitudinal movements of the high SSTs Donguy and Meyers, 1996; Fiedler and Talley, 2006 and the low SSSs zone within the ITCZ (Amador et al., 2006; Donguy and Meyers, 1996; Fiedler and Talley, 2006; McGee et al., 2007). As a result, the 101N thermocline ridge area has cyclic seasonal fluctuations in oceanic and atmospheric properties during the winter–spring and the summer–fall (Kim et al., 2011a). Monthly average SST, SSS, and wind speed data, obtained from the Tropical Atmosphere Ocean (TAO) mooring station (91N, 1401W) (available online at http:// www.pmel.noaa.gov), record this seasonal variation during the monitoring period (Fig. 3). The seasonal cycles also coincide with the cyclic variation of the satellite-derived mixed layer depth (Fig. 3). In detail, the winter–spring is characterized by a low SST, a high SSS, a strong wind speed, and a deepening of the mixed layer depth. The opposite conditions occur during the summer– fall (Fig. 3).

3. Previous studies at KOMO Station ˜ o Index (www.cpc.ncep.noaa.gov), According to the Ocean Nin ˜ o/La Nin ˜ a events occurred during the monitoring five El Nin ˜ o (between June of 2004 periods: a weak and moderate El Nin and February of 2005, and between August of 2006 and January of ˜ a (between September of 2007 and May 2007), a moderate La Nin ˜ os (between July of 1997 and May of 2008), and two strong El Nin of 1998, and between September of 2009 and June of 2010) ˜ os, particle (Fig. 3a). Except for the 1997/98 and 2009/10 El Nin flux variations during these ENSO events and during normal (i.e., non-ENSO) periods were described in Kim et al. (2010, 2011a). In summary, the total mass fluxes during the normal periods had distinct seasonal fluctuations: an average total mass flux of 25.3 ( 77.71, 1 SD) mg m  2 day  1 during the winter–spring and 14.0 ( 77.03, 1 SD) mg m  2 day  1 during the summer–fall (Fig. 4). This seasonal variability was caused by seasonal shifts in the ˜ o event had no location of the ITCZ. The 2004/05 weak El Nin recognizable effect on sinking particle fluxes. In contrast, the ˜ o event was accompanied by a notice2006/07 moderate El Nin able reduction of sinking particle fluxes (i.e., ca. 60% of the average flux of non-ENSO period). Meanwhile, the 2007/08 ˜ a caused a threefold increase in particle fluxes moderate La Nin over the average flux of non-ENSO period. These results are consistent with the general pattern of surface biological produc˜ o/La Nin ˜a tivity and sinking particle fluxes observed for El Nin conditions in the eastern equatorial Pacific (Barber and Chavez, 1983; Chavez et al., 1999; Dymond and Collier, 1988; Honjo et al., 1995; Kang et al., 2008). 4. Methods Sinking particles were collected each month from a time-series sediment trap at 4950 m depth, 90 m above the seafloor. It was

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˜ o 3.4 region (51N–51S, 1201–1701W) and given as the Oceanic Nin ˜ o Index (ONI), temporal variations Fig. 3. Sea surface temperature (SST) anomalies (a) observed in the Nin of SST (b, solid circles), sea surface salinity (SSS; b, open circles), wind speed (c, solid circles), and mixed layer depth (c, gray bars) for the monitoring period from July of 1997 to June of 2010. The meteorological data were adopted from the Tropical Atmosphere Ocean project (TAO, 91N and 1401W), and can be found online at www.pmel.noaa.gov. The mixed layer depth data are from the Global Ocean Data Assimilation System (GODAS).

found that the effect of re-suspension of bottom sediment is negligible at the sampling depth and particle flux variation reflects the surface process with one to two months lag time (Kim et al., 2011a). To prevent bacterial decay, collection bottles were filled with a sodium borate buffered 5% formalin solution. Upon recovery of the sinking particles, sample bottles were sealed and refrigerated. Each sample was split, using a McLane WSD-10 divider, into five equal aliquots. Three of the aliquots of each sample were rinsed with Milli-Q water to remove residual formalin solution. Then, the washed samples were freeze-dried and weighed for total mass flux. These freeze-dried samples were analyzed for total carbon (using a Carlo-Erba 1110 CNS elemental analyzer), calcium carbonate (CaCO3) contents (using a UIC coulometrics carbon analyzer), and for biogenic silica (BSi) (using a time-series sequential dissolution method) (Kim et al., 2011a). The precision for these analyses was less than 3%, 2%, and 5%, respectively. To determine the flux of planktonic foraminifera, an aliquot of each sample was wet-sieved through a 63 mm mesh,

rinsed with Milli-Q water, and then freeze-dried. Planktonic foraminifers were hand-picked from the 463 mm fractions under a binocular microscope, and were weighed for measurements of foraminiferal flux. In the open ocean, CaCO3 consists mainly of planktonic foraminifers (mostly 463 mm ) and coccolithophores (usually o63 mm) (Broecker and Clark, 2009; Honjo et al., 1982; Kawahata and Gupta, 2002; Poulton et al., 2007; Ziveri et al., 2007). Thus, the coccolithophore flux was estimated by subtracting the foraminiferal flux from the CaCO3 flux. For the investigation of large-scale environmental patterns associated with the observed sinking particle fluxes, supplementary data sets are also used in this study. These include SST, sea level height (SLH), outgoing longwave radiation, and chlorophyll data. The SST is taken from the extended reconstructed SST version 3 (ERSST.v3) (Smith et al., 2008) and has a 2.51 latitude  2.51 longitude global grid. The SLH is taken from the Global Ocean Data Assimilation System (GODAS) (Behringer and Xue, 2004). This has 40 vertical ocean levels and a 0.3331

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Fig. 4. Average fluxes of (a) total mass and CaCO3 and (b) biogenic silica and organic carbon at KOMO under various oceanographic conditions. Vertical solid bars represent one standard deviation. Temporal variations of (c) total mass, CaCO3, foraminiferal fluxes, (d) biogenic silica and organic carbon fluxes at KOMO and site S (1982/83). Data ˜ o are from Dymond and Collier (1988). The KOMO data other than two strong events (1997/98 and 2009/10) are from Kim et al. (2011a). for the 1982/83 El Nin

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latitude  1.01 longitude global grid. The GODAS is forced by the momentum flux, heat flux, and fresh water flux from the National Centers for Environmental Prediction (NCEP) atmospheric Reanalysis 2 data sets (Behringer et al., 1998). The satellite-based outgoing longwave radiation data are from the National Center for Atmospheric Research (NCAR) archives and are interpolated on temporal and spatial scales. Details of the interpolation technique can be found in Liebmann and Smith (1996). Finally, chlorophyll concentrations are obtained from two satellite-based ocean color sensors (Sea-viewing Wide Field-of-view Sensor or SeaWiFS, and Moderate Resolution Imaging Spectroradiometer or MODIS). These data, along with global and multi-year data for marine phytoplankton, are available through the Goddard Space Flight Center (Esaias et al., 1998; McClain et al., 1998). These data are monthly level-three data sets, binned into a regular spatial grid of 9 km  9 km. The data have been bilinearly interpolated to a 2.51  2.51 grid and span September 1997 to December 2010 (SeaWiFS from September 1997 to December 2007, and MODIS from January 2008 to December 2010). All data used in this study are presented as the monthly-mean anomaly.

5. Results ˜o event 5.1. Particle fluxes during the 1997/98 strong El Nin ˜o event, total mass fluxes reached a For the 1997/98 strong El Nin maximum of 72.2 mg m  2 day  1 (March 1998). This was three times higher than average of total mass fluxes (25.377.71 mg m  2 day  1) during the normal cold season (winter–spring season) (Table 1, Fig. 4). In general, the temporal variation of biogenic particle fluxes followed the pattern of total mass flux (Table 1, Fig. 4). Among the biogenic particles, organic carbon flux had the largest increase to 7.85 mg m  2 day  1, four times higher than average flux (1.74 mg m  2 day  1) during normal cold season (Fig. 4). The maximum fluxes of CaCO3 (38.2 mg m  2 day  1) and biogenic silica (7.88 mg m  2 day  1) were 2–3 times higher than average fluxes (CaCO3: 11.7 mg m  2 day  1, biogenic silica: 5.33 mg m  2 day  1) during normal cold season (Table 1, Fig. 4). Fluxes of planktonic foraminifera (i.e., foraminiferal flux) were less than 7 mg m  2 day  1

at their peak from February to March of 1998 (Fig. 4c). This accounted for 18% of the CaCO3 fluxes. CaCO3, besides planktonic foraminifers, consists mainly of coccolithophores (Broecker and Clark, 2009; Honjo et al., 1982; Poulton et al., 2007; Ziveri et al., 2007). Thus, coccolithophores, a primary producer, comprise most of CaCO3 fluxes during ˜o period. Particle fluxes, during the 1997/98 the 1997/98 strong El Nin ˜o, are characterized by an increase in each biological compoEl Nin nent with a considerable contribution of coccolithophores in CaCO3 fluxes. The enhanced particle fluxes, observed at the KOMO station ˜ o, are consistent with the results during the 1997/98 strong El Nin ˜o of Dymond and Collier (1988). Here, the 1982/83 strong El Nin event was observed at site S (Fig. 4c and d). At site S, total mass flux at the 3400 m water depth was 74.9 mg m  2 day  1 from 29 December 1982 to 8 April 1983. This was four times higher than ˜ o periods (15–17 mg m  2 day  1) (Fig. 4c). values following El Nin ˜ o caused an increase in Similarly, the 1982/83 strong El Nin biogenic particle fluxes at site S, where biogenic silica showed the highest rate of increment, followed by increases in CaCO3 and organic carbon (Fig. 4c and d). Diatoms were responsible for an increase in opal fluxes (Pisias et al., 1986) and caused the downward fluxes of biogenic silica and organic carbon in the eastern equatorial Pacific (Honjo et al., 1995). Therefore, diatom would be expected to regulate opal and organic carbon fluxes during the ˜ o events at the KOMO station. 1997/98 El Nin

˜o event 5.2. Particle fluxes during the 2009/10 strong El Nin ˜ o event, also The total mass flux, for the 2009/10 strong El Nin increased dramatically (i.e., a maximum of 66.1 mg m  2 day  1), ˜ o events. consistent with the 1982/83 and 1997/98 strong El Nin ˜ o, The highest total mass flux, during the 2009/10 strong El Nin ˜ o; Fig. 4c). occurred in March (similar to the 1997/98 strong El Nin ˜ o was only characterized by a However, the 2009/10 strong El Nin significant increase in the biogenic component CaCO3 (Fig. 4c ˜ o events, and d). Unlike the 1982/83 and 1997/98 strong El Nin biogenic silica and organic carbon fluxes exhibited minor increases (Fig. 4d). These increases were equal to, or less than, what was observed during the non-ENSO season (Fig. 4b and d).

Table 1 Particle flux data recorded at the KOMO station (located in the 101N thermocline ridge area) from July of 1997 to May of 1998, and from September of 2009 to June of 2010. Date open (mm/dd/yr)

07/08/97 08/08/97 09/08/97 10/08/97 11/08/97 12/08/97 01/08/98 02/08/98 03/08/98 04/08/98 05/08/98 09/01/09 10/01/09 11/01/09 12/01/09 01/01/10 02/01/10 03/01/10 04/01/10 05/01/10 06/01/10

Days

31 31 30 31 30 31 31 28 31 30 31 30 31 30 31 31 28 31 30 31 30

Fluxes in mg m  2 day  1

% of total

Total mass

Organic carbon

CaCO3

Biogenic silica

Foraminiferal

Organic carbon

CaCO3

Biogenic silica

7.16 13.0 18.9 13.4 13.8 21.4 38.6 53.1 72.2 65.8 14.7 16.9 20.9 22.1 19.6 16.7 41.9 66.1 27.3 12.7 15.4

0.71 1.61 2.64 1.84 1.71 2.30 3.97 6.20 7.85 6.28 1.44 1.51 1.61 1.58 1.52 1.17 2.11 3.14 1.92 1.02 1.17

No data 6.34 5.76 3.01 1.91 4.84 13.3 34.0 38.2 22.8 3.93 6.20 7.05 7.79 5.67 6.09 21.8 43.7 13.7 4.52 6.06

No data No data No data No data 3.83 7.07 9.38 9.70 7.88 13.7 No data 1.87 2.36 2.22 1.95 1.58 3.12 3.88 1.97 1.20 1.61

No data No data No data No data No data No data No data 5.94 6.92 No data No data No data No data No data No data 1.75 24.9 32.2 9.81 No data No data

9.94 12.4 14.0 13.7 12.4 10.7 10.3 11.7 10.9 9.53 9.74 8.91 7.71 7.13 7.73 7.02 5.04 4.76 7.02 8.00 7.58

No data 48.9 30.5 22.5 13.9 22.7 34.4 64.0 52.9 34.7 26.7 36.5 33.8 35.2 28.9 36.5 52.1 66.1 50.1 35.6 39.4

No data No data No data No data 37.8 33.0 24.3 18.3 10.9 20.9 No data 11.0 11.3 10.1 9.92 9.48 7.45 5.88 7.22 9.47 10.5

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In addition, foraminiferal flux accounted for 75% of the CaCO3 flux in February and March of 2010. The contribution of foraminiferas, ˜ o, exceeded values from the 1997/98 during the 2009/10 El Nin event (Fig. 4c).

6. Discussion The marked increases in total mass particle flux, associated ˜ o events, are with the 1997/98 and 2009/10 strong El Nin consistent with the 1982/83 results from Dymond and Collier ˜ o-modulated oceano(1988). This suggests that strong El Nin graphic conditions promote biological productivity in the 101N thermocline ridge area. However, there are differences in the response of the marine ecosystem to the 1997/98 and the 2009/ ˜ o events: there was a major increase in phytoplankton 10 El Nin (diatoms and coccolithophores) in the 1997/98 (and the 1982/83) event and an increase in zooplankton (foraminiferas) in the 2009/ 10 event. This difference is likely attributed to different oceanographic and meteorological conditions generated by the CT-type ˜ o events. (1982/83 and 1997/98) and WP-type (2009/10) El Nin Dymond and Collier (1988) attributed the marked increase in ˜o, biogenic particle fluxes at site S, during the 1982/83 strong El Nin to the strengthening of the NECC. This increased the shear mixing between the NECC and NEC, and the upwelling of nutrient-enriched subsurface water. Their argument is supported by previous studies, based on direct measurements at hydrographic stations for the ˜o events, which suggest an 1971/72 and 1982/83 strong El Nin intensification of the NECC, a lowering of sea level height, and a resulting shoaling of thermocline in the 101N thermocline ridge area (McCreary, 1976; Meyers and Donguy, 1984; Wyrtki, 1974). The sea level height (SLH) and chlorophyll anomalies, from the SeaWiFS and the TOPEX/Poseidon, predicted the same phenomenon for the 1997/ ˜o event (e.g., Christian et al., 2004; Murtugudde 98 strong El Nin et al., 1999; Wilson and Adamec, 2001) and reinforced field ˜o events. Christian et al. (2004) observations from previous El Nin attributed an increased westward transport of iron-enriched NGCUC ˜o to the NECC. In addition feeding during the 1997/98 strong El Nin to local upwelling, they suggested that NGCUC contributed to enhanced surface productivity in the central equatorial Pacific and in the 101N thermocline ridge area (Fig. 1a). The spatial distribution of SLH anomalies, at the waxing to ˜ o (October of 1997 to January mature stage of the 1997/98 El Nin of 1998), consists of a negative SLH anomaly along the projected NECC path (i.e., from the western pacific to south of the 101N thermocline ridge area, between 51N and 101N; Fig. 5a and ˜ o in Supplementary Fig. 1a–d). At the waning stage of El Nin February of 2008, the SLH anomalies dissipated in the east Pacific and the 101N thermocline ridge area (Fig. 5b). This SLH mode is interpreted to reflect the strength of the NECC, and a variable thermocline depth in the 101N thermocline ridge area (Dymond and Collier, 1988; Kessler and Taft, 1987; Meyers and Donguy, 1984; Murtugudde et al., 1999; Wilson and Adamec, 2001; Wyrtki, 1979). This is supported by current speed and conductivity–temperature–depth data (Christian et al., 2004; Johnson et al., 2000; McCreary, 1976; Meyers and Donguy, 1984; Wyrtki, 1977). The negative SLH anomalies indicate a shallower thermocline and a more intense NECC. Our results also ˜ o, there was a suggest that, until the mature stage of El Nin strengthening of the NECC and of the shoaling of thermocline depth in the study site. We also find a weakening or vanishing of the NECC at later stage (i.e., from February of 1998) (Fig. 5b and Supplementary Fig. 1e–g). This is consistent with the aforementioned studies. The strengthening of the NECC has been attributed ˜o to the development of intense westerlies under strong El Nin

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conditions (Keen, 1984; Sadler and Kilonsky, 1983; Zhang and McPhaden, 2006). The chlorophyll anomalies, at the waxing stage of the 1997/98 ˜ o, are also characterized by a weak positive band along the El Nin SLH (Fig. 5d and Supplementary Fig. 2a–d). This demonstrates the importance of the NECC for the shoaling of the thermocline depth and for supplying nutrient-enriched subsurface water through shear mixing between the NECC and the NEC (Fig. 5) (Christian et al., 2004; Dymond and Collier, 1988; Murtugudde et al., 1999; Wilson and Adamec, 2001). Murtugudde et al. (1999) and Wilson and Adamec (2001) found in satellite-derived chlorophyll concentration data a localized chlorophyll bloom between 1201W and 1801W in the 101N thermocline ridge area. This was attributed to the shoaling of the thermocline caused by a strengthening ˜ o. However, our of NECC during the peak of the 1997/98 El Nin ˜ o period, show that the chlorophyll anomaly data, from the El Nin KOMO station was located at the southern periphery of the positive chlorophyll anomaly zone (Fig. 5d–f and Supplementary Fig. 2). Furthermore, it was not possible to measure the increase in chlorophyll concentration at KOMO station. Nevertheless, changes to the chlorophyll anomaly were synchronous to changes in the particle flux, except for a one to two month lag caused by particle deposition rates. Specifically, the positive chlorophyll anomaly zone shifted northward from March of 1998, placing KOMO in a negative anomaly zone (Fig. 5f and Supplementary Fig. 2). This proceeded the particle flux decrease in May of 1998 (from its peak in February and March of 1998) by several months (Fig. 4c and d). This suggests that the strengthening of the NECC and the shoaling of the thermocline caused surface water productivity to increase. This was responsible for the increased particle flux. Our particle flux data suggest that the three- to fourfold ˜ o, increase in surface productivity, during the 1997/98 El Nin support the satellite-based evidence of upwelling in the 101N thermocline ridge area. Similar to the 1982/83 flux record of Dymond and Collier (1988), our data show a marked increase in particle flux in the 101N thermocline ridge area under strong CT˜ o conditions. This is an abnormal phenomenon when type El Nin compared with the generalized ENSO-modulated environmental conditions observed in the equatorial Pacific. However, the intense westerlies that cause the intensification of the NECC have ˜ o conditions only been reported under strong CT-type El Nin (Keen, 1984; Sadler and Kilonsky, 1983; Zhang and McPhaden, 2006). This explains why particle fluxes did not increase in the ˜o 101N thermocline ridge area under weak and moderate El Nin conditions (e.g., Honjo et al., 1995; Kim et al., 2010, 2011a). The increase in particle flux, during the 2009/10 strong El ˜ o, was also evident in our data. Specifically, we found that the Nin total mass flux was comparable to values from the other two ˜ os. However, the particle components strong CT-type El Nin differed. There was a major increase in single component for˜ o, which is likely related to aminifera in the WP-type El Nin different oceanographic and atmospheric environments modu˜ o (as exemplified in the SST lated by two different types of El Nin anomaly projections; Fig. 2). As shown in Fig. 5 and Supplemen˜ o had distinctive spatial and tary Figs.. 1 and 2, the 2009/10 El Nin temporal surface environmental factors, which evolved in a ˜ o. The negative SLH different way than the 1997/98 El Nin anomaly banding was weaker across the equatorial Pacific and ˜o in the 101N thermocline ridge area during the 2009/10 El Nin ˜ o (Fig. 5 and Supplementary than that during the 1997/98 El Nin Fig. 1). The spatial distribution of the chlorophyll anomaly showed a negative loading around the 101N thermocline ridge ˜ o (Fig. 5 and Supplementary Fig. 2). In area in the 2009/10 El Nin ˜ o, KOMO was located near the contrast, during the 1997/98 El Nin positive anomaly zone in the 101N thermocline ridge area (Fig. 5)

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Fig. 5. Monthly mean sea level height and chlorophyll anomalies for the 1997/98 and 2009/10 El Nin˜o periods in the equatorial Pacific obtained from GODAS and SeaWiFS. Shading indicates seal level height anomaly (Sea Level) and chlorophyll anomaly (CHL) normalized for the monthly average of the last 30 years. Contour shows the sea level height and chlorophyll concentrations with 0.2 m and 0.05 mg m  3 intervals, respectively, averaged for the last 30 years. Refer to the supplementary figures for full coverage from October 1997 to April 1998 and from October 2009 to April 2010.

(Supplementary Fig. 2). Thus, at the study site, the NECC was weaker and biological productivity was lower during the 2009/10 ˜ o than the 1997/98 El Nin ˜ o. Kim et al. (2011b) compared the El Nin eastward discharge rate of the equatorial Pacific Ocean between ˜ os using meridional current and 201C CT- and WP-type El Nin isotherm depth anomaly data. They found a weaker eastward ˜ o. Consistent with meridional current during the 2009/10 El Nin our SLH anomaly data, this suggests the weak development of the ˜ o, and the strengthening of the NECC during the 2009/10 El Nin NECC during the 1997/98 event. The satellite-derived chlorophyll anomaly data, which revealed a minor increase in biological components from primary producers, also agreed with our flux data. Nonetheless, an increase in the foraminiferal flux resulted in ˜ os. The blooming a similar total mass flux as during CT-type El Nin of planktonic foraminiferal species is affected by many environmental factors (e.g., water temperature, salinity, light intensity,

and food supply) (Kuroyanagi et al., 2008). To understand this phenomenon, additional oceanographic and meteorological data are needed to characterize the environmental conditions of the ˜ o. As WP-type El Nin ˜ o events have been occurring 2009/10 El Nin more frequently and intensely in recent years (Yeh et al., 2009), our 2009/10 results are relevant for understanding equatorial biological dynamics of the future.

7. Conclusion ˜ o events, at KOMO in The 1997/98 and 2009/10 strong El Nin the 101N thermocline ridge area, increased total mass fluxes to almost threefold the average for normal periods. The 1997/98 CT˜ o event had the most dramatic increases in organic type El Nin carbon and biogenic silica fluxes. In contrast, CaCO3 (especially

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foraminifera) only showed a significant increase during the 2009/ ˜ o event. Meanwhile, total mass flux increased 10 WP-type El Nin ˜ o. to similar levels as reached during the 1997/98 El Nin The higher amounts of organic carbon and biogenic silica ˜ o, are attributed fluxes, under the 1997/98 strong CT-type El Nin to increased surface water productivity. This is due to a strengthening of the NECC, which causes the upwelling of nutrientenriched subsurface water along its path. This is supported by a strong negative SLH anomaly over the equatorial Pacific, and a positive chlorophyll anomaly in the 101N thermocline ridge area. A similar increase in particle flux was observed for biological ˜ o. Thus, sinking particles during the 1982/83 strong CT-type El Nin ˜ o, increasing particle fluxes, during the strong CT-type El Nin might be regarded as a common phenomenon in the 101N thermocline ridge area. The weak negative SLH anomaly band over the equatorial Pacific, and negative chlorophyll anomaly in the 101N thermocline ridge area, indicated a weak development of the NECC during the ˜ o. These observations are consistent with 2009/10 strong El Nin the particle flux data, which show a minimal increase in biogenic silica and organic carbon. However, the drastic increase in foraminiferal flux cannot be explained by the data in this study. The blooming of planktonic foraminiferal species is affected by many environmental factors (e.g., water temperature, salinity, light intensity, and food supply). Therefore, our 2009/10 particle flux data should be supplemented with the data from future ˜ o events. studies to better understand strong WP-type El Nin

Acknowledgment We thank Dong Jin Ham and crew members of R/V Onnuri, their help for the time-series sediment trap operation. The revised manuscript benefited from constructive comments by Dr. Michael P. Bacon, editor, Dr. Debbie Steinberg, associate editor, and two anonymous reviewers. This study was supported by the Ministry of Land, Transport and Maritime Affairs of the Korean Government (PM56283), and by the Korea Ocean Research and Development Institute (PE98662).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.dsr.2012.05.008.

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