Mesoscale Eddy Variability in the Sargasso Sea and Its Impact on Upper Ocean Biogeochemistry Rodney J. Johnson, Nicholas R. Bates, Megan Roadman, Steve Bell, Nathan Buck, Vivienne Lochhead, Keven Neely, Sybille Pluvinage, Anthony H. Knap Bermuda Biological Station For Research Dennis J. McGillicuddy Woods Hole Oceanographic Institution (WHOI) Andreas Andersson, A. Christine Pequignet, Paul Lethaby University of Hawaii at Manoa Craig A. Carlson University of California at Santa Barbara

Talk Outline 1. Introduction The EDDIES project 2. Data Sets and Temporal Context The Bermuda Atlantic Time-series Study (BATS) 3. BATS Validation 18 and 19 data(1997) Survey cruises of cyclone to the east of Bermuda 4. EDDIES Transects 1 and 2 (2004) Survey cruises of cyclone to the east of Bermuda 5. EDDIES Transects 3 and 4 (2005) Survey cruises of anticyclone to the east of Bermuda

Data Resources BATS ( years ) BATS Validation #18,&19 - June/July 1997 Two 7-day survey cruises of a strong cyclone located ~240km ENE of Bermuda. A 5X5 grid of CTD stations (0-700m) with station spacing set at 40km. Biogeochemical parameters: dissolved oxygen, DIC, nutrients and chl_a Eddy Dynamics, mIxing, Export, an Species composition (EDDIES) Transect Cruises Transects 1(EDT1) &2 (EDT2) -June/July/August 2004 Two 10 day survey cruises of a cyclone located ~220km to south of Bermuda. EDT1 : NE to SW and a N-S transect through eddy center (EC) plus extensive spatial surveys at EC. EDT2 : S-N transect through EC and other spatial surveys. Transects 3 (EDT3) &4(EDT4) - July/August 2005 Two 10-day survey cruises of a mode water eddy located ~250km to the SW of Bermuda EDT3 : E-W and N-S transect through EC plus extensive high resolution sampling (10-20km) at EC. EDT4 : W-E transect through EC and extensive high resolution (10km) sampling at EC. Biogeochemical parameters(EDT1,2,3&4): dissolved O2, DIC, DOC, DON, POC/PON, HPLC -pigments, FCM, Microbial production, Thorium flux, 14C production and sediment Traps (150m).

Location map of ocean time-series sites near Bermuda Overlaid on Sea surface height data courtesy of ( Introduction

Data Sets and Temporal Context Bermuda Atlantic Time-series Study (BATS)

r=0.49 r=0.81r= r= Data Sets and Temporal Context BATS data and Dynamic Height Relationships Temp (120 m) Temp (500 m) Nitrate (120 m) Nitrate (500 m)

BVAL 18 (07/Jul/1997 to 13/Jul/1997) Target Feature: Cyclone 240 km to ENE of Bermuda Survey Grid: A 5x km 2 box with 40km station spacing 35 CTD casts (0-700m) (28 with biogeochemistry) 25 XBT drops (mid -way between stations and Eddy center) BVAL 19 (22/Jul/1997 to 29/Jul/1997) Target Feature: repeat survey of BVAL18 cyclone Survey Grid: Added a leg to the south, north and line through Eddy center 43 CTD casts (0-700m) (34 with biogeochemistry) 29 XBT drops (mid -way between stations and Eddy center) Cyclonic Mesoscale Eddy (1997)

(a) BVAL18 Temperature - 80m (b) BVAL18 Nitrate (umol/kg) - 80m (c) BVAL18 Chlorophyll a (ug/kg) - 40m Cyclonic Mesoscale Eddy (1997) Hydrographic and biogeochemical changes (a) BVAL19 Temperature - 80m (b) BVAL19 Nitrate (umol/kg) - 80m (c) BVAL19 Chlorophyll a (ug/kg) - 40m

1-D Model (Physical- Price et al, 1986; NO 3 -McGillicuddy & Robinson, 1997) results showing nitrate uptake at 80m. Model initialized with BVAL18 Eddy center conditions on 07/July. BVAL19 80m NO 3 at Eddy center shown as red circle. Total estimated Model uptake in the euphotic zone for the 35 day run was found to be 0.14 mol N m -2. Cyclonic Mesoscale Eddy (1997) Estimate of new production from nitrate changes (27/July/97) BVAL18 observed data BVAL19 observed data (07/July/97)

EDT1 Operations (July 2004) Target Feature : Cyclone C1 Transect#1: NE-SW through EC (St.111) Transect#2: S-N through EC (St. 110) ] 30 CTD Casts ( black triangles) 2 Sediment Trap Deployments [1 at EC and control at NE of C1 (red dots) 3 Production deployments 9 Thorium pump profiles EDT2 Operations (August 2004) Target Feature : Cyclone C1 Main Transect :N-S through EC (St. 264) 27 CTD Casts (black triangles) 1 Sediment Trap Deployment at EC ( red dots) 1 Production deployments at EC 9 Thorium pump profiles EDDIES 2004

Survey of temperature for cyclone C1 Station Locations Eddy center

EDDIES 2004 Survey of dissolved oxygen for cyclone C1 Eddy center Station Locations

EDDIES 2004 Survey of CTD Fluorescence for cyclone C1

EDDIES 2004 Survey of suspended particulate organic carbon (POC) for C1 Eddy center Station Locations

EDT3 Operations (July 2005) Target feature : Anticyclone A4 Transects: E-W through EC(St.2317) N-S through EC(2316) 43 CTD casts (blue solid circles) 2 Sediment Trap deployments at EC ( Array A - blue track, Array B -green track) 3 Primary production deployments at EC (red asterisk) 8 Thorium pump profiles 8 SPMR profiles EDT4 Operations (August 2005) Target feature : Anticyclone A4 Transects:W-E through EC (St.2070) 43 CTD casts (blue solid circles) 2 Sediment Trap deployments at EC ( Array A - blue track, Array B -green track) 3 Primary production deployments ( 1 at EC, 1 at Traps A and 1 at Traps B) 8 Thorium pump profiles 8 SPMR profiles Anticyclonic feature sampled on EDDIES in summer 2005 EDDIES 2005

Survey of temperature for anticyclone A4 Eddy center

EDDIES 2005 Survey of dissolved oxygen for anticyclone A4 Eddy center

EDDIES 2005 Survey of CTD fluorescence for anticyclone A4 Eddy center

EDDIES 2005 Survey of suspended POC for anticyclone A4 Eddy center

EDDIES 2004 and 2005 Comparison of dissolved oxygen with BATS climatology Low O 2 in A4 Low O 2 in C1

[BATS ( ) min,mean,max shown as solid black lines ] EDDIES 2004 and 2005 Comparison of primary production rates with BATS climatology

Mass C N 2004 Cyclone C1 EDT1 - EC EDT1- Control EDT2 -EC EDT2- B190 Control Mode Water Eddy - A4 EDT3 - EC Array A EDT3 - EC Array B EDT4 - EC Array A EDT4 - EC Array B BATS Climatology (summer ) Mass(150m)  39.0 mg m -2 day -1 Carbon (150m) 27.2  8.0 mg C m -2 day -1 Nitrogen (150m) 4.3  1.5 mg N m -2 day C1 EDT1-PP1 at Eddy Center 536 EDT1-PP2 at Eddy Center 367 EDT1-PP3 at high fluorescence site 307 (south west of eddy center) EDT2-PP1 at Eddy Center Mode Water Eddy A4 EDT3-PP1 at Eddy Center 200 EDT3-PP2 at Eddy Center 311 EDT3-PP3 at Eddy Center 307 EDT4-PP1 at Eddy Center 808* EDT4-PP2 at sediment traps A 650* EDT4-PP3 at sediment traps B 605* BATS Climatology (summer ) 426 ± 207 mg C m -2 day -1 EDDIES Integrated Primary Production (0-140m) EDDIES Sediment Trap Flux EDDIES 2004 and 2005 Comparison of primary and export production rates

Summary The three eddies were presumed to pertubate the euphotic zone in a similar manner, however very different responses were observed. BVAl18/19 cyclone revealed the ‘idealized’ cyclone structure with significant lifting of density surfaces at the eddy center and relaxation on the eddy periphery. Upwelled surfaces provided high nutrient levels in the lower euphotic zone, enhancing chlorophyll biomass. Further the observed nitrate loss between the two cruises was consistent with a simple 1-D uptake model. The 2004 Cyclone showed substantial lifting of density surfaces in the permanent thermocline but downwelling at the base of the euphotic zone. No enhancement in productivity at EC. Most striking feature was an extreme negative oxygen anomaly between 100 &400m at the EC. This O2 deficit was equivalent to a nitrate utilization of ~ 1.5mN m-2. The reduction in this signal between cruises suggests the signal may be locally produced and is a remnant of previous production in this decaying eddy. Substantial downwelling of the mode water and lifting of the seasonal thermocline was observed in the 2005 mode water eddy. Lifting of nutrient surfaces into the euphotic zone appeared to enhance local productivity. Primary production estimates on ETD4 were found to be very high at 60 & 80m (~twice BATS maximum) although sediment trap fluxes were low for both cruises. A deep negative O2 anomaly with a similar deficit as in 2004 eddy was observed between 700 and 1000m, which appeared to develop between the two cruises. The question of its origin as with the 2004 O2 anomaly warrants further investigation.

Both the 2004 and 2005 eddies were found to be very patchy especially in the vicinity of EC Observations from 2 cyclones and a mode water eddy clearly show that mesoscale eddies are far more complex than initially presumed. Further, given their patchiness, age/state issues and the inability to distinguish between anti-cyclones and mode water eddies by satellite altimetry, quantifying their net effect in the Sargasso Sea may be difficult. These eddy survey data provide an invaluable resource in helping to understand patterns of short term variability observed at the BATS site. Acknowledgements We thank NSF for the support for these projects. We thank all the EDDIES scientists and we are greatly appreciative of all the BATS technicians. A special thanks to the crews of R/V Weatherbird II and to Valery Kosnyreva for providing altimetry data.