Modeling Indian Ocean Biogeochemistry Iron Limitation and Dipole-Zonal Mode Impacts

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1 Modeling Indian Ocean Biogeochemistry Iron Limitation and Dipole-Zonal Mode Impacts Jerry Wiggert Funded by the NASA Oceanography Program

2 Outline 1) Coupled 3-D Bio-physical Model Basinwide distribution of bio-available iron 2) Biogeochemical sensitivity to spatio-temporal distribution of aeolian dust deposition Nutrient supply to offshore waters of the Arabian Sea SWM-period export production response 3) Biogeochemical variability coincident with the Indian Ocean dipole-zonal mode (IODZM) Observed chlorophyll anomalies in SeaWiFS Basinwide distribution of biogeochemical response 4) Summary & Conclusions

3 Coupled 3-D 3 D Bio-physical Model 1/2 (lon) x 1/3 (lat) 19 σ layers below active mixed layer Sponge layers along 30 S and 125 E Daily NCEP winds (climatological and interannual) T, S and Nitrate IC from NODC Atlas Aeolian Iron Boundary Condition (GOCART; Ginoux et al. 2001) Ecosystem model features of note: Explicit Iron Chemistry Two phytoplankton size classes (speciation shifts captured) Wiggert, Murtugudde & Christian, Deep-Sea Res. II., 2006

4 Seasonal Surface Chlorophyll (Monthly Mean) SeaWiFS Climatology Model Chlorophyll Jan (NEM) Aug (SWM) Wiggert, Murtugudde & Christian, Deep-Sea Res. II., 2006

5 Bio-availability of Dissolved Iron Iron limitation develops w/ upwelling of higher N:Fe waters and/or lack of nearby terrigenous sources Western Arabian Sea exhibits pronounced seasonal variation Bay of Bengal is iron replete except for Sri Lanka Dome during SWM Southern Tropics primarily iron limited Western Equatorial Band always iron limited Wiggert, Murtugudde & Christian, Deep-Sea Res. II., 2006

6 Biogeochemical sensitivity to the spatio- temporal distribution of aeolian dust deposition Compare two climatological solutions Sole difference: Apply aeolian iron flux BC derived from the dust fields of two different atmospheric transport models 1) GOCART (Ginoux et al. 2001) 2) GISS (Tegen & Fung, 1995) Canonical View: The Arabian Sea is persistently iron replete

7 Surface Chlorophyll in July GOCART GISS In GOCART solution Amplitude & phase of offshore SWM bloom shows better agreement with SeaWiFS Stronger shift to large phytoplankton Local export flux is up to 2x greater Possible Causes SeaWiF S Local aeolian mineral flux (No!) nmol Fe m -3 d -1 Wiggert & Murtugudde, JGR-Oceans, in press.

8 Biological and Chemical Response at BR2 Z ML 0.2 μmno 3 P R (=P S /P S +P L ) Wiggert & Murtugudde, JGR-Oceans, in press.

9 Potential Source Mechanisms for Offshore Bloom Iron Budget (nmol Fe m -3 d -1 ) ΔEnrichment (nmol Fe m -3 d -1 ) (GOCART-GISS) Possible Causes (contd.) Aeolian enrichment at BR2 & BR3 reinforce local deposition not responsible Mixing/entrainment into surface layer not a factor BR1 BR2 BR3 BR4 Iron budget points to lateral advection Flowlines suggest Gulf of Aden region, but velocity magnitude insufficient to reach site before August Wiggert & Murtugudde, JGR-Oceans, in press.

10 Difference in Surface Iron (nmol Fe m (GOCART - GISS) (nmol Fe m - 3 ) In GOCART solution: Difference in dissolved iron between these solutions is a convenient tracer Dissolved iron accumulates south of Arabian Peninsula (April-May) Reaches this location after 8-month propagation from Gulf of Oman New nutrient source mechanism for supporting offshore phytoplankton bloom (mechanisms recently reviewed in Wiggert, Hood, Banse & Kindle, 2005) Wiggert & Murtugudde, JGR-Oceans, in press.

11 Difference in Euphotic Zone Export (mmol N m (GOCART - GISS) (mmol N m - 2 d - 1 ) Indian Ocean export; 4.6 Tg C yr -1 higher, 54% occurs in JAS Arabian Sea contributes 80% to 2.5 Tg C yr -1 basinwide increase that occurs during SWM Wiggert & Murtugudde, JGR-Oceans, in press.

12 Anomalous biogeochemical signatures coincident with the Indian Ocean dipole- zonal mode (IODZM) Anomalous distributions of surface chlorophyll (SeaWiFS) Model results from interannually forced experiment Oceanic mechanisms behind biological anomalies Impact on basinwide export of organic matter from euphotic zone

13 Anomalous Surface Chlorophyll: Arabian Sea IODZM Typical Dec (NEM) During IODZM Chlorophyll concentration is reduced by 50% Thermocline depth (modeled) meters deeper Altimetry shows weaker upwelling Rossby wave propagating from SW coast of India (Brandt et al. 2002) Wiggert, Murtugudde & McClain, Deep-Sea Res. II., 2002

14 Interannual Variability in SSH and Wind Curl in the Southeastern Arabian Sea Anomalous Conditions during 97/98 NEM Curl-driven upwelling absent during latter part of 1997 along Indian coast Weaker upwelling Rossby Wave propagated NW across Arabian Sea Coincides with the physical model indicating an anomalously deep thermocline in the central Arabian Sea SeaWiFS-observed Chla also anomalously low

15 Anomalous Surface Chlorophyll: Eastern IO Dec (NEM) IODZM Typical

16 EOF of Surface Chlorophyll

17 IODZM Impact on Equatorial BGC Zonal Volume Transport (m 2 s -1 ) (Along 90 E) Areal Production & Nitracline Depth 2 N Spring (May) Fall (Nov) Climatological 1 N 0 1 S N 2 S 55 E 65 E 75 E 85 E 95 E / N 0 1 S S

18 Mean Impact of IODZM in 1990s

19 Areal Production Anomaly (mmol C m Mean IODZM Impact in 1990s (mmol C m - 2 d - 1 ) IODZM Impact on Basinwide Export from Euphotic Zone (200 m) IODZM (1990s Composite) - Climatological Condition: -7.4 Tg C yr -1 IODZM (1997/1998) - Climatological Condition: -9.7 Tg C yr -1

20 Summary & Conclusions Sensitivity to aeolian dust deposition distribution Dissolved iron transported from Gulf of Oman to central Arabian Sea contributes to offshore SWM (July) phytoplankton bloom 2.5 Tg C yr -1 additional basinwide euphotic zone export during SW Monsoon; 80% occurs in Arabian Sea Anomalous biogeochemical properties coincident with the Indian Ocean dipole-zonal mode Chlorophyll anomalies in eastern Equatorial IO(+), southern Bay of Bengal(+) and central Arabian Sea(-) Alteration of typical planetary wave characteristics Basinwide euphotic zone export reduced by 7-10 Tg C yr -1 (~15%) Indian Ocean s biogeochemical response to both the IODZM and aeolian dust deposition require further study

21 Arabian Sea Milestones Wiggert, Hood, Banse & Kindle, Progress in Oceanography, 2005.

22 Surface Iron Enrichment Wiggert & Murtugudde, JGR-Oceans, in press.