1. Impacts of IO mixed layer thickness on biological activity
Impacts of Indian Ocean circulation on biological activity
Jay McCreary, Raghu Murtugudde, D. Shankar, Satish Shetye, Jerome Vialard,
P. N. Vinayachandran, Jerry Wiggert,
and Raleigh Hood
Jay McCreary
Summer School on:
Dynamics of the North Indian Ocean
National Institute of Oceanography
Dona Paula, Goa
June 17 – July 29, 2010
The physical environment has an enormous impact on biological activity.
In this talk, I provide an overview of the basic processes of biophysical interaction in the IO.
A key aspect of biodynamics is the mixed layer. Its thickness compared to the depth of the euphotic zone is determines whether a bloom can occur or not. 1

2. References
McCreary, J.P., R. Murtugudde, J. Vialard, P.N. Vinayachandran, J.D. Wiggert, R.R. Hood, D. Shankar, and S.R. Shetye, 2009: Biophysical processes in the Indian Ocean, In: Indian Ocean Biogeochemical Processes and Ecological Variability, American Geophysical Union, Washington DC.
McCreary, J.P., K.E. Kohler, R.R. Hood, and D.B. Olson, 1996: A four- component ecosystem model of biological activity in the Arabian Sea. Prog. Oceanogr., 37, 193–240.
McCreary, J.P., K.E. Kohler, R.R. Hood, S. Smith, J. Kindle, A. Fischer, and R.A. Weller, 2001: Influences of diurnal and intraseasonal forcing on mixed-layer and biological variability in the central Arabian Sea. J. Geophys. Res., 106, 7139–7155.
Hood, R.R., K.E. Kohler, J.P. McCreary, and S.L. Smith, 2003: A four- dimensional validation of a coupled physical-biological model of the Arabian Sea. Deep-Sea Research II, 50, 2917–2945.
This talk is taken entirely from paper 1) listed above, a review paper of the physical processes that impact biology in the IO.

3. Biophysical interactions Near-surface processes
Overview
Biophysical interactions
Near-surface processes
Shallow overturning cells
Climatological processes
Arabian Sea, Bay of Bengal
South Indian Ocean
I begin with a broad overview of basin-wide biophysical linkages, then discuss basic processes of biological interactions. Next, I consider climatological processes (that is, the events that tend to repeat every year). I conclude with a discussion of interactions at intraseasonal and interannual time scales.
Intraseasonal processes
MJOs
Interannual processes
ENSO & IOD events

4. Overview

5. SeaWiFS annual chlorophyll composite (1999)
Somali and Oman
Central and northern AS
Sri Lanka
Western Bay
The figure shows where the major blooms in the IO are. A goal of this paper is to understand the physcis behind this biological organzation.
In contrast to the other tropical oceans, there is no annual phytoplankton bloom in the eastern, equatorial ocean, a consequence of the lack of prevailing easterly trades.
Java/Sumatra
South Indian Ocean band
Vinayachandran (2006; priv. comm.)

6. Climatological wind forcing
January
July
The wind field in the Indian Ocean is very different from that in the other oceans, accounting for the unique properties of Indian Ocean circulation and its phytoplankton distributions.
One might expect biological activity to be related to the winds. It is, through the impact of the winds on ocean circulation and properties.
Furthermore, the winds in the IO are very different from the trade-wind systems in the other tropical oceans (Atlantic and Pacific).
There are seasonally reversing monsoon winds in the Arabian Sea, the Bay of Bengal, and extending to 10°S, with a stronger ( weaker) clockwise circulation during the summer (winter).
There are no easterly winds (trades) on the equator, so that there is no equatorial upwelling. Instead, there are reversing cross-equatorial winds.
South of 10°S, the Southeast Trades are relatively more steady, but they are stronger in the southern winter (July).

7. McCreary, Kundu, and Molinari (1993)
Thermocline response
January
July
During the SWM, upwelling favorable winds lift the thermocline off Somalia and Oman, on the Indian coast, and around Sri Lanka.
During the NEM, the mixed-layer in the central and northern Arabian Sea thickens to ~100 m.
the h panels show the depth of the top of the thermocline.
A key process by which the wind impacts the ocean is Ekman drift, in which a surface drift occurs to the right (left) of the winds in the northern (southern) hemisphere due to Coriolis force (due to the earth’s rotation).
Note the presence of basin-scale features that radiate from the tip of India and the eastern boundary of the basin.
There is a thermocline ridge in a band from 5–10°S. It is driven by Ekman pumping associated with the northward weakening of the Southeast Trades, and is stronger in northern summer when the Trades are stronger.
McCreary, Kundu, and Molinari (1993)

8. Phytoplankton/thermocline depth linkage
July
There is an obvious connection between physics and biology, with regions of high phytoplankton concentrations tending to occur where the top of the thermocline rises close to the surface. This is sensible since then the subsurface nutrient supply lies within the euphotic zone. On the other hand, blooms also occur in regions where the thermocline is NOT shallow.
The fact that phytoplankton activity occurs in (near) regions where the thermocline is thin is not a coincidence.
Phytoplankton are plants, and they can only exist near the ocean surface where there is enough light (euphotic zone), and typically they consume all the nutrients there.
Thus, there is a strong nutrient gradient (nutricline) in the ocean, and new phytoplankton growth can only occur in regions where the nutricline rises to be close to the surface.

9. Biophysical interactions Near-surface processes
Upwelling
Entrainment
Detrainment
Advection

10. Upwelling, entrainment, and detrainment
Detrainment blooms tend to be highly productive because the final hm is thin and, hence, the depth-averaged light intensity is high. They are also short-lived because detrainment does not provide a source of new nutrients (Sverdrup, 1933).
During detrainment (bottom, reversed panels 5–1), hm thins due to a decrease in turbulent mixing (either when the wind weakens or there is surface heating). Initially (profile 5), there is a deep mixed layer and nutrients are high, as often occurs after wintertime cooling.
During entrainment (bottom), hm thickens due to turbulent mixing (from either strengthened winds or surface cooling). Fluid entrains into hm until hf vanishes (profiles 1–5). Thereafter, thermocline water entrains into the mixed layer (profile 5–7).
Even though entrainment can bring considerable nutrients into the euphotic zone, entrainment
blooms are not as productive as upwelling blooms because hm is thick and, hence, the depth-
averaged light intensity is low.
During upwelling (top), hm thins until it reaches its minimum thickness Hm (profiles 1–4). Then, water from the seasonal thermocline (gray shading) entrains into hm and hf thins (profiles 4–6). When hf is eliminated, upwelling from the main thermocline begins (dark shading; profile 7).
hm = mixed layer
hf = seas. therm.
h2 = thermocline
It is useful to view the near-surface ocean as consisting of three layers: a surface mixed layer, overlying a seasonal thermocline, overlying the main thermocline.
Before detrainment, the mixed layer can be so thick that phytoplankton cannot grow, because the depth-averaged light intensity over the layer is too low. In this situation, there are a lot of nutrients in the layer but no phytoplankton.
Figure 3: Schematic diagram illustrating the three types of blooms that can occur due to physics, namely, upwelling, entrainment, and detrainment blooms. The sequences of profiles indicate the changes in thicknesses of the mixed layer hm (light shading), seasonal thermocline hf (medium shading) and thermocline h2 (heavy shading) that occur during upwelling events (upper panel) and during entrainment and detrainment events (lower panel). For upwelling and entrainment events, the initial state is the profile at the left-hand edge of the panels and time increases to the right. For detrainment events, the initial state is profile 5 in the lower panel and time increases to the left. (After McCreary et al., 1996.)
Upwelling is a powerful process
for generating biological activity because it brings high-nutrient, thermocline water into a
thin, surface mixed layer, where the depth-averaged light intensity is high.

11. Subsurface phytoplankton maximum
Subsurface blooms occur when the thermocline is shallow enough to extend into the euphotic zone. In that case, phytoplankton can tap into the subsurface nutrient supply after nutrients are depleted from the mixed layer. For example, a subsurface bloom may occur at the interface between the seasonal and main thermoclines.
An entrainment event can then produce an apparent surface bloom by entraining the subsurface bloom into the mixed layer (after profile 5).

12. Ekman Pumping Advection by SMC Coastal Upwelling Advection
IRS-P4 OCM image during July, 1999
The high resolution images of OCM clearly demarcates the regions of large chlorophyll concentration. The coastal regions around India are highly productive. The southern coast of Sri Lanka is also highly productive. But the eastern coast of Sri Lanka is practically barren and whatever we see in its northern part seems to be overflows from the Palk Bay.
In the open ocean away from the coast there is a band of chl extending from the southern tip of Sri Lanka into the open ocean and there are patches of chl east of Sri Lanka.
Different physical process are important for local chl dynamics. Each one is marked.
The new process that was identified in Vinay et al. (2004, GRL) is coastal upwelling along the southern coast of Sri Lanka and the lack of it along the western and eastern coasts.

13. Biophysical interactions Shallow overturning cells

14. Meridional streamfunction from an IO GCM
Eq.
CEC
STC
Shallow cells
The CEC carries nutrients from the southern hemisphere to the upwelling regions in the northern hemisphere. Similarly, the STC can supply nutrients for the upwelling band from 5–10°S.
1) The IO’s shallow overturning cells in an OGCM solution (annual mean).
2) The STC may or may not supply nutrients for blooms along 5–10ºS, an issue still being debated
C.I. = 1 Sv
Garternicht and Schott (1997)
from global GCM (Semtner)

15. Upwelling, subduction, and inflow/outflow regions
for IO overturning cells
Somali/Omani
upwelling
Indian
Sumatra/Java
upwelling
Subduction
Indonesian throughflow
Southern Ocean
5-10°S upwelling
Agulhas Current

16. Climatological processes
Arabian Sea

17. Chlorophyll from SeaWiFs
JAN
MAY
SWM (Jul): Upwelling, filaments, mixing and entrainment; nutrient replete, high production, eutrophic.
NEM (Jan): Wind and buoyancy-driven mixing; nutrient replete, high production, but light limited.
Intermonsoon (May, Oct): Stratified conditions, low nutrients, near oligotrophic.
JUL
OCT ND
State that the strong SWM bloom along Somalia and Oman is due to upwelling.
What causes blooms in the central Arabian Sea? They are entrainment/entrainment blooms, not upwelling blooms. I discuss their biodynamics next, using a coupled biophysical model. ND
Wiggert et al. (2005)

18. Coupled biophysical model
McCreary et al. (2001)
Hood et al. (2003) i
The model is an upper-ocean model, which neglects flows in the deeper ocean. The 4 layers of the model represent: the surface mixed layer (layer 1), diurnal thermocline (layer 2), seasonal thermocline (layer 3), and thermocline (layer 4).
State that the NPZD biological model has compartments for nutrients (N), phytoplankton (P), zooplankton (Z), and detritus (waste, D).
There are equations for each variable at every grid point in the model and in each layer.
The physical component is a 4½-layer model of the Indian Ocean. Layer 1 represents the oceanic mixed layer, and its physics are based on the Kraus-Turner (1967) parameterization. The biological component is an NPZD model included in each layer of the physical model, which allows for advection within layers and transport across them.

19. The biological equations specify how nitrogen moves between compartments. Each is an advective/diffusive equation with source, sink, and vertical-mixng terms. For example, the layer-1 phytoplankton equation is
where the source/sink term is
with
and
and the vertical-mixing term is

20. Response in central AS (WHOI mooring site) Climatological forcingDB DB EB EB
Start during the Spring Intermonsoon and continue throughout the SWM. Note a) low P during the intermonsoon, followed by b) weak EB, c) rise of nutrients, c) strong DB, and d) rise of Z.
A similar sequence occurs during the NEM.
Under climatological forcing, there are unrealistically large detrainment blooms at the end of the monsoons when the mixed layer thins rapidly. There are weak entrainment blooms at the beginning of the monsoons when the mixed layer begins to thicken and nutrients are entrained into the mixed layer. During the monsoons, the mixed layer is so thick that phytoplankton growth is light limited.

21. Response in central AS (WHOI mooring site)
diurnal forcing
When forced by winds with realistic variability, the model generates a sequence of entrainment/detrinament blooms that spread blooms more evenly throughout both monsoons.
Under forcing by actual winds (1994) and with the diurnal cycle, there are a number of detrainment and entrainment blooms, so that phytoplankton growth is spread more realistically throughout both monsoons.

22. Sensitivity to mixed-layer thickness
Hood et al. (2003)
When compared with (US JGOFS Arabian Sea Process Study) data elsewhere in the basin, the model’s response was initially not good. Many model/data discrepancies were traceable to the solution’s mixed-layer thickness being too thick (purple curve).
CONCLUSION: Relatively simple biogeochemical models can capture the first-order biological variability in the Arabian Sea, but solutions are very sensitive to how well the models represent the physical state, particularly mixed-layer thickness and vertical-exchange processes.

23. Climatological processes
Bay of Bengal

24. Southwest Monsoon SeaWiFS images for 1997–2002
Vinayachandran et al. (2004; GRL)
This monthly climatology from SeaWiFS clearly shows that there is a biological response south of India during the SWM. As shown in the next slide, different physical processes account for the bloom in different locations.

25. Ekman Pumping Advection by SMC Coastal Upwelling Southwest Monsoon
IRS-P4 OCM image during July, 1999
The high resolution images of OCM clearly demarcates the regions of large chlorophyll concentration. The coastal regions around India are highly productive. The southern coast of Sri Lanka is also highly productive. But the eastern coast of Sri Lanka is practically barren and whatever we see in its northern part seems to be overflows from the Palk Bay.
In the open ocean away from the coast there is a band of chl extending from the southern tip of Sri Lanka into the open ocean and there are patches of chl east of Sri Lanka.
Different physical process are important for local chl dynamics. Each one is marked.
The new process that was identified in Vinay et al. (2004, GRL) is coastal upwelling along the southern coast of Sri Lanka and the lack of it along the western and eastern coasts.

26. Northeast Monsoon
There is also usually a bloom in the western Bay during the NEM. In the above sequence, the only exception was during 1997 when there was a bloom in the eastern Bay, likely a result of the ongoing ENSO/IOD event.

27. Coupled biophysical model
McCreary et al. (2001)
Hood et al. (2003) i
The physical component is a 4½-layer model of the Indian Ocean. Layer 1 represents the oceanic mixed layer, and its physics are based on the Kraus-Turner (1967) parameterization. The biological component is an NPZD model included in each layer of the physical model, which allows for advection within layers and transport across them.

28. Evolution of the 1996 bloom Obs Model
The model is able to reproduce the bloom at the right time and right place.

29. Bloom dynamics
An important part of the bloom dynamics is the presence of a prior subsurface (layer 3) phytoplankton maximum. As a result, the initial surface bloom is caused largely by the entrainment of subsurface phytoplankton into the mixed layer (layer 1).

30. Bloom dynamics Mixed Layer Layer 3
Prior to an increase in the wind, the mixed layer of the Bay is thin. Thus, there is sufficient light at subsurface levels (layer 3) to allow a subsurface bloom to develop, where nutrients are also available. When the winds strengthen, both nutrients and chlorophyll are entrained (or upwelled) into layer 1.
Nutrient Rich, No Light

31. Climatological processes
South Indian Ocean

32. South Indian Ocean blooms
Jan
Aug
Models and a few observations show that there is a prominent subsurface bloom in the South Indian Ocean everywhere from about 5–15ºS where the thermocline is shallow. The surface bloom is caused partly by entrainment of the subsurface bloom into the mixed layer. Recent modeling work, however, suggests that they result from new production, that is, from nutrient entrainment. The surface chlorophyll band is more intense during the southern winter (August) when the local winds, and hence entrainment, are stronger.
The blooms across the southern IO from 5–15ºS are weak. Initial work suggested that they were generated in part by the entrainment of a strong subsurface bloom into the surface layer. Recent modeling work suggests that they are rather generated by the entrainment of nutrients, that is, they are an upwelling/entrainment bloom in the open ocean.

33. Intraseasonal variability
MJOs

34. Madden-Julian oscillations (MJOs)
easterlies
100°E
MJOs are eastward-propagating, convective disturbances, typically with periods of 40–60 days.
Their impacts on rainfall, oceanic surface fluxes, and SST are well documented.
150°E
westerlies
180°
Waliser, Murtugudde, Lucas (2003, 2004)

35. Chlorophyll from SeaWiFs (NH summer)
Maps of SeaWiFs chlorophyll data composited for 13 summer MJOs from 1998–2004.
“Chlorophyll ratio” is the value relative to the seasonal mean, thus 1.20 means a 20% increase over the typical seasonal value.
Systematic changes associated with MJOs are observed over most of the tropical Indian and West Pacific Oceans.
Bands of chl anomalies stretch from the BofB to the western Pacific, sloping slightly equatorward toward the east.
A positive (orange/red) band peaks in the central Bay about day 5.

36. Mixed-layer thickness (NH summer)
Model-derived, mixed-layer-thickness anomalies associated with MJO forcing.
Positive anomalies indicate a thicker mixed layer, and thus regions that might be expected to have enhanced nutrients and hence to higher phytoplankton concentrations. This relation-ship holds at some, but not all, locations.
Assuming that changes in γ are caused by entrainment blooms, regions of increased γ and hm should correspond with hm leading γ by some time.
Indeed, this simple relationship does seem to hold in the BofB and western Pacific: red/orange and blue/green areas tend to overlap there, with P lagging hm by about 10 days. For example, the positive hm (γ) anomaly peaks in the central BofB about day –5 (+5).
This relationship, however, breaks down in the Arabian Sea.

37. Chlorophyll from SeaWiFs (NH winter)
Maps of SeaWiFs chlorophyll data composited for 14 winter MJOs from 1998–2004.
“Chlorophyll ratio” is the value relative to the seasonal mean, thus 1.20 means a 20% increase over the typical seasonal value.
Systematic changes associated with MJOs are observed over most of the tropical Indian and Pacific Oceans.
Chlorophyll is related to MJOs during the winter.
But, the authors were not able to identify any linkage to model hm.

38. Intraseasonal variability
South Indian Ocean

39. McCreary, Kundu, and Molinari (1993)
Thermocline ridge
January
July
McCreary, Kundu, and Molinari (1993)
The thermocline rises close to the surface in a band from 5º–10ºS in the western and central IO in response to Ekman suction associated with the Southeast Trades.
The model shown at the left illustrates the thermocline ridge and its seasonal variability.
Because the thermocline is so shallow, strengthened intraseasonal winds can cause thermocline variables to be entrained into the surface mixed layer.

40. Summertime variability
Chl (obs)
Chl (model)
Kawamiya and Oschlies (2001; GRL)
Observed (left panel) and modeled (right panel) chlorophyll (mg/m3) concentrations during September, Units are mg/m3. In the model, there is a band of high concentration from 10−12°S.

41. Variability along 12ºS during 1998
Observed and modeled chlorophyll (top) and sea level (bottom) during summer and fall, 1998.
Chl (obs)
Chl (model)
In both the model and observations, chlorophyll variations are associated with westward-propaga-ting disturbances.
Sea level (obs)
Sea level (model)

42. Biophysical processes
8/13
9/2
9/22
During a phase of high chlorophyll, there is upwelling, the mixed-layer thickens, and the subsurface, chlorophyll maximum is entrained to the surface.

43. Interannual processes
ENSO & IOD events

44. 1997/98 ENSO/IOD event
Oct/Nov, 1997
Maps of wind-stress anomalies (dynes/cm²) from NCEP2, SST anomalies (color; °C) from Reynolds‘ data (left) and chlorophyll anomalies (color; log mg/m³) from SeaWiFS (right) during October/November/December The wind-stress anomalies are determined from a climatology averaged from 1979–2007. The SST and chlorophyll anomalies are calculated from climatologies averaged from September 1999 – August 2007, which avoid the 1997/98 ENSO/IOD event.
There is not enough data to identify general biophysical interactions with statistical reliability. On the other hand, such biophysical interactions were clearly active during the intense 1997/98 ENSO/IOD event. In this event, there was upwelling in the eastern, equatorial ocean and along Sumatra/Java forced by anomalous southeasterly winds. In response, SST cooled and there was an upwelling phytoplankton bloom.

45. Summary

46. Summary (1)
Biophysical interactions: Near the surface, physics impacts biology through upwelling, entrainment, detrainment, and advection.
Arabian Sea: In the central Arabian Sea (away from upwelling regions), blooms are driven by mixed-layer entrainment and detrainment events. There, the response of an NPZD model is very sensitive to mixed-layer thickness, indicating that the precise simulation of the physical state is critical for the realistic simulation of biological activity.
Bay of Bengal: In the southern Bay and south of Sri Lanka, summertime blooms are driven by upwelling and advection. In the western Bay, wintertime blooms are driven in part by entrainment of subsurface phytoplankton into the surface mixed layer.

47. Summary (2)
Intraseasonal variability: The strength of phytoplankton blooms is linked to the life cycle of MJOs, and some blooms appear to be driven by MJO-induced changes in mixed-layer thickness.
South Indian Ocean: During the summer, some surface blooms are associated with the passage of Rossby waves, which shallow the thermocline and allow subsurface phytoplankton to be entrained into the surface layer.
Interannual events: During ENSO/IOD events, easterly winds develop along the equator. They cause upwelling along Sumatra and Java, lowering SST and generating a bloom there.

49. Biophysical interactions
Deeper circulations

50. Arabian Sea Oxygen Minimum Zone (ASOMZ)
OMZs occur in many regions of the world ocean underneath beneath areas of high production. The high production also generates detritus. The detritus is remineralized by bacteria at depth as it sinks, consuming oxygen and creating an OMZ.

51. Arabian Sea Oxygen Minimum Zone (ASOMZ)
A major biological puzzle is that the ASOMZ is shifted away from the location of the highest production in the western Arabian Sea. Both biological (e.g., differential sinking rates) and physical (e.g., flow of oxygenated RSW and PGW into the western Arabian Sea) might be the cause.

52. SST variability in 5−10°S band
The figure shows band-passed (30–90 day) SST anomalies averaged from 5−10°S, the latitude band of the thermocline ridge.
There are zonally elongated SST anomalies that are strongest during winter, and westward-propagating anomalies (Rossby waves) during summer and fall.

53. Time dependence of wintertime events
Shaded: SST anomaly
Arrows: wind anomaly
Dashed: OLR anomaly
A composite of 11 wintertime events when SST anomalies exceeded 1.5 sigma. A phase change of 45° is equivalent to 7−10 days.

54. Wintertime variability
SST averaged in 60–90ºE, 5−10ºS
Existing evidence indicates that blooms are triggered by some but not all of these events. Why not ALL events? A possible explanation is that the thermocline is often too deep to allow for an entrainment bloom.
A particular cool event studied by Harrison and Vecchi (2001) & Duvel et al. (2004)

55. Biophysical interactions: 1) Near the surface, physics impacts biology through upwelling, entrainment, and detrainment. 2) Shallow overturning cells, the CEC and STC, provide the water that upwells in the northern IO and along 5–10ºS. 3) Deeper circulations impact OMZs.
Arabian Sea: In the central Arabian Sea (away from upwelling regions), blooms are driven by mixed-layer entrainment and detrainment events. There, the response of an NPZD model is very sensitive to mixed-layer thickness, indicating that the precise simulation of the physical state is critical for the realistic simulation of biological activity.
Bay of Bengal: In the southern Bay and south of Sri Lanka, summertime blooms are driven by upwelling and advection. In the western Bay, wintertime blooms are driven in part by entrainment of subsurface phytoplankton into the surface mixed layer.
MJOs: The strength of phytoplankton blooms is linked to the life cycle of MJOs, and some blooms appear to be driven by MJO-induced changes in mixed-layer thickness.
South Indian Ocean: During the summer, some surface blooms are associated with the passage of Rossby waves, which shallow the thermocline and allow subsurface phytoplankton to be entrained into the surface layer.