Jay McCreary
Dynamics of Indian-Ocean shallow Dynamics of Indian-Ocean shallow overturning circulationsoverturning circulations
Jay McCrearySummer School on:
Dynamics of the North Indian Ocean
National Institute of OceanographyDona Paula, Goa
June 17 – July 29, 2010
ReferencesReferences1) Miyama, T., J. P. McCreary, T.G. Jensen, S. Godfrey, and A.
Ishida, 2003: Structure and dynamics of the Indian-Ocean Cross-Equatorial Cell. Deep-Sea Res., 50, 2023–2048.
2) (MKM93) McCreary, J.P., P.K. Kundu, and R. Molinari, 1993: A numerical investigation of dynamics, thermodynamics and mixed-layer processes in the Indian Ocean. Prog. Oceanogr., 31, 181–244.
3) (SM04) Schott, F., J.P. McCreary, and G.C. Johnson, 2004: Shallow overturning circulations of the tropical-subtropical oceans. In: Earth Climate: The Ocean-Atmosphere Interaction, C. Wang, S.-P. Xie and J.A. Carton (eds.), AGU Geophys. Monograph Ser., 147, 261–304.
QuestionsQuestions
1) What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation?
2) What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell?
3) What are their fundamental dynamics?
4) What is their impact on the Indian-Ocean heat budget?
What are the 3-d structures of these cells? How do they vary on climatic time scales?
2d structure in an idealized GCM solution
SPC
Bryan (1991)
STC
AMOC
Tropics SubtropicsSubtropicsLu et al. (1998)
Subtropical Cells (STCs) in the Pacific Ocean
The STCs carry cool subtropical thermocline water into the tropics. The two cells account for almost 30 Sv of overturning.
Rothstein et al. (1998)
surface
thermocline
upwelling
subduction
subduction
3d structure in a GCM solution
QuestionsQuestions
1) What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation?
2) What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell?
3) What are their fundamental dynamics?
4) What is their impact on the Indian-Ocean heat budget?
Wind forcing for CEC and STC
Upwelling-favorable annual-mean winds (dominated by July)
Reversing cross-equatorial winds
Relatively steady Southeast tradewinds
As a result, the IO winds circulate clockwise (anticlockwise) about the equator during the summer (winter). The annual-mean winds have the summer pattern.
Upwelling, subduction, and inflow/outflow regions in Indian Ocean
Somali/Omaniupwelling
Indianupwelling
5-10°S upwelling
Sumatra/Javaupwelling
Subduction
Indonesian Throughflow
Southern OceanAgulhas Current
Meridional streamfunction from an IO GCM
Garternicht and Schott (1997)
CECSTC
Equatorial roll
Deep cell
Models used in Miyama et al. (2003)
1) MKM 2½-layer model (0.5°)
2) TOMS 4½-layer model (0.33°)
3) JAMSTEC GCM (55 levels, 0.25°)
4) SODA reanalysisGCM + data
5) LCS model
MKM TOMS
Subsurface water crosses the equator in a western boundary, a consequence of PV conservation
Subsurface circulation of CEC (backward tracking from upwelling regions)
Subsurface circulation of CEC (backward tracking from upwelling regions)
JAMSTEC
Subsurface water crosses equator in a western boundary current, a consequence of PV conservation.
Surface water crosses equator in interior ocean, increasingly to the east for Somali, Omani, and Indian upwellings
MKM TOMS
Surface circulation of CEC (forward tracking from upwelling regions)
In GCMs, surface water tends to flow across the basin in the interior ocean and only crosses the equator in the eastern basin. Particle trajectories show equatorial rolls.
Surface circulation of CEC (forward tracking from upwelling regions)
JAMSTEC
Equator
Equatorial roll in JAMSTEC model
Surface trajectories cross equator in the eastern ocean because of equatorial roll, consistent with observed drifters.
January
July
Surface (10 m) trajectories in JAMSTEC model
Annual-mean, surface (0–75 m) circulation in SODA reanalysis
Near-surface currents cross equator in the eastern ocean because of equatorial roll, consistent with observed drifters.
3d structure of CEC in JAMSTEC model
QuestionsQuestions
1) What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation?
2) What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell?
3) What are their fundamental dynamics?
4) What is their impact on the Indian-Ocean heat budget?
STC dynamics
Wind forcing for the STC
Wind curl along the northern edge of Southeast Trades
Eq.
Basic processes for the STC
Consider the response in layer 2 of a 2½-layer model forced by a mass sink (upwelling into layer 1) south of the equator.
The water that upwells first flows eastward across the basin, a remotely forced response due to the radiation of Rossby waves from the upwelling region.
There is an additional recirculation, the so-called “β plume.”
Finally, the subsurface flow also includes the circulation of the Subtropical Gyre. As a result of all of these contributions, layer-2 STC water enters the upwelling region from the north.
Basic processes for the STC
Consider the response in layer 2 of a 2½-layer model forced by a mass sink (upwelling into layer 1) south of the equator.
CEC dynamics
a) Why does surface water cross the equator in the interior ocean?
b) What causes the equatorial roll?
Wind forcing for the CEC
The IO winds circulate clockwise (anticlockwise) about the equator during the summer (winter). The annual-mean winds have the summer pattern.
EQ
Wind (boreal Summer, annual mean)
Ekman Transport
Ekman transport appears to be involved off the equator.But, what dynamics are involved near the equator?
EQ
Wind (boreal Winter)
Ekman Transport
Basic processes for the CEC
fyy
y
L
XV
L
XyV
xx
x
1
/1
0
0
The Sverdrup transport is
Thus, for this special wind the Sverdrup and Ekman transports are equal. It follows that the concept of Ekman flow can be extended to the equator, since τx tends to zero as f does.
LyxXyYxXx /)()()( 0
Consider forcing by τx that is antisymmetric about the equator
but V can be rewritten
Analytic solution
Consider the equations for a 1½-layer model,
Then,
For a τx that is antisymmetric about the equator
and so h never changes in response to this wind! So, no geostrophic currents are ever generated, and the total flow field is entirely Ekman drift.
.0)()(
,/'
,/'
yxt
yy
xx
hvhuh
hhgfu
hhgfv
,)/(')/( 2ey
xxt wfhhgfh
,00
y
y
yLwe
Linear, continuously stratified (LCS) model
1) Model equations of motion linearized about a state of rest and Nb(z)
2) Solutions expressed as sums of 50 vertical modes
3) Horizontal resolution is 0.25°
4) Realistic Indian-Ocean coastline
5) Forced by Hellerman and Rosenstein (1983) winds
6) Spun up for 10 years
meridional velocity
Symmetric zonal wind
meridional velocity
Antisymmetric zonal wind
CEC dynamics
a) Why does surface water cross the equator in the interior ocean?
b) What causes the equatorial roll?
Section at 70 E
meridional velocity
Symmetric meridional wind
1) Total wind 2) Zonal wind 3) Meridional wind
LCS solution forced by July HR winds. Cross-equatorial flow is driven by τx (middle), and equatorial roll is driven by τy.
Roles of zonal and meridional winds
Courtesy of Toru Miyama
Meridional velocity zonally averaged between 40–100ºE. The linear model reproduces the GCM solution very well!
Comparison of LCS and GCM solutions
Courtesy of Toru Miyama
QuestionsQuestions
1) What are shallow overturning circulations in the world ocean? What is their role in the general ocean circulation?
2) What are the structures of the prominent cells in the Indian Ocean, the Subtropical Cell and the Cross-equatorial Cell?
3) What are their fundamental dynamics?
4) What is their impact on the Indian-Ocean heat budget?
So, the heat flux into the ocean is caused by oceanic upwelling. Advection then spreads cool SSTs away from the upwelling region, causing heating over a larger area.
There is a net annual-mean heat flux into the Indian Ocean, …
… that vanishes when cooling due to upwelling is dropped
from the model. In this model, then, the annual-mean heating happens entirely because of
upwelling.
How model dependent is this result? Perhaps in this model it is overemphasized because heating
in the 5–10°S band is too strong.
Subtropical CellDriven by upwelling caused by Ekman pumping at the northern edge of the Southeast Trades (5–10ºS). Subsurface water for the upwelling comes from the north, due to the formation of a “β-plume.”
Cross-equatorial CellDriven by upwelling in the northern ocean. Its source waters are all from the southern hemisphere, requiring cross-equatorial flow.
Subsurface flow crosses the equator only near the western boundary due to PV conservation.
Near-surface water crosses the equator in the interior ocean. It is driven by the antisymmetric component of the zonal wind, which drives a southward, annual-mean, cross-equatorial Ekman drift.
Because of the equatorial roll, the CEC surface branch dives below the surface as it crosses the equator. Moreover, flow right at the surface (e.g., as measured by surface drifters) can cross only near the eastern boundary.
Heat fluxThe observed annual-mean heat flux into the IO exists only because of upwelling associated with the STC and CEC.
Conclusions
Annual-mean, layer-2 circulation in MKM model
Subtropical Cell
Layer 1
Layer 2