RAPID/MOCHA/WBTS

THE SEASONAL CYCLE OF THE AMOC AT 26ºNEastern Boundary Considerations

Gerard McCarthy, Eleanor Frajka-Williams, Aurélie Duchez and David

Smeed

National Oceanography CentreSouthampton

UK

Introduction

The AMOC at 26ºN

Rayner, D., et al. (2011), Monitoring the Atlantic Meridional Overturning Circulation, Deep Sea Research II, 58, 1744–1753.

Gulf Stream + Ekman + Upper Mid Ocean = AMOC

Gulf Stream, MOC, Ekman & Upper Mid-Ocean Transports

• Interannual Variability in 09/10 shifted circulation from overturning to gyre (McCarthy et al., 2012, GRL)• Double dips in winters 09/10 and 10/11 described in Blaker et al., 2013, submitted to Climate Dynamics• AMOC timeseries and related data products are available from www.rapid.ac.uk/rpdmoc• Data from individual instruments are available from www.bodc.ac.uk

Seasonal Cycle

• Large seasonal cycle in AMOC driven by upper Mid-Ocean transport seasonality• AMOC timeseries and related data products are available from www.rapid.ac.uk/rpdmoc

Seas. Amp.

GS

3 Sv

MOC

6 Sv

Ekman

3 Sv

UMO

6.5 Sv

Basin-scale Seasonality

±1.8 Sv,SE=1.0 Sv

±2.6 Sv, SE=0.5 Sv

SD 3.5 Sv, Range 7.0 Sv

SD 3.1 Sv, Range 6.2 Sv

The Seasonal Cycle due to the Eastern Boundary

Kanzow, T., et al. (2010), Seasonal variability of the Atlantic meridional overturning circulation at 26.5°N, J. Clim., 23(21), doi: 10.1175/2010JCLI3389.1171.

Chidichimo, M. P., T. Kanzow, S. A. Cunningham, W. E. Johns, and J. Marotzke (2010), The contribution of eastern-boundary density variations to the Atlantic meridional overturning circulation at 26.5 N, Ocean Science, 6,

• Largest seasonal influence in mid-ocean transports due to the east

• Recent variability hasn’t changed this

Basinwide response to wind forcing

Zhao, J. and Johns, W. E. (2013) Wind driven seasonal cycle of the Atlantic Meridional Overturning Circulation, submitted

70ºW 60ºW 50ºW 40ºW 30ºW 20ºW

• Seasonal cycle coherent across the basin in OFES and simple two layer model

• Not solely an eastern boundary phenomenom and not dependent on location of RAPID moorings

Seasonal cycle (Sv) in (top) OFES and (bottom) two layer model

Duchez et al. (2013) Density variations around the Canary Islands and their influence in the AMOC at 26ºN, in prep

Forced Rossby wave response to wind stress curl

Observed Mid-ocean transport anomaly

• Ocean response to wind stress curl forced Rossby wave model explains the first baroclinic mode structure seen in observations

Modelled Mid-ocean transport anomaly

Local Seasonality

Local Seasonality

Observed Mid-ocean transport anomaly

• Not all of seasonal structure explained by first baroclinic mode structure

Canarian Deep Poleward Undercurrent

Vélez-Belchi, P., et al. (2013), The Canary deep poleward undercurrent. In prep

Machín, F., et al. (2009), Northward Penetration of Antarctic Intermediate Water off Northwest Africa, JPO, 39, 512-535

• AAIW (MOW) flows northward (southward) in the Canarian deep poleward undercurrent (CDPU)during the Autumn (Spring) forced by seasonal layer stretching (compression)

• This deep poleward undercurrent is unusual and infrequently observed

Machín, F., et al. (2010), Seasonal Flow Reversals of Intermediate Waters in the Canary Current System East of the Canary Islands, JPO, 40, 1902–1909

Seasonal Reversals of Intermediate Water

• Distinct seasonal reversals of flow in the Lanzarote Channel

• Intermediate (1000 m) measurements correlate salinity changes with current reversals

• High salinity Med. Outflow water (MOW): Southward flow in Dec/Jan

• Low salinity Antarctic Int. Water (AAIW): Northward flow in Summer/Autumn

Evidence of CDPU in RAPID mooring data

Monthly Sal. Anom on 8 C

500 dbar

1000 dbar

2000 dbar

Pote

ntial

Tem

pera

ture

• Rapid data at Eastern Boundary shows clear salinity signal of seasonal oscillations around 1000 dbar

• These are consistent with seasonal reversals of AAIW and MOW in the CDPU

Evidence of layer stretching

Large Scale Consequences• Understanding the role of the Poleward Undercurrent is important to understand the

part of the seasonal cycle that is not explained by the Rossby Wave model• It has important consequences for freshwater transport at 26 N as transport

fluctuations are associated with salinity changes• Changes in freshwater flux near the eastern boundary of 0.1 Sv are not picked up by

models

Courtesy of Elaine McDonagh

Observed Mid-ocean transport anomaly Integrated Freshwater flux at 26ºN

• MOC seasonal cycle is 6.7 Sv peak-to-peak

• UMO contributes the most pronounced seasonal cycle of 5.9 Sv

• Seasonal cycle in UMO is caused by heaving of the thermocline

forced by seasonal anomalies in the wind stress curl.

• Canarian deep poleward undercurrent plays a secondary role

driven by seasonal stretching of the intermediate layer

Conclusions

End

Contribution of the upper mid-ocean western and eastern boundaries to the UMO seasonal cycle

Expectation that upwelling drives stronger (weaker) thermocline flow in the autumn (spring)

Reality is that seasonally reversing intermediate flows driven by water column stretching (squeezing) are the major factor

Chidichimo, M. P. et al.,(2010), The contribution of eastern-boundary density variations to the Atlantic meridional overturning circulation at 26.5 N, Ocean Science, 6

The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299NACLIM http://www.naclim.eu