The RAPID ocean observation array at 26.5°N in the HadCM3 model

Leon Hermanson,

Rowan Sutton, Keith Haines,

Doug Smith, Joël Hirschi

Content

• Introduction to RAPID-WATCH and VALOR• The RAPID observation array• Getting an MOC from the observations• Creating an analogous array in HadCM3• Can models reproduce MOC without RAPID?

(but including other observations, eg. ARGO)• Seasonal cycles in the model and

observations• Assimilating the RAPID observations

Introduction to RAPID-WATCH

• The RAPID climate change programme ran from 2001—2008 – Aim was to improve our ability to quantify the probability and

magnitude of future rapid change in climate, with a focus on the role of the Atlantic Ocean's Thermohaline Circulation

• RAPID-WATCH follows on from RAPID and will run until 2014– Aim is to continue monitoring the MOC using the moored array

in the North Atlantic and also exploit the data being collected– Observations will be used to:

• determine and interpret recent changes in the Atlantic MOC• assess the risk of rapid climate change due to changes in the MOC• investigate the potential for predicting the MOC and its impacts on climate.

Value of the RAPID array (VALOR)

• Aim is to assess the value of the RAPID array observations for predictions of the Atlantic MOC and its impact on climate

• Will provide information for an assessment of the RAPID array late 2011

• The project will explore a range of issues concerning the design of a potential MOC prediction system

• There are four main activities:

1. Ocean synthesis

2. Hindcast experiments

3. Ensemble design

4. Pseudo-observations

The RAPID array

• Located at approximately 26°N• Between Florida and Morocco• Reasons for choosing this

latitude:– Gulf Stream is mainly found in

Florida Straits and has been measured there since 1981

– High correlation between MOC and ocean heat transport

– Steep continental slope in the West

– Strong, steady trade winds

• Downside: – Low decadal signal to noise ratio

Meridional transports at 26°N in the Atlantic

Transport (Sv)

Gulf Stream

MOC

Ekman

Upper mid-ocean

1 Sv =106 m3 s-1

Gulf Stream transport

Ekman transport

Upper mid-ocean transport

Meridional Overturning Circulation

The RAPID array and HadCM3

Resolution: 1¼° x 1¼°

26.25°N

W1 W2 MARW MARE E

Meridional velocity at 26.25 °N

76.25W76.25W

Meridional velocity at 28.8°N

76.25W76.25W

Monthly meansΔ RAPID array- - 26.25°N 28.8°N

Comparison of MOC components

NOC1.1HadCM3 control

Jan

July

Wind stress

Using all obs except RAPIDAverage correlation

0.3 (0.4)

0.6 (0.6)

0.9 (0.8)

-0.3 (0.0)

DPS2 PPE AssimilationMonthly means

Component correlations

Correlation Model RAPID

Gulf Stream with

Upper mid-ocean

-0.75 -0.15

Gulf Stream with

MOC minus Ekman

0.40 0.55

Upper mid-ocean with

MOC minus Ekman

0.30 0.75

Seasonal cycle of MOC at 26°N

Seasonal cycle of upper mid-ocean

Seasonal cycle of Gulf Streamand Ekman

Gulf Stream transport Ekman transport

Assimilating RAPID observations

• Four years of twice-daily temperature and salinity at five merged profile locations– Too high frequency– Need to split up some profiles– Differences in mean states

• Given the large amount of ARGO and other observations 2004—2008, will RAPID have an impact?– Eastern boundary poor in current assimilation (without RAPID)– In NEMO deep observations project onto MOC– Alternative methods

Salinity1951-2006

Levitus 05

Potential Temp1951-2006

HadCM3 ABW

Correlations with transport1000m

Lag -5months

Scientific summary

• The way the observed MOC is calculated is particular to the unusual bathymetry at 26°N

• As most of Florida is missing in HadCM3, the components of the MOC are difficult to reproduce

• There are no compelling reasons to move the model array to 28.8°N

• Assimilating all observations except RAPID creates an unrealistic seasonal cycle

• RAPID observations may have a large impact at eastern boundary and at deep levels

• RAPID transports could possibly be assimilated through their covariance with temperature and salinity