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 = 10 6 m 3 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 W1W2MARWMARE E

Meridional velocity at °N 76.25W

Meridional velocity at 28.8°N 76.25W

Monthly means Δ RAPID array °N 28.8°N Comparison of MOC components

NOC1.1HadCM3 control Jan July Wind stress

Using all obs except RAPID Average correlation 0.3 (0.4) 0.6 (0.6) 0.9 (0.8) -0.3 (0.0) DPS2 PPE Assimilation Monthly means

Component correlations CorrelationModelRAPID Gulf Stream with Upper mid-ocean Gulf Stream with MOC minus Ekman Upper mid-ocean with MOC minus Ekman

Seasonal cycle of MOC at 26°N

Seasonal cycle of upper mid-ocean

Seasonal cycle of Gulf Stream and 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

Salinity Levitus 05 Potential Temp HadCM3 ABW

Correlations with transport 1000m Lag -5 months

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