Coupled versus Uncoupled hindcast simulations of the MJO in the Year of Tropical Convection Ann Shelly, Prince Xavier, Dan Copsey, Tim Johns, Jose Rodríguez and Sean Milton

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3. Lag correlation analysis Lag correlations between Outgoing Longwave Radiation (OLR) and SST anomalies are calculated, to determine the relationship between convection and SST. Lag correlations are performed on all day 1, day 2, day 3 etc hindcasts (Fig. 2). In the case of the AGCM, surface air temperature at sea points is used in lieu of actual SST. To examine eastward propagation, lag correlations between a basepoint of OLR at 70E and OLR at other longitudes are calculated. Correlations are considered for a range of leads/lags between -15 days (lead) and +15 days (lag) for all longitudes in the Indian Ocean.

5. Conclusions

• An interactive ocean improves the representation of the MJO over that of an AGCM, throughout Oct 2009 - Jan 2010.Predictability of RMM1 and RMM2 is greater (not shown) and eastward propagation of the MJO is better in the CGCM than in the AGCM.

• The CGCM maintains a realistic phase relationship between SST and convection, while AGCM predictions drift towards an in-phase relationship within a few days.

• The CGCM demonstrates skill in representation of tropical ocean waves. Anomalously warm waters are shown to propagate along the trajectory of down-welling tropical waves. This process is well modelled and could be important to the lifecycle of the MJO.

• The case for using an interactive ocean model capable of simulating tropical ocean waves seems strong, given that waves can influence SST, which can subsequently feedback on the MJO. Work is underway to extend this study to encompass the DYNAMO period.

Reference: Shelly, A, Xavier, P., Copsey, D, Johns, T., Rodríguez, J., Milton,S., Klingaman, N (submitted) Coupled versus Uncoupled hindcast simulations of the MJO in the Year of Tropical Convection, Geophys. Res. Lett.

1. Background The Madden Julian Oscillation (MJO) is the dominant component of intra-seasonal variability in the tropics. It consists of large-scale coupled patterns in deep convection and atmospheric circulation, propagating eastward though the warm pool region of the tropics. An accurate simulation of the MJO in medium range hindcasts could potentially enhance hindcast predictability. The Met Office has been developing a fully coupled global model to run on 1-15 day timescales . Here, we study the impact of a full interactive ocean on MJO prediction, in comparison to an atmosphere only configuration on medium range hindcasts.

Figure 2: Experimental design for lead-lag analysis of OLR and SST over the period Oct 2009 – Jan 2010. Green lines represent the daily initialized 15 day hindcasts while the black arrows depict the data included in the lagged analysis.

2. Experimental design Coupled model (CGCM): MetUM atmosphere; NEMO3.2/CICE Ocean 3-hourly coupling frequency, 60km/85L atmosphere, 25km/75L ocean Atmosphere control (AGCM): Persisted initial Sea Surface Temperature (SST) anomaly (wrt. GISST) Initial Conditions: MetUM analyses (Atmosphere) & NEMOVAR (Ocean/Ice) Case study analysis: In order to explore the representation of the MJO in the coupled model, we analysed data from daily initialised hindcasts spanning October 2009 – January 2010, part of the Year of Tropical Convection (YOTC) campaign (Fig. 1).

4. Results

The CGCM demonstrates ability to propagate the MJO in day 5 hindcasts (Fig. 3 (b)) while the AGCM shows evidence for an almost stationary MJO (Fig 3. (c)). The CGCM improvements in predictability (not shown) and propagation may arise in part from the more realistic representation of the phase-lag relationship between convection and SST.

The observed relationship indicates that warm SST’s lead enhanced convection by 5-10days (Fig 3 (d). The CGCM maintains a realistic phase lagged relationship at day 5 (e) and out to day 15 (not shown). In the AGCM hindcasts, MJO related fluxes have no influence on SST so convection adjusts to a location where SST is favourable, resulting in an in-phase relationship within ~3 days (f).

Figure 3: Lag correlations between OLR at 70ºE and OLR at other longitudes in the range 60-100ºE, over the YOTCE period between 10ºN and 10ºS, for (a) for NOAA OLR observations (b) CGCM (c) AGCM for day 5 hindcasts. Lag correlations over YOTCE period for SST and OLR for (d) observations (OSTIA and NOAA OLR), (e) CGCM (f) AGCM for day 5 hindcasts.

Figure 4: Time-longitude plots over the period of YOTCE and YOTCF:

(Top) Depth 20º isotherm anomaly (m) for (a) hindcast Ocean Assimilation Model (FOAM) analysis (b) day 1 CGCM & (c) day14 CGCM, averaged between 2°N-2°S.

(Middle) Sea surface height anomaly (m) for (d) FOAM analysis (e) day 1 CGCM & (f) day 14 CGCM averaged between 2ºN-8ºN and 2ºS-8ºS.

(Bottom) SST anomaly field for (g) OSTIA (h) day 1 CGCM & (i) day 14 CGCM averaged between 2ºN-8ºN and 2ºS-8ºS. Diagonal lines (R1,R2,R3) in (d)-(i) represent downwelling Rossby waves propagating from East to West across the Indian Ocean.

Results presented in figure 3 indicate that SST modulates and is modulated by the MJO. Tropical ocean waves can play a role in altering the SST in the tropical warm pool, which could feedback on the MJO. Downwelling waves deepen the thermocline and raise the SST by reducing entrainment of cold sub-surface waters, while upwelling waves lead to enhanced entrainments and cooling. MJO activity can initiate and interact with oceanic Kelvin and Rossby waves. An ocean Kelvin wave is seen to propagate eastward between Oct and Dec 2009, excited by YOTCE MJO activity Fig 4 (a-c)). Westward propagating SST anomalies (Fig 4 (g-i)) correlate positively with 3 westward propagating Rossby waves (Fig 4 (d-f)) labelled R1,R2 and R3. The CGCM portrays the observed Kelvin and Rossby waves well at each lead time and simulates the westward warming of SST’s in conjunction with Rossby wave motion. These results suggest that MJO predictions on sub-seasonal timescales may be enhanced through coupling to an interactive ocean capable of both dynamically predicting SSTs and representing ocean waves.

Figure 1: Wheeler and Hendon diagram for the YOTC E and F MJO events observed Oct 2009-Jan 2010 as calculated from MetUM analyses. Dates are labeled every 5 days.