Monthly Forecasting at ECMWF Frédéric Vitart European Centre for Medium-Range Weather Forecasts

Forecasting systems at ECMWF Product ECMWF: Weather and Climate Dynamical Forecasts Medium-Range Forecasts Day 1-10(15) Monthly Forecast Day 10-32 Seasonal Month 2-7 Slide 2: The monthly forecasting system fills the gap between two currently operational forecasting systems at ECMWF: medium-range weather forecasting and seasonal forecasting. Medium-range weather forecasting produces weather forecasts out to 15 days, whereas seasonal forecasting produces forecasts out to 7 months. The two systems have different physical bases. Medium-range weather forecasting is essentially an atmospheric initial value problem. Since the time scale is too short for variations in the ocean significantly to affect the atmospheric circulation, the ECMWF medium-range weather forecasting system is based on atmospheric-only integrations. SST anomalies are simply persisted. Seasonal forecasting (2-7 months forecasts), on the other hand, is justified by the long predictability of the oceanic circulation (of the order of several months) and by the fact that the variability in tropical SSTs has a significant global impact on the atmospheric circulation. Since the oceanic circulation is a major source of predictability in the seasonal scale, the ECMWF seasonal forecasting system is based on coupled ocean-atmosphere integrations.

Bridging the gap between seasonal forecasting and NWP A particularly difficult time range: Is it an atmospheric initial condition problem as medium-range forecasting or is it a boundary condition problem as seasonal forecasting? Some sources of predictability in the monthly time scale: Sea surface temperature/Sea ice Snow cover Soil Moisture Stratospheric Initial conditions The Madden-Julian oscillation Slide 3:The time range 10 to 30 days is a very difficult time range for weather forecasts. It is not clear if it is an atmospheric initial condition problem as medium-range weather forecast or if it is a boundary condition problem as seasonal forecast. Most likely, it is a combination of both. The time range 10 to 30 days is probably too long for the atmosphere to keep a memory of its initial conditions, and too long for the ocean variability to have an impact on the atmospheric circulation. Therefore, for a long time, it was assumed that there was almost no predictability in this time scale. However, two main sources of predictability in this timescale have been discovered in the recent years: - The impact of the stratosphere on the troposphere - The Madden-Julian Oscillation (MJO)

Index Main sources of predictability on the monthly time-scale Soil Moisture Stratospheric Initial conditions Madden Julian Oscillation The ECMWF monthly forecast system Description Some examples of forecasts Skill

Impact of soil moisture Slide 5: Impact of land moisture initialization on the skill of a multi-model ensemble to predict 2-metre temperature anomalies for different time ranges (1-15 days, 16-30 days, 31-45 days and 46-60 days) and different amplitude of soil moisture anomalies in the initial conditions. Koster et al, GRL 2010

Impact of soil moisture Slide 6: Same as Slide 5 but for the skill of the multi-model to predict precipitation anomalies. Koster et al, GRL 2010

Stratospheric influence on the troposphere? Slide 7: Baldwin et al. (2003) found that the stratospheric polar vortex varies relatively slowly compared to the tropospheric circulation, and that it has an impact on the troposphere. Slide 5 shows the vertical cross section of a composite of 18 weak vortex events as a function of time. This figure displays a propagation from the stratosphere to the lowest levels of the troposphere, well beyond 10 days. This suggests that there is predictability in the monthly timescale. Baldwin and Dunkerton, 2001

Z1000 Response (Weak vortex-CTL) Stratospheric influence on the troposphere? D+1-D+10 D+11-D+20 Z1000 Response (Weak vortex-CTL) From T. Jung et al 2005 D+21-D+30 D+31-D+40 Slide 8: This slide shows the impact of the stratospheric forcing on the geopotential height at 1000 hPa in a GCM. Close to the surface, the largest and most statistically significant response is found in the north-eastern North Atlantic and parts of the Arctic, at least after more than 20 days in the integration.

Synoptic Z500 Activity D+21-D+30 Stratospheric influence on the troposphere? Synoptic Z500 Activity D+21-D+30 Slide 9: This slide shows the difference of synoptic activity in the range from D+21 to D+30 between strong and weak polar stratospheric vortex. The synoptic activity is computed by taking the standard deviation of the day-to-day Z500 changes. The largest and most statistically significant impact of the stratospheric forcing is found over Northern Europe and the north-eastern North Atlantic, highlighting the fact that extended range forecasts in the European region should benefit the most from stratosphere-troposphere coupling. From T. Jung et al 2005

Stratospheric Sudden warming- January 2009 SSW Index (T50 gradient) 15/1/2009 8/1/2009 Slide 10: Stratospheric Sudden Warming (SSW) Index. The SSW index is calculated by integrating the gradient of temperature at 50 hPa between 30N and 0N around the globe. The black line represents the analysis. Each green line represents one ensemble member. The red line represents the ensemble mean, and the blue line represents the control run. The left panel represents the monthly forecast of 8 January 2009 and the right panel the monthly forecast of 15 January 2009.

Stratospheric Sudden warming- January 2009 15/1/2009 2mtm anomaly Forecast Composite Bad SW Composite Good SW Analysis Day 19-25 Day 26-32 Slide 11: 2-metre temperature forecasts with the ensemble members which predicted correctly a SSW and with the ensemble members which did not predict a SSW. Both members with and without SSW display cold anomalies over west Europe, but the forecasts obtained from the members with SSW (right panels) are closer to the analysis (left panel) than the forecast obtained from the members without SSW, particularly over Scandinavia. Therefore it seems that the correct prediction of the SSW helped to get a better 2-metre temperature forecast over Europe, although the impact is of second order.

The Madden-Julian Oscillation (MJO) Slide 12: The MJO was first described by Madden and Julian (1971). By looking at rawinsonde data provided by a station in the central Pacific (Canton), they discovered large coherence between surface pressure, zonal winds and temperature at various levels over a broad period range that maximized between 41 and 53 days. By looking at data from other stations, they discovered a coherence in the 40-50 day range between stations far removed from one another. The sum of evidence pointed toward an oscillation, which is an eastward movement of large-scale circulation. From Madden and Julian (1972)

The Madden Julian Oscillation (MJO) MJO life cycle Slide 13 : The left panel shows the evolution of OLR anomalies during a typical MJO. The blue colour indicates increased convection (positive phase of the MJO) and the red colour indicates suppressed convection (negative phase of the MJO). The MJO starts with increased convection in the Indian Ocean. This convection propagates eastwards, and it reaches the Maritime continent by about day 9. It reaches the western Pacific by day 15, and stops at the dateline (180E). Then, the negative phase of the MJO starts, with suppressed convection over the Indian Ocean propagating eastward. The right panel shows the time evolution of OLR anomalies over a period of 3 years from observations (2001 to 2003). In this Hovmoeller diagram, MJO episodes are well visible, but they are not regular. There are periods of strong MJO activity, and periods with low MJO activity. The different MJO events display a lot of variability in intensity and main characteristics, in the same way as ENSO events do not look all the same. The non-regularity in the occurrence of the MJO, makes the forecast of the MJO non trivial. (From NASA) From http://www.bom.gov.au/bmrc/clf

The Madden Julian Oscillation (MJO) The MJO is a 40-50-day oscillation The MJO is a near-global scale, quasi-periodic eastward moving disturbance in the surface pressure, tropospheric temperature and zonal winds over the equatorial belt. The Madden-Julian Oscillation (MJO) is the dominant mode of variability in the tropics in time scales in excess of 1 week but less than 1 season. The MJO has its peak activity during Northern winter and spring. Slide 14: This is a summary of the previous slide: the MJO is a 40-50-day oscillation. The MJO is a near-global scale, quasi-periodic eastward moving disturbance in the surface pressure, tropospheric temperature and zonal winds over the equatorial belt. The Madden-Julian Oscillation (MJO) is the dominant mode of variability in the tropics in time scales in excess of 1 week but less than 1 season. The MJO has its peak activity during Northern winter and spring.

The Madden Julian Oscillation (MJO) Why is the MJO so important? Impact on the Indian and Australian summer monsoons (Yasunari 1979), Hendon and Liebman (1990) Impact on ENSO. Westerly wind bursts produce equatorial trapped Kelvin waves, which have a significant impact on the onset and development of an El-Niňo event. Kessler and McPhaden (1995) Impact on tropical storms (Maloney et al, 2000; Mo, 2000) Impact on Northern Hemisphere weather Slide 15: The MJO has a significant impact on the Indian summer monsoon (Yasunari 1979). Clouds associated with the active phase of the Indian monsoon propagate northward through the Indian Ocean and Indian subcontinent at about 1 degree latitude per day (Murakami, 1976). Yasunari (1979) associated these northward moving clouds to the MJO. The MJO has also a significant impact on the Australian monsoon. Hendon and Liebmann (1990) noticed that 27 of the 30 monsoon onsets from 1957 to 1987 coincided with the arrival of clouds associated with the MJO. Several papers have suggested an impact of the MJO on the onset and development of the 1997 El-Nino event. During an active phase of the MJO, intense near-surface westerly wind events develop over the tropical Pacific Ocean, known as westerly wind bursts (Kiladis et al, 1994). Kindle and Phoebus (1995) documented a significant impact of the westerly wind bursts on the ocean through the initiation of equatorial trapped Kelvin waves. Kessler and McPhaden (1995) suggested that westerly wind bursts have a significant impact on the onset and development of an El-Nino event. The MJO impacts the low-level vorticity and vertical wind shear over the eastern North Pacific and the Gulf of Mexico (Maloney et al, 2000), which has a significant impact on the tropical storm cyclogenesis. Mo (2000) documented that the suppression of deep convection over the Pacific during the early phase of the MJO is conducive to more easterly wind in the upper troposphere over Atlantic, which implies less vertical wind shear of the horizontal wind, and therefore more favorable conditions for cyclogenesis.

From Wheeler and Hendon, BMRC MJO Prediction Combined EOF1 Combined EOF2 Slide 17: In order to assess the skill of the monthly forecasting system to predict an MJO event, the forecasts are projected into combined EOFs of U200, U850 and OLR (see Wheeler and Hendon, 2004). Those combined EOFs have been computed using NCEP reanalysis and the two dominant combined EOFs , which represent about 12% of the total variance each, give a good representation of the MJO propagation. From Wheeler and Hendon, BMRC

MJO FORECAST Slide 18: The time evolution of the MJO of a monthly forecast is described by a multivariate MJO index (Wheeler and Hendon 2004 Mon. Wea. Rev. vol. 132, 8 p 1917-1932). The diagram represents 8 regions of the two dimensional phase space defined by the first two principal components (RMM1 and RMM2) of a combined fields (OLR, zonal wind at 850 hPa and 200 hPa) averaged between 15S and 15N. Individual ensemble member values at day 1, 5, 10, 15 and 20 are represented respectively by a red, pink orange blue and green circles. The ensemble mean values (black triangles) are joined by a solid black line and the analysis values of the preceding 30 days are joined by a grey line. The grey squares represent the analysis values of the preceding 5, 10, 15, 20, 25 and 30 days. Points representing sequential values trace anticlock-wise circles around the origin, which signifies systematic eastward propagation on the MJO. Large amplitudes (outside of the circle) signify strong cycles of the MJO, while weak activity appears as rather random motion near the origin. The solid blue line represents the verification computed from the ECMWF operational analysis.

MJO and Precipitation Observational study (precipitation) Phase 2 Slide 19: Composite of precipitation anomalies computed from station data as a function of the phase of the MJO. Phase 6 Phase 8 Donald et al. (2006,GRL)

Ting and Sardeshmukh JAS 1993 Impact of the MJO on Extratropics Slide 20: Impact of imposing a Heat source in the Tropics on Z500 circulation in the Extratropics. This shows a heat source over the maritime continent (consistent with MJO EPF1) has little impact o the Z500 circulation. Imposing a heat source over the Indian Ocean and a cooling over the west Pacific (consistent with MJO EOF2) has a significant impact on the Extratropics. Lin et al, MWR 2010 See also Simmons et al JAS 1983 Ting and Sardeshmukh JAS 1993

Impact on Europe Cassou (2008) Slide 22: Lagged relationships between the eight phases of the MJO (rows) and the four North Atlantic weather regimes (columns) from ERA40. This figure shows a significant impact of some phases of the MJO on the NAO (see Cassou 2008)

OLR anomalies - Forecast range: day 15 ERA40 28R3 29R1 31R1 32R2 29/12 05/01 12/01 days 20/01 28/01 04/04 12/02 10/04 04/05 09/06 06/07 32R3 33R1 35R1 35R3 Slide 23: Hovmoeller diagram of OLR from ERA40 (left panel) and forecasts at day 15 from 8 successive versions of IFS since the monthly forecasting system became operational. This slide shows that there has been a considerable improvement in the amplitude of the MJO since the monthly forecasting system became operational. However the propagation of the MJO is too slow over the Indian Ocean in the last model cycles. 11/07 06/08 09/08 09/09

Experiment’s setting: MJO prediction Experiment’s setting: 46 day forecasts at T255L62 coupled to HOPE 15 members Starting dates: 15 Nov/Dec/Jan/Feb/Mar/Apr 1989-2008 Model Cycle 32R3 (operational cycle from 11/07 to 06/08) Slide 24: This slide describes an experiment which has been set up to assess the skill of IFS to represent and predict MJO events. With this experiment, we can also estimate the MJO teleconnections in the model.

Ensemble mean/ reanalysis Ensemble mean/ reanalysis MJO Skill scores Bivariate RMS error Bivariate Correlation Slide 25: MJO skill scores. Left panel shows the bivariate correlation and the right panel shows the bivariate RMS error (black lines). Ensemble mean/ reanalysis Ensemble mean/ reanalysis Ensemble Spread “Perfect Model” Climatology

MJO Propagation Analysis Forecast Slide26: This slide shows a composite of the propagation of the MJO from phase 2 (convection over the Indian ocean) in the model (right panel) and in ERA Interim (right panel). In the model and in the analysis, all the cases where the MJO is in phase 2 have been composited. The plots show the percentage of cases in each phase of the MJO as a function of time. From this graphic, it appears quite clearly that the MJO propagates too slowly in the model.

Impact on Precipitation anomalies (Summer) Slide 27: Impact of the MJO on precipitation. The plots show the precipitation anomalies composited over all the cases in phases 2+3, 4+4, 6+7 and 8+1 of the MJO in the model (left panel) and in the analysis (right panel) for the period DJF 1989-2008. The model seems to represent very well the northward propagation of the MJO and its teleconnections over Central America.

Impact on Tropical Cyclone Density (Summer) Slide 28: Impact of the MJO on tropical storm activity. The plots show the anomalies of tropical storm density composited over all the cases in phases 2+3, 4+5, 6+7 and 8+1 of the MJO in the model (left panel) and in the analysis (right panel). The model seems to represent very well the impact of the MJO on tropical cyclones. Vitart, GRL 2009

Impact on the Extratropics- Z500 anomalies Slide 29: Impact of the MJO on the Extratropics. In agreement with Cassou (2008), composites of phase 3 + 10 days of the MJO project into a positive NAO (top panels), and composites of Phase 6 +10 days of the MJO project into a negative phase of the NAO in the analysis (bottom panels). In the model, the teleconnections are consistent with reanalysis (right panels), but are too weak over the Atlantic. The impact of the MJO on the NAO seems to be underestimated in the model.

Impact of MJO on Z500 anomalies 1 std< AMP < 1.5 std AMP > 2 std 1.5 std< AMP < 2 std Slide 30: Impact of the MJO on the Z500 anomalies for phase 3 + 10 days as a function of the amplitude of the MJO. Stronger MJOs have a stronger impact than weaker MJOs. Interval = 5 metres

Impact on the Extratropics – NAO+ weather regime Slide 31: The top panels show the time evolution of the percentage of days in NAO+ relative to climatology as a function of lead time from phase 3 (left panel) and phase 6 (right panel). Each blue line represents an individual ensemble member. The solid black line represents the 15-member ensemble mean. The red Line represents ERA Interim, and the green line in the top left panel represents the fast propagating MJO cases. The bottom panels show the number of ensemble members which display an increase (blue bars) or a decrease (white bars) of the frequency of NAO+ events.

Impact on weather regimes in hindcasts Phase3+10 days Phase6+10 days NAO- NAO+ Atlantic ridge Scandinavian blocking Slide 32: Impact of the MJO on the frequency of 4 Euro-Atlantic weather regimes. Red bars are for Phase 3 + 10days and blue bars are for Phase 6 + 10 days. This plot shows that the strongest impact of the MJO is on the frequency of NAO+.

T850 anomalies – NDJFM 1989-2008 Phase 3 + 10 days Phase 6 + 10 days ERA MODEL Slide 33: Impact of the MJO on 2-metre temperature anomalies. Left panels are for Phase3+10 days and the right panels for Phase6+10 days. Top panels show results obtained with ERA Interim and bottom panels show results obtained from the model. Degree C

Impact of ocean-atmosphere coupling: PC1 PC2 Slide 34: Anomaly correlation between observations and forecasts at different time lags. The left panel represents the results obtained with PC1 and the right panel the results obtained with PC2. The scores are higher than those obtained when running the atmospheric model forced by persisted SSTs (as in 10-day EPS) rather than using a coupled ocean-atmosphere model. The scores are lower than those obtained by integrating the atmospheric model coupled from day 0 and even lower than those obtained with an atmospheric model coupled to an oceanic mixed-layer model. This suggests that SSTs play a very important role in the propagation of the MJO and that the vertical resolution of SSTs is important. On the other hand, an increase of the horizontal resolution of the atmospheric model does not lead to better MJO forecasts. Coupled after day 10 Persisted SSTs Coupled Ocean ML

The ECMWF monthly forecasting system A 51-member ensemble is integrated for 32 days every week Atmospheric component: IFS with the latest operational cycle and with a T639L62 resolution till day 10 and T319L62 after day 10. Persisted SST anomalies till day 10 and ocean-atmosphere coupling from day 10 till day 32. Oceanic component: HOPE (from Max Plank Institute) with a zonal resolution of 1.4 degrees and 29 vertical levels Coupling: OASIS (CERFACS). Coupling every 3 hours. Slide 35:Description of the VarEPS-monthly forecasting system. Each week, the coupled model is integrated forward to make a 32 day forecast with 51 different initial conditions, in order to create a 51-member ensemble.

The ECMWF VarEPS-monthly forecasting system Current system (VAREPS): EPS Integration at T639 Initial condition Heat flux, Wind stress, P-E Day 10 Day 32 Coupled forecast at TL319 Slide 36: New monthly forecasting system. Previously the monthly forecasting system consisted of coupled ocean-atmosphere integrations with an atmospheric horizontal resolution at TL159. Now, the monthly forecasting system and the VarEPS system have been merged. This graphic displays the new configuration: atmosphere-only at TL639 forced by persisted SST anomalies till day 10 twice a day. After day 10, the atmospheric model at a TL319 resolution is coupled to an ocean model till day 32. The coupled model consists of the ECMWF atmospheric model (the same cycle as the deterministic forecast), coupled to an ocean general circulation model, which is a version of the Hamburg Ocean Primitive Equation model (HOPE), developed at the Max Plank Institute for Meteorology, Hamburg. The ocean model has lower resolution in the extratropics but higher resolution in the equatorial region, in order to resolve ocean baroclinic waves and processes, which are tightly trapped at the equator. The ocean model has 29 levels in the vertical. The atmosphere and ocean communicate with each other through a coupling interface, called OASIS, developed at CERFACS, France. The atmospheric fluxes of momentum, heat and fresh water are passed to the ocean every 3 hours and, in exchange, the ocean sea surface temperature (SST) is passed to the atmosphere. Ocean only integration

The ECMWF monthly forecasting system Atmospheric initial conditions: ECMWF operational analysis Oceanic initial conditions: “Accelerated” ocean analysis Perturbations: Atmosphere: Singular vectors + stochastic physics Ocean: wind stress perturbations during data assimilation. Slides 37 In order to initiate monthly forecasts, initial conditions for both the ocean and atmosphere are required. Atmospheric and land surface initial conditions are obtained from the ECMWF operational atmospheric analysis/reanalysis system. Oceanic initial conditions originate from the oceanic data assimilation system used to produce the initial conditions of the seasonal forecasting system 2. However, this oceanic data assimilation system lags about 12 days behind real-time. The lag is partially due to the fact that the SST, obtained by interpolating in time the weekly OIv2 SSTs produced by NCEP, can be up to 12 days behind real-time. A first option would be to wait for the oceanic initial condition to be created by the data assimilation system to start the forecast, as in seasonal forecasting. This would mean losing 12 days of forecast and is not therefore suitable for monthly forecasting. A second option would be to persist the SST anomalies of the latest ocean analysis. However, we have some information about the wind stress and heat fluxes during the last 12 days of the ECMWF atmospheric analysis; this information can be used to help determine the present ocean initial state. Therefore, the option that has been chosen for monthly forecasting consists in integrating the ocean model from the last ocean analysis forced by analyzed wind stress, heat fluxes and P-E. During this 'ocean forecast', the sea surface temperature is relaxed towards persisted SST, with a damping rate of 100 W/m2/K.

The ECMWF monthly forecasting system MODEL BIAS Slide 38: This figure shows the evolution of the model bias of 2-meter temperature week by week. The patterns of the model bias are roughly the same during the 4 weeks, but the intensity increases almost linearly in some areas. After 10 days of forecast, such model error cannot be ignored, and the model needs to be biased corrected as for seasonal forecasting.

The ECMWF monthly forecasting system Background statistics: 5-member ensemble integrated at the same day and same month as the real-time time forecast over the past 18 years (a total of 90 member ensemble) Initial conditions: ERA Interim It runs once every week Slide 39: Because of model errors, a drift occurs in the coupled system. In order to evaluate this model drift, the coupled model is integrated with 5 different initial conditions (5-member ensemble) at the same day and month as the real time forecast, but over the past 18 years, creating a 90-member climate ensemble.

The ECMWF monthly forecasting system Anomalies (temperature, precipitation..) - Slide 40: Anomaly maps are similar to seasonal forecasting charts, but with weekly means instead of monthly means. Over each point of the map, atmospheric variables such as 2-metre temperature, total precipitation, mean sea-level pressure or surface temperature, have been averaged over a weekly period (week 1: day 5 to 11, week 2: day 12 to 18, week 3: day 19 to 25, and week 4: day 26 to 32) and also over the 51 members of the real-time forecast and the 60 members of the back statistics. The plots display the difference between the ensemble mean of the real-time forecast and the ensemble mean of the back-statistics. The product therefore displays the shift of the forecast ensemble mean from the estimated "climatological" mean (created from ensemble runs over the past 18 years). In addition, a Wilcoxon-Mann-Whitney test (WMW-test, see for instance Wonacott and Wonacott 1977) has been applied to estimate whether the ensemble distribution of the real-time forecast is significantly different from the ensemble distribution of the back-statistics. Regions where the WMW-test displays a significance less than 90% are blank. Regions where the WMW-test displays a significance exceeding 95% are delimited by a solid contour (blue or red depending on whether the anomaly is positive or negative respectively). The blanking of "non-significant" shifts does not mean that there is no signal in the blanked regions, but only that, with the particular sampling we have, we cannot be sure that there is a signal. For this reason, there are likely to be many areas where a signal is real but remains undetected.

The ECMWF monthly forecasting system Probabilities (temperature, precipitation..) - Slide 41: Probability and tercile maps are also produced. An example of tercile map for the period day 12-18 is displayed on this slide.

Experimental product: Tropical cyclone activity The ECMWF monthly forecasting system Experimental product: Tropical cyclone activity Slide42: Forecast of tropical cyclone activity. This plot shows the probability of tropical storm strike within 300 km predicted by the monthly forecast starting on 8 April 2010 and for the period day 12-18.

Precip anomalies : 26 July 2010 – 01 August 2010

Days 12-18 Days 19-25 Verifying weeks: 3-9 May to 16-22 Aug. Precipitation over Pakistan Averaged over (34-25N 60-73E) : Days 12-18 Days 19-25 Verifying weeks: 3-9 May to 16-22 Aug.

Skill of the ECMWF Monthly Forecasting System 2-meter temperature in upper tercile - Day 12-18 ROC score Reliability diagram ROC area=0.68 0.62 Slide 39: The toughest test for monthly forecasting is a comparison with the persistence of the medium-range forecasts. If the monthly forecast does not perform better than persisting the EPS forecasts, then it is useless. The left plot shows the ROC diagram obtained with the monthly forecasting system for days 12-18 (red) and the persistence of the probabilities of days 5-11 (about the same as persisting the last week of EPS) (blue). The event is the probability that 2-meter temperature is in the upper tercile. The right panel represents the reliability diagram of the monthly forecast for day 12-18 (blue line) and persistence of day 5-11 (red line). This slide demonstrates that the monthly forecast of days 12-18 performs better than persisting the medium range forecast, and therefore can be useful. The difference is statistically significant according to WMW test. This result is also valid for all the other variables and other probabilistic events. It can be concluded that the model show some skill over the northern Extratropics at this time range, and therefore there is no reason to stop the EPS at day 10. Persistence of day 5-11 Monthly forecast day 12-18

Skill of the ECMWF Monthly Forecasting System 2-meter temperature in upper tercile - Day 19-32 ROC score Reliability diagram ROC area=0.61 0.56 Slide 40: Same as slide 31 but for the period day 19-32. Persistence of day 5-18 Monthly forecast day 19-32

ROC scores over the Northern extratropics Skill of the ECMWF Monthly Forecasting System ROC scores over the Northern extratropics 2-metre temperature Mean sea-level pressure Precipitation Slide 44: ROC diagrams of the probability that weekly (left) 2-meter temperature, (middle) mean-sea level pressure, and (right) precipitation are in the upper tercile. The diagrams have been calculated over all the land grid points of the Northern hemisphere extratropics (North of 30N) and over the 45 cases. The solid black line represents the period days 5-11, the blue line days 12-18, the red line days 19-25, the green line days 26-32. This plot shows that the ROC score varies a lot from one variable to another. For days 12-18, the ROC score remains sufficiently high to be potentially useful, particularly for temperature. Day 5-11 Day12-18 Day19-25 Day 26-32

ROC score: 2-meter temperature in the upper tercile Skill of the ECMWF Monthly Forecasting System ROC score: 2-meter temperature in the upper tercile Day 5-11 Day 12-18 Day 19-25 Day 26-32 Slide 43 Map of ROC scores of the probability that 2-meter temperature averaged over the period day 12-18 is in the upper tercile. Only the scores over land points are shown. The terciles have been defined from the model climatology. The verification period is Oct 2004-May 2008. Red areas indicate areas where the ROC score exceeds 0.5 (better than climatology). This plot shows that the coupled model performs better than climatology for the period days 12-18. For the period days 19-26, the skill is much lower than for days 12-18, as expected. The red is largely dominating overall, suggesting that the model generally performs better than climatology at this time scale. Europe seems to be a difficult region, with very low skill at this time range. Tropical regions display the strongest skill after 30 days, suggesting that the coupled model at this time range starts to behave more like seasonal forecasting.

Monthly Forecast: Performance over the Northern Extratropics ROC score: 2-meter temperature in the upper tercile Monthly Forecast Monthly Forecast Persistence of day 5-11 Persistence of day 5-18 Slide 45: ROC area of the probability of 2-metre temperature in the upper tercile averaged over a season for day 12-18 (left panel) and day 19-32 (right panel) (red curves). The blue curve represents the scores obtained when persisting the day 5-11 probabilities (left panel) of day 5-18 probabilities (right panel). At both ranges the forecast is better than the persistence. The scores were exceptionally high in DJF 2010.

Probabilistic skill scores – NDJFMA 1989-2008 Reliability Diagram Probability of 2-m temperature in the upper tercile Day 19-25 N. Extratropics Europe 0.04 0.03 -0.06 -0.09 Slide 46: Reliability of the probability that 2-metre temperature anomalies are in the upper tercile for the period day 19-25 for the Northern Extratropics (left panel) and Europe (right panel). The red (blue) line represents the reliability diagram obtained with all the cases with an MJO (no MJO) in the initial conditions. The numbers represent the Brier Skill Scores. Only land points have been included in the calculation of the reliability diagram and the Brier skill scores. This figure shows that the MJO has a major source of predictability for the time range day 19-25 in the ECMWF forecasting system. MJO in IC NO MJO in IC

Impact of the MJO on Brier Skill Scores NDJFMA 1989-2008- N Impact of the MJO on Brier Skill Scores NDJFMA 1989-2008- N. Extratropics DAY 5-11 Z500 T850 Precip DAY 12-18 DAY 19-25 DAY 26-32 Slide 47: Impact of the MJO on the Brier skill score of the probability that Z500, T850 and precipitation anomalies are in the upper tercile over the Northern Extratropics (North of 30N). The red bars are for all the cases when there is an MJO in the initial conditions. The blue bars show the scores when there is no MJO in the initial conditions. This plot shows that the MJO has a significant impact on the Northern extratropical skill scores. MJO in IC NO MJO in IC

Conclusion SSTs, Soil moisture, stratospheric initial conditions and MJO are source of predictability at the intra-seasonal time scale. In particular the MJO has a significant impact on the forecast skill scores beyond day 20. Model improvements, particularly in simulating the MJO activity are likely to be beneficial for monthly forecasting. The monthly forecasting system produces forecasts for days 12-18 that are generally better than climatology and persistence of day 5-11. Beyond day 20, the monthly forecast is marginally skilful. For some applications and some regions, these forecasts could however be of some interest.