The hydrologic cycle: a challenge to climate modelling

1. Annarita Mariotti ENEA, Italy and ESSIC, University of Maryland, USA The hydrologic cycle: a challenge to climate modelling

2. Outline • Background and climatologies • Relevance to climate variability • Observational and modelling challenges • Aspects of the variability in oceanic regions

3. Background and general relevance • Moisture is of critical importance for the Earth System: • for the water budget (effects on freshwater resources, river discharge and oceanic salinity/buoyancy) • for the heat budget (greenhouse effect of water vapor, latent heat moderation of temperature changes and redistribution of energy) • The hydrologic cycle has fundamental influences on the planet’s atmospheric and oceanic circulation and their coupling. • Its representation is essential in model simulations of climate variations.

4. Climatological Features mm/d ~80% of the global evaporation and ~70% of the Global precipitation occur over the oceans. Plotted: climatologies from ERA40 re-analyses, 1958-2000.

5. The Atmospheric Water budget ∂w/∂t + div(Qz) = E-P (water vapor conservation) E-P ~ div(Qz) (monthly timescales) ∂w/∂t Qz P E w, precipitable water; Qz=1/g∫qvdp; q specific humidity

6. Climatological Features Cross-equatorial atmospheric moisture flux North to South hemisphere in DJF and opposite in JJA. Plotted: E-P from ERA40 re-analyses; vertically integrated moisture flux from NCEP re-analyses (1958-2000).

7. Climatological Features Annual mean atmospheric moisture flux is from Southern to Northern Hemispheres, about 3Sv (Mehta et al., 2005).

8. Atlantic atmospheric water fluxes Basins draining in the Atlantic (solid grey), deserts (white), other basins (shaded).Arrows give direction of atmospheric transport (SV). Overall the Atlantic loses 0.32 Sv (Broecker, 1997).

9. Relevance to climate variability and modeling • Precipitation/diabatic heating, effect the structure of the storm-tracks and are key characteristics of tropical circulations, for example: • During ENSO events, precipitation/heating anomalies contribute to anomalous Walker cell/engendering Rossby waves • Changes in tropical precipitation and heating, related to trends in Indian Ocean SST, have possibly had far-reaching long-term effects (Sahel droughts, NAO-trends..).

10. Relevance to climate variability and modeling • Freshwater fluxes at the air-sea interface contribute to the interaction of atmosphere and ocean, for example: • At mid/high latitudes, changes in freshwater fluxes effect salinity and have potentially important effects on the Atlantic thermohaline circulation • Anomalous atmospheric moisture fluxes between the Pacific and Atlantic during ENSO events may play an important role in the inter-basin “atmospheric bridge”

11. Water cycle observations - challenges • Rainfall and clouds occur on small spatial and time scales • Over land, rain gauge observations suffer from “undercatch” especially in windy and snowy conditions • Remote sensing of frozen precipitation is still a challenge • Evaporation estimates rely on “bulk” aerodynamic formulas and assumptions to parameterize turbulent processes • Lack of observations, especially over the oceans and remote land regions; long-term datasets.

12. Water cycle observations-advances State-of-the-art for medium to large-scale climatic studies and model validation: • Precipitation: • Land-only global datasets: CRU TS2 (high resolution), PREC/L, GHCN, many cover the 20th century • Global, satellite derived: blended analyses (CMAP, GPCP), since 1979; GPROF (SMM/I based), since 1987. • Evaporation, ocean-only: • in situ: COADS-UWM, since 1960 • Satellite derived: GSSTF2 (SSMI/NCEP), 1988.

13. Water cycle modelling – challenges • In analyses, precipitation and evaporation are model dependent, and forecasts are affected by spin-up problems. • Tendency to rapidly adjust the moisture fields to be compatible with models moist physics, mainly moist convection • Overall, the global water budget is not closed • Region dependent biases exist: many models, tend to mis-represent ITCZ precipitation; storm-tracks are also problematic • Variability is often better represented

14. Water cycle modelling – Advances • Re-analyses have provided the first stable platform for comprehensive studies of the global water cycle. • At medium to large-scales, these have proven useful for water budget studies • On-going model improvements include changes in the convection schemes, cloud representation and land-surface runoff • More realistic E-P fluxes are being used in ocean simulations

15. DJF Precipitation climatology Climatologies for the period 1979-2000

16. JJA Precipitation climatology Climatologies for the period 1979-2000

17. Aspects of the variability in oceanic regions

18. ENSO and the hydrological cycle

19. ENSO and the hydrological cycle Regression of with an ENSO index over the period 1948-2002, NCEP re-analyses. .

20. The North Atlantic Oscillation NAO + NAO - Images courtesy Martin Visbeck • Dominant mode of climate variability in the Atlantic in winter (van Loon & Rogers, 1972) • Seesaw of atmospheric mass between subtropical high and subpolar low (Walker and Bliss, 1932) • Controls the position of the jet stream and the characteristics of the storm tracks (Hurrell, 1995)

21. Objectives and questions • Characterize oceanic precipitation anomalies associated with the NAO based on state-of-the-art datasets • How do the observed precipitation anomalies relate to atmospheric circulation and SST anomalies? • Is there evidence of a connection with tropical climate variability? • What is the impact on the oceanic water budgets?

22. Datasets • Indices, 1950-2002: NAO-CPC (Barnston and Livezey, 1987); NAO-station (Jones et al., 1997); AO-CPC (Thompson and Wallace, 1998) • CMAP and GPCPv.2 merged analyses of precipitation based on satellite and gauge data (2.5°, monthly, 1979-2002; Xie and Arkin, 1997 and Adler et al., 2003). • PREC/O global precipitation reconstruction (for ocean, eofs of satellite estimates with coastal/island gauge observations; for land, OI of GHCN2 gauges; Chen et al., 2003). • NCEP/NCAR reanalyses (atmospheric circulation, precipitation and evaporation, 1948-2003; Kalnay et al., 1996). • ERA40 reanalyses (precipitation, 1957-2002; Simmons and Gibson, 2000). • GISST SST data.

23. Seasonal Patterns NCEP precipitation, CPC-NAO index, 1950-2002. From Mariotti and Arkin, 2006.

24. DJF pattern/various data Precipitation from NCEP, ERA40, PREC (1958-2001) and GPCP (1979-2002); CPC NAO index

25. DJF composite patterns Prec, shaded SSTs, shaded Atmospheric data from NCEP, SST from GISST, CPC NAO index, 1950-2002

26. “Key” regions of influence

27. Oceanic water budgets - DJF 15% P clim 34% (14%, 0.01Sv) 59%(65%, -0.028Sv) 10% P clim 51% (33%, 0.012Sv) 24%(28%, -0.003) Regressed precipitation anomalies for NCEP (black), ERA40 (red), PREC(green) and GPCP (blu)

28. Summary - I • The NAO has robust manifestations in North Atlantic Ocean precipitation (“quadrupole” pattern) and circulation during winter (explained variance is over 50% in the high-lat and sub-trop Atlantic) • In summer, the signal present (“tripole”), but weaker and displaced northward (greater sensitivity to the choice of the NAO index) • Intriguing link between NAO variability and the ITCZ in the western tropical Atlantic via anomalous easterlies. • Some connection to precipitation variability in the eastern Indian Ocean, opposite in DJF and JJA, but significance is weak.

29. Summary - II • In the sub-tropical and high-latitude North Atlantic in winter interannual precipitation anomalies have been 15% and 10% of climatology per unit change of the NAO • Fresh water variations have been -0.028Sv and 0.012Sv respectively. • Decadal changes in the NAO index have been associated with drier (wetter) than usual winter conditions in the high-lat eastern Atlantic (Labrador) in the 1960s and late 1970s, with an opposite situation since the early 1980s • In summer the North Sea/Baltic region has been drier during the period 1965-1975 with the NAO being positive.

30. _______________________________________________________ DJF Key region Domain P Correl. PME Correl Tropical Atlantic [60°E,40°E;12°N,23°N] 0.59(34%)/0.58/0.61 0.37(14%)/0.42/0.37 Sub-trop. Atlantic [40°E, 5°E;30°N,45°N] 0.77(59%)/0.51/ 0.85 0.81(65%)/0.76/0.85 High-lat. east. Atl. [35°E,15°E;55°N,65°N] 0.72(51%)/0.80/0.74 0.58(33%)/0.77/0.54 Labrador Sea [67°E,57°E;55°N,72°N] 0.49(24%)/0.78/ 0.49 0.53(28%)/0.74/0.59 _______________________________________________________ JJA Key region Domain P Correl. PME Correl. Mediterranean Sea [10°W,27°W;37°N,45°N] 0.56(31%)/0.51/0.610.60(36%)/0.64/ 0.64 North Sea/Baltic Sea [20°E,35°W;50°N,60°N] 0.65(42%)/0.83/0.690.66(43%)/0.86/0.68 Sub-polar Atlantic [5°W,20°W;65°N,74°N] 0.32(10%)/0.52/0.330.36(13%)/0.54/0.38 Eastern Indian [95°W,120°W;13°S,5°S] 0.49(24%)/0.60/0.440.44(19%)/0.57/0.39 Table 2

31. _______________________________________________________ DJF Key region P Clim ΔP PME Clim ΔPME Tropical Atlantic 2.37 0.38 / 0.018 -3.89 0.22 / 0.010 Sub-trop. Atlantic 2.70 -0.44/-0.033 -0.86 -0.37/-0.028 High-lat. east. Atl. 3.39 0.38/ 0.018 0.47 0.25 / 0.012 Labrador Sea 1.40 -0.19/-0.003 0.28 -0.21 /-0.003 _______________________________________________________ JJA Key region P Clim ΔP PME Clim ΔPME Mediterranean Sea 1.53 0.32 / 0.008 -1.08 0.28 / 0.007 North Sea/Baltic Sea 2.35 -0.30 /-0.019 0.04 -0.22 /-0.014 Sub-polar Atlantic 1.32 0.10 / 0.001 0.02 0.10 / 0.001 Eastern Indian 4.17 -0.59 /-0.026 -0.77 -0.61 /-0.027 Table 3 [For comparison discharge from the former Nile is 0.0027Sv; from the Amazon is about 0.15Sv; Mediterranean evaporative sink 0.07Sv].

32. Sensitivity of the North Atlantic thermoaline circulation Stability diagram for North Atlantic Deep Water (NADW) flow as function of freshwater input (precipitation and runoff minus evaporation). The location of the present climate is not exactly known, but models generally locate it in a regime where 2 equilibria, with and without NADW formation, exist. The diagram shows that increasing the freshwater input beyond S will shut down NADW flow, while restarting it requires the freshwater input to be reduced to near zero. This diagram is based on general circulation model experiments (see Rahmstorf [1996] for details)

33. Winter rainfall and the NAO seasonal correlation NCEP reanalyses 1948-98 CRU data 1948-96

34. Mediterranean Sea water budget Annual mean components: atmosphere: E-P ~ D sea: D-R-B ~ G

35. NAO index CPC NAO black (filtered, grey) AO green STAT NAO red Table 1

36. JJA pattern/various indices NCEP precipitation and SLP, various NAO indices, 1950-2002

37. Oceanic water budgets - JJA 31%(36%, 0.07Sv) 42%(43%, -0.014Sv) 10%(13%, 0.001Sv) 24%(19%, -0.027Sv) Regressed precipitation anomalies for NCEP (black), ERA40 (red), PREC(green) and GPCP (blu)

38. Climatological Mediterranean fresh water flux Annual mean E-P~700mm/yr E from COADS/UWM, P from CMAP, R from our derivation

39. Fresh water flux variability and the NAO NCEP reanalyses, 5yr running means +300 mm/yr Potential implications for: • Mediterranean sea level • Atlantic circulation

40. JJA composite patterns Atmospheric data from NCEP, SST from GISST, CPC NAO index, 1950-2002