Oceanography of the tropical Pacific Ocean Part 2 Historical changes and future projections

Transcription

1 Oceanography of the tropical Pacific Ocean Part 2 Historical changes and future projections A. Ganachaud 1, A. Sen Gupta 2, J. Brown 3, L. Muir 3, with contributions from J. Orr, S. Wijffels, K. Ridgway, M. Hemer, C. Maes, C. Steinberg, A. Tribollet, B. Qiu, J. Kruger 1 Oceanographer, Institut de Recherche pour le Développement, Nouméa, New Caledonia 2 Climate modeling expert, Centre for Climate Change Research, University of New South Wales, Sydney, Australia 3 Climate modeling experts, Centre for Australian Weather and Climate Research, CSIRO Wealth from Oceans National Research Flagship Hobart, Tasmania, Australia Rarotonga, Oct

2 Temperature trends trend ( C/50yr) Time series from an in situ sensor in Noumea Cravatte et al Guyennon et al. 2011, submitted Rarotonga, Oct

3 Temperature trends trend ( C/50yr) Extension of the Warm Pool (10yr average) Cravatte et al Rarotonga, Oct

4 Salinity trends trend ( C/50yr) Extension of the Warm Pool (10yr average) Cravatte et al Rarotonga, Oct

5 Outline State of the Ocean 1. Provinces 2. Currents 3. Warming and stratification 4. Vertical structure 5. Nutrients and oxygen 6. Nutrient supply processes 7. Waves and Sea Level Rise Observed and Projected changes Rarotonga, Oct

6 Ocean state: Trade Winds and Warm Pool Classical view : The Trade Winds pile up warm waters in the west Rarotonga, Oct

7 Ocean state: Oceanic provinces ("biomes") Rarotonga, Oct

8 Pacific Ocean circulation Ocean currents transport nutrients, oxygen and fish larvae Rarotonga, Oct 2011 Source: Fieux, M

9 Ocean state: currents North Equatorial Current Winds create two broad westward flows in the tropical Pacific South Equatorial Current Rarotonga, Oct

10 Ocean state: currents Winds create two broad westward flows in the tropical Pacific ITCZ and SPCZ influence on the wind field give rise to two eastward counter currents Rarotonga, Oct

11 Ocean state: currents Winds create two broad westward flows in the tropical Pacific ITCZ and SPCZ influence on the wind field give rise to two eastward counter currents Archipelagoes and coasts lead to strong north south coastal currents Rarotonga, Oct

12 Ocean state: Temperature Vertical Structure Temperature across the Equator 0m 100m 500m Rarotonga, Oct

13 Ocean state: Temperature Vertical Structure 0m 250m Presence of stratification in the thermocline 500m 0m 100m 0 C 10 C 20 C 30 C Temperatures 500m Rarotonga, Oct

14 Ocean state: Nutrients DEPTH Dissolved nitrate at 100m Nutrients are mostly depleted in the euphotic zone Replenishment by remineralization of marine snow Oceanic upwelling or mixing is needed to transfer them to the surface layer Similar features for phosphate & silicate Rarotonga, Oct

15 Ocean state: Oxygen DEPTH Dissolved Oxygen at 400m Oxygen is abundant near the surface and depleted near 400m Replenishment by high latitude atmospheric input and subsurface transport by ocean currents Rarotonga, Oct

16 Ocean state: how to supply the euphotic zone??? 1. Eddies 2. Upwelling (vertical current; east equator and some islands) 3. Internal tides 4. Mixed layer... against stratification Rarotonga, Oct

17 Ocean state: Eddies Small scale circulation generated spontaneously or by interaction between the largescale flow and land Rarotonga, Oct

18 Ocean state: Eddies Small scale circulation generated spontaneously or by interaction between the large scale flow and land Rarotonga, Oct

19 Ocean state: Eddies and land effects Small scales generated spontaneously or by interaction between the large scale flow and land Boundary currents Rarotonga, Oct

20 Ocean state: Eddies and land effects Small scales generated spontaneously or by interaction between the large scale flow and land Boundary currents Upwelling Rarotonga, Oct

21 Ocean state: Eddies and land effects Small scales generated spontaneously or by interaction between the large scale flow and land Boundary currents Upwelling Vertical mixing for internal tides Rarotonga, Oct

22 Ocean state: Mixed layer Seasonal variations of the mixed layer depth pumps deep nutrients towards the sunlit zone Rarotonga, Oct

23 Variability Global Warming PDO El Nino Seasonal Eddies Internal tides Rarotonga, Oct Courtesy J. Lefèvre, IRD

24 Global Warming What changes were detected over the past ~50 years? What do IPCC projections suggest? Rarotonga, Oct

25 Observed changes to the circulation Ocean currents SEC strengthens in the south subtropical gyre EAC strengthens Little change in the north Change in Sea Level (Roemmich, pers. Comm.) Rarotonga, Oct

26 Projected changes to the circulation Ocean currents SEC weakens on the equator EUC moves upward Eastward SECC weakens Little change in the subtropical gyres Changes 2100/A2 versus 2000 Average over 13 IPCC projections Rarotonga, Oct

27 Temperature: recent changes at the surface Tropical warming of 0.1 C/decade and Warm Pool expansion Climatological average Surface Temperature trend over past 50yr Cravatte et al., 2009 Warm Pool Rarotonga, Oct

28 Temperature: recent changes... and surface projections Warming ~2.5 C Weaker warming in the southeast Pacific Multi-model projected temperature change (13 IPCC models; A2/2100 versus ) Rarotonga, Oct

29 Temperature: recent changes... and surface projections "Warm pool" as per 28 C definition Interannual variability Multi-model projected temperature change added to 20th century observations Rarotonga, Oct

30 Temperature: recent changes at the surface Tropical warming of 0.1 C/decade and Warm Pool expansion Climatological average Surface Temperature trend over past 50yr Cravatte et al., 2009 Warm Pool Rarotonga, Oct

31 Temperature: recent changes at depths Warming ~1 C down to m Weaker warming or cooling below the thermocline > Enhanced stratification Temperature change over past 50yr (Durack & Wijffels, 2010) Contours are average temperature Rarotonga, Oct

32 Observed change in density with depth Nutrient supply recent changes... and projections Warm, light water o Surface water becomes lighter more than deep water, due to: Surface warming Surface freshening o Increased stratification inhibits mixing Cold, heavy water Courtesy A. S. Gupta Rarotonga, Oct

33 Temperature: recent changes... and projections More warming ~2 C down to 80m Weaker warming or cooling below the thermocline Even more stratification Multi-model projected temperature change (13 IPCC models; A2/2100 versus ) Rarotonga, Oct

34 Nutrient supply recent changes Observed changes in nutrient concentrations oover the past 20 years: Only two time series oone suggests a decrease, the second one no trend otoo few data to be conclusive! (need continued observations!) Phosphate concentration in the mixed layer (Watanabe et al. 2005) Rarotonga, Oct

35 Nutrient supply recent changes... and projections What controls nutrient concentration: obiological activity osupply to the euphotic (sunlit) zone from deep ocean: ostratification oocean currents oturbulence in the mixed layer oupwelling oeddies Phosphate concentration in the mixed layer (Watanabe et al. 2005) Rarotonga, Oct

36 Nutrient supply recent changes... and projections Projections (2100/A2): ostratification increases by 20-30%; especially in the Warm Pool othe winter mixed layer shallows by ~20m; oequatorial upwelling decreases, but region-wide 9 S-9 N upwelling remains constant. oequatorial UnderCurrent reinforced } } Suggest reduced nutrient supply and therefore biological activity Reinforcement possible in the PEQD (Polovina et al.) Rarotonga, Oct

37 Dissolved oxygen recent changes Stramma et al o More oxygen data than nutrient o Major decrease of dissolved oxygen in the remineralization zone with westward extension of the oxygen minimum zone (OMZ) Rarotonga, Oct

38 Dissolved oxygen recent changes... and projections Stramma et al Dissolved oxygen concentration is expected to continue to decline due to high latitude ocean warming Low oxygen areas are expected to expand Rarotonga, Oct

39 Two more actors: Waves and Sea Level Rarotonga, Oct

40 Wave climate Too few observations to determine CC trends The wave "climate" is related to ENSO and other climate signals Only few projections; not resolved by IPCC models Projected increase (or decrease) to significant wave height Wang and Swail, 2006 Rarotonga, Oct

41 Sea level rise As water warms it expands As ice-sheets and glaciers melt they increase ocean volume +20 cm over 100yrs Increasing sea level: Alters oceanic ecosystems/habitat Changes shape of coastlines Changes nature and extent of mangrove Rarotonga, Oct

42 Sea level rise +20 cm New published estimate suggest IPCC AR-4 was too conservative: +80 cm to +1.4m possible Regional deviations from global mean are of O(5cm) Rarotonga, Oct

43 Conclusions on the tropical Pacific Ocean Oceaniswarming, with enhanced signal in the upper m More stratification limits nutrient supply; Mixed layer reach is reduced Some ocean currents projected to change; Equatorial divergence region projected to shrink Dissolved oxygen decreases; ocean becomes more aciditic Sea level has risen by ~20 cm; Sea level could rise by another cm Rarotonga, Oct