The Atlantic/Pacific moisture budget asymmetry: the role of atmospheric moisture transport

Transcription

1 The Atlantic/Pacific moisture budget asymmetry: the role of atmospheric moisture transport Philip Craig, David Ferreira, John Methven Lunchtime seminar Department of Meteorology, University of Reading 13 th March Copyright University of Reading

2 Motivation World Ocean Atlas Zweng et al. (2013) psu Greater sea surface salinity (SSS) in Atlantic than Pacific Up to 2 practical salinity units (psu) difference Atlantic SSS greater at all latitudes SSS asymmetry is known to be increasing (Durack & Wijffels, 2010) 2

3 The Atlantic Meridional Overturning Circulation (AMOC) Greater Atlantic SSS linked to AMOC Deep water formation in the Nordic Seas Dense water sinks in North Atlantic and upwells in the rest of the ocean Purple = upper ocean & thermocline. Red = denser thermocline & intermediate water. Orange = Indian & Pacific Deep Waters. Green = North Atlantic Deep Water. Blue = Antarctic Bottom Water. Gray = Bering Strait components & Med./Red Sea inflows Talley (2013); Updated from Talley et al. (2011), based on Schmitz (1995), Rahmstorf (2002), and Lumpkin and Speer (2007). 3

4 How does the AMOC/SSS asymmetry link to the hydrological cycle? Net evaporation across Atlantic, Pacific moisture budget is approximately balanced Basin widths (Schmitt et al., 1989): Fraction of ocean affected by dry air from continents Mountain ranges (Schmittner et al., 2011): Higher mountains block moisture transport Therefore higher Atlantic SSS, stronger AMOC See reviews by Weaver et al. (1999) and Ferreira et al. (2018) 4

5 Freshwater budget evaporation minus precipitation minus runoff (E P R) 2 ways to calculate this: Atmospheric moisture budget and river runoff Ocean freshwater transport 5

6 Freshwater budget evaporation minus precipitation minus runoff (E P R) 2 ways to calculate this: Atmospheric moisture budget and river runoff Ocean freshwater transport Annual mean ( ) ERA- Interim vertically-integrated moisture flux divergence: Net evaporation: E P > 0 Net precipitation: E P < 0 Use river runoff dataset (Dai & Trenberth, 2002) 6

7 Freshwater budget evaporation minus precipitation minus runoff (E P R) 2 ways to calculate this: Atmospheric moisture budget and river runoff Ocean freshwater transport Ganachaud & Wunsch (2003), J. Climate 7

8 Moisture budget asymmetry 1 Sv 10 6 m 3 s kg s 1 Various estimates basinintegrated: E P R (evaporation minus precipitation minus runoff) Atmospheric and oceanographic data Estimates remarkably consistent (within error bars) Atmospheric data Craig et al. (2017) Tellus A Oceanographic data BS = Bering Strait 8

9 Which part of E P R asymmetry Craig et al. (2017) Tellus A is most important? e, p and r are area averaged fluxes Evaporation more important at high latitudes Higher SSTs due to AMOC (Warren, 1983; Czaja, 2009) South of 20 N precipitation is more important Stationary eddies (Wills & Schneider, 2015) Runoff dominant in tropical Atlantic due to Amazon & Congo SSS asymmetry still holds! 9

10 Moisture transport Most of asymmetry explained by precipitation, so moisture transport is important ERA-Interim annual mean ( ) vertically-integrated moisture fluxes (arrows) & total column water vapour (colours) 10

11 Values beside arrows calculated from: All values in Sv. Values in boxes represent P E for entire drainage basin Note the change of sign! Divergence theorem: 11

12 Typical understanding Atlantic/Pacific asymmetry focuses on Central America (Broecker, 1991) Isthmus of Panama This interpretation doesn t take other fluxes into account: 1. Transport across 35ºS 2. Exchange with Arctic 3. Zonal mean zonal moisture flux, qu 12

13 1. Moisture transport across 35ºS Moisture flux per unit length: Atlantic = Sv/m Pacific = Sv/m Indian = Sv/m Atlantic is approx. 1/3 more efficient at transporting moisture across 35ºS, Indian is the least efficient q v stronger in South Atlantic 13

14 2. Moisture exchange with Arctic Pacific sector Export across Alaska/Canada balances import across Asia No net moisture exchange between Pacific and Arctic Therefore Atlantic loses 0.11 Sv more moisture to Arctic Atlantic sector Export between Greenland & Russia (0.08 Sv) close to import across Canada (0.09 Sv); net import of 0.1 Sv Net moisture exchange between Atlantic & Arctic dominated by zonal transport across Europe (0.12 Sv) 14

15 3. Zonal mean moisture flux Integrated flux across Americas quite close to [qu] Africa slightly less so South-East Asia long way off qv n [qu] Solid lines: qv Dashed lines: [qu] Americas Africa SE Asia Zonally anomalous moisture flux

16 Indian Real world: SE Asia Americas Africa Pacific Atlantic 0.16 Sv 0.23 Sv 0.16 Sv P E = Sv 0.50 Sv P E = 0.00 Sv P E = Sv P E = 0.02 Sv P E = Sv P E = Sv 0.50 Sv 0.36 Sv P E = Sv P E = Sv P E = Sv All basins net evaporative Area-averaged P E very similar! Approx Sv/m² Note that transports to Arctic/Southern Oceans unchanged 16

17 Moisture transport summary Departures from asymmetry [qu] are most important in setting moisture budget Moisture flux across Americas approximately matches Flux across Africa is 0.2 Sv less than [qu] Flux across South-East Asia is of opposing sign to [qu] [qu] Moisture exchange with Arctic and transport across 35ºS play a secondary role CMIP5 models (Levang & Schmitt, 2015) and NCEP reanalysis (Rodriguez et al., 2011) give opposite sign across SE Asia to ERA- Interim lower resolution? (Schiemann et al., 2014; Chaudhari et al., 2014) Possible role of monsoon and Walker Circulation in asymmetry? Geometry of ocean drainage basins/continents important? 17

18 Moisture origins Moisture transports between oceans basins typically understood as direct exchanges between neighbouring basins This ignores any remote moisture sources Aim to partition moistures fluxes into their origin catchments (Atlantic, Pacific, Indian, Arctic, Southern): qv n = F Atl + F Ind + F Pac + F Arc + F Sou + F Strat + F None 18

19 Methodology Trajectory model (Methven, 1997); RK4; offline trajectories; ERA-Interim Back trajectories released from catchment boundaries Origins defined by last rapid interaction with surface (de Leeuw et al., 2017) CAT I: boundary layer exit, well mixed in θ v CAT II: mixing in cloud layer, well mixed in θ e de Leeuw (2014) PhD thesis Also include stratospheric origin (PV>3 or θ > 380 K) 19

20 Trajectory length: proportion of moisture flux 30 day back trajectories, 17 model levels ( ) Released from American catchment boundary every 12 hours, July 2010 Positive (eastward/northward), Negative (westward/southward), & Net moisture fluxes 14 day trajectories Proportions of (a) trajectories, (b) magnitude of moisture flux explained with time: net, CAT I, CAT II, stratosphere CAT I is BL origin CAT II is mixing in cloud layer 20

21 14 day back trajectories released every 12 hours (00Z & 12Z) All 9 catchment boundaries (points spaced approx. 75 km apart) (101.5 million trajectories) 17 evenly spaced model levels ( ) Model set-up Interpolate attributes along trajectories: q, pressure, PV, temperature, height, BL height Also output u,v winds & surface pressure at t=0 Blue: η levels Red: trajectory z 21

22 Flux-weighted density maps Kernel density estimation weighted by qv n dp Americas Africa South-East Asia Red is positive (eastward/northward) Blue is negative (westward/southward) 22

23 Americas Positive flux is eastward/northward; negative flux is westward/southward drainage basins catchment boundary Entering Atlantic Entering Pacific Westward flux with Atlantic origin is dominant Atlantic-to-Pacific transport occurs in tropics Peak over Lake Nicaragua (Papagayo jet) SSTs in Atlantic Warm Pool (Wang et al., 2013) Remote sources are negligible! 23

24 Africa Positive flux is eastward/northward; negative flux is westward/southward Entering Indian Entering Atlantic Westward flux with Indian Ocean origin is dominant Indian-to-Atlantic transport occurs in tropics Peak over Kenya (Turkana jet) 24

25 South-East Asia Positive flux is eastward/northward; negative flux is westward/southward Entering Pacific Entering Indian Eastward flux with Indian Ocean origin is dominant Indian-to-Pacific transport occurs over Thailand Monsoon? 25

26 South-East Asia: seasonal cycle 26

27 South-East Asia: seasonal cycle Indian Ocean origin Pacific Ocean origin Indian-to-Pacific transport peaks in JJA Pacific-to-Indian transport minimum in JJA Emile-Geay et al. (2003, JGR Oceans) Annual mean ( ) seasonal ERA- Interim vertically-integrated moisture fluxes (arrows) and total column water vapour (contours) 27

28 Summary Atlantic/Pacific moisture budget asymmetry largely a result of precipitation asymmetry (Craig et al., 2017) Zonally anomalous moisture transport across SE Asia the main reason for this Secondary roles for transport across Africa, and moisture exchange with Arctic/Southern Oceans Trajectories can be used to partition moisture flux into origin ocean basins: Transport across Americas/Africa dominated by westward transport Transport across SE Asia dominated by anomalous eastward transport Seasonal cycle shows this is a result of the Asian Monsoon 28

29 Bonus Slides 29

30 Trajectory length: moisture residence time Many moisture tracing studies use 10-day back trajectories (e.g. Nieto et al., 2007; Drumond et al., 2008; Vazquez et al., 2017) Based on global mean residence time of water vapour in atmosphere (Trenberth, 1998; Numaguti, 1999): ω is precipitable water, P is precipitation, λ 1 is the depletion constant λ 1 = 8 10 days; global mean, varies a lot (van der Ent & Tuinenberg, 2017) Is 10 days long enough for this application? van der Ent & Tuinenberg (2017) 30

31 Broecker (1991, Oceanography) 31

32 Rodriguez et al. (2011, Clim. Dynam.) 32

33 Levang & Schmitt (2015, J. Climate) 33

34 Singh et al. (2016, GRL) 34

35 35

36 Pacific origin Atlantic origin 36

37 37

38 Atlantic origin Indian origin 38

39 SODA Atlantic Warm Pool JJA SST 39