Rainforest-Initiated Wet Season Onset Over the Southern Amazon

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Jonathon Amazon the southern over onset season wet Rainforest-initiated rainfall vulnera- regional enhances and drought. and to season defor- onset bility dry how season the understanding wet extends for of estation framework timing mechanistic the a interac- alter provide which may and convection, by burning the atmospheric biomass processes, mechanisms of surface efficiency the land the among highlight tions alter results may burning Our of SCMP. by biomass produced arrival season Aerosols the dry (ITCZ). before moisture late Zone mo drives Convergence 2–3 turn Intertropical onset the increase in season rapid wet which a and convection, for convergence deep scale regional rain-bearing the in at atmosphere the precon- (SCMP) season ditions pump dry-to-wet moisture the convection shallow of This destabilizes stages transition. initial and enables independent the moistens during transpiration that atmosphere multiple convection the rainforest shallow use that of we increase show the an Here, to to responds datasets rainfall. merely satellite of or unclear contributes cycle remains drive 2017) it 3, to transpiration seasonal Amazonia, January helps review over that for rainfall transpiration (received for 2017 established whether 14, water June the well approved of and is CA, much Jolla, La it Diego, San Although at California of University Thiemens, H. 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John , 05%o einlrain- regional of ∼30–50% et ´ 04%o Amazonia, of ∼30–40% oooi yaiu,Isiu ireSmnLpae 55 ai,Fac;and France; Paris, 75252 Laplace, Simon Pierre Institut Dynamique, eorologie ´ | oso onset monsoon c ui Chakraborty Sudip , 1073/pnas.1621516114/-/DCSupplemental at online information supporting contains article This 2 Submission. 1 Direct PNAS a is article This interest. paper. of the conflict wrote R.F. no and declare J.S.W. authors and The data; analyzed L.Y. and Y.S., C.R., N.E.C., S.C., ae nraayi rdcsta obn vial observations overwhelmingly available been combine the that primar- have products from late topic reanalysis humidity on advection this the based on or tropospheric whether studies transpiration Previous is lower rainforest ocean. from answered in derives be increase ily must season that dry question first onset the ET-initiated an under onset season mechanism). conven- wet moisture a surface (delaying reduce under also fluxes would onset but season mechanism), temper- onset wet tional land–ocean (accelerating the gradient For sharpen rainfall. ature of might cycle deforestation seasonal land the moistening example, how affect which burning understanding biomass by for The and processes use implications 24). the profound (23, DCMP and have the moisture occurs activating this to of key source likely low- the (pressures the as atmosphere of emerges the Moistening of (21). km season 4 transition est the humid- in tropospheric late lower until rises rare ity is season convection dry deep the but during (22), strongest are incursions front Cold unclear. (DCMP). pump moisture transition convective this deep to the refer convection, as We deep onset. mechanism season favor wet moisture to that leading Large-scale conditions ultimately Atlantic. the tropical reinforces the transport from mois- initiating transport thereby ture (21), heating upper-level midlati- deep and in from increases convection large-scale northward cause could moving (20) humid America fronts South this tude cold of Lifting by 19). air (18, near-surface buoyancy increase and may humidity transpiration air rainforest surface in increases season dry late uhrcnrbtos ..dsge eerh ...promdrsac;JSW,J.R.W., J.S.W., research; performed J.S.W. research; designed R.F. contributions: Author onl nvriy taa Y14853. NY Ithaca, University, Cornell rsn drs:Sho fItgaiePatSine oladCo cecsSection, Sciences Crop and Soil Science, Plant Integrative of School address: [email protected]. Present Email: addressed. be should correspondence whom To n h oeta o lmt n aduecagst le or alter to region. this changes in use onset land season and wet affirm- climate disrupt season, for dry potential late the the ing during rates transpiration depends high thus on pump” tran- moisture the convection during “shallow This convection sition. shallow for pri- source the moisture as mary transpiration rainforest identify unequivocally ture mois- atmospheric as in fingerprints convection Isotopic proposed. deep previously activating directly than rather deep convection, regional-scale for atmosphere the preconditions that tion the convec- shallow over activates transition first Transpiration season Amazon. dry-to-wet southern the cen- initiating a in plays role season tral dry late the during that transpiration evidence rainforest observational compelling provides analysis This Significance b ocaiytemcaim novdi ciaigteDCMP, the activating in involved mechanisms the clarify To been have DCMP the activate that processes exact The eateto topei n cai cecs nvriyof University Sciences, Oceanic and Atmospheric of Department b ihlsE Clinton E. 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ENVIRONMENTAL SCIENCES than in observations (SI Text). Enhanced rainfall and associated heating in the atmosphere directly affect reanalysis estimates of ET and MFC, potentially confounding moisture source attribu- tions based on reanalysis products. In situ observations indicate that maximum ET leads the late dry season increase in rainfall (26–28); however, it has been unclear whether modest increases in ET can contribute sufﬁcient moisture above the ABL at regional scales. The potential inﬂuences of aerosols on the dry-to-wet season transition are an additional source of uncertainty (29), because the aerosol climatologies used by most reanalyses neglect or underestimate seasonal and interannual variations in aerosol loading in this region (30, 31). It is therefore necessary to examine the dry-to-wet season transition by using observable quantities.

Stages of the Dry-to-Wet Season Transition Fig. 2 shows the evolution of atmospheric, land surface, and vegetation properties over the southern Amazon during the transition season. Reanalysis ET and MFC are included for context; all other quantities are derived from satellite observations. Each Fig. 1. Distribution of land cover based on Moderate-Resolution Imaging time series is a sequence of 5-d means composited relative to Spectroradiometer observations from 2009. The southern Amazon (5◦S to wet season onset (day 0; Materials and Methods). Negative times 15◦S, 50◦W to 70◦W) is indicated by the solid white box. indicate days before wet season onset. Wet season onset typically occurs in middle October over the southern Amazon. Specific with numerical model simulations. These products are heavily onset dates are listed in Table S1. influenced by the behavior of the underlying model in data- We divide the transition season into three stages: a pretransi- poor regions like Amazonia. Inadequate treatments of surface tion (90–60 d before onset), an early transition (60–30 d before hydrology, vegetation, and turbulent mixing near the top of onset), and a late transition (30–0 d before onset). The pretran- the atmospheric boundary layer (ABL) lead to large uncertain- sition stage marks a turning point from seasonal drying to sea- ties in reanalysis estimates of ET, rainfall, and moisture flux sonal moistening. This turning point is reflected in column water convergence (MFC) (25). For example, the increase in rainfall vapor (CWV; Fig. 2C), but does not emerge in precipitation over the southern Amazon during the dry-to-wet season transi- until the following stage (Fig. 2A). Surface air temperature (Fig. tion occurs ∼2–3 wk earlier in the European Center for Medium- 2C) and reanalysis sensible heating (Fig. S1) increase during the Range Weather Forecasting Interim Reanalysis (ERA-Interim) pretransition, consistent with a brief rise in absorbed surface

A B

C D

E F

Fig. 2. Onset-relative low-pass filtered composites of area mean precipitation from Tropical Rainfall Measuring Mission (TRMM), ET and MFC from ERA- Interim, and net absorbed surface radiation from Clouds and the Earth’s Radiant Energy System (CERES) Synoptic Radiative Fluxes and Clouds (SYN1Deg) −1 (A); vertical distributions of RH (shading) and time rates of change in equivalent potential temperature (∂θe/∂t; contour interval 0.02 K d ) computed from Atmospheric Infrared Sounder (AIRS) observations (B); surface air temperature and column water vapor (CWV) from AIRS (C); low (<700 hPa; ∼3 km above sea level), midlow (700–500 hPa; ∼3–5.5 km), midhigh (500–300 hPa; ∼5.5–10 km), high (>300 hPa; ∼10 km), and total cloud cover from CERES SYN1deg (D); solar-induced chlorophyll fluorescence (SIF) for rainforests from the Global Ozone Monitoring Instrument 2 and fire emissions of CO2 from Version 3.1 of the Global Fire Emissions Database (E); conditional instability in the lower–middle troposphere (θe850 − θe500) based on AIRS and best-fit linear slopes of δD against specific humidity (q) in the free troposphere based on TES (F). Shaded areas in A and C–F and error bars in A and E illustrate estimated uncertainties. Data sources, quality control criteria, and uncertainty calculations are provided in SI Text.

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Downloaded by guest on September 29, 2021 Downloaded by guest on September 29, 2021 n ceeae ntelwrtoopeeadbgn openetrate to begins and Moisten- troposphere increas- S3). lower the (Fig. an in convection accelerates of for ing development environment favorable the in ingly decreases confirm 2F and Condi- inhibition (CAPE) (Fig. 2D). convective energy grow (Fig. potential cover to available convective cloud continues by low instability indicated in tional increases as continued 2C) intensifies, and (Fig. convection temperature positive shallow destabi- strong air Surface continued but atmosphere. with plateaus, the along of photosynthesis, lization rainforest and aerosols. to due in heating air radiative moist of or mixing convection, radiative upward heat shallow and clouds, latent low both as with such associated of heating processes, tropospheric local exports lower from Amazon in arise net Increases therefore southern 2A). with (Fig. entire stage, moisture the and pretransition almost tropospheric the over lower (via during divergent The moistening transport). are or moisture winds advection) lateral thermal or or upward heating diabatic in (via changes Positive grow. day to bility after Positive troposphere occur. lower to the convection in moist for potential greater 2F in (Fig. troposphere difference middle the the as tropo- instability lower the conditional θ in quantify occurs We condensation if sphere. convec- triggered which be under may instability, tion conditional indicate height with Contours (θ temperature pretransition. 2B the Fig. during in begin stability atmospheric during conditions. dry increase with consistent emissions 2E), (Fig. burning frequent phase more this Biomass and and growth cumulus. ABL ABL imply shallow also the 2D) low into (Fig. in Increases cover air 2B). cloud (Fig. troposphere tropospheric free the free into air ABL dry moist mixing day- the ABL, deepens heating time sensible enhanced that suggest (RH) ity increases loading also aerosol (see as assumed reverses is often that and 2A) precipitation. occurs (Fig. as condensation radiation removed which immediately in in ABL, is detail the and in of parcel described top air are approximate rising models the idealized a from These in ascent parcel. occurs adiabatic the condensation moist in either which reversible remain representing in represent to mass lines ABL, air blue the moist and of a green top and Dotted troposphere approximate free the season from dry distillation the of Rayleigh representing behaviors masses air joint (green; dry The ET four (D). local between condensation) +90) (no to mixing 0 represent (day lines season wet the (day of stage pretransition the during observations rgte al. et Wright 3. 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ENVIRONMENTAL SCIENCES A values of δD. The evolution of moisture sources in the free troposphere over the southern Amazon can therefore be diagnosed by using linear fits of δD against specific humidity (q). These linear fits have larger positive slopes when the primary moisture source is upward mixing of ET, and smaller or negative slopes when the primary moisture source is large-scale transport. Fig. 3 shows distributions of TES measurements of lower tropospheric q and δD during the three stages of the transition and B the first 90 d of the wet season. Joint variations of q and δD pre- dicted by several theoretical models are also shown for context. These theoretical models are organized into two groups. The first group (green lines) corresponds to a local ET moisture source (δD = −30 ). The second group (blue lines) corresponds to an ocean evaporationh moisture source (δD = −80 ). The models correspond to dry mixing, reversible moist adiabatich ascent, and Rayleigh distillation (33, 37). The dry mixing model represents C intermixing of two air masses without condensation, whereas the other two models track the evolution of a single undilute air parcel undergoing lifting and condensation (see SI Text for details). During the pretransition (Fig. 3A), the peak of the joint distribution based on TES observations matches expectations for the subtropical free troposphere: dry air with low δD descending from the upper troposphere mixed with air rising from the trop- D ical ocean ABL (33). However, the distribution also contains a moist tail that is highly enriched in deuterium and can only be explained by upward mixing of local ET. The q-δD slope during the pretransition is large and positive, as expected for the case in which moistening is dominated by upward mixing of transpired water vapor. The entire distribution shifts upward and toward the right during the early transition (Fig. 3B), indicating moister air with larger δD. This distribution, which corresponds to the peak activity of the SCMP (Fig. 2B), is consistent with a dom- inant local ET moisture source. The q–δD slope for this stage remains large and positive, but is slightly reduced from the pretransition because dry, depleted observations are less common. Fig. 4. Distribution of specific humidity (Left) and δD(Right) in the free tro- The center of the distribution during the late transition is sim- posphere based on TES observations during the pretransition (day −90 to −60) (A), early transition (day −60 to −30) (B), late transition (day −30 to ilar to that during the early transition, but with an increased 0) (C), and early wet season (day 0 to +90) (D). Winds at 850 hPa (Left) and vertically integrated MFC (Right) based on ERA-Interim are also shown for each stage of the transition. The contour interval for MFC is 1 kg m−2 d−1.

of water vapor. Stable water isotopes are valuable tracers of the origin and history of air masses. Molecular differences among 16 common isotopes (such as H2 O and HDO) cause fractionation during most phase transitions: Heavier isotopes (HDO) prefer- 16 entially condense, whereas lighter isotopes (H2 O) preferen- tially evaporate (33). Evaporation from the ocean surface and condensation during transport both deplete the deuterium content of water vapor relative to its source. By contrast, no fractionation occurs during steady-state transpiration, so that the mean isotopic composition of transpired vapor is virtually identical to that of soil water (34, 35). Here, we evaluate the joint evolution of water vapor and its deuterium content (δD; Materials and Methods) in the free troposphere (750–348 hPa) over the southern Amazon using satellite observations (36). The atmosphere over the southern Amazon has two main moisture sources: rainforest ET and ocean evaporation (10). Under thermodynamic equilibrium conditions, δD in tropical ocean evaporate has an isotopic composition of approximately −70 to −80 . Estimating the isotopic composition of soil water as that of localh rainfall (SI Text), δD for rainforest ET is approximately −20 to −40 (Fig. S6). We can therefore treat ocean evaporation and rainforesth ET as isotopically distinct moisture Fig. 5. Changes in the vertical profiles of RH (∆RH) associated with sources, with rainforest ET relatively enriched in deuterium. If shallow convection under clean conditions [Cloud condensation number con- free tropospheric moistening is dominated by upward mixing of centrations (CCN) ≤ 500 cm−3], moderate aerosol pollution (500 cm−3 < local ET, then the largest specific humidities in the free tropo- CCN ≤ 1,000 cm−3), and heavy aerosol pollution (CCN > 1,000 cm−3). CCN, sphere will be associated with the highest values of δD. If moist- RH profiles, and convective occurrence are based on observations collected ening is instead dominated by transport from oceanic sources, during the Green Ocean Amazon (GOAmazon) field campaign (SI Text). ∆RH then the larger specific humidities will be associated with lower is reported in absolute differences.

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Tech- with this of contract Institute in California a Laboratory, described under Propulsion nology, Jet research Fellowship the the Science by out of Space carried was Part and NNX13AN95H. Earth Grant NASA of AGS-0937400; Department Program and the Grant Project; NNX09AD85G; GOAmazon Foundation Grant TsinghuaEnergy Program Science Team at Science National Aura 41350110225) NASA with (Grant Scientists along Young University; Research International China of for Foundation Sciences Fund Natural National and fellowship ents ACKNOWLEDGMENTS. in provided are data factors of confounding models, descriptions potential theoretical and Detailed calculations, uncertainty (50). criteria, variables control quality both sources, in uncertainties and of fits uals Linear retrievals. these on details additional mzndmi,dfie steae one y5 by southern bounded the area over 70 the averages to as spatial defined area-weighted domain, are Amazon fields analyzed All Methods and Materials needed be other would possibility. investigation in this further apply evaluate but to also Amazon, may the mechanisms of parts onset ET- season season). wet transition early initiated (the rainfor- August–October suppressing during by ET onset during est delay could recharge Atlantic moisture North tropical soil Ni Our El deep with 48). south- associated reduced (47, March–May the that unclear in imply are activity August–October results fire during forest Amazon and ern onset season March–May but during wet conditions (46), influence oceanic Ni Oscillation which by El Multidecadal mechanisms the Atlantic the like the patterns and climate Oscillation large-scale to uted of for development. constraints understanding and valuable evaluation detailed provide model would more budget isotopic A over water (45). season the changes Amazon wet climate projected southern control in that the discrepancies lead- mechanisms large 44), the and (43, of onset convection misdiagnosis shallow to realistically and than ing to changes ET unable transition vegetation rainforest remain and dry-to-wet represent models use the Many land rainforest- (42), thought. convec- to region previously the sensitive shallow this more of of in be cover sensitivity may importance land strong the to the tion Given and (3). SCMP mediated season lengthen season and dry onset wet delay bolsters the could finding hastens This deforestation ITCZ. that initi- that the hypothesis of to processes the arrival the helps atmospheric before season mo the of rain- 2–3 dry onset the that chain late on shown a depends the have also during ate the region we ET this Here, on Rainforest in 4). season depends forest. dry (2, rainforest the season of Amazon dry length southern the of the length of fate The Implications e TS nteESAr aelt 4) see (49); satellite Aura EOS the on (TES) ter and (R Water Ocean and molecules R vari- remove δ S7). 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