Presentation on theme: "What controls the seasonality of the Amazon rainfall and its interannual variations? How strong it interacts with land surface? Rong Fu and Wenhong Li."— Presentation transcript:
What controls the seasonality of the Amazon rainfall and its interannual variations? How strong it interacts with land surface? Rong Fu and Wenhong Li Earth & Atmospheric Sciences, Georgia Tech. Amazon climate and hydrology workshop, Duke, May 9-10, 2005
Length of dry season: Sombroek 2001 Malhi 2004 Ecosystem and agriculture of the Amazônia depend more on the length and rainfall of the dry season than on wet season rainfall. Yet, most of the meteorological studies have been focus only on the wet season rainfall. Regions with dry season rainfall < 100mm could potentially be converted to Savanna given enough of deforestation - Steinberg 2001
Outline: Processes that controls the wet season onset – How do land surface, moisture transport and extratropical influence wet season onset? Mechanisms that control the interannual variation of the wet season onset – - Why is the relationship between Amazon rainfall and SST anomalies so complex? The influence of Amazonian rainfall on tropical and North Atlantic atmospheric circulation (NAO, ITCZ) Key uncertainties and future challenges
Seasonal Cycle: Kousky 1979; ; Marengo et al. 2000 Kousky 1979; Horel et al. 1989; Marengo et al. 2000 What causes rapid increase of rainfall across broad latitudes during austral spring (non-ITCZ like)?What causes rapid increase of rainfall across broad latitudes during austral spring (non-ITCZ like)? Why is the northward withdraw of the rainy area (ITCZ-like) more gradual during austral fall?Why is the northward withdraw of the rainy area (ITCZ-like) more gradual during austral fall?
Change of seasonal cycles is a main source of interannual variation of annual rainfall: Liebmann and Marengo 2001 Marengo et al. 2001 JulDec What cause such strong variations of wet season onset?
What control the changes of rainfall and wet season onset in the western Amazon? Ronchail et al. 2002 El Nino related El Nino+cold SATL Oceanic influences cannot be clearly detected in the western Amazon.
Key issues-1: What is the primary forcing of the wet season onset? – Evaluated heating is most efficient to drive overturn of the large-scale circulation! Surface fluxes over Altiplano: Schwerdtfeger 1961; Gutman & Schwerdtfeger 1965 Amazonian convection drives the wet season circulation: e.g., Silva Dias et al. 1983, Kleeman 1989; Gandu & Geisler 1991; Lanters & Cook 1995; Convection increases prior to the onset of monsoon circulation: Horel et al 1989 Is Amazon convection a forcing or a result of seasonal reversal of the large-scale circulation? What causes increase of convection over Amazon during the transition from dry to wet season?
Key issues-2: What are the relative roles of land surface flux and moisture transport in determine the seasonality of the rainfall, esp. the wet season onset? – Local Recycling: e.g., Salati et al. 1979, Shuttleworth 1988, 50% – Transport from Atlantic: e.g., Gibbs 1979; Marengo 1992; Rao et al. 1996; Mechanisms that explain the various precipitation maxima.. are all apparently linked to either large-scale features … or to other local and mesoscale forcings. They do not appear to depend, to a first approximation, on type of underlying vegetation. Yet, there is a wealth of observational evidence showing that evapotranspiration accounts for more than 40% of the precipitation. Salati and Nobre 1991 Is land surface important to the wet season onset?
Key issues-3: What cause the sudden increase of rainfall across broad latitudes (~20˚) in western and southern Amazon? What is the role of cold fronts incursion? – Strong cold surges account for about 50% of the total summertime precipitation south of 25 S, about 30% over the western Amazon basin. (Kousky 1979; Kousky and Ferreira 1981; Marengo et al. 1997; Garreaud and Wallace 1998; Garreaud 1999; Garreaud 2000a; Vera and Vigliarolo 2000; Vera et al. 2002) Could cold fronts trigger the wet season onset given adequate large-scale thermodynamic conditions over the Amazon? Convective Clouds associated with cold front incursions (Garreaud & Wallace 1998) cold SummerWinter
Data Sets: Data: 15-year (1979-1993) pentad – ECMWF reanalysis data (4-times a day, 2.5 lat x 2.5 lon, 17 pressure levels) – Rain gauge data (daily): National Water and Electric Energy Agency of Brazil (ANEEL). – GPCP precipitation data (daily, 2.5 lat x 2.5 lon) – TRMM daily rainrate – Radiosonde: 7 years – ABRACOS flux tower data, 1992 and 1993 Domain (5-15S 45-75W): Onset Sept. 10 to Oct. 1. Define the wet season onset: – The pentad before which rain rate is less than 6.1 mm day –1 during 6 out of 8 preceding pentads and after which rain rate is greater than 6.1 mm day –1 during 6 out of 8 subsequent pentads (Marengo et al. 2001; Liebmann & Marengo 2001) gauge GPCP Marengo et al. 2001 ERA15 Li & Fu 2004
initiatingdevelopingOnset Kinetic energy conversion function V-index 850mb Equivalent potential temperature change with time mm/day K/day m/s pentad 10 -6 (m 2 s -3 ) ECMWF 15-yr composite results gauge GPCP onset 1000hPa45W 15S 200hPa 45W 5S Buoyancy, Li and Fu 2004 Rainfall begins to increase before the transition of the circulation. Thus, it may provides elevated heating and initiates circulation transition.
e/ t e/T*( T/ t+L c /C p * q/ t) T/ t L c /C p * q/ t e/ t What causes increase of air buoyancy near surface? How does it influence convective instability? CAPE CINE initiating pentad K/day kJ/kg maturing height temperature Positively buoyant, CAPE Negatively buoyant, CINE Increase of air humidity dominates the increase of air buoyancy near surface Increase air buoyancy rapidly reduce the convecitve inhibition energy (CINE) prior to the occurrence of moisture convergence. LFC
Sensible heat Latent Heat Net Radiation Downward Solar at surface Onset LH: W/m2 W/m2 SH: W/m2 Latent flux increase leads to higher surface air buoyancy Increase of air buoyancy
1000hPa 75W 15S 45W 15S 100hPa 45W 5S Increase of surface air buoyancy peaks prior to the moisture convergence during the initial phase of the transition. Moisture convergence Change of surface buoyancy Surface latent flux Onset Land surface latent flux initiates the transition. Initialdevelop
Wind (u,w) and Relative humidity(shaded) 10 o S onsetmaturing initiating developing onset maturing Pressure(hPa) Longitude Init.Develop. onsetmature Humidity begins to increase prior to the increase of moisture transport.
Aug - Dec. Jan-Mar. Aug. Sept - Mar. wetdry drywet Fu et al. 1999, J. Climate In contrary to Asian monsoon, circulation transition to the wet season is NOT driven by land-ocean temperature reversal in the upper troposphere. It is driven by convection in Amazon. Observations: 7 years radiosonde data
How is transition initiated and accelerated? Elevated heating Upper troposphere high forms
Synoptic episodes induce increase of rainfall across broad latitude during the transition from dry to wet season. TRMM daily rainrate, averaged over 5 days Aug. 1- Nov. 12, 1998-2001
What is the role of cold front incursions? Cold front index: SLP among the top 10% for SON season, SLP1018 hPa, T sfc >8˚C (similar to Garreaud 2000) JunDec rainfall
Rainfall associated with cold air incursion Li and Fu, 2005, Composite for 15 transition seasons (1979-93) Rainfall increases in in western Amazon 2 days after the cold front passes 25˚S and move northward into Amazon. The spatial pattern is very similar to that of rainy area during the wet season onset.
1 day before cold events 1 day after cold events Under what condition can cold fronts trigger wet season onset? Composite of all cold events prior to the wet season onsets Composite of all cold events that triggered wet season onsets. Readiness of the large-scale thermodynamic condition is central.
Withdraw Lack of extratropical influence over northern Amazon perhaps contribute to gradual northward withdraw of rainy area associated with the wet season demise.
Processes that control wet season onset: Increase of land surface latent flux initiates the rainfall increases. Increases of rainfall initiates moisture transport. Positive feedback between moisture transport and rainfall accelerates the circulation transition. Cold front incursions trigger the wet season onset when the atmosphere becomes sufficiently unstable.
YearOnset pentad Calendar dateAnnual mean rain (mm/day) 7949Aug 29-Sep 28.2 8057Oct 8-126.9 8159Oct 18-227.5 8268Dec 2-66.0 8360Oct 23-276.4 8473Dec 26-314.9 8561Oct 28-Nov 16.0 8673Dec 26-315.7 8764Nov 12-165.4 8863Nov 7-115.6 8963Nov 7-116.4 9060Oct 23-275.9 9162Nov 2-66.0 9266Nov 22-264.5 9360Oct 23-276.0 Early onset Late onset Normal onset Fu & Li 2004 Interannual variations of the wet season onsets - Can we explain them? Late onset
1979-early 1986-late 1984-late 1990-norm Early onset: more unstable (lower CINE, higher CAPE) in dry season Later onset: more stable (higher CINE, lower CAPE) in dry season Fu&Li, 2004, LBA special issue Influence of thermodynamic condition:
Normal Early late Early Onset: Higher air buoyancy and humidity in dry and transition season Late Onset: lower air buoyancy and humidity in dry and transition season
Influence of land surface fluxes: 1979-early 1986-late 1984-late 1990-norm Early onset: lower Bowen ratio in dry season Later onset: higher Bowen ratio
1984-late1990-norm The differences in atmospheric circulation: Differences during transition are not as clear as those of land surface. Abnormally dry/wet land surface during dry season delays initiation of the transition, can strongly delay/accelerate wet season onsets (e.g., 1984 and 86, 1979). 1979-early1986-late
Influence of cold air incursion: a. CINE (kJ/kg) b. CAPE (kJ/kg) 1983: normal 1979: early 1979 1982: late, 1984 Atmospheric instability cannot explain late onset in 1982. 1982: air is more unstable than normal 1983, but with delayed wet season onset.
Sept.DecOct. cold events Lack of cold air incursion as a trigger appears to delay wet season onset, even though the atmospheric thermodynamic condition was ready. Late onset normal onset
What might influence the cold air incursion? Stronger subtropical jets in 1982 may suppress cold front incursions. ENSO and S. Atlantic SSTs can influence the subtropical jet (e.g., Horel and Wallace 1981, Grimm et al. 2000). 1982Clim
Can we explain interannual variations of the wet season onsets?
YearOnset pentad Calendar dateAnnual mean rain (mm/day) 7949Aug 29-Sep 28.2 8057Oct 8-126.9 8159Oct 18-227.5 8268Dec 2-66.0 8360Oct 23-276.4 8473Dec 26-314.9 8561Oct 28-Nov 16.0 8673Dec 26-315.7 8764Nov 12-165.4 8863Nov 7-115.6 8963Nov 7-116.4 9060Oct 23-275.9 9162Nov 2-66.0 9266Nov 22-264.5 9360Oct 23-276.0 Early onset: Wetter land surface & stronger cold air incursion Late onsets: drier pre- seasonal land surface Li and Fu 2005 Late onset: weaker cold air incursions may be due to El Niño
Land surface (soil moisture & vegetation Extratropical Influence (cold fronts incursions) Pacific and Atlantic Influences (cross- equatorial flow) Why is interannual change of rainfall/wet season onset so complex? These factors can either work for or against each other to influence the wet season onset.These factors can either work for or against each other to influence the wet season onset. Importance of land surface cannot be represented by correlation and its fraction in wet season rainfall.Importance of land surface cannot be represented by correlation and its fraction in wet season rainfall.
soil moisture/ Vegetation Memory and feedback Key uncertainties in understanding rainfall seasonality and its climate changes: Soil moisture/vegetation memory and feedback: How would soil moisture& vegetation memory & feedbacks affect subsequently dry and transition season?How would soil moisture& vegetation memory & feedbacks affect subsequently dry and transition season? To what extent can soil/moisture feedback mitigate or amplify the externally forced rainfall variability?To what extent can soil/moisture feedback mitigate or amplify the externally forced rainfall variability? Interference between various external forcings: ENSO+NAO, ENSO+SALT on cold air incursion ENSO+NAO, ENSO+SALT on cold air incursion Human influences: biomass burning, land use ENSO Atlantic Cold air Onset Biomass burning Land use Moisturetransport
Implications to climate changes: Reduce forest would delay wet season onset and prolong dry season, esp. in the areas where dry season is already 3-5 months, savannization appears to be highly probable. Changes in latitudinal SST gradient in South Pacific and Atlantic: subtropical jets, extratropical cold air incursions Nobre 2004
Remote Influence of Amazon Rainfall - The influence of Amazon rainfall on NAO Amazon rainfall can amplify NAO during boreal winters. – Data: QuikSCAT and TRMM for winters of 1999-2004.
- The influence of Amazon rainfall on Atlantic ITCZ : Day -3 Day -2 Day -1 Day 0 Day 1 Day 2 Day 3 Composite of Rainfall and Ocean Surface Wind Anomalies April 2000-2003 Eastward propagation Kelvin waves Phase speed: 10-12 m/s Wang & Fu 2005, Data: TRMM & QSCAT West Phase East Phase
Future Challenges In current GCMs, dry season rainfall is too low in current climate to sustain rainforest.In current GCMs, dry season rainfall is too low in current climate to sustain rainforest. Duration of rain ~ 30 mm/mon: Obs. 3 months Models: > 5 months
Future Challenges Rainfall predictions for the 21st. century in the 11 models in CMIP-IPCC AR4: 4 models: increase rainfall significantly4 models: increase rainfall significantly 5 models: no significant rainfall change5 models: no significant rainfall change 2 models: decrease of rainfall significantly. HadCM3: stronger interannual changes.2 models: decrease of rainfall significantly. HadCM3: stronger interannual changes. How can we reduce the uncertainty in understanding and predicting long- term rainfall variabilities in past and future?