Poveda Alvarez Rueda

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    Hydro-climatic variability over the Andes of Colombia associatedwith ENSO: a review of climatic processes and their impact on one

    of the Earths most important biodiversity hotspots

    German Poveda Diana M. Alvarez

    Oscar A. Rueda

    Received: 25 October 2009 / Accepted: 11 October 2010

    Springer-Verlag 2010

    Abstract The hydro-climatic variability of the Colom-

    bian Andes associated with El NinoSouthern Oscillation(ENSO) is reviewed using records of rainfall, river dis-

    charges, soil moisture, and a vegetation index (NDVI) as a

    surrogate for evapotranspiration. Anomalies in the com-

    ponents of the surface water balance during both phases of

    ENSO are quantified in terms of their sign, timing, and

    magnitude. During El Nino (La Nina), the region experi-

    ences negative (positive) anomalies in rainfall, river

    discharges (average and extremes), soil moisture, and

    NDVI. ENSOs effects are phase-locked to the seasonal

    cycle, being stronger during DecemberFebruary, and

    weaker during MarchMay. Besides, rainfall and river

    discharges anomalies show that the ENSO signal exhibits a

    westerly wave-like propagation, being stronger (weaker)

    and earlier (later) over the western (eastern) Andes. Soil

    moisture anomalies are land-cover type dependant, but

    overall they are enhanced by ENSO, showing very low

    values during El Nino (mainly during dry seasons), but

    saturation values during La Nina. A suite of large-scale

    and regional mechanisms cooperating at the oceanatmo-

    sphereland system are reviewed to explaining the identi-

    fied hydro-climatic anomalies. This review contributes to

    an understanding of the hydro-climatic framework of a

    region identified as the most critical hotspot for biodiver-sity on Earth, and constitutes a wake-up call for scientists

    and policy-makers alike, to take actions and mobilize

    resources and minds to prevent the further destruction of

    the regions valuable hydrologic and biodiversity resources

    and ecosystems. It also sheds lights towards the imple-

    mentation of strategies and adaptation plans to coping with

    threats from global environmental change.

    Keywords Tropics Hydro-climatology Andes

    Colombia ENSO Biodiversity

    1 Introduction

    1.1 Threats from deforestation and biodiversity loss

    in the tropical Andes

    Colombia is located in northwestern South America amidst

    complex geographical and hydro-climatological features

    arising from its equatorial setting, in combination with: (1)

    strong topographic gradients of the three branches of the

    Andes crossing from southwest to northeast, (2) atmo-

    spheric circulation patterns over the neighboring tropical

    Pacific and Caribbean Sea, (3) its share of the Amazon and

    Orinoco River basins hydro-climatic dynamics, and (4)

    strong landatmosphere feedbacks.

    Since a decade ago, the tropical Andes have been

    identified as the most critical hotspot for biodiversity on

    Earth (Myers et al. 2000), or the region subject to the

    highest rates of biodiversity loss in the planet. Such situ-

    ation is caused by human encroachment, deforestation,

    land use/land change for agriculture, mining, and extensive

    cattle ranching. Current rates of deforestation amount to

    G. Poveda (&) D. M. Alvarez O. A. Rueda

    School of Geosciences and Environment,

    Universidad Nacional de Colombia, Medelln, Colombia

    e-mail: [email protected]

    D. M. Alvarez

    e-mail: [email protected]

    Present Address:

    O. A. Rueda

    Grupo HTM, Medelln, Colombia

    e-mail: [email protected]

    123

    Clim Dyn

    DOI 10.1007/s00382-010-0931-y

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    340,000 ha per annum (R. Lozano, pers. comm., 2010).

    Colombia is one of the top countries in biodiversity rich-

    ness worldwide, but the ongoing deterioration of the tropi-

    cal Andes constitutes an serious threat to the regions

    sustainable development.

    The purpose of this review is twofold. First, it aims

    at providing a scientific framework to understand the

    dynamics of the regions hydro-climatic variability atinterannual timescales, which are mainly controlled by the

    two phases of the El Nino/Southern Oscillation (ENSO)

    system: El Nino (warm phase) and La Nina (cold phase).

    This knowledge necessarily has to be taken on board to

    anticipating, mitigating and coping with the effects from

    global environmental change (Poveda and Pineda 2009),

    and their concomitant environmental, social and economic

    losses.

    Second, it constitutes a wake-up call for scientists and

    policy-makers alike, aimed at taking action and mobilizing

    resources and minds to set back the further destruction of

    the regions valuable hydrologic and biodiversity resour-ces. Such situation needs to be tackled from the political

    and institutional arenas, but also from the science of

    endangered ecosystems. Structural and non-structural

    measures, legislation, conservation programs and projects

    need to be implemented, based on scientific research of the

    regions hydrology and water resources (Poveda 2004a),

    atmospheric sciences, climatology, carbon and other trace

    gases budgets, biogeochemical cycles, atmospheric chem-

    istry, etc. Equally needed are studies about interactions

    between natural ecosystems and social systems, and on the

    increasingly relevant issue of compensation for ecosystems

    services, among others. Research must be funded to pre-

    vent further deforestation and degradation of the regions

    fragile but precious ecosystems. These tasks need to con-

    form a scientific program aimed at implementing decision-

    making tools and knowledge-based systems and actions,

    and public policies to face the urgent challenges brought

    about by deforestation and biodiversity loss over the

    tropical Andes.

    With the aim of providing a broader context, we review

    the main climatic features of the tropical Andes of

    Colombia, and provide a short literature review on the

    linkages between ENSO and the regions hydro-climate

    variability. Further sections quantify the effects of both

    phases of ENSO on the variables making part of the

    regions surface water balance.

    1.2 Hydro-climatology of the Colombian Andes

    In terms of the spatial distribution of precipitation Snow

    (1976) describes the Andes as a dry island in a sea of

    rain, but a detailed understanding of atmospheric

    dynamics and precipitation over the Andes covering a wide

    range of time and space scales is missing. The three

    branches of the Andes house a broad range of ecosystems

    and life zones including tropical rainforests, cloud forests,

    paramos, glaciers, dry forests, deserts, and large intra-

    Andean valleys in a predominant northerly direction.

    Rainfall over the Andes deserves a careful analysis,

    since the role of topography on the genesis and dynamics

    of weather patterns and rainfall cannot be overstated. Deepconvection developed over strong topographic gradients

    leads to deep convection that triggers highly intermittent

    and intense storms in space and time. Thereby, the space

    time distribution of rainfall over the tropical Andes exhibit

    quite a strong variability, evidenced by markedly different

    diurnal cycles even at nearby raingauges (Poveda et al.

    2005). The three branches of the Andes exhibit elevations

    surpassing 5,000 m, and house rapidly receding tropical

    glaciers on the verge of extinction (Poveda and Pineda

    2009), and long and skinny intra-Andean valleys. Extreme

    precipitation amounts are witnessed over the Pacific coast

    of Colombia, including one of the rainiest regions on Earth(averaging 10,00013,000 mm per year), which can be

    explained through ocean-atmosphere-topography interac-

    tions enhanced by the action of a low-level westerly jet

    (Poveda and Mesa 2000).

    On seasonal timescales, central and western Colombia

    experience a bimodal annual cycle of precipitation (Fig. 1)

    with marked high-rain seasons (AprilMay and Septem-

    berNovember), and low-rain seasons (DecemberFebru-

    ary and JuneAugust), mainly driven by the double passage

    of the intertropical convergence zone (ITCZ) (Eslava 1993;

    Meja et al. 1999; Leon et al. 2000; Poveda et al. 2007).

    Rainfall exhibits a uni-modal annual cycle (May

    October) at the northern Caribbean coast of Colombia and

    at the Pacific flank of the southern isthmus, reflecting the

    northernmost position of the ITCZ over both the continent

    and the eastern equatorial Pacific, respectively (Hastenrath

    2002; Poveda et al. 2006). Another single annual peak

    (JuneAugust) occurs at the eastern slope of the eastern

    Andes, resulting from the encounter of the moisture-laden

    trade winds from the Amazon with the Andes. The

    meridional migration of the ITCZ is strongly intertwined

    with other atmospheric phenomena, including: (1) the

    westerly low level Choco jet off the Pacific coast of

    Colombia (Poveda and Mesa 2000; Stensrud 1996; Mapes

    et al. 2003a, 2003b; Xie et al. 2008; Sakamoto et al. 2009),

    (2) mesoscale convective systems (Velasco and Frisch

    1987; Poveda and Mesa 2000; Meja and Poveda 2005), (3)

    the low level jet in the Caribbean trade winds (Poveda and

    Mesa 1999; Magana et al. 1999; Mestas-Nunez et al. 2005;

    Wang 2007; Munoz et al. 2008; Amador 2008), and (4) the

    easterly portion of the South American low level circula-

    tion embedding a low level jet, which influences the

    Colombias eastern Andes (Montoya et al. 2001), before

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    veering and heading southwesterly to southern South

    America and reaching to La Plata River basin (Marengo

    et al. 2004).

    At intra-seasonal time scales, the westerly and easterly

    phases of the 4050 day intra-seasonal oscillation (Poveda

    et al. 2005; Arias 2005), and the dynamics of tropical

    easterly waves during the boreal summer-autumn areknown to affect precipitation regimes over different

    regions of Colombia (Martnez 1993). At shorter time-

    scales, the diurnal cycle of maximum rainfall exhibits

    sharp differences even among nearby raingauges (Poveda

    et al. 2005), while hourly and 15-min rainfall exhibit

    fractal behavior in space and time (Hurtado and Poveda

    2009; Poveda 2010).

    1.3 ENSO-driven interannual variability

    El Nino/Southern Oscillation (ENSO) is the main forcing

    mechanism of interannual climate variability from hours to

    seasons to decades. In general, the warm phase of ENSO

    (El Nino) begins during the boreal spring, exhibiting a

    strong phase locking with the annual cycle, and encom-

    passing two calendar years characterized by increasing sea

    surface temperature (SST) anomalies during the boreal

    spring and fall of the onset year (Year 0), peaking in winter

    of the following year (Year ? 1). Anomalies then decline

    in spring and summer of the ensuing year (Year ? 1).

    Details of ENSO dynamics and their impacts worldwide

    can be found at http://www.cdc.noaa.gov/enso/.

    The hydro-climatic effects of ENSO in the tropical

    Americas have been investigated by Hastenrath (1976,

    1990), Hastenrath et al. (1987), Waylen and Caviedes

    (1986), Ropelewski and Halpert (1987), Aceituno (1988,

    1989), Kiladis and Diaz (1989), Marengo (1992), Poveda

    and Mesa (1997), Marengo and Nobre (2001), Ronchail

    et al. (2002); Poveda and Salazar (2004), Ropelewsky and

    Bell (2008), Grimm and Tedeschi (2009), Nobre et al.

    (2009), Misra (2009) and Xavier et al. (2010), among

    others. Physical mechanisms of ENSO-related hydro-

    climatic anomalies over the region are discussed by Poveda

    et al. (2006). In particular, the effects of ENSO on

    Colombia are studied by Poveda (1994, 2004b); Poveda

    and Mesa (1997), Gutierrez and Dracup (2001), Waylen

    and Poveda (2002), Poveda et al. (1999, 2001a, 2003,

    2006), Tootle et al. (2008), and Aceituno et al. (2009),

    among others.

    This work reviews a suite of hydro-climatic anomalies atinterannual timescales, with emphasis on the Colombian

    Andes during the extreme phases of ENSO. For estimation

    purposes, anomalies of precipitation, river discharges, soil

    moisture, and vegetation index (NDVI) are statistically

    linked with different ENSO indices, and their spacetime

    consistence is discussed in Sects. 24. Section 5 summa-

    rizes the physical mechanisms of the regions ocean-

    atmosphereland surface system that cooperate to explain

    the identified ENSO-driven hydro-climatic anomalies in

    the tropical Andes of Colombia.

    2 Precipitation

    2.1 EOF and correlation analysis

    Figure 2 shows iso-correlations between 3-month running

    means of sea surface temperature (SST) anomalies over the

    Indo-Pacific and the first Principal Component of monthly

    standardized records of 88 raingauges along the Andes of

    Colombia. High quality data, with very few missing

    monthly records were provided by IDEAM and Empresas

    Publicas de Medellin. The highest correlations appear over

    the Nino-4 and Nino-3 regions, but also over the eastern-

    most fringe of the Pacific Ocean by the tropical Americas.

    Nevertheless, correlations shown in Fig. 2 are lesser than

    those with the first Principal Component of river discharges

    in the Andes of Colombia (Fig. 8 of Poveda and Mesa

    1997), which evidences that ENSO signal is stronger for

    river flows and weaker in rainfall records. Such conclusion

    can be explained by the higher temporal persistence in

    the former ones, and a higher intermittency of rainfall in

    time, but also because river discharges result from the

    Fig. 1 Annual cycle of average

    precipitation during the

    19721998 period at diverse

    raingauges located in the central

    Andes of Colombia, within the

    the 1150N7460N latitudinal

    band. From Poveda et al. 2001c

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    http://www.cdc.noaa.gov/enso/http://www.cdc.noaa.gov/enso/
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    cooperative effects of rainfall, evapotranspiration, soil

    moisture and infiltration in river basins, which contribute

    altogether to filter out rainfalls high frequency variability.

    The said study of Poveda and Mesa (1997) showed that

    ENSOs influence on anomalies of river discharges

    appears earlier (later) over the western (eastern) Andes,proceeding in a wave-like westerly propagating fashion.

    Such conclusion was obtained through cross-correlation

    analysis between 3-month running means of the Southern

    Oscillation Index (SOI) and standardized monthly river

    flows. No explanation has been provided for such

    behavior. Here, we contribute towards that explanation by

    examining whether it is also the case for rainfall ano-

    malies. Figure 3 shows cross-correlations between 3-month

    running averages of the Southern Oscillation Index (SOI)

    and standardized rainfall records at five raingauges in

    Colombia, for the period 19581994. The SOI corre-

    sponds to the traditional (Standardized TahitiStandard-

    ized Darwin) sea level pressures, as is defined by the US

    Climate Prediction Center (http://www.cpc.ncep.noaa.gov/

    data/indices/). Cross-correlations indeed exhibit maximum

    values at earlier (later) time lags over the western (eastern)

    Andes, confirming the wave-like westerly-propagating

    ENSO signal on monthly rainfall anomalies. As a conjec-

    ture, such spatial rainfall dynamics could be attributed to

    the intra-seasonal oscillation over the region (Arias 2005),

    or by a combination of the direct effects of ENSO on sea

    surface temperatures over the eastern Pacific, thus weak-

    ening the strength of the winds of the Choco low level jet,

    and by long-distance teleconnections affecting the Amazon

    and eastern Colombia during both phases of ENSO, which

    are reviewed in Sect. 5.

    2.2 ENSOs effect on the diurnal cycle of rainfall

    Rainfall in the Colombian Andes exhibit clear-cut diurnal

    (24 h) and semi-diurnal (12 h) cycles, with seasonally

    shifting diurnal maxima, while peak hours are extremely

    sensitivity to raingauge location (Poveda et al. 2005). The

    effects of both phases of ENSO on the amplitude of the

    diurnal cycle are consistent throughout the Andes, as the

    hourly and daily precipitation decreases during El Nino,

    and increases during La Nina. For illustration, we used an

    hourly data set of 55 raingauges located on the Andes of

    Colombia, covering the period 19721999, with no morethan 5% of missing records. Figure 4 shows the diurnal

    cycle of rainfall at 19 selected raingauges over the northern

    Correlation map SSTs vs. PC No. 1 monthly rainfall in Colombia

    -10

    -10

    -10

    0

    0

    10

    10

    10

    10 20

    -40

    -30

    -30-20-1

    0-30

    -30

    -30

    -20

    -2

    0

    -10

    -10

    0

    0

    10

    10

    -1000

    020

    0

    20 -40

    -40

    -20

    -50

    20 3

    0

    Fig. 2 Iso-correlations (%) between sea surface temperatures and the first principal component of the Colombian standardized monthly rainfall

    at 88 raingauges, for the 19581998 period

    Fig. 3 Behavior of cross-correlations between 3-month running

    averages of the Southern Oscillation Index (SOI) and standardized

    rainfall at five raingauges in Colombia, estimated for the period

    19581994. From west to east: Ansermanuevo, La Bella, Cabrera,

    Monterredondo and Ramiriqu. Negative lags correspond to the SOI

    leading the hydrology, and the y-scale in each diagram goes from

    -1.0 to 1.0. Notice that the peaks of correlations ( P[ 0.95) occur

    later in raingauges located farther east

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    http://www.cpc.ncep.noaa.gov/data/indices/http://www.cpc.ncep.noaa.gov/data/indices/http://www.cpc.ncep.noaa.gov/data/indices/http://www.cpc.ncep.noaa.gov/data/indices/
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    Fig. 4 Average diurnal cycle of rainfall intensity at selected raingauges over the northern Andes during El Nino (red), and La Nina (blue). The

    study period corresponds to 19721999. Error bars are not shown for clarity

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    Andes, as well as the consistent effect of both phases of

    ENSO on the diurnal cycle of rainfall. Overall, a clear-cut

    decrease in rainfall intensity is witnessed during El Nino

    (red), and an increase and La Nina (blue).

    2.3 River discharges

    2.3.1 Average monthly river flows

    Figure 5 shows the evolution of bi-monthly averaged

    standardized anomalies of the Nare River at Santa Rita

    (Department of Antioquia) alongside the (negative) Mul-

    tivariate ENSO Index. Very good quality river discharges

    data set was provided by Empresas Publicas de Medellin. A

    statistically significant correlation of 0.60 indicates that El

    Nino (La Nina) is strongly associated with negative

    (positive) monthly river discharge anomalies. Seasonal

    correlations (next section) exhibit even larger values. Suchstrong association provides an excellent prediction tool of

    average monthly river discharges in Colombia (Poveda

    et al. 2003, 2008), and makes ENSO an excellent early

    warning system for multiple applied sectors in Colombia

    including disaster preparedness and mitigation, hydro-

    power generation (Poveda et al. 2003), agriculture (Poveda

    et al. 2001a), water supply, fluvial transport, infrastructure

    construction, and human health outcomes of malaria and

    dengue (Poveda and Rojas 1996; Poveda et al. 2001b),

    among others. In spite of that knowledge, the ongoing La

    Nina (October 2010) has affected more than 1,000,000

    people, causing 92 deaths and 122,000 flooded houses atmore than 400 municipalities in 28 out 32 Departments

    country-wide.

    The strong association between ENSO and river dis-

    charges anomalies are reflected in their probability distri-

    bution functions (PDF). Frequency histograms were

    estimated for different phases of ENSO, using the classi-

    fication defined by NOAA (http://www.cpc.noaa.gov/

    products/analysis_monitoring/ensostuff/ensoyears.shtml),

    and taking the hydrological year from June (Year 0) to May

    (Year ? 1). Figure 6 illustrates the frequency histograms

    for La Vieja River (Cartago, Valle del Cauca), with data

    from 1958 to 1996 provided by IDEAM. The identified

    changes in the PDFs of river discharges confirm the col-

    lapse of stationarity as one of the fundamental tenets in

    hydro-climatological time series analysis.

    Fig. 5 Simultaneous evolution

    of bi-monthly averaged

    standardized anomalies of the

    Nare River at Santa Rita

    (Antioquia; 6200N, 75100W),

    along with the negative of the

    Multivariate ENSO Index

    (MEI). The correlation

    coefficient is 0.60, statistically

    significant at 99%

    Fig. 6 Frequency histograms of monthly river flows of La Vieja

    River at Cartago (Valle del Cauca; 4460N, 75540W), during ENSO

    phases: Normal (top), El Nino (center), and La Nina (bottom). Each

    panel contains the estimated values of the sample mean (m) and

    standard deviation (sd). The study period corresponds to 19581996

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    http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtmlhttp://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtmlhttp://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtmlhttp://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml
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    2.3.2 ENSOs effects on the annual cycle

    ENSO affects the amplitude of the annual cycle, although

    not so the phase. Figure 7 shows the estimated average

    annual cycle of the Cauca River at Salvajina, during both

    phases of ENSO for the 19581998 period, estimated with

    monthly data provided by IDEAM. The amplitude increa-ses during La Nina, and decreases during El Nino, although

    the phase remains the same.

    Furthermore, the effects of ENSO on river flows vary

    with the seasonal cycle. Figure 8 shows estimates of sea-

    sonal lagged correlations between the Multivariate ENSO

    Index (MEI) and river discharges throughout Colombia,

    with very good quality data sets provided by IDEAM and

    Empresas Publicas de Medellin. In general, very large

    negative simultaneous and lagged seasonal correlations

    appear, in particular for the MEI during September

    November (SON) and river flows in DecemberFebruary

    (DJF).

    2.3.3 Different flavors of El Nin o-related anomalies

    The relationship between ENSO and the regions hydro-

    climatology is rather complex. Although both phases of

    ENSO exhibit robust dynamical features, they vary in

    duration, magnitude and timing (Trenberth 1997), and so do

    ENSO-related hydrological anomalies. Figure 9 shows the

    evolution of standardized discharge anomalies (averages

    depicted with thicker line) during past El Nino events, at

    four separated rivers in central Colombia, estimated with

    data provided by IDEAM. El Nino-driven hydrological

    anomalies differ in timing, amplitude and duration, although

    their averages (thicker line) exhibit the featured robust

    negative anomalies. Such behavior of El Nino-driven

    hydrologic anomalies demands continuous research to

    understand the physical mechanisms driving their relation-

    ship, which in turn can contribute to develop much better

    river discharges forecasting methods, a highly relevant task

    for planning and management of hydropower generation

    (Poveda et al. 2003 and 2008), among other sectors.

    2.3.4 Maximum annual and monthly flows

    Extreme hydrological events are also affected by both

    extreme phases of ENSO, such that in general, droughts

    (floods) are amplified during El Nino (La Nina). Figure 10

    shows the annual cycle of average maximum daily flows at

    diverse river gauges throughout Colombia, for the

    19702000 period. Good quality data sets were provided byIDEAM. Colors denote ENSO phases: El Nino (red), La

    Nina (blue). The hydrological year is considered from June

    (Year 0) to May (Year ? 1), as those months are the least

    impacted by the onset or demise of El Nino and La Nina. As

    in the case for average monthly flows, the annual cycle of

    average maximum daily flows indicates that ENSO effects

    are larger and felt earlier over the western Andes, whereas

    effects are smaller and felt later over the eastern Andes.

    We have discussed that ENSO imposes a non-stationarity

    and persistent dynamics in time series of river discharges,

    which invalidates the stationarity and independence

    hypotheses required by traditional probabilistic estimationof annual peak flows. Thus, estimation of floods needs to be

    conditioned on ENSO phase. Figure 11 shows the ENSO

    phase-dependant PDFs of maximum annual (hourly) river

    flows of the Negro River at Colorados (Cundinamarca),

    during the 19602006 period, estimated with the procedure

    introduced by Waylen and Caviedes (1986).

    3 Soil moisture

    Soil moisture plays an important role in tropical South

    America climate dynamics at seasonal and interannual

    timescales (Poveda and Mesa 1997), and makes part of

    ENSO-related hydro-climatic anomalies, owing to its role

    in controlling land surface-atmosphere interactions, through

    processes like evapotranspiration, latent heat and sensible

    heat fluxes, and atmospheric boundary layer dynamics.

    Soil moisture data gathered at different land-cover types

    over the tropical Andes show a remarkable dynamics at

    seasonal and interannual timescales. Soil moisture data

    consists in averaged daily records at Cenicafe research

    station (5000N, 75360W, 1,425 m a.s.l.) on the the Central

    Andes of Colombia during two consecutive extreme phases

    of ENSO: El Nino 199798, and La Nina 19982000. Data

    were gathered at three different land cover types: second-

    ary forest, sunlit coffee, and shade coffee. Data shows that

    annual and interannual (ENSO) cycles are strongly cou-

    pled. Figure 12 shows the time series of 10-day average

    soil moisture content for the three land cover types at

    20-cm depth, along with their sample frequency histograms.

    Values of 40-cm depth soil moisture contents (not shown)

    are a bit larger than those at 20-cm, due to the stronger

    effects of evapotranspiration at 20-cm depth. During the

    Fig. 7 Annual cycle of average flows of the Cauca River at Salvajina

    (Valle del Cauca; 4450N, 75500W) during ENSO phases: La Nina

    (blue), Normal (black), and El Nino (red). The study period

    corresponds to 19581998

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    Fig. 8 Estimates of seasonal lagged correlations between the MEI

    and river discharges throughout Colombia. First row MEI in March

    May (MAM) versus river flows in MAM and ensuing seasons, second

    row MEI in JuneAugust (JJA) and river flows in JJA and ensuing

    seasons, third row MEI in SeptemberNovember (SON) and river

    flows in SON and ensuing seasons, and fourth row MEI in December

    February (DJF) and river flows in DJF and ensuing seasons. Filled

    circles denote statistically significant correlations with respect to the

    circles shown at the bottom. From Poveda et al. 2002

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    Fig. 9 Time evolution of standardized anomalies at four noted river gauging stations, during past El Nin o events depicted with different color

    marks, for the previous year (-1), onset year (0), and following year (?1). The black thick line denotes average of anomalies

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    less rainy seasons (JulySeptember 1997 and December

    1997March 1998), soil moisture reached minimum values

    owing to 19971998 El Nino. During the ensuing

    19982000 La Nina, soil moisture did not exhibit the

    normal annual bi-modality, reaching saturation values

    throughout the whole year. Under sunlit coffee, soil

    moisture exhibited much more pronounced deficits than

    under shade coffee and forest, which indicates that the

    former is more prone to water stress. Thus, El Nino-related

    dry spells might be mitigated via land cover and land use,

    which is also a relevant conclusion bearing on the possible

    effects of climate change.

    A detailed analysis of the statistical parameters of

    10-day soil moisture time series indicates that:

    1. Estimated values of the mean, l, indicate that shade

    coffee exhibits greater soil moisture values than forest

    and sunlit coffee.

    2. Estimated values of the mean, l, and standard devi-

    ation, r, for sunlit coffee evidence larger dispersion

    Fig. 10 Annual cycle of average maximum daily flows during El Nin o (red), La Nina (blue) for selected rivers throughout Colombia. The

    abscissa axis denote the annual cycle from June (Year 0) to May (Year ? 1). The average study period corresponds to 19702000

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    and unimodal probability distribution functions (PDF).This behavior can be attributed to higher intermittency

    of soil moisture for sunlit coffee.

    3. Higher values of the standard deviation, r, and the

    kurtosis, j, for sunlit coffee indicate more extreme

    values of soil moisture, and therefore fatter tails,

    higher intermittency, a lower capacity of soil water

    retention, and bimodal PDFs.

    4. Soil moisture contents at secondary forest and shade

    coffee exhibit similar values and temporal behavior,

    with values around the mean, owing to a larger water

    regulation capacity. Water stress also depends on land-

    cover type.

    These observations show that soil moisture constitute an

    active key variable of climate variability during ENSO in

    the tropical Andes, owing to its strong control of evapo-

    transpiration, and therefore on recycled precipitation, as

    well as of percolation (Rueda et al. 2010).

    4 Vegetation activity (NDVI) as a surrogate

    for evapotranspiration

    Up to now we have reviewed the effects of ENSO on the

    regions precipitation, river discharges, and soil moisture

    dynamics. The effects on actual evapotranspiration should

    be quantified to cover all the variables involved in the

    surface water balance. Towards that end, we use the Nor-

    malized Difference Vegetation Index (NDVI) as a surro-

    gate measure for evapotranspiration. The NDVI is a

    satellite-derived index defined as the ratio of (NIR - Red)

    and (NIR ? Red), where NIR is the surface-reflected

    radiation in the near-infrared band (0.731.1 lm), and Red

    is the reflected radiation in the red band (0.550.68 lm).

    NDVI represents the photosynthetic capacity or photo-

    synthetic active radiation (PAR) absorption by green

    leaves, and therefore it is linked to evapotranspiration and

    plant growth. Theoretically, NDVI takes values in the

    range from -1 to 1, but the observed range is usually

    smaller, with values around 0 for bare soil (low or no

    vegetation), and values of 0.9 or larger for dense

    vegetation.The NDVI data set was obtained from the NASA Global

    Inventory Modeling and Mapping Studies (GIMMS NDVI)

    at 8 km spatial resolution during the July 1981November

    2002 period (Tucker et al. 2005). The GIMMS NDVI data

    set exhibits diverse improvements with respect to previous

    NDVI data sets, including corrections for: (1) residual

    sensor degradation and sensor inter-calibration differences,

    (2) distortions caused by persistent cloud cover in tropical

    evergreen broadleaf forests, (3) solar zenith angle and

    viewing angle effects, (4) volcanic aerosols; (5) missing

    data in the Northern Hemisphere during winter using

    interpolation; and (6) short-term atmospheric aerosoleffects, atmospheric water vapor effects, and cloud cover.

    Estimates of lagged seasonal correlations between the

    Southern Oscillation Index (SOI) and NDVI data for the

    July 1981November 2006 period are shown in Fig. 13.

    Redish colors represent high positive correlations, while

    bluish colors represent high negative correlations, indicat-

    ing that NDVI is strongly reduced during El Nino, but

    enhanced during La Nina. A detailed analysis of seasonal

    correlations evidences high positive simultaneous correla-

    tions (panels on the main diagonal), in particular during

    DecemberFebruary (DJF), and September-November

    (SON), although less over the eastern and southern parts of

    Amazonia during MarchMay (MAM). One season lagged

    correlations (above the main diagonal and left bottom

    panels) indicates that SOI in MAM exhibit high positive

    correlations with NDVI in JJA from north-east Brazil to the

    Andes, and high negative correlation between the SOI in

    JJA and NDVI in SON in northern South America. The

    SOI in SON exhibits high positive correlations with NDVI

    in DJF all over tropical South America.

    Figure 13 denotes the continental-scale effect of ENSO

    on vegetation activity over tropical South America.

    Assuming that NDVI is an appropriate surrogate for

    evapotranspiration, the observed correlations are in agree-

    ment with the observed anomalies in precipitation, river

    discharges, and soil moisture during both phases of ENSO

    over the region.

    5 Physical mechanisms associated with El Nino

    The previously discussed ENSO-related hydro-climato-

    logical anomalies in the tropical Andes of Colombia result

    Log-NormalDistribution

    0

    500

    1000

    1500

    2000

    2500

    3000

    0,0 0,2 0,4 0,6 0,8 1,0

    Non-excedenceProbability

    Nio Nia Normal

    Data Data Data

    Disc

    harge(m3/s)

    Fig. 11 Log-Normal probability distribution functions for annual

    floods of the Negro River at Colorados (Cundinamarca; 530N,

    74340W), fitted for the three phases of ENSO. Study period is

    19602006

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    from diverse physical mechanisms co-operating at the

    regions oceanatmosphereland surface system, summa-

    rized as follows:

    1. The reduction of the SST gradient over the eastern

    Pacific between El Nino 1 ? 2 region and the Colom-

    bian Pacific weakens the winds of the Choco jet, thus

    reducing moisture advection inland (Poveda and Mesa

    2000; Poveda et al. 2001a), as well as the number and

    intensity of mesoscale convective systems (MCS)

    (Velasco and Frisch 1987; Zuluaga and Poveda 2004;

    Mejia and Poveda 2005). In general, the opposite

    situation occurs during La Nina, with the concomitant

    intensification of the Choco jet winds and number of

    MCSs. Negative anomalies in moistureadvection by the

    Choco jet winds contribute to explain negative rainfall

    anomalies reported over central and western Colombia.

    2. Perturbations in the tropical atmospheric circulation

    patterns during El Nino lead to the establishment of an

    Fig. 12 Time series of 10-day

    soil moisture content under

    three different land cover types

    at Cenicafe research station

    (5000N, 75360W, 1,425 m

    a.s.l.) along with their sample

    frequency histograms. Panels

    a, b, and c correspond to 20-cm

    soil moisture at forest, shade

    coffee, and sunlit coffee,

    respectively. Statistical

    parameters of the series are

    shown at the bottom right of

    each panel as follows: mean (l),

    variance (r2), standard

    deviation (r), and kurtosis (j)

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    anomalous Hadley cell over tropical South America.

    The subdued ascent of moist air and associated

    reduction in convective precipitation explain the

    anomalously high surface pressure over the region,

    particularly during DecemberFebruary, as noted by

    Rasmusson and Mo (1993) during 19821983,

    19861987, and 19911992 El Nino events, and

    diagnosed by Yasunari (1987) and Aceituno (1988).

    Diverse characteristics such as position, and horizon-

    tal and vertical structure of thermal forcing over the

    tropical Pacific during boreal winters appear as

    important determinants of the phase and amplitude

    of ENSO-related anomalies over the tropical Ameri-

    cas (Ambrizzi and Magana 1999).

    3. Atmospheric pressure changes over tropical South

    America during El Nino contribute to the shift the

    centers of convection within the ITCZ over the

    eastern Equatorial Pacific towards the south-west of

    their normal positions (Pulwarty and Diaz 1993).

    4. It has been suggested that precipitation anomalies

    over the region during ENSO events are caused by an

    anomalous eastward shift of the Walker cell, which

    would produce an anomalous rising motion over the

    equatorial eastern Pacific and a sinking motion over

    the tropical Atlantic (Kousky et al.1984). Although

    there have been attempts to describe the entire zonal

    circulation of the tropics (Flohn and Fleer 1975;

    Wang 1987), it seems that the Walker cell is not well

    defined beyond the Pacific region (Hastenrath 1991,

    p. 210). It has been recognized that ENSO influences

    the large-scale eastwest and meridional circulations

    in the global tropics that have implications over

    Fig. 13 Estimates of seasonal lagged correlations between the

    Southern Oscillation Index (SOI) and the Normalized Difference

    Vegetation Index (NDVI) over tropical South America. First row SOI

    in DecemberFebruary (DJF) versus NDVI in DJF and ensuing

    seasons, second row SOI in MarchMay (MAM) and NDVI in DJF

    and ensuing seasons, third row SOI in JuneAugust (JJA) and NDVI

    in DJF and ensuing seasons, and fourth row SOI in September

    November (SON) and ensuing seasons. Correlations are quantified

    according to the color bar on the bottom. The study period is July

    1981 through November 2006

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    tropical South America (Misra 2008; Grimm 2003,

    2004).

    5. Weakened feedbacks between precipitation and sur-

    face convergence in tropical South America are

    associated with the aforementioned anomalies in the

    Hadley cell circulation (Numaguti 1993) and in the

    trade winds over the Caribbean. Also, during ENSO

    there is a large-scale anomalous upper-level diver-gence over continental tropical South America.

    6. The featured wave-like westerly propagation ENSO

    signal on hydrological anomalies cause a disruption

    of landatmosphere interactions, owing to the strong

    coupling between precipitation, soil moisture, vege-

    tation, and evapotranspiration anomalies (Nepstad

    et al. 1994; Jipp et al. 1998; Poveda and Mesa 1997;

    Zeng 1999; Poveda et al. 2001a; Poveda and Salazar

    2004; Nobre et al. 2009). A reduction in evapotrans-

    piration also contributes to diminish the amount of

    recycled precipitation. Diminished cloudiness pro-

    motes increased solar irradiance and surface tempera-tures, thus reinforcing dry conditions. Even in wet

    tropical climates, water stress can be imposed on

    tropical forests, as in the case of strong El Nino

    events (Oren et al. 1996; Marengo et al. 2008).

    7. Land surface-atmosphere feedbacks are important

    mechanisms to explaining anomalies in precipitation

    and upper level divergence over northern South

    America (Poveda and Mesa 1997; Misra 2009).

    8. The interannual anomalies in precipitation (Lau and

    Sheu 1988; Hsu 1994; Kousky and Kayano 1994) are

    associated with negative anomalies in soil moisture

    (Nepstad et al. 1994; Jipp et al. 1998; Poveda andMesa 1997; Fisher et al. 2008). The hydrological

    connection between soil moisture and river dis-

    charges validates the conclusions drawn from the

    isocorrelation maps shown in Fig. 8.

    9. Negative anomalies in evapotranspiration in tropical

    South America (Nepstad et al. 1994; Vorosmarty

    et al. 1996; Poveda and Mesa 1997; Malhi et al.

    2002; Meir et al. 2009; Phillips et al. 2009; Meir and

    Woodward 2010) lead to further precipitation defi-

    cits, as large proportions (2550%) of rainfall in the

    Amazon basin have been estimated as derived from

    evapotranspiration recycling (Shuttleworth 1988;Elthair and Bras 1994; Trenberth et al. 2003). This

    is a crucial aspect of the land-atmosphere feedback

    mechanisms during ENSO over tropical South

    America.

    10. Negative anomalies in evapotranspiration over tropi-

    cal South America during El Nino may also

    contribute to weaken the pumping effect of atmo-

    spheric moisture exerted by the Amazon forest, a

    physical mechanism put forward recently by

    Gorshkov and Makarieva (2007), and Makarieva

    et al. (2009).

    11. During the boreal summer of Year 0, the northeast

    trade winds intensify (weaken) during El Nino (La

    Nina). However, in concordance with the noted

    changes in surface pressures during the boreal winter

    (3), the winds weakens and even reverse in Year ? 1,

    triggering a change in sea surface temperatures overthe Caribbean and the tropical North Atlantic (Has-

    tenrath 1976; Curtis and Hastenrath 1995).

    12. SSTs positive anomalies and the strength of the trade

    winds over the Caribbean play an important role in

    decreasing the intensity and number of tropical

    easterly waves and tropical storms (Frank and Hebert

    1974; Gray and Sheaffer 1991), thus contributing to

    diminish precipitation over the Caribbean and north-

    ern South America, including Colombia.

    6 Final remarks

    The ENSO-driven hydro-climatic variability of the tropical

    Andes of Colombia at interannual timescales was

    reviewed. The strong seasonality of such an influence has

    been quantified on precipitation, average and extreme river

    discharges, soil moisture, and NDVI as a surrogate of

    evapotranspiration. Extreme phases of ENSO constitute the

    main driver of hydro-climatic anomalies, resulting from the

    combined effects of SSTs anomalies off the Pacific coast

    off Colombia, in addition to atmospheric teleconnections

    and land surfaceatmosphere feedbacks. The Nino 3 andNino 4 regions over the central tropical Pacific exhibit the

    highest correlations with rainfall in the tropical Andes.

    Seasonal cross-correlation analyses confirm that El Nino

    (La Nina) produces drier (wetter) than normal and more

    prolonged dry (wet) seasons in the Andes of Colombia.

    River discharge and rainfall data show that the effects of

    ENSO appear earlier (later) and stronger (weaker) over the

    western (eastern) Andes. Seasonal correlations indicates

    that ENSO indices become important and valuable tools to

    forecast many hydro-climatological variables in the region.

    We have also shown that soil moisture dynamics is a key

    component of climate variability over the tropical Andesfrom seasonal to interannual timescales. Both El Nino and

    La Nina affect the dynamics of soil moisture on the region,

    and their effect depends on land cover type. The coupling

    between the vegetation-soil system and land cover strongly

    modulates (space-) time hydro-climatic variability in the

    tropical Andes, and therefore El Nino-related dry spells

    might be ameliorated via land cover and land use. This in

    turn suggests an appropriate adaptation strategy to cope

    with the effects of climate change.

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    Besides ENSO, other macro-climatic phenomena affect

    the hydro-climatic variability of the tropical Andes. Among

    them, there are significant statistical correlations between

    the NAO and Colombias hydrology (Poveda et al. 1998),

    as well as with the PDO and sea surface temperatures over

    the tropical Atlantic (Poveda 2004b). The nonlinear inter-

    actions of such macro-climatic phenomena with the ITCZ,

    the Choco and Caribbean low-level jets, and with otherphysical mechanisms acting at intra-annual timescales

    (intra-seasonal oscillation, tropical easterly waves, etc.),

    coupled with land surface-atmosphere interactions produce

    the featured hydro-climatic variability pattern at the

    Colombian Andes. This knowledge contributes to improve

    hydro-climatic predictability, with important practical

    implications for agriculture, hydropower generation, fluvial

    transport, natural hazards and disasters, and human health

    outcomes in the region.

    Our study sheds light to understand how the interannual

    hydro-climatic variability could affect a suite of biological

    and ecological processes in two critical regions, namely thetropical Andes and the headwaters of the Amazon, a river

    basin of global hydro-ecological and biodiversity impor-

    tance. A fundamental research programme for the region

    will have to deal with implications for biodiversity and

    ecosystems functioning arising from feedbacks between

    global warming, deforestation, land use/land change, and

    the reviewed ENSO-related interannual hydro-climatic

    variability.

    Acknowledgments This research was supported by COLCIEN-

    CIAS and Universidad Nacional de Colombia through the GRECIA

    Research Programme. We thank Instituto de Hidrologa, Meteoro-loga y Estudios Ambientales de Colombia (IDEAM), Empresas

    Publicas de Medelln (EPM), and Cenicafe for providing hydrological

    data sets. NDVI data set was provided by C.J. Tucker and J. Pinzon

    from the NASA Goddard Space Flight Center. We are grateful to

    H.A. Moreno, O.O. Hernandez, C.D. Hoyos, V. Toro, A. Ceballos,

    and L.A. Acevedo for their help with some figures, and to Peter

    Bunyard, the Editor, Dr. Edwin K. Schneider, and the anonymous

    reviewers for their valuable comments and insights to improve the

    manuscript.

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    http://dx.doi.org/10.1029/2001JD000623http://dx.doi.org/10.1098/rstb.2007.0015http://dx.doi.org/10.1111/j.1469-8137.2010.03390.xhttp://dx.doi.org/10.1029/2008GM000882http://dx.doi.org/10.1029/2008GM000882http://dx.doi.org/10.1029/2008GM000882http://dx.doi.org/10.1029/2008GM000882http://dx.doi.org/10.1111/j.1469-8137.2010.03390.xhttp://dx.doi.org/10.1098/rstb.2007.0015http://dx.doi.org/10.1029/2001JD000623
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