10.1007-s00382-010-0931-y

Hydro-climatic variability over the Andes of Colombia associatedwith ENSO: a review of climatic processes and their impact on oneof the Earth’s most important biodiversity hotspots

German Poveda • Diana M. Alvarez •

Oscar A. Rueda

Received: 25 October 2009 / Accepted: 11 October 2010 / Published online: 30 October 2010

� Springer-Verlag 2010

Abstract The hydro-climatic variability of the Colom-

bian Andes associated with El Nino–Southern 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. ENSO’s effects are phase-locked to the seasonal

cycle, being stronger during December–February, and

weaker during March–May. 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 ocean–atmo-

sphere–land 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 region’s 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 land–atmosphere 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, Medellın, Colombia

e-mail: [email protected]

D. M. Alvarez


Present Address:O. A. Rueda

Grupo HTM, Medellın, Colombia


123

Clim Dyn (2011) 36:2233–2249

DOI 10.1007/s00382-010-0931-y

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 region’s

sustainable development.

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

at providing a scientific framework to understand the

dynamics of the region’s hydro-climatic variability at

interannual 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 region’s 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

region’s 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 region’s hydro-climate

variability. Further sections quantify the effects of both

phases of ENSO on the variables making part of the

region’s 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. Deep

convection 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 10,000–13,000 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 (April–May and Septem-

ber–November), and low-rain seasons (December–Febru-

ary and June–August), mainly driven by the double passage

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

Mejıa 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

(June–August) 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; Mejıa 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

Colombia’s eastern Andes (Montoya et al. 2001), before

2234 G. Poveda et al.: Hydro-climatic variability over the Andes of Colombia associated with ENSO

123

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 40–50 day intra-seasonal oscillation (Poveda

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

easterly waves during the boreal summer-autumn are

known to affect precipitation regimes over different

regions of Colombia (Martınez 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 at

interannual 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 space–time

consistence is discussed in Sects. 2–4. Section 5 summa-

rizes the physical mechanisms of the region’s ocean-

atmosphere–land 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

1972–1998 period at diverse

raingauges located in the central

Andes of Colombia, within the

the 1�150N–7�460N latitudinal

band. From Poveda et al. 2001c

G. Poveda et al.: Hydro-climatic variability over the Andes of Colombia associated with ENSO 2235

123

http://www.cdc.noaa.gov/enso/

cooperative effects of rainfall, evapotranspiration, soil

moisture and infiltration in river basins, which contribute

altogether to filter out rainfall’s high frequency variability.

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

ENSO’s 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 1958–1994. The SOI corre-

sponds to the traditional (Standardized Tahiti–Standard-

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 ENSO’s 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 1972–1999, with no more

than 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

00

10

10

10

10 20

-40

-30

-30-20-10-30

-30

-30

-20

-20

-10

-10

0

0

10

10

-10

00

0

20

0

20 -40

-40

-20

-50

20

30

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

at 88 raingauges, for the 1958–1998 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

1958–1994. From west to east: Ansermanuevo, La Bella, Cabrera,

Monterredondo and Ramiriquı. 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


123

http://www.cpc.ncep.noaa.gov/data/indices/

http://www.cpc.ncep.noaa.gov/data/indices/

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 1972–1999. Error bars are not shown for clarity


123

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. Such

strong 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 at

more 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; 6�200N, 75�100W),

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; 4�460N, 75�540W), 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 1958–1996


123

http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml

http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml

2.3.2 ENSO’s 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 1958–1998 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 December–February

(DJF).

2.3.3 Different flavors of El Nino-related anomalies

The relationship between ENSO and the region’s 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

1970–2000 period. Good quality data sets were provided by

IDEAM. 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 estimation

of 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 1960–2006 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 (5�000N, 75�360W, 1,425 m a.s.l.) on the the Central

Andes of Colombia during two consecutive extreme phases

of ENSO: El Nino 1997–98, and La Nina 1998–2000. 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; 4�450N, 75�500W) during ENSO phases: La Nina

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

corresponds to 1958–1998


123

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, secondrow MEI in June–August (JJA) and river flows in JJA and ensuing

seasons, third row MEI in September–November (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. Filledcircles denote statistically significant correlations with respect to the

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


123

Fig. 9 Time evolution of standardized anomalies at four noted river gauging stations, during past El Nino events depicted with different colormarks, for the previous year (-1), onset year (0), and following year (?1). The black thick line denotes average of anomalies


123

less rainy seasons (July–September 1997 and December

1997–March 1998), soil moisture reached minimum values

owing to 1997–1998 El Nino. During the ensuing

1998–2000 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 Nino (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 1970–2000


123

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

region’s 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 (0.73–1.1 lm), and Red

is the reflected radiation in the red band (0.55–0.68 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 1981–November

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 aerosol

effects, atmospheric water vapor effects, and cloud cover.

Estimates of lagged seasonal correlations between the

Southern Oscillation Index (SOI) and NDVI data for the

July 1981–November 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

December–February (DJF), and September-November

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

Amazonia during March–May (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

Niño Niña Normal

Data Data Data

Dis

char

ge(m

3 /s)

Fig. 11 Log-Normal probability distribution functions for annual

floods of the Negro River at Colorados (Cundinamarca; 5�30N,

74�340W), fitted for the three phases of ENSO. Study period is

1960–2006


123

from diverse physical mechanisms co-operating at the

region’s ocean–atmosphere–land 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 moisture advection 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

(5�000N, 75�360W, 1,425 m

a.s.l.) along with their sample

frequency histograms. Panelsa, 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)


123

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 December–February, as noted by

Rasmusson and Mo (1993) during 1982–1983,

1986–1987, and 1991–1992 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 east–west 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 December–February (DJF) versus NDVI in DJF and ensuing

seasons, second row SOI in March–May (MAM) and NDVI in DJF

and ensuing seasons, third row SOI in June–August (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


123

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 land–atmosphere 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 and

Mesa 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 (25–50%) 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 over

the 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 surface–atmosphere feedbacks. The Nino 3 and

Nino 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 Andes

from 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.


123

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 Colombia’s 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 other

physical 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 the

tropical 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 Hidrologıa, Meteoro-

logıa y Estudios Ambientales de Colombia (IDEAM), Empresas

Publicas de Medellın (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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