Nino Years Research Articles

Asymmetric evolution of El Niño and La Niña: the recharge/discharge processes and role of the off-equatorial sea surface height anomaly

Observed oceanic and atmospheric anomalies in the tropical Pacific are analyzed to understand the symmetric and asymmetric characteristics between El Nino and La Nina evolutions. It is noted that the evolutions are largely symmetric for El Nino and La Nina prior to and in their mature phase, but become asymmetric afterwards. It is further demonstrated that this asymmetry is due to the fact that, on average, the discharge process associated with El Nino is stronger than the recharge counterpart associated with La Nina. The symmetric and asymmetric evolution for different phases of El Nino-Southern Oscillation (ENSO) is consistent with the associated recharge and discharge processes that are linked with the evolution of anomalous ocean surface current and gradient of sea surface height anomalies (SSHAs). It is suggested that the evolution of the meridional gradient of SSHAs in the central and eastern equatorial Pacific is different between El Nino and La Nina years due to relatively different rates of the SSHA changes on and off the equator. During the decay of a warm event, the equatorial SSHA quickly switches from positive to negative, meanwhile the off-equator areas are occupied by positive SSHAs. Such changes on the equator and in the off-equatorial regions make the concavity of the meridional SSHA favorable for growth and persistence of westward surface zonal current anomaly in the equatorial Pacific, then for the ENSO phase transition. During the decay of a cold event, the slow decrease of the negative SSHA on the equator combined with the disappearance of the negative SSHA in the off-equatorial regions makes the meridional concavity reverse easily and is unfavorable for the steady growth of the eastward surface zonal current anomaly and for the phase transition. Thus, the asymmetric evolution of ENSO is associated with both the atmosphere and ocean anomalies on the equator and off the equator.

Variability of the western Pacific warm pool structure associated with El Niño

Sea surface temperature (SST) structure inside the western Pacific warm pool (WPWP) is usually overlooked because of its distinct homogeneity, but in fact it possesses a clear meridional high–low–high pattern. Here we show that the SST low in the WPWP is significantly intensified in July–October of El Nino years (especially extreme El Nino years) and splits the 28.5 °C-isotherm-defined WPWP (WPWP split for simplification). Composite analysis and heat budget analysis indicate that the enhanced upwelling due to positive wind stress curl anomaly and western propagating upwelling Rossby waves account for the WPWP split. Zonal advection at the eastern edge of split region plays a secondary role in the formation of the WPWP split. Composite analysis and results from a Matsuno–Gill model with an asymmetric cooling forcing imply that the WPWP split seems to give rise to significant anomalous westerly winds and intensify the following El Nino event. Lead-lag correlation shows that the WPWP split slightly leads the Nino 3.4 index.

Potential modulations of pre-monsoon aerosols during El Niño: impact on Indian summer monsoon

The potential role of aerosol loading on the Indian summer monsoon rainfall during the El Nino years are examined using satellite-derived observations and a state of the art fully interactive aerosol-chemistry-climate model. The Aerosol Index (AI) from TOMS (1978–2005) and Aerosol Optical Depth (AOD) from MISR spectroradiometer (2000–2010) indicate a higher-than-normal aerosol loading over the Indo-Gangetic plain (IGP) during the pre-monsoon season with a concurrent El Nino. Sensitivity experiments using ECHAM5-HAMMOZ climate model suggests that this enhanced loading of pre-monsoon absorbing aerosols over the Indo-Gangetic plain can reduce the drought during El Nino years by invoking the ‘Elevated-Heat-Pump’ mechanism through an anomalous aerosol-induced warm core in the atmospheric column. This anomalous heating upshot the relative strengthening of the cross-equatorial moisture inflow associated with the monsoon and eventually reduces the severity of drought during El Nino years. The findings are subject to the usual limitations such as the uncertainties in observations, and limited number of El Nino years (during the study period).

Applications of Remote Sensing and Geographic Information System to Identify Rice Planting Season During El Nino Years: Case Study in the Pringsewu District, Province of Lampung

Spatial Information about rice planting season (RPS) in a wide areas, particularly during periods of El Nino, is important to support an information about the availability of rice continously. Application of remote sensing and geographic information system (GIS) technology can support it's information continuously and accurate. In this study, we attempted to identify the rice planting season during El Nino years of 1997, 2006 and 2015 in the Pringsewu district, Lampung and we compared with meteorological drought index. Spatial information of the RPS obtained through Interpretation of multitemporal Landsat data aquired in 1997, 2006 and 2015 using normalized difference vegetation index (NDVI) and the humidity index. While standardized precipitation index (SPI) is used as a indicator of meteorological drought. This study has shown that the application of remote sensing and GIS could accurately monitor the rice planting season during the periods of El Nino in 1997, 2006 and 2015. The fallow land dominated during the El Nino years and there were no significant difference between years. While drought information based on SPI values showed different results between years of El Nino events. In this paper we also discussed the relationship between distribution of fallow land and meteorological drought in a spatial perspective.

Teleconnections of ENSO and IOD to summer monsoon and rice production potential of India

Regional trend of summer monsoon precipitation has been analyzed for broad physical regions of India namely, (i) Indo-Gangetic plain, (ii) Central and East India, (iii) Coastal and Peninsular India and (iv) Western India. A significantly drying trend has been found in the two regions namely, Indo-Gangetic plain and Central and East India with comparative seasonal rate of drying higher in the latter region. A complex relation between the regional trend of summer monsoon precipitation, global teleconnection parameters and rice production of the regions have been studied. El Niño-Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) have a significant role in the precipitation anomaly of Indo-Gangetic plain unlike Central and East India where the ENSO only plays role as global teleconnection parameter. Rice production of Central and East India has been found to be affected adversely during the El Nino years. Central and East India is found to be the worst affected region compared to the Indo-Gangetic plain with respect to its fragile rainfed rice production potential and strong adverse teleconnection of El Nino on the rice production in this zone.

The influence of the El Niño Southern Oscillation on heat waves in India

The present study investigated the relationship between the El Nino Southern Oscillation (ENSO) and heat wave characteristics, such as duration, annual number of heat wave days and maximum temperature of heat wave events, in India. El Nino is associated with an eastward shift in the Walker circulation and breakdown of circulation patterns, resulting in delay in the onset of the Indian Summer Monsoon (ISM). Because most heat waves in India occur during the pre-monsoon season, the present study showed that heat waves during El Nino years were longer and hotter, and it is argued that this is related to a delay in the onset of the ISM. Further, it was found that the shift in the circulation pattern due to El Nino resulted in (1) the weakening of southwesterlies in the Arabian Sea, and (2) the occurrence of a large number of clear sky days over India. These explain the occurrence of warmer and longer duration heat waves during El Nino years. These results imply that, if El Nino activity increases in the future, heat waves in India are likely to intensify.

ENSO related SST anomalies and relation with surface heat fluxes over south Pacific and Atlantic

The role of surface heat fluxes in Southern Pacific and Atlantic Ocean SST anomalies associated with El Nino Southern Oscillation (ENSO) is studied using observation and ocean reanalysis products. A prominent dipole structure in SST anomaly is found with a positive (negative) anomaly center over south Pacific (65S–45S, 120W–70W) and negative (positive) one over south Atlantic (50S–30S, 30W–0E) during austral summer (DJF) of El Nino (LaNina). During late austral spring-early summer (OND) of El Nino (LaNina), anomalous northerly (southerly) meridional moisture transport and a positive (negative) sea level pressure anomaly induces a suppressed (enhanced) latent heat flux from the ocean surface over south Pacific. This in turn results in a shallower than normal mixed layer depth which further helps in development of the SST anomaly. Mixed layer thins further due to anomalous shortwave radiation during summer and a well developed SST anomaly evolves. The south Atlantic pole exhibits exactly opposite characteristics at the same time. The contribution from the surface heat fluxes to mixed layer temperature change is found to be dominant over the advective processes over both the basins. Net surface heat fluxes anomaly is also found to be maximum during late austral spring-early summer period, with latent heat flux having a major contribution to it. The anomalous latent heat fluxes between atmosphere and ocean surface play important role in the growth of observed summertime SST anomaly. Sea-surface height also shows similar out-of-phase signatures over the two basins and are well correlated with the ENSO related SST anomalies. It is also observed that the magnitude of ENSO related anomalies over the southern ocean are weaker in LaNina years than in El Nino years, suggesting an intensified tropics-high latitude tele-connection during warm phases of ENSO.

The influence of large-scale climate phenomena on precipitation in the Ordos Basin, China

Large-scale atmospheric circulations significantly affect regional precipitation patterns. However, it is not well known whether and how these phenomena affect regional precipitation distribution in northern China. This paper reported the individual and coupled effects of El Nino–Southern Oscillation (ENSO), Indian summer monsoon (ISM), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) on annual precipitation for the Ordos Basin, an arid and semi-arid basin, currently with major industries of coal, fossil oil, natural gas, and halite in north central China. Our results showed that ENSO and ISM exerted substantial impact on annual precipitation while the impact of PDO and AMO was relatively limited. There were 24 and 15 out of 33 stations showing significant differences (p < 0.1) in annual precipitation (from 1950 to 2013) for ENSO and ISM, respectively. The median precipitation across the basin during El Nino years was 21.49 % higher than that during La Nina years and 17.28 % higher during the positive phase of ISM years compared to the negative phase of the ISM years. The impacts of ENSO and ISM on precipitation were enhanced during a PDO cold phase but weakened in a PDO warm phase. The impact of ENSO was still enhanced by an AMO warm phase. The effects of climatic phenomena on precipitation showed a strong spatial difference in the Ordos Basin. The impact of ENSO was mostly evident around the edges of the basin while the impact of ISM decreases from south to north. The deserts (i.e., Hobq Desert and Mu Us Sandy Land) in the center of the basin were less affected by these large-scale climatic phenomena. An improved understanding of such relationships would be helpful in water resource planning and disaster management for the Ordos Basin.

Numerical simulation of the Kuroshio intrusion into the South China Sea by a passive tracer

Owing to lack of observational data and accurate definition, it is difficult to distinguish the Kuroshio intrusion water from the Pacific Ocean into the South China Sea (SCS). By using a passive tracer to identify the Kuroshio water based on an observation-validated three-dimensional numerical model MITgcm, the spatio-temporal variation of the Kuroshio intrusion water into the SCS has been investigated. Our result shows the Kuroshio intrusion is of distinct seasonal variation in both horizontal and vertical directions. In winter, the intruding Kuroshio water reaches the farthest, almost occupying the area from 18 degrees N to 23 degrees N and 114 degrees E to 121 degrees E, with a small branch flowing towards the Taiwan Strait. The intrusion region of the Kuroshio water decreases with depth gradually. However, in summer, the Kuroshio water is confined to the east of 118 degrees E without any branch reaching the Taiwan Strait; meanwhile the intrusion region of the Kuroshio water increases from the surface to the depth about 205 m, then it decreases with depth. The estimated annual mean of Kuroshio Intrusion Transport (KIT) via the Luzon Strait is westward to the SCS in an amount of -3.86x10(6) m(3)/s, which is larger than the annual mean of Luzon Strait Transport (LST) of -3.15x10(6) m(3)/s. The KIT above 250 m accounts for 60%-80% of the LST throughout the entire water column. By analyzing interannual variation of the Kuroshio intrusion from the year 2003 to 2012, we find that the Kuroshio branch flowing into the Taiwan Strait is the weaker in winter of La Nina years than those in El Nino and normal years, which may be attributed to the wind stress curl off the southeast China then. Furthermore, the KIT correlates the Nino 3.4 index from 2003 to 2012 with a correlation coefficient of 0.41, which is lower than that of the LST with the Nino 3.4 index, i.e., 0.78.

Lightning activity with rainfall during El Nino and La Nina events over India

This paper appraises the association of lightning flash count (FC) with rainfall using the satellite-borne Lightning Imaging Sensor’s (LIS) data along with gridded rainfall data (0.5o × 0.5o) for Indian summer monsoon seasons over 10 years (2001–2010). During strong El Nino years, 2002 and 2009, FCs were greater in magnitude by about 26.5 % and 37 %, than the long-term average, respectively, while during weak El Nino year (2004), it was more by 8 %. During the same years, the rainfall was deficient by about 10 % than the long-term average. Similarly, a rise in aerosol optical depth (AOD) over its average value (by about 15 % and 20 %) reduces the ratio of rainfall to FC (RLR) by 41 % and 44 % for strong El Nino years 2002 and 2009, respectively, and for weak El Nino year (2004), a 6.5 % rise in AOD lowers the RLR by 20 %. Bowen ratio more by 11 % and 17 % of its average value reduces the RLR by 41 % and 44 % for strong El Nino years 2002 and 2009, respectively, and, also, Bowen ratio higher by 8 % for 2004 declines RLR by 20 %. On the other hand, Bowen ratio less by 9 % and 6 % raises the RLR by 19 % and 56 % for moderate La Nina year (2007) and strong La Nina year (2010), respectively. Results for the daily rainfall, AOD and Bowen ratio over Indian regions, are discussed for strong El Nino and La Nina years. Correlations of FC with AOD and Bowen ratio of 0.66 and 0.71, respectively, while, that of FC with ONI of 0.56 indicates numerous (fewer) break days during El Nino (La Nina) years.

The role of El Niño in the global energy redistribution: a case study in the mid-Holocene

It has been shown that El Nino events contribute to discharge the warm pool excess of energy out of the tropical Pacific. In a different climate, the energetic budget in the tropical Pacific is altered, which might have an effect on the El Nino amplitude and/or occurrence and thereby on the role of El Nino on energy redistribution. The mid-Holocene period (6 ka BP) offers a good example of changes in the distribution of incoming solar energy. In particular, the equator-pole gradient was weaker compared to the modern period. We analyze long stable simulations of the mid-Holocene and the pre-industrial era and discuss the mean- and El Nino-related energy transports in the two climates. We show that the role of global energy pump played by the tropical Pacific is reduced in the mid-Holocene in our simulation, both in long-term mean and during El Nino years. We demonstrate that this is not only a direct response to insolation forcing but this is further amplified by changes in internal processes. We analyze the relative role of El Nino events in the Pacific discharge in the two climates and show that it is reduced in the mid-Holocene, i.e. the fraction of the Pacific discharge that is due to El Nino is reduced. This is mainly due to reduction in the occurrence of El Nino events. This work gives a new approach to address El Nino changes, from the perspective of the role of El Nino in global energy redistribution.

Spatial and Temporal Variations in Eastern U.S. Hydrology: Responses to Global Climate Variability

AbstractCoastal ecosystems are dependent on terrestrial freshwater export which is affected by both climate trends and natural climate variability. However, the relative role of these factors is not clear. Here, both climate trends and internal climate variabilities at different time scales are related to variations in terrestrial freshwater export into the eastern United States (U.S.) coastal region. For the recent 35‐year period, the intensified hydro‐meteorological processes (annual precipitation or evapotranspiration) may explain the observed streamflow variability in the northeast. However, in the southeast, streamflow is positively correlated with climate variability induced by the Pacific Ocean conditions (El Nino‐Southern Oscillation [ENSO] and Pacific Decadal Oscillation) rather than Atlantic Ocean conditions (Atlantic Multi‐decadal Oscillation and North Atlantic Oscillation). The centroid location for volume of terrestrial freshwater export integrated along the eastern U.S. has a positive temporal trend and is negatively correlated with ENSO conditions, suggesting the northward trend in freshwater export to U.S. eastern coast may be disturbed by the natural climate variability, especially ENSO conditions, i.e., the center of freshwater mass moves southward (northward) during El Nino (La Nina) years. The results indicate the spatial and temporal variations in freshwater export from the eastern U.S. are affected by both climate change and inter‐annual climate variability during the recent 35‐year period (1980‐2014).

A Synoptic-Scale Perspective of Heavy Rainfall over Chennai in November 2015

The South Indian city of Chennai experienced three phases of heavy rainfall that resulted in devastating flood during November and early December of 2015. We find that propagating convective systems from the west Pacific Ocean intensified further over the warm Indian Ocean before moving north towards Indian land region. This northward propagation was guided by two highs of mid-troposphere to the east and west of the Indian region. While the high to the east was typical of an El Nino year, that the west was associated with global phase shift of upper tropospheric Rossby wave. Similar highs to the west were present during other years of heavy rainfall along the east coast of peninsular India.

Climatology and trend of cold waves over India during 1971-2010

Using Cold Wave (CW) information of 86 stations from Indian main land during the cold weather season (November to March) for the last 40 years (1971-2010), various statistical aspects of cold waves (CWs) and severe cold waves (SCWs) such as climatology and trend were examined. The link of CWs/SCWs with ENSO phases (La Nina &amp; El Nino) was also examined. It was observed that many stations from north, northwest, east and central India together named as core CW zone (CCZ) experienced highest number of CW/SCW waves with relatively higher frequency during January. Noticeable decrease (increase) in the frequency and spatial coverage of CW/SCW days compared to their climatological values were observed during the El Nino (La Nina) years. There were significant decreasing trends in the CW/SCW days over most of the stations from CCZ. The total number of CW/SCW days/per decade over CCZ showed noticeable decrease during the recent decades 1991-2000 and 2001-2010, which coincided with the warmest decades for the country as well as for the globe. Associated with intense and persistent CW/SCW events, large human mortality were reported during some years of the study period.

Indian summer monsoon rainfall variability in response to differences in the decay phase of El Niño

In general the Indian summer monsoon (ISM) rainfall is near normal or excess during the El Nino decay phase. Nevertheless the impact of large variations in decaying El Nino on the ISM rainfall and circulation is not systematically examined. Based on the timing of El Nino decay with respect to boreal summer season, El Nino decay phases are classified into three types in this study using 142 years of sea surface temperature (SST) data, which are as follows: (1) early-decay (ED; decay during spring), (2) mid-summer decay (MD; decay by mid-summer) and (3) no-decay (ND; no decay in summer). It is observed that ISM rainfall is above normal/excess during ED years, normal during MD years and below normal/deficit in ND years, suggesting that the differences in El Nino decay phase display profound impact on the ISM rainfall. Tropical Indian Ocean (TIO) SST warming, induced by El Nino, decays rapidly before the second half of the monsoon season (August and September) in ED years, but persists up to the end of the season in MD years, whereas TIO warming maintained up to winter in ND case. Analysis reveals the existence of strong sub-seasonal ISM rainfall variations in the summer following El Nino years. During ED years, strong negative SST anomalies develop over the equatorial central-eastern Pacific by June and are apparent throughout the summer season accompanied by anomalous moisture divergence and high sea level pressure (SLP). The associated moisture convergence and low SLP over ISM region favour excess rainfall (mainly from July onwards). This circulation and rainfall anomalies are highly influenced by warm TIO SST and Pacific La Nina conditions in ED years. Convergence of southwesterlies from Arabian Sea and northeasterlies from Bay of Bengal leads to positive rainfall over most part of the Indian subcontinent from August onwards in MD years. ND years are characterized by negative rainfall anomaly spatial pattern and weaker circulation over India throughout the summer season, which are mainly due to persisting El Nino related warm SST anomalies over the Pacific. Atmospheric general circulation model simulation supports our hypothesis that El Nino decay variations modulate ISM rainfall and circulation.

An unidentified heavy rainfall station ‘Tamhini’ in the northern region of Western Ghats of India

ABSTRACTTamhini, a station located in the leeward side of northern parts of Western Ghats (WG) of India, receives a mean‐monsoon rainfall of 6498.4 mm. The station has no record of any meteorological agencies in India. Therefore, the existence of such heavy rainfall (HR) station remained unknown. In this study, many special features of Tamhini rainfall have been discovered hitherto unknown to monsoon meteorologists. Statistical analysis of daily rainfall data of monsoon season (June–September) of Tamhini for the period 1975–2013 has been carried out. The analysis showed that in spite of lying on the leeward side of main WG, Tamhini's monsoon rainfall is more than that of stations lying on the windward side of the WG. This is in contrast to widely accepted view that rainfall is lower on the leeward side of mountains. Tamhini rainfall ranks number one in the north Peninsula and fifth in India. It is more compared to Cherrapunji monsoon rainfall (second highest rainfall‐receiving station in India) in 8 non‐El Nino years. It has the highest 1‐day (695 mm) and 3‐day (1055 mm) rainfalls in the Peninsular WG region. There are many extended HR spells during the season with a mean duration of 10 days. Other stations in the same area viz. Mulshi‐camp, Shirgaon, Davdi and Ambawane also receive high monsoon season rainfalls. Area‐mean rainfall of the region has been studied to understand the frequency distribution of HR spells. Such spells are of concern as total accumulated water during these spells is conducive for occurrence of floods. No significant trends in Tamhini and area‐mean seasonal and HRs are observed. The discovery of this station has shown the occurrence of high rainfall on the leeward side, which is a unique feature in the orographic rainfall distribution in the world.

Impact of El Niño and Climate Change on Rainwater Harvesting in a Caribbean State

The water situation of the Pusey district in St Catherine parish of Jamaica is acute because of the district’s hilly terrains which made connections to centralised public water supply difficult. Residents depend on rainwater harvesting (RWH) systems to meet potable needs, like many other catchments across Jamaica. Rainwater collecting practices and water use habits of the residents were surveyed and the present and future RWH capacity was evaluated using the available 18 years (1996 to 2013) rainfall data and downscaled PRECIS model A2 and B2 climate change scenarios. In addition, the effect of El Nino episodes on rainfall patterns was evaluated. The coefficients of variations for annual rainfall were found to be higher for the El Nino years than in normal years. In two of the El Nino years (1997 and 2009), rainwater harvesting capacity is negatively impacted as rainfall annual total is (42 % and 34 %) lesser than the average annual rainfall. The ability of RWH to meet potable needs in 2030s and 2050s will be reduced based on predicted shorter intense showers and frequent dry spells. A storage tank of 2.5 to 4.0 m3 per household (4 persons) is proposed to meet water demand during the maximum consecutive dry days, and January and February water shortage periods. Design of efficient RWH systems and provision of government subsidy on storage tanks will enable the residents to capture more rainwater to meet their daily domestic needs.

Flood frequencies and durations and their response to El Niño Southern Oscillation: Global analysis

Summary Floods are one of the most serious forms of natural hazards in terms of the damages they cause. In 2012 alone, flood damages exceeded $19 billion. A large proportion of the damages from several recent major flood disasters, such as those in South India and South Carolina (2015), England and Wales (2014), the Mississippi (2012), Thailand (2011), Queensland (Australia) (2010–2011), and Pakistan (2010), were related to the long duration of those flood events. However, most flood risk studies to date do not account for flood duration. In this paper, we provide the first global modelling exercise to assess the link between interannual climate variability and flood duration and frequency. Specifically, we examine relationships between simulated flood events and El Nino Southern Oscillation (ENSO). Our results show that the duration of flooding appears to be more sensitive to ENSO than is the case for flood frequency. At the globally aggregated scale, we found floods to be significantly longer during both El Nino and La Nina years, compared to neutral years. At the scale of individual river basins, we found strong correlations between ENSO and both flood frequency and duration for a large number of basins, with generally stronger correlations for flood duration than for flood frequency. Future research on flood impacts should attempt to incorporate more information on flood durations.

Interannual variation of tropical cyclone energy metrics over North Indian Ocean

There is decreasing trend in the tropical cyclone (TC) number over the North Indian Ocean (NIO) in recent years, though there is increasing trend in the sea surface temperature (SST) which is one of the main environmental parameters for the development and intensification of TCs. Hence, a study has been performed to understand whether any trend exists in other TC parameters such as velocity flux (VF), accumulated cyclone energy (ACE) and power dissipation index (PDI). The interseasonal and interannual variations of VF, ACE and PDI for the NIO as a whole and Bay of Bengal (BOB) and Arabian Sea (AS) are analysed based on the data of 1990–2013 (24 years). Role of large scale features like El Nino southern oscillation (ENSO) and Indian Ocean dipole (IOD) have also been analyzed. The mean ACE per year for TCs [maximum sustained wind of 34 knots (kt) or more] over the NIO is about 13.1 × 104 kt2 including 9.5 × 104 kt2 over the BOB and 3.6 × 104 kt2 over the AS. The mean PDI per year for TCs over the NIO is about 10 × 106 kt3 including 3 × 106 kt3 over the AS and 7 × 106 kt3 over the BOB. The VF, ACE and PDI of TCs are significantly less over BOB during post-monsoon season (Oct.–Dec.) of El Nino years than in La Nina and normal years. The VF for TCs over the BOB during post-monsoon season is significantly less (higher) during positive (negative) IOD years. There is significant decreasing trend at 95 % level of confidence in ACE and PDI of TCs over AS during post-monsoon season and PDI over the BOB and NIO during pre-monsoon season mainly due to similar trend in average intensity of TCs and not due to trends in SST over Nino regions or IOD index.

AMAZONIAN VÁRZEA FORESTS: ADAPTIVE STRATEGIES OF TREES AS TOOLS FOR FOREST MANAGEMENT

Amazonian varzea forests are characterized by a high diversity of species and adaptations against extended flooding. Waterlogging and submergence can last up to 210 days per year, with a water column of up to 6-7 m. The present paper gives an insight into the current knowledge of morpho-anatomical, phenological and physiological responses to flooding in varzea trees, into patterns of regeneration and seedling recruitment, and into differences found along the flooding gradient, and between populations of selected species. This knowledge may serve as a basic tool for forest management. The high selective logging already caused a substitution of timber species, with high damages in the remaining stands, calling for rigorous management plans. Since the regular inundation induces the formation of annual rings, and tree growth responds to the prolonged vegetation period with significantly wider ring widths in El Nino years as compared to neutral years, tree ring analysis can be used also for the development of models to predict tree growth and to determine minimum logging diameters and cutting cycles for timber species. Keywords: Amazonia, tree ecophysiology, establishment, environmental gradients, productivity, speciation, tree-ring analysis, El Nino.