El Nino-Southern Oscillation Signal Research Articles

The relationships between Pacific and Atlantic Ocean sea surface temperatures and Colombian streamflow variability

An evaluation of Pacific and Atlantic Ocean sea surface temperatures (SSTs) and Colombian streamflow was performed to identify coupled regions of SST and Colombian streamflow variability. Applying the Singular Value Decomposition (SVD) statistical method, SSTs (and Colombian streamflow for 10 stations) were evaluated for (a) the Pacific Ocean, (b) the Atlantic Ocean, and (c) the combined Pacific and Atlantic Ocean for a 41 year period of record (1960–2000). A lead-time approach was adopted such that spring–summer seasonal (April through September) SSTs were evaluated with streamflow for the following calendar-year (January through December). The El Niño-Southern Oscillation (ENSO) has been acknowledged in previous research efforts as an influence of Colombian streamflow. The ENSO signal identified that warmer (cooler) equatorial SSTs resulted in lesser (greater) streamflow. In general, the use of predefined published indices, such as ENSO, is appealing for water managers and forecasters. However, the utilization of SSTs for entire regions (Pacific and Atlantic Oceans) eliminates any spatial bias as to which oceanic SST region (or regions) impact hydrology. This will assist in the identification of regions that may not be represented in existing indices and could lead to improved forecasting. The SVD 1st temporal expansion series of Pacific (and the combined Pacific and Atlantic) Ocean SSTs and streamflow for several stations resulted in correlation values greater than that of well known climate indices (e.g., ENSO), which could result in improved streamflow predictability.

Teleconnection between the early immigration of brown planthopper ( Nilaparvata lugens Stål) and ENSO indices: implication for its medium- and long-term forecast

Teleconnection between the early immigration of brown planthopper (BPH) and El Nino/Southern Oscillation (ENSO) indices from January of two years previously to the current June was investigated to make long-term forecast. The teleconnection results were as follows: ENSO indices which were significantly correlated with the early immigration of BPH were primarily sea surface temperature anomalies (SSTA) in N3, N4 and N3.4 regions, accounting for 71.8% of the total. Significant ENSO indices from two years and one year before the immigration events had a proportion of about 84%, while those in the current year only accounted for 16.7%. There was significantly negative correlation between the early immigration of BPH and SSTA in each Nino region from two years before to the previous spring, whereas there was significantly positive correlation between these two factors during the period from the previous winter to the current spring. The significant correlation between the early immigration of BPH and SSTA in each Nino region in the last summer and autumn did not show any obvious tendencies. The relationship between the southern oscillation index (SOI) and the early immigration of BPH was opposite to that between the immigration and SSTA in each Nino region. The above mentioned significant ENSO indices were used as key factors to build forecasting models for the early immigration of BPH by step-wise multiple linear regression analysis. Finally, 12 integrated forecasting models were obtained, which could make predictions 3–27 months ahead and had a predictive accuracy of 88.9%.

Manifestation of the Arctic Oscillation and El Niño-Southern Oscillation in the tropical stratosphere

Characteristics of the Arctic Oscillation and El Nino-Southern Oscillation effects manifested on interannual scales in the equatorial stratosphere are determined. Wavelet analysis of local phase shifts, coherence, and correlation is used to obtain correlation portraits of the largest factors of climate variability against the background of coherent variations in the equatorial stratospheric wind speed at the 50- and 15-hPa pressure levels. It is shown that the Arctic Oscillation and El Nino-Southern Oscillation signals may reach the tropical stratosphere. The signals are easily identified in a wide range of scales, including quasi-biennial, 3-to 5-year, and 10-to 11-year periods. The results obtained reflect a coherent pattern of the manifestation of these signals at the selected stratospheric levels. It is found that the El Nino-Southern Oscillation effect at periods close to 10–11 years reaches the stratospheric level rather rapidly, in the same or next month, while the effects of the Arctic Oscillation index are delayed by nine months. The estimates obtained show that a phase shift of almost 180° in the Arctic Oscillation index relative to the equatorial stratospheric wind occurred in almost all of the range of interannual periods in 1978 and 1992. For the El Nino-Southern Oscillation, an increase in local correlations on a scale of 3-to 5-year variations was observed in 1980–1990, a 180° phase shift occurred in 1992, and the correlation with stratospheric wind increased in 1992–2004. The estimates obtained are indicative of a change in the atmospheric circulation pattern that took place in the Northern Hemisphere in 1978–1991.

Experimental drought threat forecast for Florida

Short-term climate variations including the El Niño/Southern Oscillation (ENSO) cause anomalies in temperature and precipitation that could produce or contribute to extreme events including droughts and wildfires. Wildfire threat potential in the Southeast is based in part upon the Keetch–Byram Drought Index (KBDI), and the KBDI uses daily temperature and rainfall measurements. The KBDI responds to changing climate and weather conditions on time scales of days to months, and this index is high during dry warm weather patterns and low during wet cool patterns. In the Southeastern United States, these dry periods typically occur during the winter and spring of La Niña years while wet periods occur during the winter and spring of El Niño years. This study computes forecast probabilities of KBDI exceeding thresholds by using temperature and precipitation data generated by a stochastic weather generator and initial KBDI from the observed dataset. The weather generator produces multiple realizations of ENSO's impact on local temperature and precipitation. Relatively high probabilities of exceeding thresholds tend to occur during the winter and spring months of La Niña years in both Jacksonville and Belle Glade. Low probabilities occur during the winter and spring months of El Niño years. The distribution is reversed during late spring and early summer months, but the distribution is smaller during the summer than during the winter. Verification indicates high skill during January, February, and May and low skill in April and June. Although other wildfire decision-support tools exist, foresters in the Southeastern United States commonly use the KBDI since its development during the 1970s. This study generates probabilistic KBDI forecasts based on initial KBDI and the ENSO signal.

Multidecadal variability in the transmission of ENSO signals to the Indian Ocean

Since 1980, transmission of El Niño‐Southern Oscillation (ENSO) signals into the Indian Ocean involves an equatorial, and a subtropical North Pacific (NP) Rossby wave pathway. We examine the robustness of the amount of energy that leaves the Pacific via each of the pathway using the Simple Ocean Data Assimilation with the Parallel Ocean Program (SODA‐POP) reanalysis and a multi‐century coupled model control experiment. We find that in the pre‐1980 period, little ENSO signal is transmitted to the Indian Ocean and does not involve the subtropical NP pathway. Such multidecadal variability is periodically produced by the climate model. Examinations reveal that when ENSO is weak as determined by Niño3.4, their meridional extent is narrow, the associated discharge‐recharge does not involve the subtropical NP pathway; further, weak ENSO events have a low signal‐to‐noise ratio, making the transmission hard to detect. The dynamics of multidecadal variability in ENSO strength awaits further investigation.

Coralline alga reveals first marine record of subarctic North Pacific climate change

While recent changes in subarctic North Pacific climate had dramatic effects on ecosystems and fishery yields, past climate dynamics and teleconnection patterns are poorly understood due to the absence of century‐long high‐resolution marine records. We present the first 117‐year long annually resolved marine climate history from the western Bering Sea/Aleutian Island region using information contained in the calcitic skeleton of the long‐lived crustose coralline red alga Clathromorphum nereostratum, a previously unused climate archive. The skeletal δ18O‐time series indicates significant warming and/or freshening of surface waters after the middle of the 20th century. Furthermore, the time series is spatiotemporally correlated with Pacific Decadal Oscillation (PDO) and tropical El Niño‐Southern Oscillation (ENSO) indices. Even though the western Bering Sea/Aleutian Island region is believed to be outside the area of significant marine response to ENSO, we propose that an ENSO signal is transmitted via the Alaskan Stream from the Eastern North Pacific, a region of known ENSO teleconnections.

ENSO signals in South America: rains and floods in the Paraná River region during colonial times

Several studies show that the El Nino-Southern Oscillation (ENSO) is an important factor in determining interannual rainfall variability in South America. This signal is detected in the region including Northeastern Argentina, Uruguay and part of southern Brazil, in the form of excessive rains and big floods in the regional rivers. To check that this relationship was similar in the past the objectives of this paper were to construct a time series of large floods in the Parana River region from documentary records, during the sixteenth to eighteenth centuries, and to evaluate the relationship between that historical record of extreme floods, the ENSO documentary evidence (Ortlieb, The Documented Historical Record of El Nino Events in Peru: An Update of the Quinn Record (Ortlieb, sixteenth through nineteenth centuries, Diaz and Markgraf, (eds.), El Nino and the southern oscillation. Multiscale variability and global and regional Impacts. Cambridge University Press, pp. 207-295, 2001; Quinn and Neal, The historical record of El Nino events, Bradley and Jones (eds.), Climate since a.d. 1500, Routledges, pp. 623-648, 1992) and the temperature index of the Pacific Ocean (Mann ME et al., Global temperature patterns in past centuries: an interactive presentation, IGBP pages/world data center for paleoclimatology data contribution series #2000-075. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA, 2000). Considering that the period 1904-2000, where 11 out of 16 floods occurred during El Nino events, it can be concluded that the proportion of years with exceptional flow volume in the Parana River in years with El Nino events in the seventeenth and eighteenth centuries was relatively lower than that of the twentieth century. The reason for this difference is discussed.

El Niño-Southern Oscillation impacts translated to the watershed scale: Estuarine salinity patterns along the Texas Gulf Coast, 1982 to 2004

The importance of El Niño-Southern Oscillation (ENSO) on regional-scale climate variability is well recognized, although the communication of this atmospheric signal onto the watershed and ultimately estuarine salinity structure at the bay system level is less understood. The Texas coast, situated in a climatic gradient, is an ideal location to study ENSO influences on estuaries. The seven major estuaries found on the coast share similar physical properties, yet each one is maintained by a hydrologically isolated watershed. Inflow differences maintain estuaries spanning the range from positive, to neutral, to negative. Three monthly measures of ENSO indices (Niño 3.4 sea surface temperature anomaly, atmospheric Southern Oscillation sea level pressure anomaly, and Pacific Decadal Oscillation sea surface temperature anomaly) were related to estuarine-wide average salinities during the period 1982–2004. Empirical Orthogonal Function analysis revealed that most of the variation in salinity is related primarily to five fundamental frequencies (1.02, 1.94, 3.55, 5.33, and 10.67 years), and these periodicities closely correspond with frequencies related to the ENSO measures. Cross-correlation analysis showed the ENSO signals are communicated to the salinity structure of Texas estuaries within a 4- to 6-month timeframe. Despite having very different underlying salinity regimes, the seven estuaries of the Texas coast appear to be operating in near unison to the low frequency forcing signals of ENSO. Current water management in Texas focuses much attention on maintaining estuarine salinity conditions at a monthly, bi-monthly, or seasonal scale. At this temporal resolution, water resource management efforts may not adequately address the important ENSO-driven periodicities governing coastal estuarine salinity patterns.

Inter-hemispheric linkages in climate change: paleo-perspectives for future climate change

Abstract. The Pole-Equator-Pole (PEP) projects of the PANASH (Paleoclimates of the Northern and Southern Hemisphere) programme have significantly advanced our understanding of past climate change on a global basis and helped to integrate paleo-science across regions and research disciplines. PANASH science allows us to constrain predictions for future climate change and to contribute to the management of consequent environmental changes. We identify three broad areas where PEP science makes key contributions. 1. The pattern of global changes. Knowing the exact timing of glacial advances (synchronous or otherwise) during the last glaciation is critical to understanding inter-hemispheric links in climate. Work in PEPI demonstrated that the tropical Andes in South America were deglaciated earlier than the Northern Hemisphere (NH) and that an extended warming began there ca. 21 000 cal years BP. The general pattern is consistent with Antarctica and has now been replicated from studies in Southern Hemisphere (SH) regions of the PEPII transect. That significant deglaciation of SH alpine systems and Antarctica led deglaciation of NH ice sheets may reflect either i) faster response times in alpine systems and Antarctica, ii) regional moisture patterns that influenced glacier mass balance, or iii) a SH temperature forcing that led changes in the NH. This highlights the limitations of current understanding and the need for further fundamental paleoclimate research. 2. Changes in modes of operation of oscillatory climate systems. Work across all the PEP transects has led to the recognition that the El Niño Southern Oscillation (ENSO) phenomenon has changed markedly through time. It now appears that ENSO operated during the last glacial termination and during the early Holocene, but that precipitation teleconnections even within the Pacific Basin were turned down, or off. In the modern ENSO phenomenon both inter-annual and seven year periodicities are present, with the inter-annual signal dominant. Paleo-data demonstrate that the relative importance of the two periodicities changes through time, with longer periodicities dominant in the early Holocene. 3. The recognition of climate modulation of oscillatory systems by climate events. We examine the relationship of ENSO to a SH climate event, the Antarctic cold reversal (ACR), in the New Zealand region. We demonstrate that the onset of the ACR was associated with the apparent switching on of an ENSO signal in New Zealand. We infer that this related to enhanced zonal SW winds with the amplification of the pressure fields allowing an existing but weak ENSO signal to manifest itself. Teleconnections of this nature would be difficult to predict for future abrupt change as boundary conditions cannot readily be specified. Paleo-data are critical to predicting the teleconnections of future changes.

Reconstructing El Niño–Southern Oscillation (ENSO) from high‐resolution palaeoarchives

AbstractEl Niño–Southern Oscillation (ENSO) is one of the most important coupled ocean–atmospheric phenomena to cause global climate variability on interannual timescales. Efforts to understand recent, apparently anomalous ENSO behaviour are hampered by the phenomenon's unstable (non‐stationary) nature and the limitations inherent in palaeoclimate records. In this paper, the complexities associated with isolating ENSO signals in observational and palaeoclimate records are reviewed. The utility and limitations of high‐resolution (tree‐ring, coral, speleothems, ice and documentary) proxy data for ENSO reconstruction are discussed. To overcome the regional biases contained within each palaeoclimatic source, it is necessary to compare complementary signals derived from multiple proxy climate records. To date, there have been limited attempts to reconstruct large‐scale ENSO using these ‘multiproxy’ methodologies. A critique of the complexities associated with previous approaches of reconstructing discrete ENSO events and atmospheric/oceanic indices is provided. Abundant potential remains to better characterise teleconnection patterns, propagation signatures and non‐stationary features of large‐scale ENSO behaviour. If key uncertainties in ENSO dynamics (such as the response of extreme events to natural/human forcing) are to be adequately assessed, then complementary attempts must be made to model the historic synoptic conditions with apparent changes in reconstructed indices. Copyright © 2006 John Wiley & Sons, Ltd.

ENSO and the South China Sea summer monsoon onset

AbstractThis paper investigates the relationship between the onset date of the South China Sea summer monsoon (SCSSM) and the El Niño/Southern Oscillation (ENSO). The monsoon onset date (MOD) is defined on the basis of the switch of the 850‐hPa zonal winds over the South China Sea (SCS) from easterly to westerly for two consecutive pentads. The ENSO signal is represented by the ocean heat content (OHC), which is proportional to the depth of the 20 °C isotherm.It is found that, in years associated with a warm (cold) ENSO event or the year after, the monsoon tends to have a late (an early) onset and the intensity of the SCSSM also tends to be weaker (stronger). During a 2‐year period prior to the onset, anomalies of OHC have an obvious eastward propagation. The 850‐hPa flow east of the Philippines, specifically the strength of the subtropical high, is also found to be critical in determining the MOD. The link between these two results appears to be the propagation of cold (warm) subsurface water into the western North Pacific (WNP), which strengthens (weakens) the subtropical high, and hence a late (an early) SCSSM onset. Copyright © 2006 Royal Meteorological Society.

ENSO influence on Europe during the last centuries

El Nino/Southern Oscillation (ENSO) affects climate not only in the Pacific region and the tropics, but also in the North Atlantic-European area. Studies based on twentieth-century data have found that El Nino events tend to be accompanied in late winter by a negative North Atlantic Oscillation index, low temperatures in northeastern Europe and a change in precipitation patterns. However, many questions are open, for example, concerning the stationarity of this relation. Here we study the relation between ENSO and European climate during the past 500 years based on statistically reconstructed ENSO indices, early instrumental station series, and reconstructed fields of surface air temperature, sea-level pressure, precipitation, and 500 hPa geopotential height. After removing years following tropical volcanic eruptions (which systematically mask the ENSO signal), we find a consistent and statistically significant ENSO signal in late winter and spring. The responses to El Nino and La Nina are close to symmetric. In agreement with studies using twentieth-century data only, the ENSO signal in precipitation is different in fall than in late winter. Moving correlation analyses confirm a stationary relationship between ENSO and late winter climate in Europe during the past 300 years. However, the ENSO signal is modulated significantly by the North Pacific climate. A multi-field cluster analysis for strong ENSO events during the past 300 years yields a dominant pair of clusters that is symmetric and represents the ‘classical’ ENSO effects on Europe.

Oceanic‐atmospheric variability and western U.S. snowfall

A study of the influences of interdecadal and interannual oceanic‐atmospheric influences on April 1 Snow‐Water Equivalent (SWE) in the western U.S. is presented. SWE data was identified at 323 Natural Resources Conservation Service (NRCS) SNOTEL (SNOwpack TELemetrysites) stations for the period of 1961 to 2004 and for 121 SNOTEL stations for the period 1941 to 2004. The phases (cold/negative or warm/positive) of Pacific Ocean [El Niño ‐Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO)] and Atlantic Ocean [Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO)] oceanic‐atmospheric influences were identified for the year prior to the SWE data set. Statistical significance testing of SWE data set, based on the interdecadal and interannual oceanic‐atmospheric phase (warm/positive or cold/negative) was performed by applying the nonparametric rank‐sum test. The results show that in addition to the well established ENSO signal in the northwest, the PDO and AMO influence SWE variability. Additionally, the coupled effects of the oceanic‐atmospheric influences were evaluated on the basis of the long‐term phase (cold/negative or warm/positive) of the interdecadal (PDO, AMO, NAO) influences and the interannual ENSO. Finally, the coupled effects of the oceanic‐atmospheric influences were evaluated on the basis of the long‐term phase (cold/negative or warm/positive) of the interdecadal (AMO, PDO, NAO) phenomena. Regions in the west were identified that responded to the interdecadal/decadal climatic coupling. By utilizing the April 1 SWE and the long lead‐time approach for the oceanic‐atmospheric variables, useful information can be provided to snow forecasters and water managers.

Climate–growth relationships of tropical tree species in West Africa and their potential for climate reconstruction

AbstractMost tropical regions are facing historical difficulties of generating biologically reconstructed long‐term climate records. Dendrochronology (tree‐ring studies) is a powerful tool to develop high‐resolution and exactly dated proxies for climate reconstruction. Owing to the seasonal variation in rainfall we expected the formation of annual tree rings in the wood of tropical West African tree species. In the central‐western part of Benin (upper Ouémé catchment, UOC) and in northeastern Ivory Coast (Comoé National Park, CNP) we investigated the relationship between climate (precipitation, sea surface temperature (SST)) and tree rings and show their potential for climate reconstruction. Wood samples of almost 200 trees belonging to six species in the UOC and CNP served to develop climate‐sensitive ring‐width chronologies using standard dendrochronological techniques. The relationship between local precipitation, monthly SST anomalies in the Gulf of Guinea, El Niño‐ Southern Oscillation (ENSO) and ring‐width indices was performed by simple regression analyses, two sample tests and cross‐spectral analysis. A low‐pass filter was used to highlight the decadal variability in rainfall of the UOC site. All tree species showed significant relationships with annual precipitation proving the existence of annual tree rings. ENSO signals could not be detected in the ring‐width patterns. For legume tree species at the UOC site significant relationships could be found between SST anomalies in the Gulf of Guinea indicating correlations at periods of 5.1–4.1 and 2.3 years. Our findings accurately show the relationship between tree growth, local precipitation and SST anomalies in the Gulf of Guinea possibly associated with worldwide SST patterns. A master chronology enabled the reconstruction of the annual precipitation in the UOC to the year 1840. Time series analysis suggest increasing arid conditions during the last 160 years which may have large impacts on the hydrological cycles and consequently on the ecosystem dynamics and the development of socio‐economic cultures and sectors in the Guinea‐Congolian/Sudanian region.

Propagation of ENSO temperature signals into the middle atmosphere: A comparison of two general circulation models and ERA‐40 reanalysis data

The vertical propagation of the El Niño–Southern Oscillation (ENSO) temperature signal has been analyzed in two general circulation models, the Whole Atmosphere Community Climate Model and the Middle Atmosphere European Center–Hamburg Model, and in the ERA‐40 reanalysis data set. Monthly mean data have been used, and composite differences (El Niño–La Niña) have been computed. Our results show that the ENSO signal propagates into the middle atmosphere by means of planetary Rossby waves. Significant wave‐like anomalies are observed up to around 40 km. This propagation is strongly influenced by the zonal mean zonal winds, being most effective in midlatitudes of the Northern Hemisphere because ENSO events tend to peak in northern winter, when stratospheric winds are westerly in the Northern Hemisphere, and allow vertical propagation of Rossby waves. In addition, zonal mean temperature anomalies are observed in the middle atmosphere in the tropics and at polar latitudes of the Northern Hemisphere. These anomalies are the result of changes in the residual mean meridional circulation: Our analysis reveals that during an El Niño event, vertical wave propagation and divergence of Eliassen‐Palm flux are enhanced, forcing a stronger residual circulation in the stratosphere, which cools the tropics and warms the higher latitudes. This pattern is highly significant in the models during certain months but much less in the ERA‐40 data, where other sources of variability (in particular the quasi‐biennial oscillation) also influence the residual circulation.

Inter ENSO variability and its influence over the South American monsoon system

Abstract. Previous studies have discussed the interannual variability of a meridional seesaw of dry and wet conditions over South America (SA) associated to the modulation of the South Atlantic Convergence Zone (SACZ). However, they did not explore if the variability inter ENSO (El Niño Southern Oscillation) can be related to the phase changes of this dipole. To answer this question, an observational work was carried out to explore the atmospheric and Sea Surface Temperature (SST) conditions related to the same ENSO signal and to opposite dipole phases. Rotated Empirical Orthogonal Function (REOF) analysis was applied over normalized Chen precipitation seasonal anomalies in order to find the dipole mode in the Austral Summer (December to February). The fourth rotated mode, explaining 6.6% of the total variance, consists of positive loading over the SACZ region and negative loading over northern Argentina. Extreme events were selected and enhanced activity of SACZ during the Summer season (SACZ+) was identified in nine years: five during La Niña events (LN) and two in El Niño episodes (EN). On the other hand, inhibited manifestations of this system (SACZ-) were identified in seven years: four in EN and two during LN. Power spectrum analysis indicated that the interannual variability of the precipitation dipole seems to be related to the low frequency and to the quasi-biennial part of ENSO variability. The ENSO events with the same signal can present opposite phases for the dipole. The results suggest that the displacement of the convection over Indonesia and western Pacific can play an important role to modulate the seesaw pattern.

How unusual was late 20th century El Niño-Southern Oscillation (ENSO)? Assessing evidence from tree-ring, coral, ice-core and documentary palaeoarchives, A.D. 1525-2002

Abstract. Multiple proxy records (tree-ring, coral, ice and documentary) were examined to isolate ENSO signals associated with both phases of the phenomenon for the period A.D. 1525-2002. To avoid making large-scale inferences from single proxy analysis, regional signals were aggregated into a network of high-resolution records, revealing large-scale trends in the frequency, magnitude and duration of pre-instrumental ENSO using novel applications of percentile analysis. Here we use the newly introduced coupled ocean-atmosphere ENSO index (CEI) as a baseline for the calibration of proxy records. The reconstruction revealed 83 extreme or very strong ENSO episodes since A.D. 1525, expanding considerably on existing ENSO event chronologies. Significantly, excerpts of the most comprehensive list of La Niña events complied to date are presented, indicating peak activity during the 16th to mid 17th and 20th centuries. Although extreme events are seen throughout the 478-year reconstruction, 43% of the extreme ENSO events noted since A.D. 1525 occur during the 20th century, with an obvious bias towards enhanced El Niño conditions in recent decades. Of the total number of extreme event years reconstructed, 30% of all reconstructed ENSO event years occur post-1940 alone suggesting that recent ENSO variability appears anomalous in the context of the past five centuries.

ENSO and rainfall along the Zongo valley (Bolivia) from the Altiplano to the Amazon basin

The location of eight rainfall stations in the Zongo valley (Eastern Cordillera ‐ Bolivia) between 1195 and 4750 meters above sea level (masl) gives a unique opportunity to study the time-space rainfall variability between two regions, i.e. the Altiplano and the Amazonian lowlands of Bolivia, that are differently related to El Ni ˜ no-Southern Oscillation (ENSO) events in the Pacific. In the central part of the lowlands, a La Ni ˜ na/dry signal is observed during the peak of the rainy season (February) and around wintertime (May and September). Going toward the Andes and on the foothills up to 1500 masl, no ENSO signal has been found. On the median slopes of the Zongo valley, between 1800 and 3500 m, a strong Ni ˜ no/dry signal is depicted on an annual timescale and during summer. In February, a Ni ˜ na/wet signal is also significant. Going westward through the high Andes and the eastern Altiplano, this signal weakens. On the western edge of the Altiplano, an El Ni ˜ no/dry and La Ni ˜ na/wet signals are clearly identified. In the central lowlands and on the western Altiplano, ENSO indexes account for nearly 25% of rainfall variability, and for as much as 50% along the median slopes of the eastern Cordillera, in the Zongo valley. This finding deserves further study as previous researcheshave shown that in the Andes the best relationship between rainfall and equatorial Pacific sea-surface temperature (SST) is in the western Altiplano, far from the Amazonian source of moisture. Copyright!2006 Royal Meteorological Society.

Influence of Pacific Climate Patterns on Low-Flows in British Columbia and Yukon, Canada

This study investigated the low-flow response (low-flow frequencies, magnitudes and locations) of rivers in British Columbia and Yukon, Canada to the Pacific Decadal Oscillation (PDO) and the El Niño/Southern Oscillation (ENSO). Using stream discharge data from four hydrometric stations in each of four regions of BC (Southern Coast, Southern Interior, Northern Coast and Northern Interior) and the southern Yukon, coherent responses of low streamflows to PDO and ENSO were identified and examined. Low streamflows were defined as flows less than the 10th percentile on a given day compared to the historical streamflow series for that day. It was found that PDO and ENSO both influenced low streamflows in all study regions except Yukon. The PDO signal influenced low-flows more significantly and consistently than the ENSO signal. However, the PDO signal was modulated by ENSO, either strengthening or weakening low-flows depending on geographic location. ENSO had inconsistent impacts on low-flows with large differences between geographic areas. At the study watersheds in BC, correlation analysis showed that 14 and 13 of the 16 rivers showed significant associations between PDO index and low-flow frequencies and low-flow magnitudes, respectively. Temporal location of low-flow is affected by PDO in coastal BC but not in the interior and Yukon. None of the rivers in Yukon showed significant correlations between low-flows and Pacific climate patterns.

Coupled oceanic‐atmospheric variability and U.S. streamflow

A study of the influence of interdecadal, decadal, and interannual oceanic‐atmospheric influences on streamflow in the United States is presented. Unimpaired streamflow was identified for 639 stations in the United States for the period 1951–2002. The phases (cold/negative or warm/positive) of Pacific Ocean (El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO)) and Atlantic Ocean (Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO)) oceanic‐atmospheric influences were identified for the year prior to the streamflow year (i.e., long lead time). Statistical significance testing of streamflow, based on the interdecadal, decadal, and interannual oceanic‐atmospheric phase (warm/positive or cold/negative), was performed by applying the nonparametric rank‐sum test. The results show that in addition to the well‐established ENSO signal the PDO, AMO, and NAO influence streamflow variability in the United States. The warm phase of the PDO is associated with increased streamflow in the central and southwest United States, while the warm phase of the AMO is associated with reduced streamflow in these regions. The positive phase of the NAO and the cold phase of the AMO are associated with increased streamflow in the central United States. Additionally, the coupled effects of the oceanic‐atmospheric influences were evaluated on the basis of the long‐term phase (cold/negative or warm/positive) of the interdecadal (PDO and AMO) and decadal (NAO) influences and ENSO. Streamflow regions in the United States were identified that respond to these climatic couplings. The results show that the AMO may influence La Niña impacts in the Southeast, while the NAO may influence La Niña impacts in the Midwest. By utilizing the streamflow water year and the long lead time for the oceanic‐atmospheric variables, useful information can be provided to streamflow forecasters and water managers.