Warm ENSO Research Articles

The Influence of the El Niño–Southern Oscillation on Cloud-to-Ground Lightning Activity along the Gulf Coast. Part II: Monthly Correlations

Abstract The El Niño–Southern Oscillation (ENSO) cycle is known to influence weather and climate along the Gulf Coast region, causing anomalously high precipitation during El Niño winters. This region is also known for having the highest lightning flash density in the United States. An 8-yr dataset (1995–2002) of cloud-to-ground (CG) lightning flashes was analyzed to determine if the ENSO cycle influences lighting activity along the Gulf Coast region. Simple Pearson’s correlations were computed between concurrent monthly pairings of Niño-3.4 sea surface temperature (SST) and CG lightning flash deviation values from the study area. The correlation results are mapped and analyzed for links to meteorological features. Statistically significant correlation values greater than 0.8 were noted over large swaths of the study area during each winter month. The highest correlations were arranged in banded swaths and associated with regions of low flash densities during December and February. In January, areas of high correlation were spatially coincident with areas of enhanced flash density. Both the enhanced CG flash regions and high correlation values and patterns are indicative of a southerly shift in the midlatitude storm track known to occur during warm ENSO events. During the spring and summer, most of the region has weak correlation with ENSO except for August, which has a large area of negative correlations. These findings indicate that lightning increases during La Niña summers. Correlation patterns in late fall are similar to those of winter. The ENSO–lightning relationship has implications for hazard assessment and can be a useful tool for long-term seasonal planning.

The Influence of the El Niño–Southern Oscillation on Cloud-to-Ground Lightning Activity along the Gulf Coast. Part I: Lightning Climatology

Abstract Cloud-to-ground (CG) lightning flashes from the National Lightning Detection Network are analyzed to determine if the El Niño–Southern Oscillation (ENSO) cycle influences lighting activity along the Gulf Coast region. First, an updated climatology of lightning was developed for the region. Flash density maps are constructed from an 8-yr dataset (1995–2002) and compared with past lightning climatologies. Second, lightning variability is compared with the phases of ENSO. Winter lightning distributions are compared with one published study of ENSO and lightning days in the Southeast. Flash density patterns are, overall, consistent with past U.S. lightning climatology. However, the peak flash density for the annual mean was less than observed in previous climatologies, which could be due to the disproportionately large percentage of cool ENSO periods compared to previous lightning climatologies. The highest annual lightning counts were observed in 1997, which consisted of mostly warm ENSO seasons; the 1997–98 El Niño was one of the strongest on record. The lowest lightning counts were observed in 2000, which had mostly cool or neutral phases of ENSO including the lowest Niño-3.4 anomaly of the study period. Analysis of winter season lightning flash densities substantiated the role of the ENSO cycle in winter season lightning fluctuations. Winter lightning activity increased dramatically during the 1997–98 El Niño. The lowest winter flash densities are associated with cool ENSO phases. Although 8 yr is inadequate to establish a long-term pattern, results indicate that ENSO influences lightning and that further study is warranted. As more years of lightning data are acquired, a more complete climatology can be developed.

Coastal upwelling along the southwest coast of India – ENSO modulation

Abstract. An index of El Niño Southern Oscillation (ENSO) in the Pacific during pre monsoon season is shown to account for a significant part of the variability of coastal Sea Surface Temperature (SST) anomalies measured a few months later within the wind driven southwest coast of India coastal upwelling region 7° N–14° N. This teleconnection is thought to result from an atmospheric bridge between the Pacific and north Indian Oceans, leading to warm (cold) ENSO events being associated with relaxation (intensification) of the Indian trade winds and of the wind-induced coastal upwelling. This ENSO related modulation of the wind-driven coastal upwelling appears to contribute to the connection observed at the basin-scale between ENSO and SST in the Arabian Sea. The ability to use this teleconnection to give warning of large changes in the southwest coast of India coastal upwelling few months in advance is successfully tested using data from 1998 and 1999 ENSO events.

Decadal variability of the IOD-ENSO relationship

Using three kinds of over 100-year sea surface temperature (SST) datasets as well as three-dimensional wind data from NCEP/NCAR, this paper documents the decadal variability of the Indian Ocean Dipole (IOD)-El Nino/Southern Oscillation (ENSO) relationship. During 1948–1969, positive (negative) IOD and warm (cold) ENSO events were more independent of each other. But after 1970, they tended to occur in the same year. ENSO would influence the whole life span of IOD, and IOD also affects the developing phase of ENSO. Considering the climatological background SST, low-level winds and also equatorial vertical circulations, it is revealed that the decadal variability of the IOD-ENSO relationship may be caused by the enhanced Walker circulation with increased rising motion over the Maritime Continent after 1970. Warmer SST around the Maritime Continent gives rise to anomalous low-level convergence and intensified convection there, which apparently increases the SST linkage between the eastern Indian Ocean and the western Pacific and thereby the interaction between the IOD and ENSO event.

Global responses of terrestrial productivity to contemporary climatic oscillations

The terrestrial biosphere is subjected to a wide range of natural climatic oscillations. Best known is the El Niño-southern oscillation (ENSO) that exerts globally extensive impacts on crops and natural vegetation. A 50-year time series of ENSO events has been analysed to determine those geographical areas that are reliably impacted by ENSO events. Most areas are impacted by changes in precipitation; however, the Pacific Northwest is warmed by El Niño events. Vegetation gross primary production (GPP) has been simulated for these areas, and tests well against independent satellite observations of the normalized difference vegetation index. Analyses of selected geographical areas indicate that changes in GPP often lead to significant changes in ecosystem structure and dynamics. The Pacific decadal oscillation (PDO) is another climatic oscillation that originates from the Pacific and exerts global impacts that are rather similar to ENSO events. However, the longer period of the PDO provided two phases in the time series with a cool phase from 1951 to 1976 and a warm phase from 1977 to 2002. It was notable that the cool phase of the PDO acted additively with cool ENSO phases to exacerbate drought in the earlier period for the southwest USA. By contrast in India, the cool phase of the PDO appears to reduce the negative impacts of warm ENSO events on crop production.

A Chronology of El Niño Events from Primary Documentary Sources in Northern Peru*

AbstractThe authors present a chronology of El Niño (EN) events based on documentary records from northern Peru. The chronology, which covers the period 1550–1900, is constructed mainly from primary sources from the city of Trujillo (Peru), the Archivo General de Indias in Seville (Spain), and the Archivo General de la Nación in Lima (Peru), supplemented by a reassessment of documentary evidence included in previously published literature. The archive in Trujillo has never been systematically evaluated for information related to the occurrence of El Niño–Southern Oscillation (ENSO). Abundant rainfall and river discharge correlate well with EN events in the area around Trujillo, which is very dry during most other years. Thus, rain and flooding descriptors, together with reports of failure of the local fishery, are the main indicators of EN occurrence that the authors have searched for in the documents. A total of 59 EN years are identified in this work. This chronology is compared with the two main previous documentary EN chronologies and with ENSO indicators derived from proxy data other than documentary sources. Overall, the seventeenth century appears to be the least active EN period, while the 1620s, 1720s, 1810s, and 1870s are the most active decades. The results herein reveal long-term fluctuations in warm ENSO activity that compare reasonably well with low-frequency variability deduced from other proxy data.

Influence of the Pacific Decadal Oscillation and El Niño Southern Oscillation on Operation of the Capilano Water Supply Reservoir, Vancouver, British Columbia

The magnitude and duration of low reservoir inflows in the summer and early fall are critical for water supply planning and operations for the Greater Vancouver Regional District (now Metro Vancouver). The influence of climate fluctuations on summer and fall reservoir inflows was investigated and a reservoir operation model developed to simulate Capilano Reservoir operations. Flow records for the gauge on the Capilano River above Intake were adjusted to account for releases from the upstream Palisade Lake. It was found that reservoir inflows for the June to September period are affected by the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The 15 lowest June to September inflows over the period of gauge records have all been associated with warm or neutral ENSO conditions typically combined with warm or neutral PDO conditions. Annual reservoir drawdown, defined as the annual extended period of no spill, was used as an indicator of stress on reservoir operations. Simulation of reservoir inflows and outflows from 1914 to 2005 showed that the longest reservoir drawdown durations were almost always associated with warm ENSO and warm PDO conditions. The shortest periods of drawdown were normally associated with both cool ENSO and cool PDO conditions. Due to the storage effects, reservoir drawdown is a good indicator of periodic climate variations because of amplification, similar to the sensitivity of glaciers to climate episodes.

The 1950-1998 warm ENSO events and regional implications to river flow variability in Southern Africa

The variability of annual river runoff and its possible association with the 1950-1998 seasonal El Niño/Southern Oscillation (ENSO) is investigated in 502 rivers gauged in 9 countries of the Southern African region. We found some evidence of possible links between available surface water resources in terms of mean annual runoff and warm ENSO events. This was revealed by the existence of strong and nearly-strong positive linear correlation between annual discharges and the warm seasonal ENSO indices explained by the sea level pressure (SLP) data. Of the 502 rivers we considered, 150 rivers exhibit strong positive correlation between the December to February quarter ENSO indices and the annual runoff – with 25% of the variance in annual runoff being accounted for by the warm ENSO events. A relatively weaker positive correlation also occurred in 174 rivers we considered. The strong positive correlation occurs in parts of Zambia, Namibia, Mozambique and the lowveld in South Africa. In these parts of Southern Africa, there is evidence of a general decline in annual runoff after the mid-1970s compared to the period before it. These revelations are explored and are found to be partly explained by the high frequency of drought-related warm ENSO phenomena that occurred during the same period. Water SA Vol.32 (4) 2006: pp.459-463

Effects of the El Niño–Southern Oscillation and the Quasi‐Biennial Oscillation on polar temperatures in the stratosphere

Reanalysis data are used to study the effects of the Quasi‐Biennial Oscillation (QBO) and the El Niño–Southern Oscillation (ENSO) on the stratosphere. During the boreal winter in the Arctic, Warm ENSO (WENSO) months are found to be significantly warmer and Cold ENSO (CENSO) months significantly colder than climatology. The QBO is also found to have a large effect on the Arctic stratosphere during the late fall/early winter; Westerly QBO (WQBO) poles are colder, and Easterly QBO (EQBO) poles are warmer. In the first half of the 50 years of interest, WENSO and EQBO have had a tendency to be correlated in time, and thus their signals are difficult to disentangle. In order to isolate each effect from the other, composites are taken of QBO months under near‐neutral ENSO conditions, which show a clear effect in late fall/early winter. Because of the bimodality of QBO, producing a meaningful composite of ENSO months under near‐neutral QBO is difficult, as the number of available months is quite small. To distinguish ENSO from QBO and to further study the QBO, we compare composites of months with four different combinations of QBO and ENSO anomalies, which confirms that ENSO does have a significant effect on the polar vortex. These groupings are also studied after removing the 2 years following one of the three major volcanic eruptions during the 50 years of data and during the post‐1979 satellite era only as well. These composites show distinct ENSO and QBO effects of comparable magnitude.

El Niño‐Southern Oscillation, rainfall, temperature and Normalized Difference Vegetation Index fluctuations in the Mara‐Serengeti ecosystem

AbstractUnderstanding long‐term climatic variability is basic to wise management and conservation of biodiversity. We analysed temporal variations in the local rainfall, temperature, Normalized Difference Vegetation Index and the hemispheric El Niño‐Southern Oscillation (ENSO), using the Southern Oscillation Index and how they co varied in the Mara‐Serengeti ecosystem of Kenya and Tanzania. Local rainfall showed a striking temporal variability and an evident 5‐year quasi‐periodicity in the ecosystem. Severe droughts were a recurrent/persistent feature of the ecosystem but extreme floods were relatively infrequent. The timings of droughts and floods coincided with strong episodes in the activities of the ENSO phenomenon. Above‐average rainfall often accompanied cold ENSO episodes and below‐average rainfall warm ENSO events, contrary to past generalizations suggesting that warm ENSO events are only associated with above‐average rainfall whereas cold ENSO events with below‐average rainfall in equatorial East Africa. Both minimum and maximum temperatures were below‐normal during cold ENSO episodes and above‐normal during warm ENSO events. Rising temperatures and declining rainfall throughout the 1990s and early 2000s, with unprecedently prolonged and strong ENSO episodes, engendered progressive habitat desiccation and reduction in vegetation production in the ecosystem. This exacerbated the debilitating effects of adverse weather on local plant and animal communities, resulting in high mortalities of ungulates.

How ENSO impacts precipitation in southwest central Asia

A linkage between ENSO and the hydroclimatic variability of the southwest central Asia region (SWCA) is established through observational analysis of precipitation, moisture flux and sea level pressure data, with further support from an atmospheric model of intermediate complexity. Enhanced precipitation in SWCA during warm ENSO events results from an anomalous southwesterly moisture flux coming from the Arabian Sea and tropical Africa, which is generated along the northwestern flank of the high pressure anomaly over the Indian and western Pacific Oceans, part of the canonical ENSO sea‐saw pressure anomalies. The ENSO impact on SWCA precipitation is found to be greatest in the transition seasons of autumn and spring, but the dynamical impact on pressure and circulation persists throughout the year. This connection was particularly strong in recent decades. Model sensitivity experiments further show that this is driven primarily by tropical Pacific SST anomalies and associated large‐scale sea‐level pressure changes, while the Indian Ocean SST has opposite effects.

Regional Climate Model–Simulated Timing and Character of Seasonal Rains in South America

Abstract The potential of an experimental nested prediction system to improve the simulation of subseasonal rainfall statistics including daily precipitation intensity, rainy season onset and withdrawal, and the frequency and duration of dry spells is evaluated by examining a four-member ensemble of regional climate model simulations performed for the period 1982–2002 over South America. The study employs the International Centre for Theoretical Physics (ICTP) regional climate model, version 3 (RegCM3), driven with the NCEP–NCAR reanalysis and the European Centre–Hamburg GCM, version 4.5. Statistics were examined for five regions: the northern Amazon, southern Amazon, the monsoon region, Northeast Brazil, and southeastern South America. RegCM3 and the GCM are able to replicate the distribution of daily rainfall intensity in most regions. The analysis of the rainy season timing shows the observed onset occurring first over the monsoon region and then spreading northward into the southern Amazon, in contrast to some previous studies. Correlations between the onset and withdrawal date and SSTs reveal a strong relationship between the withdrawal date in the monsoon region and SSTs in the equatorial Pacific, with above-average SSTs associated with late withdrawal. Over Northeast Brazil, the regional model errors are smaller than those shown by the GCM, and the strong interannual variability in the timing of the rainy season is better simulated by RegCM3. However, the regional model displays an early bias in onset and withdrawal over the southern Amazon and the monsoon regions. Both RegCM3 and the GCM tend to underestimate (overestimate) the frequency of shorter (longer) dry spells, although the differences in dry spell frequency during warm and cold ENSO events are well simulated. The results presented here show that there is potential for added value from the regional model in simulating subseasonal statistics; however, improvements in the physical parameterizations are needed for this tropical region.

A numerical study on the winter monsoon and cold surge over East Asia

By using the improved regional climate model (RegCM_NCC), a numerical study has been undertaken for the East Asia region over a period of 5 years (1998–2002) in an effort to evaluate the model’s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model’s ability to simulate the East Asian winter monsoon.

The Role of the Indonesian Throughflow in the Indo–Pacific Climate Variability in the GFDL Coupled Climate Model

AbstractThe impacts of the Indonesian Throughflow (ITF) on the tropical Indo–Pacific climate, particularly on the character of interannual variability, are explored using a coupled general circulation model (CGCM). A pair of CGCM experiments—a control experiment with an open ITF and a perturbation experiment in which the ITF is artificially closed—is integrated for 200 model years, with the 1990 values of trace gases. The closure of the ITF results in changes to the mean oceanic and atmospheric conditions throughout the tropical Indo–Pacific domain as follows: surface temperatures in the eastern tropical Pacific (Indian) Ocean warm (cool), the near-equatorial Pacific (Indian) thermocline flattens (shoals), Indo–Pacific warm-pool precipitation shifts eastward, and there are relaxed trade winds over the tropical Pacific and anomalous surface easterlies over the equatorial Indian Ocean. The character of the oceanic changes is similar to that described by ocean-only model experiments, though the amplitude of many features in the tropical Indo–Pacific is amplified in the CGCM experiments.In addition to the mean-state changes, the character of tropical Indo–Pacific interannual variability is substantially modified. Interannual variability in the equatorial Pacific and the eastern tropical Indian Ocean is substantially intensified by the closure of the ITF. In addition to becoming more energetic, El Niño–Southern Oscillation (ENSO) exhibits a shorter time scale of variability and becomes more skewed toward its warm phase (stronger and more frequent warm events). The structure of warm ENSO events changes; the anomalies of sea surface temperature (SST), precipitation, and surface westerly winds are shifted to the east and the meridional extent of surface westerly anomalies is larger.In the eastern tropical Indian Ocean, the interannual SST variability off the coast of Java–Sumatra is noticeably amplified by the occurrence of much stronger cooling events. Closing the ITF shoals the eastern tropical Indian Ocean thermocline, which results in stronger cooling events through enhanced atmosphere–thermocline coupled feedbacks. Changes to the interannual variability caused by the ITF closure rectify into mean-state changes in tropical Indo–Pacific conditions. The modified Indo–Pacific interannual variability projects onto the mean-state differences between the ITF open and closed scenarios, rectifying into mean-state differences. These results suggest that CGCMs need to reasonably simulate the ITF in order to successfully represent not just the mean climate, but its variations as well.

ENSO와 한국의 수문변량들간의 계절적 관계 분석

최근 들어 세계의 연중 기후 변화에 주된 요인으로써 엘니뇨와 같은 현상이 매우 잦아졌다. 많은 기상수문학자들이 강수와 유량에 대한 엘니뇨 남방진동의 영향에 대해 연구하고 있지만, 수문변량들은 큰 지역적 변동을 갖고 있기 때문에 결정적인 인과관계를 찾아내는데 있어서 많은 어려움이 있다. 본 연구에서는 엘니뇨-남방진동과 한국에서의 수문변량들 간의 계절석 관계를 고찰하였다. 엘니뇨-난방진동을 정량적으로 표현해 주는 지수로써 남방진동지수를 사용하였고, 월강수량 자료, 월평균기온 자료 그리고 댐의 월유입량 자료를 표준정규분포를 가지는 표준정규지수로 변환하여 사용하였다. 계절적 관계를 파악하기 위해 난방진동지수와 수문변량의 월 자료는 봄 (3월-5월), 여름(6윌-8월), 가을(9월-11월) 그리고 겨울 (12윌-2월)로 분류되었다. ENSO episode에 대한 수문변량들의 조건부 초과확률과 분포형태를 바탕으로 분석을 수행한 결과 전반적으로 Warm ENSO episode의 경우 강수량 증가와 기온 상승과 관련이 있고, Cold ENSO episode의 경우 강수량 감소와 기온 하강과 관계가 있다. 그러나 일부 지역에서는 이러한 전반적인 결과와 상이한 결과가 나타나기도 하였다. Climatic abnormal phenomena involving El Nino and La Nina have been frequently reported in recent decades. The interannual climate variability represented by El Nino-Southern Oscillation (ENSO) is sometimes investigated to account for the climatic abnormal phenomena around the world. Although many hydroclimatologists have studied the impact of ENSO on regional precipitation and streamflow, however, there are still many difficulties in finding the dominant causal relationship between them. This relationship is very useful in making hydrological forecasting models for water resources management. In this study, the seasonal relationships between ENSO and hydrologic variables were investigated in Korea. As an ENSO indicator, Southern Oscillation Index (SOI) was used. Monthly precipitation, monthly mean temperature, and monthly dam inflow data were used after being transformed to the standardized normal index. Seasonal relationships between ENSO and hydrologic variables were investigated based on the exceedance probability and distribution of hydrologic variables conditioned on the ENSO episode. The results from the analysis of this study showed that the warm ENSO episode affects increases in precipitation and temperature, and the cold ENSO episode is related with decreases in precipitation and temperature in Korea. However, in some regions, the local relationships do not correspond with the general seasonal relationship.

Teleconnections between ENSO and North Atlantic in an ECHO-G simulation of the 1000–1990 period

[1] The North Atlantic sea level pressure anomalies (SLPA) associated with El Nino Southern Oscillation (ENSO) have been analyzed in a quasi-millennial (1000–1990) simulation with the ECHO-G model. In November–December, the ENSO-related SLPA over the North Atlantic area are weak, while a realistic pattern already appears over the North Pacific and North America. In January–March, the SLPA over North Atlantic are stronger and realistic from the North Pacific to Europe: the Aleutian low is strengthened (weakened), SLPA are positive (negative) over the north-central and north-eastern parts of North America, and SLPA display a negative (positive) NAO-like pattern over North-Atlantic during warm (cold) ENSO events, as in observations. The results also confirm the existence of a strong inter-event SLPA associated with warm and cold ENSO events, especially over the North Atlantic, while the relationship is stationary at multidecadal timescales. It seems that neither the intensity nor geographical longitude of the equatorial Pacific sea surface temperature anomaly (SSTA) and intensity of tropical Atlantic SSTA, nor the volcanic forcing, simply introduced here as a decrease of the solar constant, significantly induce an inter-event variability, which seems, in this run, mostly of atmospheric origin.

Variability of Indian monsoon-ENSO relationship in a 1000-year MRI-CGCM2.2 simulation

There is a close relationship between interannual variability of the Indian summer monsoon rainfall and the El Nino/Southern Oscillation (ENSO) (drought conditions over India accompany warm ENSO events and vice versa). However, recent observations suggest a weakening of this ENSO-monsoon relationship that may be linked to global warming. We report here an analysis of the ENSO-monsoon relationship within the framework of a 1000-year control simulation of the MRI-coupled general circulation model (GCM), MRI-CGCM2.2. An overall correlation between the June-July-August (JJA) Nino3.4 sea surface temperature and the JJA Indian monsoon rainfall is –0.39, with reasonable circulation characteristics associated with the modeled ENSO. The simulated ENSO-monsoon relationship reveals long-term variations, from –0.71 to +0.07, in moving 31-year windows. This modulation in the ENSO-monsoon relationship is associated with decadal variability of the climate system.

Predictability of stream flow and rainfall based on ENSO for water resources management in Sri Lanka

Summary We investigate the viability of using El Nino-Southern Oscillation (ENSO) and sea surface temperature (SST) data to predict seasonal streamflow for one of the major rivers in Sri Lanka, the Kelani, using correlation analysis, contingency tables, and principal component analysis. The agricultural seasons in Sri Lanka are Yala (April-September) and Maha (October-March). The correlation between the Kelani River streamflow during Yala and ENSO indices (r = � 0.41) is significant at 99% level. In addition, the Kelani streamflow during Yala has a correlation with the Central Indian Ocean SST (r = � 0.40) that is also sig- nificant at the 99% level. The first principal component of the Indo-Pacific Ocean SST is reminiscent of the SST associated with the ENSO mode. A prediction scheme based on this mode for the streamflow during Yala has a skill characterized by a correlation of 0.5 in a cross-validated mode. The prediction of streamflow during Maha is best carried out separately for the two halves of the season. During the El Nino phase, the rainfall during Maha is enhanced during the first half of the season (October-December) and diminished in the second half (Jan- uary-February). Rainfall rather than streamflow has a better relationship with ENSO from October to December. During the second half of the Maha season, rainfall declines with both warm and cold ENSO phases and any prediction scheme has to take into account this non-linear relationship. Overall, useful skill for seasonal streamflow predictions has been demonstrated for the Yala season and skill for seasonal rainfall predictions for the first and second half of the Maha season has been elucidated. a 2006 Elsevier B.V. All rights reserved.

Coupled Ocean–Atmosphere Response to Seasonal Modulation of Ocean Color: Impact on Interannual Climate Simulations in the Tropical Pacific

Abstract The ability to use remotely sensed ocean color data to parameterize biogenic heating in a coupled ocean–atmosphere model is investigated. The model used is a hybrid coupled model recently developed at the Earth System Science Interdisciplinary Center (ESSIC) by coupling an ocean general circulation model with a statistical atmosphere model for wind stress anomalies. The impact of the seasonal cycle of water turbidity on the annual mean, seasonal cycle, and interannual variability of the coupled system is investigated using three simulations differing in the parameterization of the vertical attenuation of downwelling solar radiation: (i) a control simulation using a constant 17-m attenuation depth, (ii) a simulation with the spatially varying annual mean of the satellite-derived attenuation depth, and (iii) a simulation accounting for the seasonal cycle of the attenuation depth. The results indicate that a more realistic attenuation of solar radiation slightly reduces the cold bias of the model. While a realistic attenuation of solar radiation hardly affects the annual mean and the seasonal cycle due to anomaly coupling, it significantly affects the interannual variability, especially when the seasonal cycle of the attenuation depth is used. The seasonal cycle of the attenuation depth interacts with the low-frequency equatorial dynamics to enhance warm and cold anomalies, which are further amplified via positive air–sea feedbacks. These results also indicate that interannual variability of the attenuation depths is required to capture the asymmetric biological feedbacks during cold and warm ENSO events.

Epochal Changes in ENSO–Streamflow Relationships in Sri Lanka

Abstract In an effort to use climate predictions for streamflow and malaria hazard prediction, the decadal variability of the El Niño–Southern Oscillation (ENSO) influence on streamflow and rainfall in the Kelani River in Sri Lanka was investigated based on records from 1925 to 1995. In the last half century, the warm ENSO phase was associated with decreased annual streamflow and the cold ENSO phase with increased streamflow. The annual streamflow had a negative correlation (warm ENSO associated with low streamflow) with the concurrent ENSO index of Niño-3.4 that was significant at the 5% level. This negative correlation with Niño-3.4 is enhanced to a 1% significance level if the aggregate streamflow from January to September alone is considered. There has been a transition in correlation between January–September streamflow and ENSO between the 1950s and 1970s from near or above zero to negative values that have 95% significance levels reminiscent of an epochal shift. This shift was evident when considering the period when the southwest monsoon dominates (April–September) or when correlations were undertaken between the seasonal streamflow and rainfall and the ENSO index in the month prior to each season. Since the relationship between ENSO and Sri Lankan streamflow has strengthened in recent decades the potential for ENSO-based prediction is retained. The epochal shift may also explain why malaria epidemics ceased to co-occur frequently with El Niño episodes after 1945.