Abstract: It is well established that variations in extratropical North Pacific wind stress fields can influence the state of the tropical Pacific 12-15 months prior to the maturation of boreal winter El Nio/Southern Oscillation (ENSO) events. While most research has focused on accompanying variations in the North Pacific trade winds and underlying sea surface temperatures that subsequently shift equatorward via anomalous air-sea interactions-e.g. meridional mode dynamics-observational and numerical model analyses indicate empirical and dynamical links exist between these same trade-wind variations and concurrent changes in subsurface temperatures across the equatorial Pacific, which can also serve as a key initiator of ENSO events. This paper shows that within an observationally-constrained ocean reanalysis dataset this initiation mechanism-termed the trade-wind charging (TWC) mechanism-is induced by the second leading mode of boreal winter zonal wind stress variability over the tropical Pacific and operates separately from ENSO-induced recharge/discharge of the equatorial Pacific heat content. The paper then examines the characteristics and evolution of the ENSO and TWC modes. Results indicate that the oceanic evolution for both modes is consistent with wind stress induced vertically-integrated, meridional mass transport into and out of the equatorial Pacific-i.e. a charging and discharging of the equatorial Pacific-despite having distinctly different wind stress anomaly patterns. The process-based similarity between these two modes of tropical Pacific wind stress variability suggests that both can produce a charging/discharging of the equatorial Pacific, however one (the ENSO mode) represents part of the ENSO cycle itself and the other (the TWC mode) represents a separate forcing mechanism of that cycle.

Abstract: The interannual, equatorial Pacific, 20 degrees C isotherm depth variability since 1980 is dominated by two empirical orthogonal function (EOF) modes: the "tilt" mode, having opposite signs in the eastern and western equatorial Pacific and in phase with zonal wind forcing and El Nino-Southern Oscillation (ENSO) indices, and a second EOF mode of one sign across the Pacific. Because the tilt mode is of opposite sign in the eastern and western equatorial Pacific while the second EOF mode is of one sign, the second mode has been associated with the warm water volume (WWV), defined as the volume of water above the 20 degrees C isotherm from 5 degrees S to 5 degrees N, 120 degrees E to 80 degrees W. Past work suggested that the WWV led the tilt mode by about 2-3 seasons, making it an ENSO predictor. But after 1998 the lead has decreased and WWV-based predictions of ENSO have failed. The authors constructed a sea level-based WWV proxy back to 1955, and before 1973 it also exhibited a smaller lead. Analysis of data since 1980 showed that the decreased WWV lead is related to a marked increase in the tilt mode contribution to the WWV and a marked decrease in second-mode EOF amplitude and its contribution. Both pre-1973 and post-1998 periods of reduced lead were characterized by "mean" La Nina-like conditions, including a westward displacement of the anomalous wind forcing. According to recent theory, and consistent with observations, such westward displacement increases the tilt mode contribution to the WWV and decreases the second-mode amplitude and its WWV contribution.

Abstract: Drought index is a useful tool to assess and respond to drought. However, current drought indices could not fully reveal land use effects and they have limitations in applications. Besides, El Niño Southern Oscillation (ENSO), strongly influences the climate of Florida. Hence, understanding ENSO patterns on a regional scale and developing a new land use drought index suitable for Florida are critical in agriculture and water resources planning and management. This paper presents a 32 km high resolution land use adapted drought index, which relies on five types of land uses (lake, urban, forest, wetland, and agriculture) in Florida. The land uses were obtained from National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR) data from 1979 to 2002. The results showed that Bowen ratio responded to land use and could be used as an indicator to monitor drought events. Then, an innovative regional land use drought index was developed from the normalized Bowen ratio, which could reflect not only the level of severity during drought events resulting from land use effects, but also La Niña driven drought impacts. The proposed new index may help scientists answer the critical questions about drought effect on various land uses and potential feedbacks of changes in land use and land cover to climate.

Abstract: Much of the year-to-year climate variability on the Earth is associated with El Nino and the Southern Oscillation (ENSO). This variability is generated ˜ primarily by a coupled ocean-atmosphere instability near the eastern edge of the western equatorial Pacific warm pool. Here, I discuss the physics of this variability, including its phase locking to the seasonal cycle. ENSO growth typically occurs from April/May to November, and by July the perturbation is usually strong enough that it persists to the beginning of the following year, when ENSO events usually end. Consequently, predicting ENSO is easy from July to February but is more challenging across the April/May transition to the next event. I discuss precursors of this transition and recent results from dynamical and statistical models used for ENSO forecasting.

Abstract: Agriculture is an economic activity directly affected by drought. With the constant increase of food demand and the need for high efficiency in food production, drought effects gain attention. The Agricultural Reference Index for Drought (ARID) was developed to quantify drought and better understand its effect on agriculture. ARID values range from 0 to 1 where zero is transpiration occurring at potential rate and one is full water deficit. El Niño Southern Oscillation (ENSO) is the main factor of climate variability around the world and affects the climate of two states located in midlatitudes, Florida, USA and Rio Grande do Sul, Brazil. An historical assessment of the relationship between ARID and ENSO will assist with establishing the role played by ENSO in the seasonal variation of soil moisture, which is critical to drought forecasting based on ENSO. The main objectives of this study were to investigate ARID's temporal and spatial variability in Florida and Rio Grande do Sul, and provide an assessment of ENSO-induced potential anomalies in ARID. Daily ARID values in Florida were calculated using data obtained from the National Weather Service COOP (Cooperative Observer Program) weather stations. In Rio Grande do Sul, ARID was calculated using daily weather data from the Instituto Nacional de Meteorologia (INMET) weather stations, Brazil. Daily ARID values were compiled into monthly averages and categorized according to ENSO phase based on the Multivariate ENSO Index (MEI). Results showed that typical ARID values for both regions vary throughout the year. The highest values were observed in the warm season due to increased evapotranspiration. During this period, ARID values varied from 0.3 to 0.8 in Florida and 0.2 to 0.7 in Rio Grande do Sul. In the cold period, South Florida had the highest ARID values, varying from 0.3 to 0.7, while for the rest of state and Rio Grande do Sul the values varied from 0 to 0.5. ENSO showed stronger influence from November to March in Florida, and November and May in Rio Grande do Sul. For both regions, El Niño caused ARID values below normal, indicating higher soil moisture while during La Niña the values were above normal. This study contributes to establish historical ARID records across the southeastern United States and southern Brazil, and can assist farmers to adjust management practices according to expected water stress conditions.

Abstract: Research has shown that there is significant diversity in the location of the maximum sea surface temperature anomaly (SSTA) associated with the El Nio Southern Oscillation (ENSO). In one extreme, warm SSTA peak near the South American coast (often referred to as Eastern Pacific of EP El Nio), and at the other extreme, warm SSTA peak in the central Pacific (Central Pacific or CP El Nio). Due to the differing tropical Pacific SSTA and precipitation structure, there are differing extratropical responses, particularly over North America. Recent work involving the North American Multi-Model Ensemble (NMME) system for intra-seasonal to inter-annual prediction on prediction of the differences between El Nio events found excess warming in the eastern Pacific during CP El Nio events. This manuscript investigates the ensemble and observational agreement of the NMME system when forecasting the North American response to the diversity of ENSO, focusing on regional land-based 2-meter temperature and precipitation. NMME forecasts of North American precipitation and T2m agree with observations more often during EP events. Ensemble agreement of NMME forecasts is regional. For instance, ensemble agreement in Southeast North America demonstrates a strong connection to NINO3 precipitation and SSTA amplitude during warm ENSO events. Ensemble agreement in Northwest North America demonstrates a weak connection to NINO4 precipitation and SSTA amplitude during warm ENSO events. Still other regions do not show a strong connection between ensemble agreement and strength of warm ENSO events.

Abstract: Large scale climate phenomena can provide valuable information for regional climate and streamflow in many parts of the world. Several climate phenomena may impact a given area and their value for providing information on streamflow is dependent on first establishing the local relationship. This study was conducted to establish the individual and coupled impacts of the El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) on streamflow in the Apalachicola-Chattahoochee-Flint (ACF) river basin. Differences in annual and seasonal streamflow using two unimpaired streamflow datasets based on the phase(s) of ENSO, the PDO, and the AMO were evaluated using the nonparametric rank-sum test. Few statistical differences were found for the individual impacts of ENSO and the PDO on annual and seasonal streamflow; differences based on ENSO were largely confined to the southern portion of the basin. Significant differences in annual streamflow based on the AMO were largely confined to the northern half of the basin. Differences in seasonal streamflow based on the AMO were found for much of the year in the northern portion of the basin but were confined to the winter season in the southern portion. Significant differences in annual and seasonal streamflow were found between the La Nina/positive AMO phase and the El Nino/negative AMO phase, between the positive AMO/negative PDO phase and the negative AMO/negative PDO phase, and there appears to be a modulation of the impacts of La Nina by the phase of the AMO. A greater number of stations and a greater magnitude of significant differences were found for the coupled impacts than for the individual impacts of ENSO, the PDO, and the AMO; indicating the importance of the coupled impacts on regional streamflow when establishing the role of annual, decadal, and multidecadal climate variability.

Abstract: The El Nino/Southern Oscillation (ENSO) is a global climate phenomenon with strong effects on Florida's weather patterns. ENSO has been shown to have predictable effects on streamflow, rainfall, and crop yield; however, the relationship between N and P loading and ENSO has not been previously explored. Nutrient loads for a Lake Okeechobee sub-basin for 1965-2001 were simulated with the Watershed Assessment Model (WAM) and compared to measured P loads. The NS coefficients for simulated and measured monthly P loads were 0.73 for the calibration period and 0.63 for the validation period, which indicates "satisfactory" to "good" model performance. With a probable error range (PER) of +/-27.8% for measured P loads, the modified NS coefficients increased to 0.94 for the calibration period and 0.93 for the validation period. Results showed that ENSO strongly affected simulated seasonal and monthly phosphorus runoff. El Nino years produced seasonal peak loads of P runoff into Lake Okeechobee significant at the 99% level during the spring (February April), which indicates dominance of positive load anomalies. La Nina years produced significant seasonal peak loads in the summer (May-July) but with much greater variability in loads. Neutral years exhibited less predictable seasonal loading, although simulated P loads were generally similar to measured long-term means. Nutrient loading patterns during specific ENSO phases were comparable to previously explored precipitation and streamflow patterns in south Florida. This research has potential for use by land and water managers who can add short-term ENSO-based climate forecasts to their toolbox for reducing nutrient runoff.