Abstract:This study aims to determine trends in the long-term annual mean and monthly total precipitation series using nonparametric methods (i.e. the Mann-Kendall and Sen's T tests). The change per unit time in a time series having a linear trend was estimated by applying a simple non-parametric procedure, namely Sen's estimator of slope. Serial correlation structure in the data was accounted for determining the significance level of the results of the Mann-Kendall test. The data network used in this study, which is assumed to reflect regional hydroclimatic conditions, consists of 96 precipitation stations across Turkey. Monthly totals and annual means of the monthly totals are formed for each individual station, spanning from 1929 to 1993. In this case, a total of 13 precipitation variables at each station are subjected to trend detection analysis. In addition, regional average precipitation series are established for the same analysis purpose. The application of a trend detection framework resulted in the identification of some significant trends, especially in January, February, and September precipitations and in the annual means. A noticeable decrease in the annual mean precipitation was observed mostly in western and southern Turkey, as well as along the coasts of the Black Sea. Regional average series also displayed trends similar to those for individual stations.

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The relationship between the E1Nifio/Southern Oscillation (ENSO) and unimpaired streamflow over the contiguous United States is studied. The extreme phases of the Southern Oscillation have been linked to fairly persistent classes of atmospheric anomalies over the low and middle latitudes at regional and global scales. Of particular interest in this investigation is the identification of regions of land that appear to have strong and consistent ENSO-related streamflow signals. The first harmonic extracted from a 24-month ENSO composite at each station is assumed to be the ENSO-related signal appearing in streamflow anomalies. These regions were identified by the similarity in phase of the harmonic vectors. The vectorial display of these harmonics over a map of the United States provides the areal extents of ENSO influence on streamflow. Coherent and significant streamflow responses to hypothesized ENSO forcing are found in four regions of the United States: the Gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest. Once an ENSO event sets in, a long-range forecasting utility may be available for these regions. The results of this analysis, which are consistent with previous studies on precipitation and temperature, demonstrate the mid-latitude hydrologic response to the tropical ENSO phenomena. most extensive region in the world of water warmer than 28øC is located in the western tropical Pacific-Indonesia region. This large warm water pool is associated with heavy rainfall and is considered a major source of atmospheric heating, which drives large-scale convective circulation patterns [Rasmusson, 1987]. Normally, the trade winds (easterlies) are driven from the eastern to the western Pacific by the differences in atmospheric pressures. These winds tend to move water westward along the equator, maintaining a buildup of warm surface water and creating, in turn, a higher sea level elevation in the western Pacific. When the Southern Paper number 93WR00744. .0043.1397/93/93WR-00744505.00 and the trade winds usually weaken (or become westerlies as the case in 1982-1983). The warm water pool extends eastward, piling up off the coast of Peru and southern Ecuador. This is the sign of the development of an E1 Nifio event [Nicholls, 1987; Vogel, 1989]. E1 Nifio events have been observed and recorded since 1726 [Cane, 1983]. They occur approximately once every 4 years; however, the time interval between successive events varies from 2 to 10 years. No two events are completely alike: They evolve according to a consistent pattern, but differ in timing, intensity, extent, and duration. Quinn et al. [1978] introduce four E1 Nifio categories: strong, moderate, weak, and very weak. However, there are noticeable differences within each category. The strongest El Nifio episode in this century, in 1982-1983, had a unique pattern in terms of the sequence of the waxming and the time of onset [Cane, 1983]. The El Nifio/Southern Oscillation (ENSO) is a warm event in the tropical Pacific Ocean and is considered a sig...

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Abstract:In the present study, the trends in the reference evapotranspiration (ET O ) estimated through the Penman-Monteith method were investigated over the humid region of northeast (NE) India by using the Mann-Kendall (MK) test after removing the effect of significant lag-1 serial correlation from the time series of ET O by pre-whitening. During the last 22 years, ET O has been found to decrease significantly at annual and seasonal time scales for 6 sites in NE India and NE India as a whole. The seasonal decreases in ET O have, however, been more significant in the pre-monsoon season, indicating the presence of an element of a seasonal cycle. The decreases in ET O are mainly attributed to the net radiation and wind speed, which are also corroborated by the observed trends in these two parameters at almost all the times scales over most of the sites in NE India. The steady decrease in wind speed and decline in net radiation not only balanced the impact of the temperature increases on ET O , but may have actually caused the decreases in ET O over the humid region of northeast India.

In an earlier study of the teleconnection between streamflow and the warm (El niño) phase of the El Niño/Southern Oscillation (ENSO) cycle, we found a strong relationship evident in four regions of the United States: the Gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest. In this present study we have examined the same four regions for a relationship between streamflow and the cold (La niña) phase of the Southern Oscillation (SO). Invariably, we found evidence of strong and consistent streamflow responses to La niña events within the study regions. In each of the four regions, the strongest La niña signal occurred at the same time of year as had the El niño signal in their respective years. The sign of the seasonal streamflow anomaly associated with the La niña events is the opposite of that associated with the El niño events. This documents the existence of the biennial tendency related to the SO in the streamflow anomaly, which is expected, since La niña/El niño are opposite phases of the ENSO cycle. Finally, the relationships between streamflow and La niño/El niño were found to be statistically significant, based on the hypergeometric distribution. The results of this study demonstrate coherent, consistent, and significant midlatitude streamflow responses to the tropical SO phenomenon. This confirms the results of previous climatological studies that have examined the extratropical teleconnections from a hydrological and meteorological perspective.

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