The interannual variability of summer and winter rainfall and streamflow in the Gulf of California continental watershed is compared. Varimax-rotated empirical orthogonal function (EOF) analysis is applied to 15 streamflow series, in the period from 1960 to 1990, and two regions are defined: a central region and a southern region. Results show that in both regions, between 1944 and 1999, the long-term rainfall variability is well explained by the long-term streamflow variability in both seasons, the result being statistically significant at the 95% level. We conclude that regional streamflows in that period are climate driven. This conclusion is reinforced when we show that the large-scale circulation (700 hPa heights) explains: (i) wet and dry conditions in both regions; (ii) conditions of wet and dry years with the same signal of El Niño and La Niña events; and (iii) long-term periods in association with the Pacific decadal oscillation (PDO). When the PDO is in its warm phase, summers are likely to be dry with an El Niño event and wet with a La Niña event. In the cool phase of the PDO, summers are influenced by more localized events (i.e. the position of the subtropical continental ridge). In winter, warm and cool phases of the PDO are likely to be associated with wet and dry winters respectively.
Severe droughts have caused serious impact on water supply and agriculture in São Francisco River Basin (SFRB), Brazil, especially during the rainy season. Observational evidence suggests that droughts in this region could be driven by some large‐scale ocean‐atmospheric patterns. This study provides a general description of linkage between the ocean‐atmospheric circulation patterns and the droughts during the rainy season (November–December), in the SFRB's upper sub‐basins. The NCEP reanalysis I and monthly gridded precipitation dataset of the Global Precipitation Climatology Centre from 1948 to 2010 were used. SPI was used as drought index. All analyses were performed in a domain that covered South America and southern‐tropical portions of the Atlantic and Pacific Oceans. Both the wind speed and vertical velocity fields at 200 hPa and specific humidity at 700 hPa were analysed. Results indicate that more extensive short‐duration droughts are linked to: (1) El Niño conditions highly concentrated inside the Niño 3.4 region; (2) ITCZ with a northernmost‐than‐normal position; (3) weaker‐than‐normal convective activity over the SFRB coupled to a weaker‐than‐normal upper‐level westerly jet stream. For decadal scale, the atmospheric anomalies patterns are best correlated to long‐duration extensive droughts than the oceanic anomalies patterns (r = −0.84 and −0.14, respectively). This suggests that droughts in SFRB are strongly dependent on large‐scale moisture transport and upper‐level atmospheric circulation.
Abstract:Previous hydroclimatological analysis of the North American Monsoon (NAM) region of northwest México revealed significant sub-regions of coherence in seasonal precipitation and streamflow. In this work, interannual variations in regionalized streamflow and rainfall-runoff relationships are explored. Modulation of the monsoon hydroclimatology by large-scale forcing mechanisms such as tropical, North Pacific, and Caribbean Sea-surface temperature perturbations results in nonlinear responses in seasonal runoff that varies by region. Analyses reveal that the El Niño-Southern Oscillation (ENSO) exerts a modest but statistically significant influence on NAM streamflow. Different sea-surface temperature and sea-level pressure indices exhibit markedly different lag-correlation structures in different NAM streamflow regions and these relationships vary significantly according to the decadal scale sea-surface temperature (SST) variability in the North Pacific. The structure of correlation patterns between eastern Pacific SST and regionalized NAM streamflow anomalies complicates the application of common ENSO indices as streamflow predictors. The occurrence of ENSO also has a significant impact on regional runoff fractions and on the partitioning of streamflow between the summer and winter seasons. The summer ENSO influence is explained, in part, by changes in the lower tropospheric pressure and wind fields, which result in modest increases in atmospheric moisture over the streamflow regions during La Niña episodes versus El Niño episodes. The results suggest that improved monitoring and prediction of ENSO variability, through careful development of eastern Pacific SST indices in the pre-monsoon period should provide useful guidance for seasonal forecasts of regional monsoon streamflow.
ABSTRACT:At a global level, warming caused by the increase in greenhouse gases has been reported in different parts of the world. This warming resulted in a reduction in the diurnal temperature range (DTR), caused by a faster rate of increase in minimum temperatures. However, the tendency toward warming is not obvious in Zacatecas, Mexico. The tendencies in maximum and minimum temperatures are partly linked to the state of the low frequency variability of large-scale atmospheric flow patterns, as depicted by the 700-mb geopotential height anomalies. Analyses of maximum and minimum temperatures from 23 climate stations, from 1963 to 2002, show that the DTR in southern Zacatecas is increasing from a rise in maximum temperatures and drop in minimum temperatures, while the DTR in northern Zacatecas is decreasing from a faster rate of decline in maximum temperatures than the increase in minimum temperatures. The regional series related to the F1 and F2 leading modes, after applying a Varimax-rotated empirical orthogonal function analysis to the 23 DTR series, acceptably reproduce the north-south dipole of DTR changes, while the point-correlation analysis between the two regional series, the 700-mb geopotential height anomalies and the indices of Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation indicate that the DTR in Zacatecas, Mexico is related to fluctuations in subtropical and extratropical circulations.
Sea surface temperature (SST) harmonic and empirical orthogonal function (EOF) analyses covering 18 years were performed for the area located from 114°to 105°W and from 18°to 25°N. The results indicate that the influence of the annual signal predominates over the semiannual signal, and the closer to the coast, the stronger the annual harmonic. Several interannual anomalies arose that are connected with the main global indexes, especially the Oceanic Niño Index. Pearson correlations between the first temporal mode of the SST and regional rainfalls in Nayarit indicate that maximum correlations (r>0.7) are observed when there is a +1-month lag between the series. However, this result indicates that SST is delayed with 1 month after rainfall occurrence, which shows that the dominant influence in this relationship is not the SST forcing.
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