inter- annual trends and multi-decadal variability. They used records on tropical disturbances back to the 1 940s 

 for basin-wide Atlantic cases, and the turn of the 20* cenfur)' for U.S. land-falling systems. \'arious 

 environmental factors were considered including Caribbean sea level pressures, 200-niillibar zonal winds, 

 stratospheric Quasi-Biennial Oscillation (described below). El Nino-Southem Oscillation (ENSO, described 

 below), African West Sahel rainfall and Atlantic sea surface temperatures. AH variables indicated significant, 

 concurrent relationships to the frequenc}', intensity and duration of Adantic hurricanes. The ENSO was found 

 to be significandy linked to changes in damages by tropical cyclones that impact the United States. 



Gray et al (1993) found that various relationships can be determined if data are stratified into spatial location 

 and by disturbance intensity. The linear trend in hurricane activity of the Atlantic basin is vety weak (Landsea 

 et al., 1999). Multi-decadal variabilitj' is more characteristic of the region. This is suggested in that intense 

 Atlantic hurricanes (50 m/s or more) were common in the 1940s through the 1960s, but much reduced from 

 the 1970s through the early 1990s (Landsea, 1993). In 1995 and 1996, there occurred a return to high levels of 

 tropical cyclone activity s imil ar to earUer active decades. There were similarities in hurricane dilation variations 

 with longer-lived systems (about 25-40 tropical cyclone days per year) in the 1 940s through the 1 960s and fewer 

 hurricane days (around 10-25 days per year) in the decades of the 1970s through eady 1990s. 



While the Atlantic basin exhibits multi-decadal variation, the United States Gulf Coast from Texas to the 

 Florida panhandle expresses much weaker intense hurricane strike variability (Landsea et al, 1999). The 

 variation pattern is quite different as above average acti\'ity occurred only in the 1910s and reduced activity only 

 in the late 1940s and early 1950s. Thus, some components of the Adantic basin may exhibit markedly different 

 intense hurricane variation. 



ENSO events induce moderate-sized changes in the frequency and intensity of Adantic basin tropical cyclones 



(Landsea et al, 1999). The ENSO state can be characterized by the sea surface temperature (SST) anomalies in 

 the eastern and central equatorial Pacific (Philander, 1989). Warm SSTs in this region are referred to as El Nino 

 events, and cool SSTs are known as La Niria events. The state of ENSO can also be characterized by the 

 Southern Oscillation Index (SOI), the standardized difference in sea level pressiure between Tahiti and Darwin, 

 Australia. High (low) pressure at Darwin and low (high) pressure at Tahiti corresponds to El Nino (La Nina) 

 events. 



Figures 9 and 10 present schematics 



of the winter tropospheric jet 



streams and major 



cyclone/anticyclone features of the 



El Niiio and La Nina events, 



respectively. The schematics show 



the El Nino Pacific jet stream 



transporting moisture into the 



southern States thus favoring wetter 



conditions there, while under the La 



Nina structure the storm track is 



fiirther north. ENSO events alter 



the global atmospheric circulation 



patterns and are able to affect 



tropical cyclone frequencies. The 



mechanisms for the latter are the 



alteration of the lower tropospheric 



source of vorticity (measure of 



rotation in a fluid) and the vertical 



wtad shear profiles (Gray, 1968, 



1979). ENSO fluctuates on the scale of a few years (Philander 1989) 



Figure 9: El Nifio winter atmospheric features. 



El Niiio events are associated with fewer numbers of Adantic basin tropical cyclones (recall that El Nino leads 

 to more nontropical storms particularly during the cool season). La Nina events cause 36 percent more 

 named/subtropical storms than El Niiio events. Their mean intensities are also 6 percent stronger (Landsea et 

 al, 1999). Though ENSO events modulate hurricane landings in the Caribbean region and the United States, 

 the impacts are only weakly significant for the Gulf Coast intense hurricanes (and not significant for weaker 

 storms). 



D-6 V Recent Trends In Precipitation Occurring on the Nueces River Watershed of South Texas 



