Precipitation variations will be analyzed in the context of such elements of the seasonal cycle as the mid-summer drought minimum that is evident in the annual cycle of precipitation from Mexico to the central Rockies, changes in SST in the warm pool region of the Caribbean, Gulf of Mexico and the eastern North Pacific (ENP), and the associated restructuring of the atmospheric flow field over North America between the cold and warm seasons. We will also consider how ENSO and other factors affect the interannual signatures of precipitation in this region, playing particular attention to the interplay of time scales of large-scale precipitation variability ranging from interannual to interdecadal.

PROJECT GOALS
Our goals are to: (1) characterize summer hydroclimatic regimes in western North America on time scales from seasonal to interdecadal, using a variety of data sets with daily to monthly resolution, (2) evaluate summer teleconnections and interseasonal linkages, and (3) extend the available record of winter and summer analyses by using summer-precipitation proxies based on tree ring networks in the western United States.

METHODOLOGY
Precipitation variations will be analyzed in the context of such elements of the seasonal cycle as the mid-summer drought minimum that is evident in the annual cycle of precipitation from Mexico to the central Rockies, changes in SST in the warm pool region of the Caribbean, Gulf of Mexico and the eastern North Pacific (ENP), and the associated restructuring of the atmospheric flow field over North America between the cold and warm seasons. We also consider how ENSO and other factors affect the interannual signatures of precipitation in this region, playing particular attention to the interplay of time scales of large-scale precipitation variability ranging from interannual to interdecadal. ocean-atmosphere interactions over the Atlantic will also be considered since they are an important source of interannual and longer-term climate variability affecting part of our study region

RESULTS AND ACCOMPLISHMENTS
In the first year of the project we have concentrated on defining the large-scale patterns of hydroclimatic variability in North America and for comparison, in other parts of the world. Historical streamflow variations are an important part of this study as independent corroborations of historical precipitation variations and in their own right. The characteristics of streamflow variability on spatial and temporal scales sufficient to address the North American monsoon system are rather poorly documented in the literature. Therefore, we began by acquiring and analyzing historical streamflow time series from all over the world (Dettinger and Diaz, 2000, revised). Analyses of the North and South American interplays of interannual ENSO variations with decadal PDO variations were made to address the influences of these primary two climate modes on precipitation and streamflow by Dettinger et al. (2000a) and by Dettinger et al. (2000b). In many rivers, these two climate modes together explain about one-third to one-half of all interannual and slower hydrologic variation. Our analyses demonstrated both the decadal variations of ENSO teleconnectivity to the hydrology of the Americas and the very large scale spatial symmetries about the Equator shared by the two climate modes. These results provide a more complete basis for interpretting historical teleconnections and their paleoclimatic extensions, a basis that recognizes that ENSO teleconnections are not stationary processes.

A recent effort by Biondi et al. (submitted) uses a tree ring network from southern California and Baja Mexico to reconstruct the PDO index; the results indicate prominent interdecadal variability back to A.D. 1661. Coupled with an existing reconstruction of ENSO by Stahle et al 1998, this record provides an estimate of the frequency of reinforcing and opposing phases of PDO and ENSO over this three-and-a-half century time span.

On much shorter time scales, Dettinger et al. (2000c) analyzed recent summer lightning-strike variations as a supplement to precipitation networks. Modest interannual variations of lightning with ENSO status were detected but, on intraseasonal time scales, important tropically derived variations of lightning strikes in the southwestern US were identified. These intraseasonal lightning variations appear to be associated with, and another indication of, intraseasonal monsoon surges into the region. The lightning surges extend northward from Mexico to Idaho over the course of 3-6 days roughly every 20 to 30 days in about half the summers from 1985–94.

Biondi, F., Gershunov, A. and D. R. Cayan, 2000: North Pacific decadal climate variability since AD 1661. [Submitted to Science.]

Cayan, D. R., Kammerdiener, S., Dettinger, M. D., Caprio, J. M., and D. H. Peterson, 2000: Changes in the onset of spring in the western United States. [In preparation].

Dettinger, M.D., Cayan, D.R., McCabe, G.M., and Marengo, J.A., 2000a: Multiscale streamflow variability associated with El Niño/Southern Oscillation: In Diaz, H.F., and Markgraf, V. (eds.), El Niño and the Southern Oscillation--Multiscale Variability and Global and Regional Impacts, Cambridge University Press, 113–146.

Dettinger, M.D., Battisti, D.S., McCabe, G.J., Jr., and Garreaud, R.D., 2000b: Interhemispheric effects of interannual and decadal ENSO-like climate variations on the Americas: In Markgraf, V. (ed.), Present and Past Inter-hemispheric Climate Linkages in the Americas and their Societal Effects, Academic Press (in press).

Dettinger, M.D. and Diaz, H.F., 2000: Global characteristics of streamflow seasonality and variability. Revision sent to Journal of Hydrometeorology.

Dettinger, M.D., Cayan, D.R., and Brown, T.J., 2000c, Summertime intraseasonal and interannual lightning variations in the western United States: Proc., 24th Annual NOAA Climate Diagostics and Prediction Workshop, Tucson, AZ, 1-5 November 1999.

Nemani, R. R., White, M. A., Cayan, D. R., Jones, G. V., Running, S. W., and J. C. Coughlan, 2000: Asymmetric climatic warming improves California vintages. [Submitted to Climate Research.]

Principal Investigators:

Henry F. Diaz
hfd@cdc.noaa.gov
phone: (303) 497 6649
fax: (303) 497 7013