One of the summer precipitation regimes in the United States exhibits an inverse relationship between precipitation anomalies in the central United States and the Arizona and New Mexico (AZNM) monsoon. We focus on this rainfall regime and corroborate its seasonal evolution with longer data sets.
Composites based on rainfall anomalies from 1901 to 1996 indicate that the regime with a dry (wet) AZNM monsoon is most likely to occur after a wet (dry) Southwest (SW) winter, while the Pacific Northwest tends to be dry(wet).
Both summer and winter precipitation regimes are related to sea surface temperature anomalies (SSTA).The SSTA during the winter prior to a poor AZNM monsoon and a wet central United States show positive anomalies extending from the central Pacific to the west coast of the Americas with negative anomalies in the North Pacific. This configuration corresponds to a basin-wide mode of SSTA which extends from January through July. A new result is that the US rainfall regime with wet winter conditions in the SW and dry summer conditions during the AZNM monsoon is rooted in different locations of the Pacific ocean during summer and winter. The enhanced convection in the central Pacific favors heavy winter precipitation in the SW and dry conditions in the Pacific Northwest. We reconcile results from previous studies that have related US precipitation in winter to both the tropics and negative SSTA centered at (160oW, 32oN) by noting that both features are elements of large scale SSTA mode that extends over most of the North Pacific basin.