Recent literature using satellite observations indicates that the onset time of relatively heavy summer rainfall in Arizona generally occurs several days after the sea surface temperature (SST) in the northern Gulf of California (N.-GC) reaches or exceeds 29.5°C. Recent simulations using the MM5 model also confirm this result, showing a dramatic increase in boundary layer moisture, convective available potential energy (CAPE), and updraft velocities over the GC region when N.-GC SSTs increase from 29°C to 30°C. Studies using the Reynolds-Smith Optimal Interpolation SST dataset fail to reveal such a SST-rainfall sensitivity, since climatological July GC SSTs in this dataset are underestimated by 2 to 5°C, especially in the N.-GC.
Our work shows that the timing or onset of this threshold SST may be predicted 1-2 months in advance from the sea surface topography in the eastern tropical Pacific. By defining different ocean regions, sea surface height anomalies (SHA) in May were evaluated for each region for 10 years of TOPEX/POSEIDON satellite data. Combining these measurements into a SHA Index and regressing this index against the N.-GC 29.5°C onset time, the resulting correlation coefficient r = -0.94, explaining 88% of the variance. The same r value was obtained from EOF analysis. The forecasted time window for the onset of heavy rainfall has an uncertainty (std. dev.) of 3.4 days. While SHA was evaluated by careful visual inspection of satellite imagery, in the near future we hope to refine this result using the digital data.
This forecast of the N.-GC SST onset may also be used to forecast whether Arizona rainfall during June-August will be dry-to-average or wet-to-average. The latter condition indicates dry-to-average conditions in the Midwest. This is based on a 20 year SST data set valid for the GC (weekly temporal & 18 km spatial resolution) and statewide AZ precipitation records, and on a strong anti-correlation between Arizona and Midwest summer rainfall amounts.
This relationship between the N.-GC SST onset and summer rainfall amounts may derive from two factors: (1) longer monsoon seasons tend to have more rainfall events, and (2) the positioning of the mid-level anticyclone over North America during summer may depend on the poleward propagation of warm water up the west coast of Mexico. As these higher SSTs advance poleward out of the eastern Pacific warm pool, deep convection commences over the adjacent land, and subsiding air to the north of the convection positions the center of the poleward advancing anticyclone. Hence the latitude of the anticyclone (affecting whether a moist circulation develops) may depend on the advance of the warmest GC SSTs. Observational and MM5 modeling evidence will be presented. Note that the summertime circulation over North America is dominated by the position of this anticyclone.
The mechanism behind the apparent intrusion of high SST from the warm pool into the GC in May-June is not well understood at present, but it may be related to equatorward winds off the Pacific coast and to the behavior of a "dome" in sea surface height off the coast of southern Mexico. It is observed that in May-June, the winds and the California Current slacken, the dome diminishes and the SST intrusion occurs. Ocean topography and SST from satellites indicate that warm pool water during March-April circulates clockwise around this dome (decoupled from the coast), and as the dome begins to weaken in late May-early June, it also bridges with the coast near Cabo Corrientes, with geostrophic currents directed toward the Cape and into the GC. A dramatic reconfiguration of the 28.5°C isotherm occurs during June, perhaps in response to this change in surface currents. During the first week of June, this isotherm extends far to the west of southern Mexico but south of 18°N, whereas during the last week of June, this isotherm typically describes a narrow band "hugging" the coast, extending deep into the GC.
Although the causal relationships between these facts are not clear, they can be used for prediction purposes; their study will open new research opportunities in the oceanographic and atmospheric sciences.