3.2. Relative Delay in Arizona Monsoon Onset

We now focus on the observation that the warming of the N. gulf lagged behind that of gulf waters to the south by usually 2-3 weeks (SST > 26°C), and what this may imply in terms of the monsoon onset in AZNM. Higgins et al. (1999) show that the mean calendar date of monsoon onset systematically advances from south-to-north, consistent with an SST onset mechanism, presuming that warmer water incrementally advances northwards up the coast during spring-early summer. We have plotted the Higgins et al. results as latitude of monsoon onset vs. days beginning June 1st in Fig. 15. The rainfall data set was based on the period 1963-1988 using daily rainfall samples. Due to different mean monsoon rainfall totals in different regions (similar to the ones used here), Higgins et al. used the following threshold criteria to define monsoon onset in AZNM as 0.5 mm/day for 3 days, in the N. and C. gulf as 1.0 mm/day for 5 days, and in the S. gulf as 2.0 mm/day for 5 days. To optimize any relation between N. gulf SSTs and the U.S. southwest, only Arizona was considered north of 31 deg. latitude in Fig. 15. It is seen that the mean time lag for onset between the N. gulf and Arizona is 11.5 days, over twice the time lag relative to adjacent regions further south. But once onset occurs in southern Arizona, there is relatively little delay for the monsoon onset to spread further north. These observations are consistent with this study in that the N. gulf warming lagged behind the gulf regions to the south, and that the primary monsoon period in Arizona occurred after the N. gulf SSTs exceeded 29°C. Hence, it is tempting to view the Higgins et al. results as an advance of warm water up the coast, producing conditions favorable for convection as SSTs climb beyond a threshold value.

Fig. 15. Mean time of monsoon onset in western mainland Mexico and Arizona, replotted from Higgins et al.(1999) as a function of latitude, for a period 1963-1988. The 2°lat x 2.5°long grid boxes in Higgins et al. which had latitudes corresponding to the regions addressed in this study are labeled accordingly.

To partially test this hypothesis, the data from this study was evaluated in a similar manner. However, both rainfall and SST information were plotted with respect to time and latitude to look for relationships between the timing of N. gulf warming and rainfall. Shown in Fig. 16 are mean values (and standard deviations) for the times at which gulf SSTs exceeded 26°C and were 29.5°C (solid curves), based on the five seasons. Each point represents one of the gulf regions, identified by the latitude of the region's center. These SSTs were chosen since a prerequisite for convection appeared to be SST > 26° C, and the monsoon is well developed when SSTs 29.5°C. Of the June through August rainfall in AZNM, 69% of this was associated with N. gulf SSTs > 29.5°C. Hence, we also plot the mean time (and standard deviations) at which 33% of the June-August rainfall total was attained or exceeded for each land region. This time is denoted R1/3 and is indicated by the dashed curve.

Fig. 16 Mean times at which weekly SSTs in the four gulf regions (see Fig. 5) exceeded 26 °C and became > 29.5°C (solid curves), and the mean times at which 33% of the June-August rainfall total was attained or exceeded, for the four terrestrial regions studied (dashed curve labeled R1/3). Vertical bars are standard deviations.

All individual years exhibit the general trend of 26°C water moving up the coast over time, noting that any time past this threshold, rainfall may commence. The tendency for the 29.5°C curve was similar for all years, with the timing of these SSTs being similar in the pre-, S., and C. gulf, but delayed in the N. gulf. There is a similarity between the 29.5°C curves and the R1/3 curves, with the delay in the R1/3 curves corresponding to AZNM rather than the N. gulf (recall that prevailing winds are southerly). Moreover, the mean time delay associated with 29.5°C SSTs in the N. gulf is closely reflected in the mean time delay for R1/3 in AZNM, being 18 days and 14 days, respectively. Averaging the pre-, S., and C. gulf regions together, the mean 29.5°C delay time for the N. gulf is 13.5 days. The mean R1/3 delay time of 14 days here is similar to the corresponding onset delay time of 1.5 days reported in Higgins et al. (1999), as shown in Fig. 15. This suggests, along with Fig. 12 and Fig. 14, that SSTs in the N. gulf are critical to the timing of monsoon rainfall in AZNM, and that the usual relative time delay for monsoon onset in Arizona, as observed in Fig. 15, may be attributed to a relative delay in the warming of the N. gulf. Finally, the temporal proximity of the R1/3 curves to the 29.5°C curves suggests that the timing of the indicated rainfall fraction is sensitive to the attainment of SSTs near 29.5°C.

If N. gulf SSTs are a critical factor determining moisture availability in the regions downwind (flows out of the N. gulf are typically toward the north or northeast), then it stands to reason that the warming of N. gulf SSTs may influence the western extent of the monsoon, since this body of water is furthest west. The results of this section support this idea. In addition, the results from Higgins et al. (1999) also show that there is no delay in the monsoon onset in western New Mexico, relative to regions further south. While Arizona may be effected mostly by the N. gulf, New Mexico could be more effected by SSTs further south.

e. Arizona monsoon season strength and N. gulf SSTs

Fig. 18. Evolution of northern gulf SSTs and SST range averaged over weekly intervals for the 5 weak Arizona monsoon seasons 1993-1997. Dashed and dotted horizontal bars indicate the two wettest June-August seasons, 1984 and 1999, respectively. The 29°C threshold is e xceeded earlier during the two wettest seasons.
Fig. 19. Same as Fig. 18, except showing the other relatively wet Arizona monsoon seasons. Three 1990 bars, beginning on day 42, are superimposed over the 1992 pattern. Except for 1988, N. gulf SSTs exceeding 29°C occur earlier relative to the weaker seasons.

This analysis was repeated for the moderately wet June-August periods of 1986, 1988, 1990 and 1992, shown in Fig. 19. For 1986 and 1992, the 29°C threshold was exceeded in early July, over a week prior to any of the weak years, and the mid-July (12-18th) SST for 1990 exceeds the weak monsoon SST range for this period. Note that the horizontal bar for 1990 for this period is superimposed on the 1992 pattern in Fig. 19. An exception to this trend is the 1988 season, which exhibits SSTs comparable to or less than the SSTs associated with the weaker monsoon seasons. Hence, 5 of the 6 relatively wet monsoon years , for which we have SST data, exhibit higher SSTs earlier in the season relative to the 5 weaker monsoon years studied. The fact that one year, 1988, did not exhibit this behavior, indicates that other factors are also important in determining how wet a monsoon season in Arizona will be. For instance, the meridional temperature gradient and thermal wind circulation, set up by diabatic heating as discussed in Section 2d, may play an important role.

One can also ask, for the 17 year record considered above, were there weaker monsoon years (relative to the six wet years considered) for which N. gulf SSTs were above the weak monsoon SST range (shown in Figs. 18 and 19) during the first half of July? The answer is yes: 1989 and 1998. In summary, we can say that 14 out of 17 years exhibited the following behavior for June-August Arizona rainfall: when N. gulf SSTs during the first half of July were above the SST range for the drier monsoon years 1993-1997, rainfall during June-August was relatively high. When N. gulf SSTs during the first half of July were within or below this SST range, June-August rainfall was normal or below normal.

These results are related to findings from Higgins et al. (1999), which show that early monsoon onsets in AZNM tend to be anomalously wet monsoon seasons. Our results indicate that heavier rains in AZNM follow after a threshold SST of 29°C in the N. gulf is reached, and that if this threshold is exceeded relatively early, the June-August rainfall is likely to be higher than normal.

f. Arizona and New Mexico: Different factors influencing monsoon rainfall?

Fig. 20. July-September statewide precipitation for Arizona, 1895-1999.

Fig. 21. July-September statewide precipitation for New Mexico, 1895-1999.

Figures 20 and 21 show statewide precipitation for Arizona (AZ) and New Mexico (NM) from 1895 to 1999 for the monsoon season, July-September. The question can be asked, "are wet monsoon seasons for AZ also wet for NM, and vice-versa. To answer this, we considered the period 1950 to 1999, and considered wet AZ years to be those exceeding 6 inches rain in Fig. 20, and wet NM years to be those attaining or exceeding 8 inches rain in Fig. 21.

Fig. 22. Deviation from normal for July-September precipitation,based on the wettest Arizona seasons in Fig. 20 for the period 1950 to 1999, relative to the long term mean 1950-1995. The color legend gives standard deviations.
Then using the precipitation data set from NOAA's Climate Diagnostics Center, standardized precipitation anomalies are plotted over the U.S. for the AZ wet years in Fig. 22 and for NM wet years in Fig. 23, for the months July-September. The color scale is in units of standard deviation (S), where S is relative to the 1950-1995 long-term mean.

Fig. 23 Same as Fig. 22, but for New Mexico seasons in Fig. 21.

In Fig. 22, it is seen that in wet AZ years, wet conditions are extensive throughout the southwest, extending into Utah, Nevada and California, while eastern NM is slightly drier than normal. In Fig. 23, in wet NM years, AZ is near normal for the monsoon season, and anomalously wet conditions are confined to NM and parts of Colorado and Texas. Hence, a strong monsoon year for NM will generally not benefit other parts of the desert southwest.

We have repeated this analysis, but for the purpose of identifying dry monsoon seasons in AZ as less than 4 inches of rainfall in Fig. 20, and dry seasons in NM as less than 6 inches rainfall in Fig. 21. For the driest NM years (not shown), AZ was slightly drier but near normal, and for the driest AZ years, NM was moderately dry in the west, and slightly wetter than normal in the east.

It is also of interest that the wettest conditions in AZ (Fig. 22) correspond to the driest conditions in the mid-west. However, the same is not true when NM is wettest (Fig. 23). This suggests that the anti-correlation between monsoon and mid-western summer precipitation observed by Higgins et al. (1997; 1998) and Barlow et al. (1998) may primarily result from conditions which produce wet monsoons in AZ. This study suggests that one of these conditions may be relatively high SSTs in the N. gulf during the first half of July. It is also of interest that when NM is wettest (Fig. 23), Idaho and surrounding areas are anomalously dry. The ability to forecast wet conditions in AZ and NM may also make it possible to forecast anomalously dry conditions in the mid-west and northwest, respectively.

In summary, while both AZ and NM are effected by the monsoon, the means by which they are effected appears to be different, since wet monsoons in AZ tend not to be anomalously wet in NM, and vice-versa. Based on this work, we suggest that a principal moisture source for wet AZ monsoon seasons is the N. Gulf of California, while this moisture source may be less important for wet NM seasons. Also, as shown in Higgins et al. (1999), the monsoon begins in western NM about a week earlier on average than in western AZ. If the NM monsoon moisture comes from the Pacific (which various satellite imagery suggest), it apparently comes from lower regions of the gulf or the eastern Pacific off southern Mexico. In these regions, SSTs warm earlier than in the N. gulf, and may explain the earlier onset times in NM. Of course, this idea needs testing and further study.