Knowledge of the causes behind the relative delay in N. gulf SSTs may have relevance to the ultimate predictability of the NA monsoon. A potentially powerful influence on N. gulf SSTs are the islands which separate the N. and C. gulf, as shown in Fig. 5, due to (1) mixing of the water column in the island vicinity and (2) presenting an obstacle to the flow of warm water up the gulf. Both factors could delay the warming of the N. gulf. Regarding (1), Paden et al. (1991) present findings that tidal mixing occurs over the relatively shallow sills in the island region, especially in mid-to-late spring, affecting the upper 300-500 m. Tidal mixing over such depths pumps heat away from the surface, deep into the water column, hence cooling SSTs in the island region by 2-4°C. The same degree of SST cooling around the island region was found in this study. Simpson et al. (1994) show that, in addition to tidal or vertical mixing, current advection over the sills must occur to explain the observed temperature reductions. Paden et al. found that this pool of relatively cool water around the islands was advected northwards over much of the N. gulf, while SSTs south of the islands remained warmer owing to advection of water from the southern gulf which was not influenced by tidal mixing.
Eventually, N. gulf SSTs become as warm or warmer as gulf SSTs further south, typically by mid-July to early August. As suggested by Paden et al. (1991), increased solar insolation will lead to increased stratification of the water column (due to warmer SSTs near the surface), resulting in less mixing. With less mixing, SSTs in the N. gulf climb rapidly to their peak values, as high as 32°C. However, the maximum net heat flux into the gulf through the surface, mostly due to solar insolation, occurs during June (Castro et al. 1994), around June 10th (Ripa 1997). Yet the SST differences between the N. and C. gulf are minimized about one to two months after June 10th, suggesting that during the one-to-two months following June 10th, stratification of the water column is being inhibited by some other process. It is conjectured here that this other process is the advection of the upper surface layer through the island region. Once advection decreases, mixing and associated cooling should decrease (Simpson et al. 1994), allowing N. gulf SSTs to become comparable to or exceed those further south. This would produce favorable conditions for heavy rainfall in the AZNM region, assuming N. gulf SSTs were 29°C.
Evidence for this conjecture is found in Ripa (1997) and Beier (1997), which indicates surface current velocities through the island region peak about June 28th, with reduced advection thereafter possibly promoting increased stratification of the water column in the N. gulf. Using sea level measurements in the C. gulf, Ripa (1997) estimated the annual harmonic of the geostrophic surface velocity (top 70 m), usfc. Peak values of usfc (towards the gulf's tip) occur around June 28th, while peak sea levels at Guaymas (mainland coast) and Santa Rosalia (opposite Guaymas on Baja coast) occur August 9th and 27th, respectively. After late September, usfc reverses, with net flow toward the gulf entrance. This reversal coincides with the reversal in gulf surface currents modeled by Beier (1997), from cyclonic in summer to anticyclonic in winter. Using a two-dimensional linear two-layer model initialized with the same hydrographic observations used by Ripa (1997), Beier found the annual cycle of an internal wave, initiated in the mouth of the gulf as a baroclinic Kelvin wave by the Pacific Ocean, was primarily responsible for both sea level changes and upper layer (top 70m) currents in the gulf, although wind stress was also important. Since the annual cycle of the internal wave and wind direction were in phase, their effects were additive. While upper current velocities reached 70 cm/s, current velocities of 30-40 cm/s were more typical. Such velocities appear sufficient to induce mixing and to influence SSTs in the N. gulf. Moreover, current velocity reductions occurring after June may also affect N. gulf SSTs by reducing both mixing over the sills and advection to the N. gulf, thus allowing N. gulf SSTs to increase by responding more directly to solar insolation.
Regarding factor (2), gulf circulations are strongest and cyclonic during spring/summer (Bray 1988; Merrifield and Winant 1989; Paden et al. 1991; Castro et al. 1994; Collins et al. 1997; Ripa 1997), and can be explained by means of an internal wave trapped against the coast, propagating up the mainland coast, and back down the Baja California coast (Beier 1997). The gulf circulations predicted by Beier (1997) indicate two cyclonic gyres in summer, separated by the island region which in effect blocks some of the warm water advected up the mainland coast from entering the N. gulf. Hence, horizontal heat advection is expected to contribute less to SSTs in the N. gulf than in other gulf regions (see also fig. 8 and fig. 12 in Ripa 1997).
In summary, a working hypothesis is proposed here as a possible aid for future research. As described above, it postulates that AZNM rainfall is strongly dependent on N. gulf SSTs, and that tidal mixing and mixing from horizontal advection over the sills in the island region are largely responsible for the relatively cooler SSTs in the N. gulf during much of the warming phase. Partial blocking by the sills of the poleward advection of warm water may also be a factor. These factors, by producing relatively cooler N. gulf SSTs, may be responsible for the relative delay observed for the monsoon onset in AZNM. The eventual increase of N. gulf SSTs may be due to a decrease in current advection through the island region, allowing the water column to stratify and respond more directly to solar insolation. The complex interactions of these and other factors will need to be addressed in more detail before confident predictions of N. gulf SSTs can be made, from which long-range predictions of monsoon strength and timing in AZNM may ultimately be made.
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