Outgoing longwave radiation (OLR) measured by satellites can be used as a proxy for deep convection in the tropics, and in the subtropics during summer, since this produces extensive cirrus anvil clouds which emit at relatively cold temperatures. Thus, the lowest OLR values over such regions generally indicate convective activity and cirrus anvil coverage (e.g. Zhang 1993). The OLR data used is available on a daily basis from twice daily NOAA AVHRR soundings (e.g. Gruber and Krueger 1984). The grid size is 2.5 x 2.5 degrees for the period 1974 to present. This data provides a reasonable indicator of the extent of convection. The SST data used are based upon an optimum interpolation analysis and adjusted for biases using the method of Reynolds (1988) and Reynolds and Marsico (1993). This analysis is produced weekly on a one-degree grid using in situ and satellite derived SSTs (hence blended data). However, this was not the SST data set used in the three year study, which has higher spatial resolution.
The evolution of SSTs and tropical convection inferred from OLR are shown in Fig. 1 and Fig. 2 for the monsoon and surrounding regions. Monthly averaged OLR and blended SST data, from 1974-1993, were used to generate the 20 year mean values. The SST isotherms are shown in color. Due to the 1 degree resolution of SST data, we do not have full confidence in the absolute SST magnitudes in the Gulf of California (especially its upper half), but the spatial and temporal behavior of the SST data should be realistic enough to recognize the general behavior of the system. We regard the 240 W m-2 OLR isoline as an estimate of the monsoon's extent, since OLR < 240 W m-2 generally appears associated with deep convection at these latitudes (Zhang 1993). Clear skies dominate the warm pool region in March and April, allowing solar insolation to expand the warm pool. By April, a 29°C isotherm extends off southern Mexico from 108°W to 92°W, and south to 7°N. In May, convection and cirrus build-up in the south may cool SSTs (Ramanathan and Collins 1991). In June, the 29°C water is closer to the coast, and is found further up the coast. This continues into July, with the 29°C isotherm extended deep into the gulf. Also in July, the 240 W m-2 OLR isoline makes a dramatic advance northwestward, indicating the onset of the monsoon. The question can be asked, "does the advance of the 29°C isotherm play a role in initiating the monsoon?". Although the monthly time resolution is crude, there is still the suggestion that the poleward advance of warmer SSTs into the Gulf of California corresponds with the poleward advance of OLR, or tropical convection, hence suggesting a possible link between SSTs and the monsoon onset. We have found that the poleward propagation of SSTs >29°C is as regular an event as the monsoon singularity, occurring to varying extents each year in July. As discussed below, this phenomena appears to be driven by both solar insolation and a coastal current.
Fig. 1 and Fig. 2 - combined in one color-coded image. Fig. 1. Long-term mean (1974-93) seasonal evolution of SST fields (°C) in the eastern Pacific. Color coding: red = 29°C; orange = 28°C and each color change represents 1°C cooling.
Fig. 2. Long-term mean (1974-1993) seasonal evolution of OLR fields (W m-2) in the eastern Pacific and monsoon region (black lines).
Perhaps the best evidence for a spring-summer warm current altering SSTs to the north is given in Castro et al. (1994), who used most of the available hydrographic data (extending to 400 m depth) and meteorological data to carry out a seasonal heat balance for the gulf. In simple terms, the gulf may gain heat at the surface from solar radiation and sensible heat flux, and gain heat horizontally through import of warmer water from the Pacific (i.e. advection). It may lose heat at the surface through evaporation, radiation, sensible heat flux, and horizontally lose heat via advection. Through constructing a heat budget for the surface and horizontal fluxes, the advection terms (longitudinal heat fluxes, or LHF) were solved for, implicating the role of currents in heat transport. From late March to early July, the surface heat flux (solar and sensible) was not nearly enough to account for the observed heating rate of the gulf waters, giving negative LHFs. A net gain of heat had to be supplied through horizontal processes (advection) to account for the observed heat contents in the gulf during that period. From August to February, LHFs were larger but opposite in sign, indicating the water columns were losing heat by horizontal transport. These larger values reflect the fact that the net "vertical" energy exchange at the surface was positive, and that this net heat gain must be exported out of the gulf through horizontal advection (along with heat advected during March-July) to explain the observed heat contents during this period.
The Castro et al. results indicate an intrusion of warmer water from the eastern Pacific warm pool occurs from mid-March through mid-July on average, heating the upper waters over the entire gulf length, over and above that which would result from solar and sensible heating. This is consistent with the recent finding (Torres-Orozco, 1993) that the amount of Tropical Surface Water (TSW) at the entrance to the gulf increases from winter to summer. Broadly in phase with this accumulation of TSW is the annual cycle of the along-gulf surface tilt, with sea levels highest in the upper gulf during summer (Roden and Groves 1959; Ripa 1997). The annual changes in sea level throughout the gulf, and the annual changes in surface-layer velocity and mean heat content through the gulf's mouth, appear primarily due to forcing by the Pacific Ocean which excites a baroclinic Kelvin wave at the mouth of the gulf (Ripa 1997; Beier 1997).