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Wendell Nuss
Department of Meteorology
589 Dyer Rd., Room 254
Naval Postgraduate School, Monterey, CA 93943

A. Regional pressure and temperature effects

The Monterey Bay National Marine Sanctuary is situated on the eastern edge of a North Pacific ocean high pressure system, which determines the predominant meteorological conditions for the region. The North Pacific high pressure system is the region of high sea-level pressure that occurs over the eastern North Pacific Ocean in the climatological mean as shown in Figure 1 (Mass and Bond 1996).icon This region of high pressure shifts north during the summer and south during the winter and is largely a result of the large-scale subsidence that occurs over the subtropical regions of the world. This subsidence, in combination with the cool ocean surface temperatures, acts to produce a well-defined atmospheric mixed layer near the surface over the ocean. The mixed layer is capped by a very strong temperature inversion, where the temperature change across the inversion is often 10 to 20°C (Beardsley et al. 1987, Bridger et al. 1993, Brost et al. 1982, Lester 1985, Leipper 1994, Neiburger et al. 1961). Although the height and strength of this temperature inversion varies over space and time, it is typically located at a height between 300 and 1000 m above the ocean surface.

Within this atmospheric mixed layer, clouds, fog and temperatures characteristic of the underlying ocean surface are prevalent (Beardsley et al. 1987, Bridger et al. 1993, Elliot and O'Brien 1977, Leipper 1994). Consequently, the region is typified by persistent clouds and cool temperatures throughout most of the dry season (Renard 1996, Simon 1977). The position of the high pressure offshore in combination with lower pressure in the warm inland areas produces a relatively strong cross-coast pressure gradient, which results in west-northwest to northwesterly winds throughout much of the year (Mass and Bond 1996, Renard 1996). Although substantial variations from these typical conditions occur both seasonally and from day to day, the Monterey Bay coastal climate is characterized by considerable day-to-day persistence and seasonal mean temperature variations of about 5°C (Renard 1995), typical of other maritime climates.

B. Local meteorological conditions

The monthly averaged winds, temperatures, rainfall, and other meteorological conditions attest to the small amplitude in the seasonal cycle and persistent weather conditions of the Monterey Bay region. The mean wind direction is from the west northwest/northwest during the dry-season months (May - October) when the North Pacific high pressure system is located at its more northern location (Halliwell and Allen 1987). As this high pressure region shifts south during the wet-season months (November - April), the mean wind direction changes to a more westerly direction (Halliwell and Allen 1987, Dorman et al. 1994, Dorman and Winant 1995). The predominant west-northwest coastal winds are important in forcing and maintaining the coastal ocean upwelling (Nelson 1977, Winant et al. 1987).

The wind speed at the Monterey climate station ranges from 3.5 to 4 m/sec (8-9 knot) averages in April-June to 2-3 m/sec (4-6 knot) averages in January (Renard 1995) due largely to the seasonal changes in the intensity of the North Pacific high pressure system. Mean temperatures range from 16-18°C during the summer/early fall to 10-13°C during the winter/early spring, although considerable day to day variations exist (Renard 1995, Renard 1996).

Rainfall is limited almost entirely to the winter season (November - April) when midlatitude storms are prevalent. The rainfall amounts vary widely over the region due to topographic influences on the rainfall and range from near 48 cm in Monterey to over 150 cm in the Santa Cruz or Santa Lucia mountains (Madruga 1994). Cloud cover is a maximum during the dry season when the atmospheric mixed layer is well defined (Renard 1996, Leipper 1994).

C. Deviations from general meteorological conditions

Significant deviations from these general meteorological conditions occur due to the normal passage of migratory weather systems across the area. The interaction of these migratory weather systems with the coast and mountains is very important for producing local variations in the climate as well as the distribution of specific meteorological conditions on any given day.

icon During the winter months, extratropical storm systems (Figure 2) and their associated fronts cross the region with an average frequency of 3-4 events per month (Renard 1996, Dorman and Winant 1995, Dorman et al. 1994). These frontal cyclones account for most of the winter precipitation and produce the strongest coastal winds, as well as producing a substantial effect on the coastal upwelling (Enriquez and Friehe 1994). The southeasterly to southwesterly low-level wind directions in advance of these storms interact with the southern slopes of coastal mountain ranges to produce maximum rainfall on these slopes (Madruga 1994). Consequently, the rainfall on the southern, windward slope of the Santa Cruz mountains is considerably greater than on the north-facing slopes of the Santa Lucia mountains south of Monterey. Strong cross-coast pressure gradients frequently develop ahead of approaching fronts to produce winds in excess of 15 m/sec (30 knots) along the coast and over the Monterey Bay, similar to that observed along other mountainous coastlines (Overland and Bond 1994).

During the summer months, migratory weather systems generally produce little rainfall or significant winds but act to produce the periods of sunny, warm weather that punctuate the persistent fog and cool weather. These periods of sunny, warm weather are associated with the development of higher pressure over the interior part of the state, usually at latitudes north of Monterey, which results in offshore directed low-level winds (Mass and Bond 1996). These offshore winds maintain clear skies and warm temperatures more characteristic of inland portions of the state.

The most pronounced variability in the meteorological conditions in the Monterey Bay region is due to the significant diurnal fluctuations in winds, temperatures and clouds that occur throughout the year. Diurnal wind speed variations can exceed 15 m/sec (30 knots; Round 1993, Renard 1996), which is larger than all but the strongest storm systems. These diurnal fluctuations are primarily the result of the large surface heating differences between the Central Valley and the coastal marine atmosphere, and resemble the classic sea-breeze circulation (Banta et al. 1994).

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