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Wine Works: The Art & Science of Winemaking: 05/01/2014

Why Sonoma Valley: The local effects

Last month I attempted to explain my understanding of how the revolving planet sets forces in motion that affect the climate of Sonoma Valley; namely our dry summers and nourishing wet winters. This time around, I would like to focus on the local factors that set Sonoma Valley apart as an ideal environment to grow world-class wine grapes. The aforementioned dry summers spare us from the molds and mildews that can damage the grape crop. But that is just the beginning.

Grapes are the raw material of world-class wine. Other ingredients in wine – the yeast used to instigate fermentation, perhaps a little naturally occurring acid addition, the preservative and antioxidant sulfur dioxide, and the flavors imparted from barrel aging – have a lesser impact. A few columns ago, we established that wine grapes need to be both high in sugar and high in acid to be “in balance.” The weather during the growing season is instrumental in establishing this sugar/acid balance. So what’s this ideal weather pattern and how did it get here?

After a winter of rain and cold, when the vines are dormant and nourished, comes a spring and summer of warmth that allows for grape development. For wine grapes to be high both in sugar and acid, nights must be cool. A normal summer day here is typically in the mid-80s and is followed by a night in the 50s. An average 30-degree variation in temperature within 24 hours is what sets this area apart.

The cause begins with the falling dry air, the high-pressure system that plants itself over California in summer. The prevailing breeze off the Pacific Ocean from the northwest is forced to the south. The coastal water beneath it is affected by the “Ekman transport,” which turns surface water to the right of the prevailing winds in the northern hemisphere. The surface of the ocean pulls away from the coast and much colder water rises from below to take its place. This phenomenon is known as “upwelling.” The water temperature at the coast at Ocean Beach (San Francisco) in August is typically eight to 10 degrees colder than the surface water 100 miles off shore. The sudden drop in surface water temperature causes the air above it to cool in a burst of fog. But what brings it onshore to chill the nights over Sonoma County?

To go here we must look at the geography of California, specifically to the great Central Valley of California to our east. This valley is 400 miles long running between Redding to the north and Bakersfield to the south. It’s surrounded by mountains on all sides. The exception is to the west, where all of the rain that falls on this huge valley drains to the sea through the San Joaquin Delta, the Carquinez Straits, San Pablo Bay, and on out the Golden Gate. A typical summer day in the Central Valley sees a rising temperature during the day. With that expanding and rising air, the dense foggy air off the coast of San Francisco is sucked inland through this gap to fill the void. The fog is also pulled through other low spots along the coast to join the river of cold air heading east. In Sonoma County, it is pulled over the American Estero, through Petaluma, where it joins the fog over San Pablo Bay or shoots through Stagecoach Canyon and southern Sonoma Valley.

Sonoma Valley’s access to this phenomenon provides the great temperature differential needed to grow exceptional wine grapes. It also accounts for the many micro-climates throughout the valley that allow a wide variety of wine grapes to grow successfully here. The cool, afternoon west wind of the Carneros region at the southern edge of Sonoma Valley provides the climate needed for Pinot Noir and Chardonnay. Sonoma Mountain blocks this wind, directing it south to the San Pablo or San Francisco bays. Thus the afternoons are warmer around the towns of Sonoma, Glen Ellen and Kenwood. Here, the climate is ideal for Cabernet Sauvignon, Merlot, Zinfandel and Sauvignon Blanc.

The topography of Sonoma Valley further affects the flavors of the grapes grown here. The slopes of Mt. Veeder face west and receive late afternoon sunshine. This is the situation along Nelligan Road above Kenwood. This area produces world-class Cabernet Sauvignon and Zinfandel known for bright, fruity yet tannic, deep wines. The west side of the valley, by contrast, falls under the shade of Sonoma Mountain in the afternoon. These wines tend to be more herbal in character. Some of the best Sauvignon Blanc that I’ve tasted has come from this area.

In short, every vineyard has a story: a combination of climate and setting that, when rolled up together, forms the “terroir” for that spot. If every vineyard is unique, it is also true that not all vineyards are great. The grower must understand the terroir and select a grape variety that can make great wine from that location. He or she then must provide the care needed to nurture those grapes so that they arrive at the winery in good condition. It is then left to the winery to appreciate the fruit and treat it with the respect it deserves.

Mark Stupich is Cellar Master & Winemaker, Kenwood Vineyards


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