Garua

Garua is the local term for the cool, misty season in the Galapagos Archipelago, typically lasting from June through November. It is characterized by persistent low stratocumulus cloud decks, fine drizzle, and cool temperatures despite the equatorial latitude.

Mechanism

The garua season is driven by a combination of large-scale and local processes:

1. Cool upwelling: During the austral winter, the southeast trade winds intensify and the Pacific Equatorial Undercurrent strengthens upwelling of cold, deep water around the western islands. Sea surface temperatures drop markedly below typical tropical values.

2. Marine temperature inversion: The cool SSTs stabilize the lower atmosphere, producing a persistent temperature inversion in the lower troposphere. Below the inversion, moisture is trapped in a shallow marine boundary layer.

3. Stratocumulus formation: The trapped moisture condenses into extensive Stratocumulus cloud sheets. These clouds produce fine drizzle (garua) that provides little measurable rainfall at standard rain gauges but delivers significant moisture through fog interception at vegetated highland surfaces.

Ecological significance

The garua is critical for highland ecosystems on the Galapagos. While the arid lowlands receive almost no moisture during this season, the highlands above approximately 300 m are bathed in fog. Epiphytic vegetation, mosses, and ferns intercept cloud droplets, creating a moist microhabitat that supports distinct ecological communities.

The DARWIN project investigated the hypothesis that occult precipitation delivers a significant fraction of total water input to highland ecosystems (see DARWIN publication thread).

Measurement challenges

Quantifying garua precipitation is inherently difficult because:

• Standard tipping-bucket rain gauges underestimate or miss fog precipitation entirely
• Fog collectors measure fog water flux but do not directly represent ecosystem water input
• Drizzle-sensitive instruments can extend detection beyond standard gauges, but very fine fog remains difficult to quantify directly

The weather station network in the DARWIN project was designed to capture the transition from lowland dryness to fog-influenced highland conditions.

Interannual variability

The intensity and duration of the garua season is strongly modulated by ENSO. During El Nino events, warmer SSTs weaken the inversion and suppress garua conditions, replacing them with convective rainfall. During La Nina, the inversion strengthens and the garua intensifies, extending fog cover to lower elevations.

See also: Precipitation, Galapagos Islands, Cloud frequency in the Galapagos, Ecoclimatic cloud zonation in the Galapagos