San Diego Bay vs. La Jolla: a five-year water-temperature comparison

A comparison of two thermometers, not two entire coastlines

We compare NOAA CO-OPS station 9410170, San Diego, used for the site's San Diego Bay page, with station 9410230, La Jolla. The dataset contains coverage-qualified hourly observations from the five complete years 2021–2025. Using named stations keeps the geographic claim precise: neither instrument measures every location within its surrounding water body.

The distinction is useful locally. Someone interested in exposed La Jolla ocean conditions should not silently substitute a San Diego Bay reading simply because both locations are in the same metropolitan area. Conversely, a La Jolla value is not a measurement of a particular bay marina. These station identities remain attached to the historical comparison just as they do on the live pages.

The difference grows from winter into late summer

The San Diego station's pooled mean exceeds La Jolla's in all 12 months. The smallest difference is January's 0.312°C; the largest is September's 2.357°C. Converted to temperature differences and rounded, those are 0.6°F and 4.2°F. Fahrenheit differences use a factor of 1.8, without adding 32.

Both records peak in August. However, San Diego Bay's September mean remains only about 0.5°F below its August mean, while La Jolla's September mean is about 0.8°F below its peak. The result is a slightly larger difference between stations in September than in August. These calculations use unrounded source means; small apparent discrepancies can result from subtracting the rounded table entries.

The seasonal amplitude also differs. San Diego's coldest-to-warmest pooled monthly mean spans about 13.0°F (7.2°C), from January to August. La Jolla's corresponding span is about 9.8°F (5.4°C), from February to August. This measures the shape of each five-year seasonal cycle; it does not measure the full temperature extremes experienced at either station.

Source: Water Temperature Live analysis of NOAA CO-OPS hourly observations. All five years qualify in each month at both stations, using a 70% station-month-year coverage threshold. Differences are computed before rounding. These are differences of unpaired pooled means.

Why an average difference is not an always-warmer rule

San Diego Bay's August 10th–90th percentile interval is 20.3–23.9°C. La Jolla's is 15.9–23.2°C. The intervals overlap from 20.3 to 23.2°C, so the two historical distributions are not cleanly separated. A higher mean does not require every San Diego observation to exceed every La Jolla observation.

More importantly, overlapping distributions do not establish how often one station was warmer at the same instant. Answering that would require joining the records by timestamp, deciding how to handle missing pairs and calculating the distribution of paired differences. We have not performed that test here. The defensible finding is the difference between each station's independently pooled seasonal means.

The pattern does not identify its cause

An exposed coast and an enclosed bay present different oceanographic settings, but the monthly table alone does not isolate the responsible processes. Upwelling is one candidate for coastal cooling: NOAA describes colder water rising as displaced surface water is replaced. This analysis does not measure wind stress, vertical mixing, solar heat input, water exchange or sensor-depth effects, and cannot assign percentages of the observed difference to them.

Even nearby scientific temperature records require care. Scripps' Shore Stations field methods describe daily manual surface samples and a separate near-bottom collection at Scripps Pier. Those are different measurements from the NOAA hourly series used here. Bringing in additional records could improve a future physical analysis, but only after documenting the depth, location, sampling time and method for each.

How to use the result without losing its limits

Both stations have five qualifying years for every month, but the exact sets of sampled hours differ. La Jolla contributes 43,785 hourly values and San Diego 43,776. For example, July contains 3,711 La Jolla values and 3,708 San Diego values. A pooled average weights every available qualifying hour equally; it does not first give each annual monthly mean identical weight. The small count differences are disclosed rather than treated as perfect synchronization.

For a seasonal overview, the table establishes that the regional label 'San Diego water temperature' conceals a meaningful station difference. For a current comparison, open the San Diego Bay and La Jolla pages and inspect observation times as well as values. The comparison tool provides a convenient side-by-side view; an unavailable reading should remain unavailable rather than being replaced with a historical mean.