Climatic Setting

Nearshore sea temperatures have been recorded every day of the year at the HMS since 1920. These records have always been taken by hand with a standard thermometer, so the entire record is technologically consistent. Yearly mean, maximum, and minimum shoreline temperature increased significantly during the period from 1920 to 1995 (Fig. 3.5). Linear regressions for annual mean, maximum, and minimum shoreline temperature for HMS versus year indicate a significant increase of annual sea surface temperature of 0.79°C, 1.26°C,and 0.66°C, respectively, between 1931 (the time of Hewatt's study) and 1995.

In addition to the gradual long-term temperature increases, sea temperature conditions during the 13-year period preceding our study were consistently warmer than the same period preceding Hewatt's study, with a difference in mean temperature of 0.99°C, and a maximum difference in seasonal temperature of 1.94°C in late July (Fig. 3.6).

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Year

Figure 3.5. Plot of annual maximum, mean, and minimum shoreline ocean temperature recorded at HMS from 1920 to 1995. Gray vertical bars indicate strong El Nino-Southern Oscillation (ENSO) years. Slopes for maximum, mean, and minimum temperature regressions are 0.0199, 0.0125, and 0.0104, respectively. Adapted from Sagarin et al. 1999.

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Year

Figure 3.5. Plot of annual maximum, mean, and minimum shoreline ocean temperature recorded at HMS from 1920 to 1995. Gray vertical bars indicate strong El Nino-Southern Oscillation (ENSO) years. Slopes for maximum, mean, and minimum temperature regressions are 0.0199, 0.0125, and 0.0104, respectively. Adapted from Sagarin et al. 1999.

Figure 3.6. Monthly averaged shoreline ocean temperatures for HMS for the 13 years preceding this study (black line, above) and the 13 years preceding Hewatt's study (dashed line, below). Dashed lines are + or -standard error. Adapted from Sagarin et al. 1999.

Air temperature records, on the other hand, do not show a clear trend. There are no continuous air temperature records for Monterey, because the weather station was moved several times during this century. Therefore, temperature data from nearby stations (Santa Cruz, Watsonville, Salinas) with continuous records were compared to discontinuous records from the Monterey station (using least squares linear regression) to generate a continuous time series of predicted air temperature for Monterey. The linear regression slopes of the predicted minimum and maximum Monterey air temperatures were positive, but nonsignificant (0.001 and 0.006, respectively; R2 values = 0.0363 and 0.0009, respectively) (Fig. 3.7).

El Niño-Southern Oscillation events of varying intensities occurred in the 13-year periods preceding and during each study (Fig. 3.8). Quinn et al. (1987) provide a long-term historical record with which to compare the magnitude and frequency of El Niño events. Six of 13 years preceding Hewatt's survey were classified as El Niño years including a strong two-year event (1925-1926). Five of 13 years preceding our surveys were El Niño years, including the event of 1982-83, considered to be the strongest of the century before our surveys (Hansen 1990). Both studies were conducted

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Figure 3.7. Air temperature for Monterey Bay from National Climatic Data Center data. Plot of predicted Monterey air temperature from 1929 to 1995 based on regressions from three nearby stations. Slopes of maximum and minimum temperature linear least squares regressions are 0.0017 (R2 = 0.0023) and 0.0057 (R2 = 0.0355), respectively. Adapted from Sagarin et al. 1999.

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Figure 3.7. Air temperature for Monterey Bay from National Climatic Data Center data. Plot of predicted Monterey air temperature from 1929 to 1995 based on regressions from three nearby stations. Slopes of maximum and minimum temperature linear least squares regressions are 0.0017 (R2 = 0.0023) and 0.0057 (R2 = 0.0355), respectively. Adapted from Sagarin et al. 1999.

VERY STRONG

STRONG

MODERATE

WEAK/MODERATE

1917

1919

1921

1923

1925

1927

1929

1931

1933

VERY STRONG

STRONG

MODERATE WEAK/MODERATE

1980

1982

1984

1986

1988

1990

1992

1994

1996

Figure 3.8. Strength of El Nino-Southern Oscillation (ENSO) events prior to Hewatt's study (top) and prior to this study (bottom), based on Quinn et al. (1987) and Vernon Kousky (pers. comm. 1998). Hatched areas represent survey periods for each study. Adapted from Sagarin et al. 1999.

during El Niño years, including weak (1931), moderate (1994, 1995 Vernon Kousky, pers. comm. 1998), and strong (1932) events.

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