Marine Heatwaves

Summary of CBEFS Marine Heatwave Forecasting

Marine heatwaves occur when the water temperature is elevated for a prolonged period of time and can be detrimental to marine life, with the frequency of marine heatwaves in the Bay increasing over time. Elevated water temperatures are forecast in CBEFS using two different methods. The first is based on the definition of a marine heatwave and the second alternate method is based simply on the water temperature being high. The below bullets describe the two calculations. The alternate forecast is provided in a dropdown menu near the bottom of the page.

1. Marine Heatwave: Using this definition a marine heatwave is forecast if the water temperature is forecast to be above the 90% occurrence temperature for 5 consecutive days. The 90% occurrence temperature is the temperature where 90% of the historical daily-averaged water temperatures were less than this value on a given day. That is, 90% of the years between 1985 and 2023 had a daily-averaged water temperature less than this value on a given day of the year. The 90% occurrence temperature varies by day of the year (cooler in winter and warmer in summer) and based on location in the Bay. This definition can result in marine heatwaves occurring during the winter because it is based on seasonally-varying water temperatures. The 90% temperature was determined using long-term model simulations suitable for determining the climatological (average) conditions throughout the Bay.
2. Alternate High-Temperature Marine Heat Wave: Using this definition a high temperature alert is forecast if the surface water temperature is forecast to be above 82.5 degrees Fahrenheit for 5 consecutive days. Using this method a high temperature alert will only be forecast in the summer. Please note that we are still evaluating the specific temperature threshold to use for forecasting a high temperature alert and the 82.5 Fahrenheit temperature could change in the future.

For CBEFS to forecast a marine heatwave, the area of water experiencing a marine heatwave must be above 1000 acres, approximately 0.04% of the Bay surface area. This allows the forecasts to focus on more significant marine heatwave events and not small-scale localized elevated water temperatures.

The duration of marine heatwaves is measured in the number of days that an area has been in a marine heatwave. Maps provide the number of days that currently forecasted areas have been in a marine heatwave. Additional maps show the longest duration marine heatwave so far this year and the total number of days so far this year that an area has been in a heatwave.

Forecast Using Alternate High-Temperature Definition

This section provides the forecast of water temperature being higher than 82.5 degrees Fahrenheit. Maps show current locations of forecast high temperatures (if any) and a line plot shows the forecast area of the Bay through time. Please note high temperature forecasting began in March 2025, so forecast areas are not available prior to that.