Marine heatwaves detection code

Brief description

marineHeatWaves is a python module which implements the Marine Heatwave (MHW) definition of Hobday et al. (2016).

The code was written by Eric C. J. Oliver.

Contributors to the Marine Heatwaves definition and its numerical implementation include Alistair J. Hobday, Lisa V. Alexander, Sarah E. Perkins, Dan A. Smale, Sandra C. Straub, Jessica Benthuysen, Michael T. Burrows, Markus G. Donat, Ming Feng, Neil J. Holbrook, Pippa J. Moore, Hillary A. Scannell, Alex Sen Gupta, and Thomas Wernberg.

Hobday, A.J. et al. (2016), A hierarchical approach to defining marine heatwaves, Progress in Oceanography, 141, pp. 227-238, doi: 10.1016/j.pocean.2015.12.014

Full description

The Hobday et al. (2016) definition detects marine heatwaves from daily seawater temperature time series. A marine heatwave is defined as a "discrete prolonged anomalously warm water event at a particular location" with each of those terms (anomalously warm, prolonged, discrete) quantitatively defined and justified for the marine context. Specifically, "discrete" implies the marine heatwave is an identifiable event with clear start and end dates, "prolonged" means it has duration of at least five days, and "anomalously warm" means the water temperature is above a climatological threshold, defined here to be the seasonally-varying 90th percentile. Marine heatwaves are therefore identified as periods of time when temperatures were above the threshold for at least five consecutive days; and two successive events with a break of two days or less between then were considered a single continuous event. The seasonally varying climatological mean and 90th percentile threshold are calculated for each day of the year using the pool of daily temperature values across all years within an 11-day window centred on the day, which are then smoothed using a 31-day moving window. The use of a seasonally-varying threshold allows for the detection of summer heatwaves as well as winter warm spells.

Marine heatwaves detected using this definition are assigned a set of properties. These properties include duration, several measures of intensity, and onset and decline rates.  These metrics are defined as follows: "duration" is the time between the event's start and end dates, "mean intensity" and "maximum intensity" are the average and maximum temperature anomaly (measured relative to the climatological, seasonally-varying mean) over the duration of the event respectively, "cumulative intensity" is the integrated temperature anomaly over the duration of the event, and  "onset rate" and "decline rate" are the rates of temperature increase and decline from the start date to the peak of the event and from the peak to the end date respectively.  In addition, annual time series are calculated for each MHW property. For duration, the intensity metrics, and depth this is the average of these properties across all events in each year; years with no events are marked as missing data. We also calculate frequency, being the count of events in each year, and total MHW days, being the sum of the durations of all events in each year.