Western Canada daily temperatures

Introduction

On the 8th of July 2021 many news outlets reported on the astonishingly high maximum temperatures being experiences in Western Canada, centered around the town of Lytton and extending to coastal regions including Vancouver, Victoria Island and Seattle.

An example of one of the reports.

https://www.theguardian.com/weather/2021/jul/08/weatherwatch-canada-records-its-highest-temperature

High temperatures in Western Canada sound strange. The area is associated with a temperate maritime climate in most people’s minds, including the residents of the area. Mean temperatures are not extreme. So is this evidence that climate change is leading to a tipping point? I downloaded data from the ghcnd meterological stations operating around the town of Lytton using the R package rnoaa

Data and model fit

Daily temperatures

In order to interrogate the results use the mouse to select different regions of the dygraph. This allows the large data set to be “eye balled” quite quickly. None of the the meteorological stations have complete records. A formal analysis would require some sophisticated statistics to fill in the data gaps.However when all the time series are placed together some general patterns can be spotted. To smooth the results using a rolling average change the number in the bottom left box and press enter.

Map of the meteorological stations in the ghcnd data base.

Mixed effect GAM model

Trends

The model is fitted to the monthly mean tmax values taken from all the stations using the R package gamm4. The station ID is used as a random effect (offset) in order to combine data from stations with differing overall mean temperatures over the time span of the data. Thus the monthly figure is centred around this value (zero on the y axis) and the annual figure shows differences from the overall mean once monthly variability has been accounted for. The model in effect “glues together” the individual time series from each station while accounting from differences between them then shows the overall trends.

First derivatives

The first derivative helps to show inflections in the rate of change. Periods in which the derivative is positive are underging warming while periods with a negative first derivative are cooling.

Interpretation

The historical data show much greater variability in the maximum daily temperatures than might be expected. Heat waves are common in the region, but usually short lived. The temperatures recorded in July 2021 broke previous records. However seen in context they were less extreme than intuition might suggest. The extracted trend shows that warming has not been constant over the last century. Although the overall trend is increasing maximum temperatures there have been short lived periods of decrease in the region.