AIM : Investigate number, intensity and duration of MHW events in coastal NZ, including Chatham, Stewart, Auckland and Campbell islands.

Document Note : The maps, plots and app within this document are interactive so make sure you give them a play like zooming in and out in the maps but also on the plots. Clicking on the legend allows to only select and display the time series needed.

Table of contents

  1. Year of Maximum Intensity MHW

  2. National Coastal Trends

  1. Regional Trends
  1. Bibliography

Summary of Preliminary Results

  • MHW events are getting more numerous. Especially in Jan, Mar, May, Jul, Aug, Oct, Dec? Inceasing recently for all seasons, bump years 2000 only in Autumn, Winter and Spring.

  • MHW Events are getting more intense (in terms of cumulative intensity). Especially in Jan, Feb, Mar, Apr, Dec? The 3 regions the most affected recently are the Chatham, the West Coast of the South Island and Fiorldand. Mainly in Summer and Autumn.

  • MHW Events are getting more intense (in terms of max intensity). Especially in Jan, Mar, Dec? The West Coast of the South Island and Fiordland are the regions experiencing the most intense MHW events. Summer MHW more intense, but spring/Autumn not far.

  • MHW Events are getting longer. Especially in Jan, Feb, Mar, Apr, May, Jun, AUg, Nov, Dec? The Chatham Islands, the West Coast of the South Island and Fiordland, are experiencng longer events recently. Summer and Autumn.

  • Global trends: Positive Sen’s slope showing positive trend + significant for metrics Mean Cumulative Intensity, Mean Duration and Number of Events (p-value<0.05), not significant for Mean Maximum Intensity.

Data and Methodology for MHW event detection

Data - Downloading and Preparing NOAA OISST Data By Robert W Schlegel and AJ Smit From https://robwschlegel.github.io/heatwaveR/articles/OISST_preparation.html

Event Detection - Uses heatwaveR:: (Schlegel and Smit 2018), translation of GitHub python functions from Eric C.J. Oliver (published in paper Holbrook et al. (2019) A global assessment of marine heatwaves and their drivers) https://robwschlegel.github.io/heatwaveR/articles/gridded_event_detection.html

Year and Maximum Intensity of strongest MHW

We show here the Years of the most intense MHW events (in terms of cumulative intensity) in coastal NZ. All coastal pixels are used for the summarisy in part 2. The polygons represent the regional zones where information is summarised in part 3. Note the presence of the red (2016), grey (2017) and green (2018) colours showing the importance of recent events in coastal NZ since 1981.

#NZ_polys <- st_read('NZ_IslandsMainland_polys2.shp',crs=CRS("+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"),quiet=T)

NZ_polys <- st_read('14_Coastal_NZ_Biogeographic_Regions.shp',crs=CRS("+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"),quiet=T)

# MHW Coastal Summaries
#MHW_coastal <- read_csv('MHW_Events_NZ_Coastal.csv',col_types = cols()) %>% dplyr::filter(date_peak < "2021-01-01") #Removed year 21

MHW_coastal_regions <- read_csv('MHW_Events_NZ_Kermadec_Coastal_Biogeographic_Regions.csv',col_types = cols()) %>% dplyr::filter(date_peak < "2021-01-01") #Removed year 21 #read_csv('MHW_Events_NZ_Coastal_Biogeographic_Regions.csv',col_types = cols())


MHW_coastal_YearMax <- MHW_coastal_regions %>% mutate(year = lubridate::year(date_peak)) %>% group_by(lon,lat,year) %>% 
  summarise(max_intensity = max(intensity_cumulative)) %>% 
  dplyr::filter(max_intensity==max(max_intensity))
## `summarise()` has grouped output by 'lon', 'lat'. You can override using the `.groups` argument.
MHW_coastal_YearMax_sf <- st_as_sf(MHW_coastal_YearMax,coords = c('lon','lat'), crs=crs(NZ_polys))

## Map
pal <- c(brewer.pal(9,'Purples'),brewer.pal(9,'Blues'),brewer.pal(9,'Oranges'),brewer.pal(9,'Reds'),brewer.pal(3,'Greens'))

p_intensity <- ggplot() +
  geom_sf(data=NZ_polys$geometry,fill=NA) +
  geom_sf_text(data=NZ_polys,aes(label = Region),size=3, colour = "black",nudge_x = c(4,1,-2,-4,-3,3,-3,5,4,0,0,0,0,0),
    nudge_y = c(0,-2,0,0,1,-1,-1,0,0,1,-1,1,0,-2)) +
  #geom_sf(data=MHW_coastal_YearMax_sf,aes(size = scale(max_intensity))) +#Size of point by intensity
  geom_sf(data=MHW_coastal_YearMax_sf,aes(col = year)) +#Colour of point by intensity
  #scale_color_viridis(option = "magma",direction = -1,name = "Cumulative Intensity (DegC)")
scale_colour_gradientn(colours=pal) +
  ggtitle('Year of Most Intense Coastal MHW events and coastal pixels included') +
  borders('world2',xlim=c(163,185),ylim=c(-55,-28)) + xlim(c(163,185)) + ylim(c(-55,-28)) + 
    theme(panel.background = element_rect(fill = 'aliceblue'), panel.grid.major = element_blank(),legend.position = "bottom") + 
  annotation_scale(location = "br",pad_x = unit(0.05, "in"),pad_y = unit(0.05, "in"), width_hint = 0.2)  +
  annotation_north_arrow(location = "br", which_north = "true", 
                         pad_x = unit(0.01, "in"), pad_y = unit(0.3, "in"),
                         style = north_arrow_fancy_orienteering) + 
  xlab('') + ylab('') 
ggplotly(p_intensity)

Figure 1 - Year of Most Intense MHW events (in terms of cumulative intensity) around Coastal New Zealand. Note the red (2016), grey (2017) and green (2018) colours.

We show here the corresponding cumulative intensity of the strongest MHW recorded at these coastal pixels.

## Map

p_intensity <- ggplot() +
  geom_sf(data=NZ_polys$geometry,fill=NA) +
  geom_sf_text(data=NZ_polys,aes(label = Region),size=3, colour = "black",nudge_x = c(4,1,-2,-4,-3,3,-3,5,4,0,0,0,0,0),
    nudge_y = c(0,-2,0,0,1,-1,-1,0,0,1,-1,1,0,-2)) +
  #geom_sf(data=MHW_coastal_YearMax_sf,aes(size = scale(max_intensity))) +#Size of point by intensity
  geom_sf(data=MHW_coastal_YearMax_sf,aes(col = max_intensity)) +#Colour of point by intensity
  scale_color_viridis(option = "magma",direction = -1,name = "Cumulative Intensity (DegC.Days)") +
  ggtitle('Most Intense Coastal MHW events and coastal pixels included') +
  borders('world2',xlim=c(163,185),ylim=c(-55,-28)) + xlim(c(163,185)) + ylim(c(-55,-28)) + 
    theme(panel.background = element_rect(fill = 'aliceblue'), panel.grid.major = element_blank(),legend.position = "bottom") + 
  annotation_scale(location = "br",pad_x = unit(0.05, "in"),pad_y = unit(0.05, "in"), width_hint = 0.2)  +
  annotation_north_arrow(location = "br", which_north = "true", 
                         pad_x = unit(0.01, "in"), pad_y = unit(0.3, "in"),
                         style = north_arrow_fancy_orienteering) + 
  xlab('') + ylab('') 
ggplotly(p_intensity)

Figure 1 bis - Cumulative intensity of Most Intense MHW events around Coastal New Zealand.

Cumulative intensity or max intensity?

What makes a MHW particularly destructive? Is it a short but very abrupt event (very high maximum intensity but short duration and small cumulative intensity) or a sustained but slightly mellower event (lower maximum intensity but longer duration and higher cumulative intensity)? Let’s just plot here the same 2 maps than previously but showing the maximum max intensity ever recorded in signel events as well as their date (year).

MHW_coastal_YearMax2 <- MHW_coastal_regions %>% mutate(year = lubridate::year(date_peak)) %>% group_by(lon,lat,year) %>% 
  summarise(max_intensity = max(intensity_max)) %>% 
  dplyr::filter(max_intensity==max(max_intensity))
## `summarise()` has grouped output by 'lon', 'lat'. You can override using the `.groups` argument.
MHW_coastal_YearMax2_sf <- st_as_sf(MHW_coastal_YearMax2,coords = c('lon','lat'), crs=crs(NZ_polys))

## Map

p_intensity <- ggplot() +
  geom_sf(data=NZ_polys$geometry,fill=NA) +
  geom_sf_text(data=NZ_polys,aes(label = Region),size=3, colour = "black",nudge_x = c(4,1,-2,-4,-3,3,-3,5,4,0,0,0,0,0),
    nudge_y = c(0,-2,0,0,1,-1,-1,0,0,1,-1,1,0,-2)) +
  #geom_sf(data=MHW_coastal_YearMax_sf,aes(size = scale(max_intensity))) +#Size of point by intensity
  geom_sf(data=MHW_coastal_YearMax2_sf,aes(col = max_intensity)) +#Colour of point by intensity
  scale_color_viridis(option = "magma",direction = -1,name = "Max Intensity (DegC)") +
  ggtitle('Most Intense Coastal MHW events and coastal pixels included') +
  borders('world2',xlim=c(163,185),ylim=c(-55,-28)) + xlim(c(163,185)) + ylim(c(-55,-28)) + 
    theme(panel.background = element_rect(fill = 'aliceblue'), panel.grid.major = element_blank(),legend.position = "bottom") + 
  annotation_scale(location = "br",pad_x = unit(0.05, "in"),pad_y = unit(0.05, "in"), width_hint = 0.2)  +
  annotation_north_arrow(location = "br", which_north = "true", 
                         pad_x = unit(0.01, "in"), pad_y = unit(0.3, "in"),
                         style = north_arrow_fancy_orienteering) + 
  xlab('') + ylab('') 
ggplotly(p_intensity)

Figure 1 bis - Maximum intensity of Most Intense MHW events around Coastal New Zealand.

And associated year

## Map
pal <- c(brewer.pal(9,'Purples'),brewer.pal(9,'Blues'),brewer.pal(9,'Oranges'),brewer.pal(9,'Reds'),brewer.pal(3,'Greens'))

p_intensity <- ggplot() +
  geom_sf(data=NZ_polys$geometry,fill=NA) +
  geom_sf_text(data=NZ_polys,aes(label = Region),size=3, colour = "black",nudge_x = c(4,1,-2,-4,-3,3,-3,5,4,0,0,0,0,0),
    nudge_y = c(0,-2,0,0,1,-1,-1,0,0,1,-1,1,0,-2)) +
  #geom_sf(data=MHW_coastal_YearMax_sf,aes(size = scale(max_intensity))) +#Size of point by intensity
  geom_sf(data=MHW_coastal_YearMax2_sf,aes(col = year)) +#Colour of point by intensity
  #scale_color_viridis(option = "magma",direction = -1,name = "Cumulative Intensity (DegC)")
scale_colour_gradientn(colours=pal) +
  ggtitle('Year of Most Intense Coastal MHW events and coastal pixels included') +
  borders('world2',xlim=c(163,185),ylim=c(-55,-28)) + xlim(c(163,185)) + ylim(c(-55,-28)) + 
    theme(panel.background = element_rect(fill = 'aliceblue'), panel.grid.major = element_blank(),legend.position = "bottom") + 
  annotation_scale(location = "br",pad_x = unit(0.05, "in"),pad_y = unit(0.05, "in"), width_hint = 0.2)  +
  annotation_north_arrow(location = "br", which_north = "true", 
                         pad_x = unit(0.01, "in"), pad_y = unit(0.3, "in"),
                         style = north_arrow_fancy_orienteering) + 
  xlab('') + ylab('') 
ggplotly(p_intensity)

Figure 1 bis - Year of Most Intense MHW events (in terms of max intensity) around Coastal New Zealand. Note the red (2016), grey (2017) and green (2018) colours.

Bibliography

Holbrook, Neil J, Hillary A Scannell, Alexander Sen Gupta, Jessica A Benthuysen, Ming Feng, Eric CJ Oliver, Lisa V Alexander, et al. 2019. “A Global Assessment of Marine Heatwaves and Their Drivers.” Nature Communications 10 (1): 1–13.
Schlegel, Robert W., and Albertus J. Smit. 2018. “heatwaveR: A Central Algorithm for the Detection of Heatwaves and Cold-Spells.” Journal of Open Source Software 3 (27): 821. https://doi.org/10.21105/joss.00821.