Long-term warming and record-breaking marine heatwaves in the Hauraki Gulf, northern New Zealand

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New Zealand Journal of Marine and Freshwater Research
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Long-term warming and record-breaking marine
heatwaves in the Hauraki Gulf, northern New
Zealand
Nick T. Shears, Melissa M. Bowen & François Thoral
To cite this article: Nick T. Shears, Melissa M. Bowen & François Thoral (11 Mar 2024):
Long-term warming and record-breaking marine heatwaves in the Hauraki Gulf,
northern New Zealand, New Zealand Journal of Marine and Freshwater Research, DOI:
10.1080/00288330.2024.2319100
To link to this article: https://doi.org/10.1080/00288330.2024.2319100
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SHORT COMMUNICATION
Long-term warming and record-breaking marine heatwaves
in the Hauraki Gulf, northern New Zealand
Nick T. Shears
a
, Melissa M. Bowen
b
and François Thoral
c,d
a
Leigh Marine Laboratory, Institute of Marine Science, University of Auckland, Auckland, New Zealand;
b
School of Environment, University of Auckland, Auckland, New Zealand;
c
School of Science, University of
Waikato, Tauranga, New Zealand;
d
National Institute of Water and Atmospheric Research, Wellington, New
Zealand
ABSTRACT
As global oceans warm, long-term temperature records are
critical in understanding and interpreting warming trends and
the significance of marine heatwaves (MHWs) in coastal
environments. Daily measurements of sea surface temperature
(SST) have been taken since 1967 at the Leigh Marine Laboratory,
Hauraki Gulf, northern New Zealand. We analyse long-term trends
in SST anomalies and MHWs at Leigh from 1967 to 2023. We find
a significant long-term warming trend in annual SST anomaly,
with warming evident in autumn and winter, but not summer
and spring. SST has been consistently and anomalously warm
over the last decade, with 2022 being the warmest year in the
57-year record and having record high temperatures for six
months. There were no long-term trends in the number of MHW
days and cumulative intensity of MHWs annually, but break point
analysis revealed a significant and sharp increase in both metrics
since 2012. 2022 had the greatest number of MHW days (313
days) on record, including the two longest and cumulatively
intense MHWs which had numerous unpredicted impacts on
coastal ecosystems. The long-term data analysed here provides a
unique long-term perspective on warming trends in the Hauraki
Gulf and demonstrates the unprecedented nature of recent MHWs.
ARTICLE HISTORY
Received 7 December 2023
Accepted 10 February 2024
HANDLING EDITOR
Jessica Ericson
KEYWORDS
Climate change; coastal
marine environments; Leigh
Marine Laboratory; long-
term temperature records;
sea surface temperature;
tropicalisation
Introduction
Global oceans are warming with 2023 having the highest global sea surface temperature
(SST) on record (Cheng et al. 2024). These warming trends and increases in marine heat-
waves (MHWs) are having a multitude of impacts on marine species (Oliver et al. 2018;
Smith et al. 2023). In Aotearoa New Zealand the occurrence of a series of large marine
heatwaves since 2017 (Salinger et al. 2019,2020,2023) is broadly consistent with these
global trends and the effects of these recent MHW events in New Zealand on marine
© 2024 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License
(http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any
medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. The terms on which
this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.
CONTACT Nick T. Shears n.shears@auckland.ac.nz
Supplemental data for this article can be accessed online at https://doi.org/10.1080/00288330.2024.2319100.
NEW ZEALAND JOURNAL OF MARINE AND FRESHWATER RESEARCH
https://doi.org/10.1080/00288330.2024.2319100

species and ecosystems is rapidly emerging (Thomsen et al. 2019; Tait et al. 2021; Bell
et al. 2023; Cornwall et al. 2023; Montie and Thomsen 2023).
Recent analysis of satellite-derived SST from 1982 to 2021 has found that MHWs have
become stronger, longer and more frequent across much of coastal New Zealand (Montie
et al. 2023). Furthermore, the 2021/2022 MHW was the longest with highest cumulative
intensity in the 40-year satellite record for the northeastern and Fiordland regions (Bell
et al. 2023; Salinger et al. 2023). While these analyses of satellite-derived SST are highly
valuable in providing a large-scale picture of warming trends and their variability around
New Zealand, analysis of SST trends over the satellite era (since 1982) has been shown to
overestimate the occurrence and strength of warming trends around New Zealand due to
changes in the dominant ENSO phases over the satellite period (Shears and Bowen 2017).
The presence and strength of trends in SST can be greatly influenced by the length of time
over which data are analysed and in general shorter-term studies in New Zealand have
reported much higher rates of warming (e.g. Broekhuizen et al. 2021) than observed in
longer-term studies that are less influenced by interannual variability and climate
cycles (Shears and Bowen 2017; Cook et al. 2022).
Long-term daily SST records from the Leigh Marine Laboratory in the Hauraki Gulf,
northern New Zealand (since 1967), and from the Portobello Marine Laboratory, in
southern New Zealand (since 1953) (Shears and Bowen 2017; Cook et al. 2022),
provide a unique long-term perspective to understand and interpret warming trends.
Analyses of these two data sets over the last half century found limited seasonal
warming at Leigh and weak but significant long-term warming at Portobello. This
coincided with an increase in MHW duration and annual MHW days at Portobello
and no evidence of increases in MHWs at Leigh with a decrease in MHW maximum
intensity (Cook et al. 2022). The long-term rates of warming reported in these studies
are generally lower than seen in other western boundary current regions over the last
50–100 years (Ridgway 2007; Wu et al. 2012; Shears and Bowen 2017), and the contrast-
ing trends between northeastern and southern New Zealand are broadly consistent with
climate-related changes in large-scale circulation patterns around New Zealand (Shears
and Bowen 2017; Cook et al. 2022).
The rates of warming and trends in MHWs in northern New Zealand based on the
long-term SST record at Leigh is in stark contrast with recent analyses of satellite-
derived SST (Balemi and Shears 2023; Bell et al. 2023; Montie et al. 2023). While the
increased warming trends and reportedly unprecedented MHWs documented in
recent studies may reflect a more current analysis that captures recent MHW events,
they may also be influenced by the comparatively shorter time series (since 1982). The
recent novel impacts of the 2021/2022 MHW in the Hauraki Gulf (Bell et al. 2023; Sal-
inger et al. 2023) and other long-term changes emerging in northeastern New Zealand
(Balemi and Shears 2023) suggest that temperatures are increasing at rates faster than
previously thought. An updated analysis of long-term trends in SST and MHWs at
Leigh is therefore necessary to help put these recent observations and findings based
on the satellite era into a longer-term perspective. Here we analyse SST data from
January 1967 to January 2024 to provide an updated analysis of the long-term trends
in SST at the Leigh Marine Laboratory. We also carry out a MHW analysis to investigate
whether MHWs are increasing and how the recent 2021/2022 MHW events compare to
those that have occurred previously in the long-term record.
2N. T. SHEARS ET AL.

Methods
Daily sea surface temperature measurements from the Leigh Marine Laboratory
(36°16.12′S 174°48.01′E) were analysed from 1st January 1967 to 29th January 2024.
Daily measurements were originally taken at 9 am from the rocky shore with a ther-
mometer but since 2011 have been taken using an automatic data logger (Shears and
Bowen 2017; Cook et al. 2022; Shears et. al. 2023). The infilled data set of Cook et al.
(2022) was used to account for gaps in the 9 am daily data up until October 2011, but
logger data from a nearby site (∼600 m to the west; 36°15.96′S; 174°47.80′E) was used
to fill the two subsequent gaps when the logger was lost or failed (6/3/2013–13/6/2013
and 26/2/2014–20/5/2014) (as in Shears and Bowen 2017). The raw infilled 9 am daily
temperature data set is plotted in Figure S1(A) and all temperature data sets are available
in Shears et al. (2023).
All analysis of long-term trends in SST were carried out on monthly SST anomalies,
calculated for the full time series using the 30-year period from January 1983 to Decem-
ber 2012 as the reference (based on Oliver et al. 2018). Long-term trends in mean annual,
seasonal (Summer: December–February, and Autumn: March–May Winter: June–
August, Spring: September–November) and monthly SST anomalies were analysed up
until the end of 2023. Non-parametric trend analysis using Mann–Kendall and Theil–
Sen (Sen’s slope hereafter) was used to test for trends at each temporal scale. Tests for
autocorrelation were carried out and where necessary a modified Mann–Kendall trend
test was used to account for autocorrelation (as in Montie et al. 2023). Non-parametric
Pettitt change point tests were used to test for changes in temporal dynamics (as in
Thoral et al. 2022). Non-parametric trend analysis was then repeated before and after sig-
nificant change points. Non-parametric trend analyses were carried out using the ‘trend’
R package and the ‘modifiedmk’package was used for the modified Mann–Kendall tests.
Marine heatwave (MHW) analysis of the daily temperature record (to 29th January
2024) was carried out using the ‘heatwaveR’R package (version v0.4.6). The 30-year cli-
matology period from January 1983 to December 2012 was used (based on Oliver et al.
2018) and MHWs were defined as a period of 5 days or more when water temperatures
are greater than the 90th percentile of the daily climatology (Hobday et al. 2016). The
total number of MHW days and cumulative intensity of MHWs (
o
C x days) were calcu-
lated annually (six events spanned two years so were split according to number of days in
each year). Non-parametric trend analysis and Pettitt change point tests were carried out
on both MHW metrics as for SST trend above (Thoral et al. 2022).
Results
Long-term annual and seasonal trends in mean SST at Leigh are summarised in Figure 1,
which shows an upward trend in annual, autumn and winter SST, but no long-term
increase in SST for summer and spring. These general trends were supported by analysis
of SST anomalies (Figure S2; Table 1A). Mean annual SST anomaly increased signifi-
cantly from 1967 to 2023, with a marginally significant change point in 2009 followed
by a period of rapid warming with comparatively low interannual variability. Long-
term seasonal warming was evident in autumn and winter, but not summer and
spring (Figure S3; Table 1A), and significant warming occurred for the months of
NEW ZEALAND JOURNAL OF MARINE AND FRESHWATER RESEARCH 3

February, and April through to September (Figure S4; Table 1A). The highest rates of
warming were evident in May and June (0.23 and 0.19°C decade
−1
respectively). 2022
had the highest annual SST anomaly, the warmest Autumn, Winter and Spring, and
record high temperatures for March, April, May, July, August and September.
MHW analysis detected 156 MHW events at Leigh over the past 57 years, with the
two longest and greatest cumulative intensity MHWs occurring in 2022 (113 and 137
days, Figure 2A). Near continuous MHW conditions occurred from November 2021 to
November 2022 with temperatures generally 1–2°C above the long-term mean (Figure
2B). The MHWs with the highest maximum intensity occurred in the early 1970s, but
these were comparatively short and had a moderate cumulative intensity (Figure S4).
There was no significant long-term trend in the number of MHWs, total MHW days
or cumulative intensity of MHWs per year (Figure 3,Table 1B). There was a signifi-
cant change point in 2012 for total MHW days and the cumulative intensity of
MHWs, which was followed by significant increase in these metrics. Overall, 2022
had the greatest total number of MHW days (313 days) and the highest cumulative
intensity of MHWs.
Figure 1. Long-term trends in mean annual and seasonal sea surface temperature (SST) at the Leigh
Marine Laboratory, northern New Zealand (1967–2023). See Figure S1 for plot of raw data and
smoothed monthly means. Note that statistical analysis of long-term trends was carried out on SST
anomalies (see Figure S2 and Table 1).
4N. T. SHEARS ET AL.

Table 1. Annual, seasonal and monthly trend and break point analysis of SST anomaly (A) and MHW metrics (B).
Full time series (1967–2023) Change Point Pre-break Post-break
(A) SST Anomaly Zc P_Value Sen’s slope Year P_Value Sen’s slope P_Value Sen’s slope P_Value
Annual 2.97 0.0030 0.011 2009 0.0651 −0.004 0.5860 0.060 0.0003
Season
Summer [−1967] 1.49 0.1350 0.007 2006 0.3660 −0.019 0.1000 0.047 0.2560
Autumn 4.11 <0.0001 0.014 1998 0.0294 −0.011 0.3070 0.025 0.0219
Winter 4.80 <0.0001 0.012 1997 0.0069 −0.012 0.2478 0.031 0.0302
Spring 0.88 0.379 0.003 2016 0.5316 −0.006 0.2840 0.106 0.0635
Month
Jan 1.52 0.1280 0.005 2007 0.6970 −0.016 0.2860 0.055 0.1740
Feb 3.22 0.0013 0.016 2007 0.1370 −0.009 0.5670 0.047 0.1490
Mar 1.80 0.0712 0.007 2012 0.1430 −0.008 0.3070 0.099 0.1930
Apr 4.22 <0.0001 0.015 2007 0.0201 −0.003 0.7280 0.059 0.0912
May 7.14 <0.0001 0.023 1994 0.0002 −0.017 0.1990 0.027 0.0160
Jun 6.14 <0.0001 0.019 1997 0.0010 −0.010 0.4550 0.031 0.0302
Jul 3.99 <0.0001 0.014 1997 0.0211 −0.015 0.2408 0.038 0.0606
Aug 3.43 0.0006 0.009 1997 0.0573 −0.015 0.1440 0.021 0.0874
Sep 2.38 0.0172 0.004 1997 0.0561 −0.018 0.0664 −0.005 0.7387
Oct 0.85 0.3956 0.001 2015 0.5044 −0.005 0.2280 0.104 0.1179
Nov 0.01 0.9917 0.000 1975 0.4980 0.127 0.2514 0.010 0.1500
Dec 0.58 0.5592 0.003 2016 0.3603 −0.010 0.1455 0.079 0.7105
B–MHW metrics Zc P_Value Sen’s slope Year P_Value Sen’s slope P_Value Sen’s slope P_Value
No. of MHWs 1.65 0.0996 0.000 2012 0.0901 0.000 0.2880 0.333 0.1033
Total MHW days 1.72 0.0859 0.333 2012 0.0262 −0.154 0.1357 11.833 0.0164
Cum_Int (
o
C x days) 1.54 0.1249 0.346 2012 0.0354 −0.300 0.0920 16.799 0.0236
Sen’s slope indicates rate of change in each metric per year from non-parametric trend analysis. For the full time series Zc and p-values are corrected for autocorrelation. Bold values are sig-
nificant at the alpha = 0.05 level and italicised values are marginally significant ( p= 0.05–0.10).
NEW ZEALAND JOURNAL OF MARINE AND FRESHWATER RESEARCH 5

Discussion
Our updated analysis of long-term trends in SST and MHWs at the Leigh Marine Lab-
oratory highlights the record-breaking nature of recent warming and MHW events in
the Hauraki Gulf, but also the more nuanced and less severe rates of long-term
warming compared to that reported based on the satellite record (Sutton and
Bowen 2019; Montie et al. 2023). In contrast to previous analyses of SST at Leigh
(Shears and Bowen 2017; Cook et al. 2022), we find a significant increase in annual
SST anomaly (0.11°C decade
−1
), and strong evidence for warming in autumn and
winter (particularly the months of April–July). Contrary to general expectations
based on the occurrence of recent summer MHWs (Salinger et al. 2023) and analysis
of satellite data (Balemi and Shears 2023), we found no long-term increase in summer
SST. Instead, the highest summer temperatures and most intense MHWs at Leigh
occurred in the early 1970s (Cook et al. 2022), which is not captured in the satellite
Figure 2. Duration and cumulative intensity (
o
C x days) of marine heatwave (MHW) events at the
Leigh Marine Laboratory (1st January 1967–29th January 2024) Aand sea surface temperature
(SST) time series of the 2021/2022 MHWs (B; SST climatology (1983–2012; blue), 90th percentile
MHW threshold (green) and SST time series (black), red shading indicates intensity of MHW and hori-
zontal black bars indicate duration of each event).
6N. T. SHEARS ET AL.

record and leads to overestimate of summertime warming trends and MHWs. For
example, when the Leigh data is constrained to the satellite era (1982–present;
Figure S5) we see considerably higher rates of annual warming (0.23°C decade
−1
)
and significant warming for each season including summer (ranging from 0.13 to
0.34°C decade
−1
). This emphasises the importance of long-term SST records in under-
standing warming trends and how the evidence for and strength of warming from
shorter-term studies needs to be treated cautiously.
Figure 3. Annual number of MHW events, number of MHW days and cumulative intensity of MHWs at
the Leigh Marine Laboratory from 1967 to 2023. Solid black line indicates linear regression and blue
line is loess smoother. Vertical line indicates break-point years in the time series based on Petitt tests
(solid: significant, dashed: non-significant).
NEW ZEALAND JOURNAL OF MARINE AND FRESHWATER RESEARCH 7

Annual SST anomaly at Leigh exhibited high interannual variability up until 2009 fol-
lowed by a period of rapid increase with low interannual variation. This culminated with
2022 being the warmest year on record (0.38°C higher than 1999, the previous warmest
year), with record high temperatures for six months of the year and the most MHW days
in one year (313 days, compared to 198 days in 1999). Near continuous marine heatwave
conditions persisted from November 2021 to November 2022 with temperatures typically
between 1 and 2 degrees above average. Across the whole time series, we found a margin-
ally significant (p= 0.086) increase in the total number of MHW days per year, which
contrasts with Cook et al. (2022) analysis of the data to the end of 2020. There was
also a significant breakpoint in 2012, which was the last year in the 57 year record
without a MHW (Figure 3). 2012 was followed by a rapid increase in both MHW days
and cumulative MHW intensity highlighting that the warming pattern seen over the
last decade and the recent MHWs at Leigh are indeed unprecedented since the record
started in 1967.
Global rates of ocean warming have generally been greatest in western boundary
current regions (Wu et al. 2012) and these are the areas where some of the greatest
impacts on marine ecosystems have been observed (Vergés et al. 2014). However,
our results demonstrate how warming and its impacts are emerging in regions that
historically have had large interannual variability with little long-term warming
(Shears and Bowen 2017). The large interannual fluctuations in temperature at
Leigh are correlated with changes in winds over the South Pacific and may be due
to advection by ocean currents, changes in the ocean heat convergence around the
country communicated by Rossby waves and changes in the heat exchange as atmos-
pheric circulation changes (Bowen et al. 2017). There is a lack of large riverine inputs
close to Leigh that could influence coastal temperatures and coastal upwelling has little
influence on variability in SST at Leigh (Bowen et al. 2017). The record temperatures
recorded in 2022 coincided with another peak in the wind stress curl (Figure S1B and
C). In addition, this was the sixth warmest year globally (and the warmest La Nina
year on record) (Blunden et al. 2023) and the warmest year measured by the New
Zealand seven station records that began in 1909 (Meyers 2023). These events likely
added an unprecedented level of background heating to the regional drivers of
higher temperatures at Leigh and elevated the ocean to levels of a nearly continuous
MHW throughout 2022.
While the mechanisms driving the long-term warming trends, and both the seasonal
and regional variation observed, require further investigation, recent ecological changes
and observations on shallow reefs in the Hauraki Gulf during the 2022 MHWs provide
some novel insights into how these ecosystems may be impacted with future warming
(Figure 4). The warm summer temperatures in 2022 coincided with extensive blooms
of the red filamentous algae (Gayliella sp.) on shallow reefs (Figure 4A) and extended
blooms of the epi-benthic dinoflagellate Ostreopsis siamensis (N.S. pers. obs), which is
typically associated with warm water events and has negative impacts on sea urchins
(Shears and Ross 2009). Sustained periods of warm, rather than extreme, temperatures
through autumn and into winter likely lead to prolonged heat stress and impacts on
temperature sensitive species, such as observed with sponges (Bell et al. 2023;Figure
4B). The warmer temperatures through autumn and winter likely also facilitated
8N. T. SHEARS ET AL.

Figure 4. Recent warming trends and the unprecedented marine heatwaves in 2022 in the Hauraki
Gulf coincided with extensive blooms of red filamentous algae on shallow reefs A, the occurrence of
necrotic and ‘melting’sponges (B;Ecionemia alata; Bell et al. 2023), increases in the occurrence of the
invasive colonial ascidian Symplegma brakenhielmi (C, shown growing on the tail of a rock lobster
Jasus edwardsii), and increased observations in 2023 of an unknown tropical Diadema species D
and juveniles (<20 mm test diameter) of the subtropical sea urchin Centrostephanus rodgersii (D–
yellow circle) that is increasing in northern New Zealand (Balemi and Shears 2023, unpubl. data).
Photos by N. Shears taken at the Mokohinau Islands (23/2/2022; A), Tāwharanui Marine Reserve
(2/5/2022; B, C) and the Poor Knights Islands Marine Reserve (19/7/2023; D).
NEW ZEALAND JOURNAL OF MARINE AND FRESHWATER RESEARCH 9

increases in subtropical and tropical species that would otherwise be limited by cool
winter temperatures (Figure 4C,D).
The El Niño conditions that developed and contributed to record high SST globally in
2023 (Cheng et al. 2024) would typically be expected to provide some relief from warm
temperatures in this region, as they usually coincide with lower temperatures at Leigh
(Bowen et al. 2017; Shears and Bowen 2017). However, as the climate warms there is
increased uncertainty in the regional responses to ENSO (Yeh et al. 2018) and despite
El Niño conditions in 2023, it was the 4th warmest year in the long-term record at
Leigh (Figure 1). Throughout 2023, temperatures remained above the climatological
mean with a number of small MHWs and another MHW developed in early January
2024 that persisted at the time of final data collection for this paper (29/1/2024; Figure
S6). These recent patterns are consistent with predicted increases in extreme warming
events and MHWs in waters around Aotearoa New Zealand (Law et al. 2018; Behrens
et al. 2022), but also suggest the traditional indicators of coastal ocean temperatures
such as ENSO are becoming less useful as the oceans warm. While our study documents
a concerning and persistent level of warming in recent years, it also highlights the critical
value of long-term temperature records in providing historic context to understand
warming trends and the significance of current events, as well as interpreting emerging
impacts and changes occurring in marine ecosystems.
Acknowledgements
We greatly acknowledge the late Dr Bill Ballantine who started the long-term collection of temp-
erature data at the Leigh Marine Laboratory, and Jo Evans and subsequently John Atkins who
maintained the record for many years. We are grateful to the many staffand students involved
in the historic and ongoing data collection, and also offer our deepest condolences and gratitude
to the family of Yue Gui (Alice) who passed away while taking the daily measurements. Funding to
FT from MBIE: Toka ākau toitu Kaitiakitanga –building a sustainable future for coastal reef eco-
systems UOWX2206.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Data availability statement
The long-term daily sea surface temperature measurements are available in Shears et al. (2023).
This data set includes the original manual 9 am daily measurements, logger data from 2011 to
2023, and the infilled daily data set from 1967–2023 that was analysed in this study. The data
are openly available in Figshare at https://doi.org/10.17608/k6.auckland.24773262.
ORCID
Nick T. Shears http://orcid.org/0000-0002-1551-582X
Melissa M. Bowen http://orcid.org/0000-0003-1425-5082
François Thoral http://orcid.org/0000-0003-0457-1735
10 N. T. SHEARS ET AL.

References
Balemi CA, Shears NT. 2023. Emergence of the subtropical sea urchin Centrostephanus rodgersii as
a threat to kelp forest ecosystems in northern New Zealand. Frontiers in Marine Science.
10:1224067.doi:10.3389/fmars.2023.1224067.
Behrens E, Rickard G, Rosier S, Williams J, Morgenstern O, Stone D. 2022. Projections of future
marine heatwaves for the oceans around New Zealand using New Zealand’s earth system model.
Frontiers in Climate. 4:19. doi:10.3389/fclim.2022.798287.
Bell JJ, Smith RO, Micaroni V, Strano F, Balemi CA, Caiger PE, Miller KI, Spyksma AJ, Shears NT.
2023. Marine heat waves drive bleaching and necrosis of temperate sponges. Current Biology. 33
(1):158–163. doi:10.1016/j.cub.2022.11.013.
Blunden J, Boyer T, Bartow-Gillies E, editors. 2023. State of the climate in 2022. Bulletin of the
American Meteorological Society. 104(9):Si–S501.
Bowen M, Markham J, Sutton P, Zhang X, Wu Q, Shears NT, Fernandez D. 2017. Interannual
variability of sea surface temperature in the southwest Pacific and the role of ocean dynamics.
Journal of Climate. 30:7481–7492. doi:10.1175/JCLI-D-16-0852.1.
Broekhuizen N, Plew DR, Pinkerton MH, Gall MG. 2021. Sea temperature rise over the period
2002–2020 in Pelorus Sound, New Zealand–with possible implications for the aquaculture
industry. New Zealand Journal of Marine and Freshwater Research. 55(1):46–64. doi:10.1080/
00288330.2020.1868539.
Cheng L, Abraham J, Trenberth KE, Boyer T, Mann ME, Zhu J, Wang F, Yu F, Locarnini R, Fasullo
J, Zheng F. 2024. New record ocean temperatures and related climate indicators in 2023.
Advances in Atmospheric Sciences. 1–15. doi:10.1007/s00376-024-3378-5.
Cook F, Smith RO, Roughan M, Cullen NJ, Shears NT, Bowen M. 2022. Marine heatwaves in
shallow coastal ecosystems are coupled with the atmosphere: insights from half a century of
daily in situ temperature records. Frontiers in Climate. 4:207. doi:10.3389/fclim.2022.1012022.
Cornwall CE, Nelson WA, Aguirre JD, Blain CO, Coyle L, Archino D, Desmond R, Hepburn MJ,
Liggins CD, Shears L, et al. 2023. Predicting the impacts of climate change on New Zealand’s
seaweed-based ecosystems. New Zealand Journal of Botany. 1–28. doi:10.1080/0028825X.
2023.2245786.
Hobday AJ, Alexander LV, Perkins SE, Smale DA, Straub SC, Oliver EC, Benthuysen JA, Burrows
MT, Donat MG, Feng M, Holbrook NJ. 2016. A hierarchical approach to defining marine heat-
waves. Progress in Oceanography. 141:227–238. doi:10.1016/j.pocean.2015.12.014.
Law CS, Rickard GJ, Mikaloff-Fletcher SE, Pinkerton MH, Behrens E, Chiswell SM, Currie K. 2018.
Climate change projections for the surface ocean around New Zealand. New Zealand Journal of
Marine and Freshwater Research. 52(3):309–335. doi:10.1080/00288330.2017.1390772.
Meyers T. 2023. New Zealand [in “State of the climate in 2022”]. Bulletin of the American
Meteorological Society. 104(9):S463–S465. doi:10.1175/2023BAMSStateoftheClimate_Chapt
er7.1.
Montie S, Thomsen MS. 2023. Long-term community shifts driven by local extinction of an iconic
foundation species following an extreme marine heatwave. Ecology and Evolution. 13(6):
e10235. doi:10.1002/ece3.10235.
Montie S, Thoral F, Smith RO, Cook F, Tait LW, Pinkerton MH, Schiel DR, Thomsen MS. 2023.
Seasonal trends in marine heatwaves highlight vulnerable coastal ecoregions and historic
change points in New Zealand. New Zealand Journal of Marine and Freshwater Research. 1–
26. doi:10.1080/00288330.2023.2218102.
Oliver EC, Donat MG, Burrows MT, Moore PJ, Smale DA, Alexander LV, Benthuysen JA, Feng M,
Gupta AS, Hobday AJ, et al. 2018. Longer and more frequent marine heatwaves over the past
century. Nature Communications. 9(1):1–12. doi:10.1038/s41467-018-03732-9.
Ridgway KR. 2007. Long-term trend and decadal variability of the southward penetration of the
East Australian current. Geophysical Research Letters. 34:L13613. doi:10.1029/2007GL030393.
Salinger H, Diamond J, Behrens E, Fernandez D, Fitzharris BB, Herold N, Johnstone P, Kerckhoffs
H, Mullan AB, Parker AK, et al. 2020. Unparalleled coupled ocean-atmosphere summer
NEW ZEALAND JOURNAL OF MARINE AND FRESHWATER RESEARCH 11

heatwaves in the New Zealand region: drivers, mechanisms and impacts. Climatic Change.
162:485–506. doi:10.1007/s10584-020-02730-5.
Salinger H, Renwick J, Behrens E, Mullan AB, Diamond HJ, Sirguey P, Smith RO, Trought MC,
Cullen NJ, Fitzharris BB. 2019. The unprecedented coupled ocean-atmosphere summer heat-
wave in the New Zealand region 2017/18: drivers, mechanisms and impacts. Environmental
Research Letters. 14:044023. doi:10.1088/1748-9326/ab012a.
Salinger MJ, Diamond HJ, Bell J, Behrens E, Fitzharris BB, Herod N, McLuskie M, Parker AK, Ratz
H, Renwick J, et al. 2023. Coupled ocean-atmosphere summer heatwaves in the New Zealand
region. Weather and Climate. 42(1):18–41. doi:10.2307/27226713.
Shears N, Evans J, Atkins J. 2023. Long-term sea surface temperature measurements from the
Leigh Marine Laboratory, northern New Zealand. The University of Auckland. Dataset.
doi:10.17608/k6.auckland.24773262.
Shears NT, Bowen MM. 2017. Half a century of coastal temperature records reveal complex
warming trends in western boundary currents. Scientific Reports. 7:1–9. doi:10.1038/s41598-
017-14944-2.
Shears NT, Ross PM. 2009. Blooms of benthic dinoflagellates of the genus Ostreopsis; an increas-
ing and ecologically important phenomenon on temperate reefs in New Zealand and worldwide.
Harmful algae. 8:916–925.
Smith KE, Burrows MT, Hobday AJ, King NG, Moore PJ, Sen Gupta A, Thomsen MS, Wernberg
T, Smale DA. 2023. Biological impacts of marine heatwaves. Annual Review of Marine Science.
15:119–145. doi:10.1146/annurev-marine-032122-121437.
Sutton PJ, Bowen MM. 2019. Ocean temperature change around New Zealand over the last 36
years. New Zealand Journal of Marine and Freshwater Research. 53:305–326. doi:10.1080/
00288330.2018.1562945.
Tait LW, Thoral F, Pinkerton MH, Thomsen MS, Schiel DR. 2021. Loss of giant kelp, Macrocystis
pyrifera, driven by marine heatwaves and exacerbated by poor water clarity in New Zealand.
Frontiers in Marine Science. 8:721087. doi:10.3389/fmars.2021.721087.
Thomsen MS, Mondardini L, Alestra T, Gerrity S, Tait LW, South PW, Lilley SA, Schiel DR. 2019.
Local extinction of bull kelp (Durvillaea spp.) due to a marine heatwave. Frontiers in Marine
Science. 6:84. doi:10.3389/fmars.2019.00084.
Thoral F, Montie S, Thomsen MS, Tait LW, Pinkerton MH, Schiel DR. 2022. Unravelling seasonal
trends in coastal marine heatwave metrics across global biogeographical realms. Scientific
Reports. 12:1–13. doi:10.1038/s41598-022-11908-z.
Vergés A, Steinberg PD, Hay ME, Poore AG, Campbell AH, Ballesteros E, Heck KL, Booth DJ,
Coleman MA, Feary DA. 2014. The tropicalization of temperate marine ecosystems: climate-
mediated changes in herbivory and community phase shifts. Proceedings of the Royal Society
B: Biological Sciences. 281(1789):20140846. doi:10.1098/rspb.2014.0846.
Wu L, Cai W, Zhang L, Nakamura H, Timmermann A, Joyce T, McPhaden MJ, Alexander M, Qiu
B, Visbeck M, et al. 2012. Enhanced warming over the global subtropical western boundary cur-
rents. Nature Climate Change. 2(3):161–166. doi:10.1038/nclimate1353.
Yeh SW, Cai W, Min SK, McPhaden MJ, Dommenget D, Dewitte B, Collins M, Ashok K, An SI,
Yim B-Y, Kug JS. 2018. ENSO atmospheric teleconnections and their response to greenhouse
gas forcing. Reviews of Geophysics. 56:185–206. doi:10.1002/2017RG000568.
12 N. T. SHEARS ET AL.