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Mean Annual
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Duration
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Mean Annual
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Number MHW
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Mean Annual
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SST
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Sen’s
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Slope pvalue
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Sen’s
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Slope pvalue
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Sen’s
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Slope pvalue
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West Palm Beach 5.6 0.0005 0.7 0.0002 0.14 0.001
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Miami Beach 10 <0.0001 1.1 0.0001 0.15 0.0005
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Biscayne Bay 7.2 0.0002 0.9 <0.0001 0.1 0.021
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Key Largo 7.9 0.0002 0.9 <0.0001 0.16 0.0002
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Marathon 9.3 0.0007 1.0 0.0007 0.18 0.0002
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North Key West 4.6 0.0486 0.6 0.022 0.14 0.036
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South Key West 7.5 0.0007 0.8 0.0008 0.18 <0.0001
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Dry Tortugas 7.6 0.0011 0.8 0.0011 0.18 <0.0001
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Fort Myers 4.6 0.0088 0.5 0.0053 0.14 0.045
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Tampa 4.9 0.0006 0.5 0.0003 0.12 0.021
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Different trends were computed between the north and south coastal areas of the
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Florida Keys. Although the northern coasts of the Florida Keys (southern WFS) summed
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more events during the entire study period (Figure 7c), showing larger annual numbers
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of MHWs especially before 2008, the general Sen’s Slopes are weaker in the North Key
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West (Figure 8f) than in the South Key West area (Figure 8g) for all variables (number
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of events, duration and mean SST; Table 2). The broader area of Dry Tortugas showed
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significantly high interannual slopes (Figure 8h) with more than 11 events lasting approximately 110 days in 2015; very high numbers were also computed for 2019 and 2020. The
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southern coastal area of Marathon (Figure 8e) was also characterized by high Sen’s Slope of
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MHW events (1.0 event/decade) and the strongest interannual trend of the event durations
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among all areas (9.3 days/decade). Very strong trends were also computed for Key Largo
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(0.9 events/decade, 7.9 days/decade and 0.16 ◦C/decade). The coastal areas of the Dry
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Tortugas and Florida Keys, and especially along the Straits of Florida (southern coasts),
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are characterized by very strong increasing trends with high frequencies of MHW events,
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especially during the last seven years (2015–2021; Figure 8). The highest number of events
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over the Florida Keys before 2015 were computed for 1997 and 1998 (Figure 8) during the
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El Niño event, causing extensive coral bleaching ([55]; see Section 4.3). The increasing
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numbers of MHWs along the southern coastline of the Florida Keys during the last decade
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agree with the stronger and statistically significant trends of SST computed over the same
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areas (Figure 5).
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Water 2022, 14, 3840 16 of 28
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4. Discussion
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4.1. Effects of Atmospheric Conditions on SST Variability
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The air temperature variability averaged annually and over the entire study domain
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is presented in Figure 3a, showing a similar but slightly sharper interannual trend than
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the SST increase during the 40-year period. The air temperature and the the respective
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net heat flux variability are both well correlated with the formation of MHWs showing all
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significant increasing trends (Figure 9). The highest mean annual air temperatures and
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the strong positive (downward) heat fluxes generally coincide with the MHW peaks. The
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Sen’s Slope of all trends are statistically significant (pvalue < 0.0001), with lower significance
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for the heat fluxes (pvalue = 0.0142), which also show smaller correlation with the MHW
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frequency (RP = 0.44) in comparison to the air temperature (RP = 0.84).
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coasts), are characterized by very strong increasing trends with high frequencies of MHW
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events, especially during the last seven years (2015–2021; Figure 8). The highest number
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of events over the Florida Keys before 2015 were computed for 1997 and 1998 (Figure 8)
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during the El Niño event, causing extensive coral bleaching ([55]; see Section 4.3). The
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increasing numbers of MHWs along the southern coastline of the Florida Keys during the
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last decade agree with the stronger and statistically significant trends of SST computed
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over the same areas (Figure 5).
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4. Discussion
|
4.1. Effects of Atmospheric Conditions on SST Variability
|
The air temperature variability averaged annually and over the entire study domain
|
is presented in Figure 3a, showing a similar but slightly sharper interannual trend than
|
the SST increase during the 40-year period. The air temperature and the the respective net
|
heat flux variability are both well correlated with the formation of MHWs showing all
|
significant increasing trends (Figure 9). The highest mean annual air temperatures and the
|
strong positive (downward) heat fluxes generally coincide with the MHW peaks. The
|
Sen’s Slope of all trends are statistically significant (pvalue < 0.0001), with lower significance
|
for the heat fluxes (pvalue = 0.0142), which also show smaller correlation with the MHW
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frequency (RP = 0.44) in comparison to the air temperature (RP = 0.84).
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Figure 9. Annual variability (continuous lines) and trends (dashed lines) of the mean annual number
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of all Marine Heat Waves (MHWs; red line), the air temperature (°C; black line), and the surface net
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heat flux (J/m2
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; blue line). The Sen’s Slopes and the Pearson correlation coefficients between both
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atmospheric variables (air temperature and heat flux) and the number of MHWs are presented. The
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pvalues of the MK trend and correlation tests are also shown.
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The correlation coefficients between the SST and air temperature timeseries show a
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strong spatial variability over the South Florida region (Figure 10a). High correlations
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were computed over the inner WFS with very large correlation coefficients (>0.90) at the
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broader Tampa and Fort Myers bays, confirming the determining role of air temperature
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on the SST variability over the western Florida coastal zone; the inner parts of these two
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bays revealed the weakest impact of air temperature on SST (smaller correlation
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Figure 9. Annual variability (continuous lines) and trends (dashed lines) of the mean annual number
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of all Marine Heat Waves (MHWs; red line), the air temperature (◦C; black line), and the surface net
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heat flux (J/m2
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; blue line). The Sen’s Slopes and the Pearson correlation coefficients between both
|
atmospheric variables (air temperature and heat flux) and the number of MHWs are presented. The
|
pvalues of the MK trend and correlation tests are also shown.
|
The correlation coefficients between the SST and air temperature timeseries show a
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strong spatial variability over the South Florida region (Figure 10a). High correlations were
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computed over the inner WFS with very large correlation coefficients (>0.90) at the broader
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Tampa and Fort Myers bays, confirming the determining role of air temperature on the SST
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variability over the western Florida coastal zone; the inner parts of these two bays revealed
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the weakest impact of air temperature on SST (smaller correlation coefficients). The impact
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of air temperature on SST gradually reduces towards the WFS shelf slope (0.85–0.75), an
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area where the Gulf of Mexico mesoscale ocean circulation patterns usually prevail [34].
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Loop Current interactions with the WFS shelf control the physical characteristics over
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the slope contributing to the upwelling of colder waters toward the surface layers [30] or
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supply warmer Loop Current waters over the shelf through advection [56]. Very strong
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