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