Seasonal Signals in Satellite Derived Chlorophyll-a Concentration in the Gulf of Mexico OCG561 Term Paper

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1 Seasonal Signals in Satellite Derived Chlorophyll-a Concentration in the Gulf of Mexico OCG561 Term Paper K.C. Rosburg Graduate School of Oceanography, University of Rhode Island, Narragansett, RI Abstract Current literature is lacking in basin-wide chlorophyll-a studies in the Gulf of Mexico, specifically those studying the seasonality of large-scale blooms. Here, time-series and correlation analysis are used to investigate the relationship between temperature, Loop Current area, and the seasonal cycles of satellite derived chlorophyll-a concentration. High correlation between temperature and Chl-a, and low correlation between Loop Current area and Chl-a is found, but evidence of other factors indicates the need for further studies of the involved processes. The influence of hurricanes, mixed layer depth, and winds should be considered in the future. 1 Introduction The Loop Current (LC) is the main driver of circulation in the Gulf of Mexico (GOM). The LC enters the Gulf of Mexico from the Caribbean Sea as a continuation of the Yucatan Current and circulates anti-cyclonically with speeds up to 1.8 m s 1 (G.Z. Forristal et al., 1992), forming a large ring in the south-eastern GOM. As time progresses, the ring grows, becoming increasingly unstable. Eventually the ring, or Loop Current Eddy (LCE), separates from the LC and drifts westward (Figure 1). Separation occurs non-periodically on irregular intervals between 3 17 months (Sturges and Leben, 2000). Some GOM water exits through the Straights of Florida to form the Florida Current. Loop Current eddies are of particular importance in the GOM. Their high velocities can shut down oil-drilling operations at high costs. Loop Current eddies have warm cores and can modify the intensity of passing hurricanes (Yablonsky and Ginis, 2012). The Loop Current, Loop Current eddies, and associated peripheral cold-core frontal eddies also 1

2 31 N (a) 09 Mar 2010 (b) 22 May 2010 (c) 17 Jul N 23 N 19 N LC YC Formation/Intrusion Florida Florida LCE LCE FC FC FC LC YC YC Separation Retreat 95 W 90 W 85 W 80 W 95 W 90 W 85 W 80 W 95 W 90 W 85 W 80 W Sea Surface Height (cm) Florida Figure 1: HYCOM Sea Surface Height maps depicting the three stage Loop Current Eddy cycle: (a) northward intrusion of the Loop Current (LC) marking the start of the formation process, (b) necking down and separation of the Loop Current Eddy (LCE), and (c) southward LC retreat and westward migration the Loop Current Eddy. Yucatan and Florida currents are marked by YC and FC, respectively, and the LC boundary is outlined in black. impact biology. Warm-core features in the Gulf of Mexico (e.g. LC, LCEs) are downwelling zones and are low in nutrients (Biggs, 1992). Likewise, chlorophyll standing stock, primary production, and zooplankton biomass are lower in these warm-core features than the rest of the Gulf. Higher zooplankton and micronekton standing stock, and higher hyperiid abundance is found in cold-core cyclones than in the warm-core features (Gasca et al., 2001; Zimmerman and Biggs, 1999). Large basin-wide studies of chlorophyll-a abundance in the Gulf of Mexico are seemingly absent from literature. Instead, many small-scale coastal region studies have been conducted (e.g. Biggs and Moller-Karger 1994). This is true despite the availability of long-term high-resolution satellite chlorophyll-a imagery. Motivation for this study comes from the observation of seasonal cycles in movies of satellite derived chlorophyll-a (provided by Leben, R.R.; reproduced in Figure 2). I use a high-resolution two-year satellite chlorophyll-a record to examine seasonal signals in surface chlorophyll-a concentration over the entire Gulf of Mexico. I then compare this record to a series of other satellite derived time-series to investigate the physical controls on basin-wide seasonal chlorophyll-a signal. The goal is to provide an analysis of large-scale chlorophyll cycles in the Gulf of Mexico, which is as of writing absent from literature. In Section 2, I discuss the datasets and processing there of, and the methods used in analysis. Section 3 covers results, and Section 4 provides a discussion of results and conclusions. 2

3 31 N 29 Aug Mar Sep N 23 N 19 N 95 W 85 W 95 W 85 W Chl a (mg m 3 ) W 85 W Figure 2: Satellite derived cholophyll-a concentration in the Gulf of Mexico on 29 Aug (left), 17 Mar (center), and 03 Sep (right). 2 Data & Methods Remotely sensed chlorophyll-a (Chl-a) concentrations were used in this study. These data were derived from satellite ocean color via MODIS-Aqua with 1 km resolution and processed by Goddard Space Flight Center using algorithms to remove influence from nonchlorophyll turbidity (e.g. gelbstoff, sediment, etc.). The data were obtained from servers at the University of Rhode Island Graduate School of Oceanography (GSO) as daily-averaged global Chl-a maps. Daily Gulf of Mexico region Chl-a maps were created by extracting data from the original global maps. The repeat time of MODIS-Aqua is around 10 days, so a 10-day mean time-series was created in order to reduce data sparseness and the impact of clouds on data quality. The dataset spans the time period 1 January December Global maps are available for a longer period, but the length of readily available sea surface height (SSH) products limited the duration of this study. Two Chl-a time-series were created using this method. One for the entire Gulf of Mexico (unmasked). This showed consistantly high Chl-a concentrations near the coasts (not pictured), but yielded a high standard deviation about the mean (Figure 3). To remedy this, a second dataset was created, in which coastal biology was ignored by masking Chl-a concentrations above 0.35 mg m 3. Doing so yielded a lower standard deviation (Figure 3), so the masked dataset is used for this study. 3

4 Two other satellite datasets were used in this study. Loop current area was calculated from the Colorado Center for Astrodynamics Research (CCAR) satellite altimetry (sea surface height (SSH)) product. The CCAR product melds SSH data from Jason-1, Envisat, and OSTM/Jason-2 satellites with a resolution of approximately 6 km. These data were provided by Robert Leben at CCAR. Following Leben (2005), the Loop Current is defined as the area enclosed by the 17 cm SSH contour. For ease, I use the number of enclosed grid points as a proxy for area. Basin-wide sea surface temperature (SST) records were derived from NOAA s Geostationary Operational Environment Satellite (GOES-13 East) imager, with a spatial resolution of 4 km. GOES data was also obtained from GSO s Satdat3 server. To match processing of Chl-a data, a time-series of 10-day mean basin-averaged SST was generated. St. Dev. (mg m 3 ) GOM Masked Jan 2009 May 2009 Sep 2009 Jan 2010 May 2010 Sep 2010 Figure 3: Unmasked (green line) and coastally-masked (black line) basin-mean chlorophyll-a abundance. Green and white shading indicates standard deviation about the mean for unmasked and masked timeseries, respectively. 3 Results Seasonal cycles were observed in both Chl-a and SST time-series, but were less evident in Loop Current area time-series (Figure 4). 10-day mean basin-averaged satellite sea surface temperature varied from approximately 21 C to 30 C, peaking in August and reaching a minimum in January/February. Detrended 10-day mean basin-averaged satellite chlorophyll-a concentration followed an inverse pattern, peaking in winter and early spring, and becoming minimal during the late-spring and summer. The number of grid points 4

5 enclosed by the 17 cm SSH contour (proxy for Loop Current area) is cyclic, but is not necessarily consistent enough to be described as seasonal or annual. This is corroborated by a strong lack of agreement in the literature (e.g. Sturges and Leben 2000; Chang and Oey 2012). These time-series comparisons suggest an inverse relationship between temperature and Chl-a concentration, as well as a possible temperature range for the basin-wide bloom (increase) in Chl-a concentration. From Figure 4, it was hypothesized that the bloom could exist between 21.6 C and 25.6 C. The lower bound is close to the minimum temperature during the study period, and can be thrown out. The upper bound, perhaps, presents itself as a maximum threshold temperature above which the bloom cannot form. Correlations analysis showed a negative correlation between SST and Chl-a for both masked and unmasked datasets (Table 1). The masked dataset had a stronger correlation than the unmasked dataset: and -0.57, respectively. Loop Current area showed little to no correlation with chlorophyll-a concentration for both datasets, with correlations below 0.15 for both datasets. For comparison, the masked and unmasked Chl-a records were considered in the correlations analysis and showed to be well correlated to each other (0.78). Oddly (and unexpectedly), sea surface temperature and Loop Current area were weakly negatively correlated, but this is beyond the scope of this paper. The high correlation between masked Chl-a and SST confirms an inverse relationship between SST and Chl-a, and rules out a strong influence from Loop Current variability. Detrended Chl a Concentration (mg m 3 ) Jan 2009 May 2009 Sep 2009 Jan 2010 May 2010 Sep Sea Surface Temp. ( C) LC Area (grid pt.) Figure 4: Time-series comparison of detrended masked (green) basin-averaged 10-day mean satellitederived chlorophyll-a concentration (mg m 3 ), basin-averaged 10-day mean sea surface temperature (red; C), and Loop Current area (gray; no. grid points). 5

6 Table 1: Matrix of correlations between unmasked Chl-a, masked Chl-a, basin-averaged sea surface temperature, and Loop Current area. GOM Masked GOM LC Chl-a Chl-a SST Area GOM Chl-a Masked Chl-a GOM SST LC Area 1.00 Chlorophyll-a concentration decreases quadratically with temperature (Figure 5). A scatter plot of SST versus Chl-a concentration shows quadratically decreasing chlorophylla concentration with increasing temperature. Above about 26 C the curve levels out and Chl-a remains low and constant ( 0.1 mg m 3 ). The range of chlorophyll-a concentration is low (0.18 mg m 3 ), but the standard deviation is only 0.05 mg m 3. The shape of the curve in Figure 5 suggests a possible threshold temperature, but not equal to that hypothesized here: more analysis than can be fit into the five page maximum is required. Chl a Concentration (mg m 3 ) Quadratic Fit Hypothesized Temp. Range R 2 = 0.81 r = SST ( C) Figure 5: X-Y scatter plot depicting the relationship between sea surface temperature and coastally-masked chlorophyll-a concentration. 4 Discussion and Conclusions A seasonal cycle in satellite derived Gulf of Mexico-wide chlorophyll-a concentration was observed, with increasing Chl-a in winter months. Two physical controls on the seasonality of Chl-a were examined. The variability of the Loop Current has been ruled out as a 6

7 major player in the seasonality of Chl-a, but the high negative correlation between Chl-a and temperature suggest that basin-wide sea surface temperature is an important physical control on chlorophyll-a concentration in the Gulf of Mexico. Results are not conclusive enough to show, and it is unlikely given the complexity of the physical processes in the Gulf of Mexico, that temperature is the sole, or most important, physical driver of Chl-a seasonal cycles. More likely is the complex interplay of multiple physical and biological processes, and further examination is needed to fully understand the process(es). Results have neither confirmed or ruled out the possibility of a temperature threshold for the seasonal basinwide Chl-a bloom, and more research is needed to do so. I conclude by suggesting other processes that may play a role in the basin-wide seasonal Chl-a cycle that are of interest, but were not able to be investigated in this paper. Given the role that mixed layer depth is believed to play in the North Atlantic Bloom onset, it is possible also that the mixed layer depth is wholly or partially responsible for the onset of the basin-wide bloom in the Gulf of Mexico. Alternatively, the Gulf of Mexico is prone to numerous hurricanes each year, so the possibility of hurricane controlled bloom formation or termination also exists. Hurricanes modify the structure of the pycnocline, and hence can create upwelling/downwelling zones in their wake, which may impact surface biology. The likelihood of this, however, is low. Hurricane season usually begins later in the year than the formation of the basin-wide bloom, but hurricanes may remain as a bloom termination mechanism. Again, more research is necessary to understand the large-scale seasonality of chlorophyll-a in the Gulf of Mexico. Acknowledgments K.C. Rosburg would like to acknowledge Peter Cornillon and Robert Leben for some of the data used in this study, and Susanne Menden-Deuer for guidance and help in analysis. References Biggs, D.C., Nutrients, plankton, and productivity in a warm-core ring in the western gulf of mexico. Journal of Geophysical Research: Oceans 97, doi: /90jc Biggs, D.C., Moller-Karger, F.E., Ship and satellite observations of chlorophyll stocks in interacting cyclone-anticyclone eddy pairs in the western gulf of mexico. Journal of Geophysical Research: Oceans 99, doi: /93jc

8 Chang, Y.L., Oey, L.Y., Why does the loop current tend to shed more eddies in summer and winter? Geophysical Research Letters 39, L doi: /2011gl Gasca, R., Castellanos, I., Biggs, D.C., Euphausiids (crustacea, euphausiacea) and summer mesoscale features in the gulf of mexico. Bulletin of Marine Science 68, G.Z. Forristal, K.J. Schaudt, C.K. Cooper, Evolution and kinematics of a loop current eddy in the gulf of mexico during J. Geophys. Res., Leben, R.R., AltimeterDerived loop current metrics, in: Circulation in the Gulf of Mexico: observations and models. American Geophysical Union, Washington, DC. volume 161 of Geophys. Monograph Ser., pp Sturges, W., Leben, R., Frequency of ring separations from the loop current in the gulf of mexico: A revised estimate. J. Phys. Oceanogr. 30, doi: / (2000)030. Yablonsky, R.M., Ginis, I., Impact of a warm ocean eddy s circulation on hurricaneinduced sea surface cooling with implications for hurricane intensity. Mon. Wea. Rev. 141, doi: /mwr-d Zimmerman, R.A., Biggs, D.C., Patterns of distribution of sound-scattering zooplankton in warm- and cold-core eddies in the gulf of mexico, from a narrowband acoustic doppler current profiler survey. Journal of Geophysical Research: Oceans 104, doi: /1998jc