Changing trends and relationship between global ocean chlorophyll and sea surface temperature

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1 Available online at Procedia Environmental Sciences 3 (0) The 8th Biennial Conference of International Society for Ecological Modelling Changing trends and relationship between global ocean chlorophyll and sea surface temperature J.F. Feng *, L.Zhu a Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), College of Environmental Science and Engineering, Nankai University, Tianjin 30007, China Abstract With the MODIS Aqua satellite-derived ocean chlorophyll concentration and sea surface temperature(sst) time series data, recent changing trends were evaluated by linear regression analysis. Furthermore, the time frequency space and regions in time frequency space where the chlorophyll and SST time series show high common power and phase angle were explored and found with Continuous wavelet transform (CWT) and Cross wavelet transform(xwt) methods. Results showed that during , the global ocean chlorophyll concentration is increasing while the SST is decreasing. There is a stable one year frequency in the global ocean chlorophyll for the whole time period but a half year frequency only in To the SST, there are a stable one year and half year frequency during There is a stable one and an unstable 0.5 year resonance frequency between global ocean chlorophyll and SST. The phase angle between global ocean chlorophyll and SST increased from 30 to 80 for one year resonance frequency and phase relationship changed from in-phase to anti-phase for 0.5 year frequency after Published by Elsevier B.V. Selection and/or peer-review under responsibility of School of Environment, 0 Published by Elsevier Ltd. Beijing Normal University. Open access under CC BY-NC-ND license. Keywords: Global change; Ocean chlorophyll; Sea surface temperature; Wavelet analysis. Introduction As the trophic base of marine ecosystems, marine phytoplankton accout for nearly 50% of global primary production, and have a key role in global biogeochemical cycles. Changes in phytoplankton biomass may have significant effects on ecosystem structure and functioning[]. Marine ecosystem are subject to mutiple stressors in different scals such as global warming and large-scale climate cycles(el * Corresponding author. Tel.: address: fengjf@nankai.edu.cn Published by Elsevier B.V. Selection and/or peer-review under responsibility of School of Environment, Beijing Normal University. Open access under CC BY-NC-ND license. doi:0.06/j.proenv

2 J.F. Feng and L. Zhu / Procedia Environmental Sciences 3 (0) Nin o Southern Oscillation, Pacific Decadal Oscillation and North Atlantic Oscillation)[]. Also human activities will cause the variablity of phytoplankton with the increasing nutrient input and fishing, especially in coastal and shelf regions. Identifying changing trends in global marine phytoplankton and its relationship with global climate change is crucial to improve our understanding of biological responses to environmental factors such as climate forcing and human exploitation. Global phytoplankton shows a decling trend over the past century, and this trend is related to increasing sea surface temperatures[3]. But there are still some dicussions about the data and methods used by Boyce[4-5] due to the absent of consistent observations. So long-term consistent data records are essential to understand changes in marien ecosystems. Satellite ocean data provide a precious chance, and there are several operational global ocean colour sensors (SeaWiFS, MERIS, MODIS-Aqua, MODIS-Terra) which present standard prodcuts[6-8]. By comparing the global ocean chlorophyll from the CZCS ( ) and SeaWiFS ( ) records, chlorophyll concentrations decreased in the northern high latitudes while chlorophyll in the low latitudes increased[9]. But from 998 to 003, global ocean chlorophyll has increased 4.%[0]. The short-term changes of global ocean chlorophyll may not consistent or even opposite to the long-term change trends. Many studies have shown that changes of global phytoplankton biomass are realted to global cilmate change[-4]. But there are few research focused on the frequency and phase angle between them. In this paper, with the MODIS Aqua satellite-derived ocean chlorophyll concentration and sea surface temperature(sst) time series data, recent changing trends were evaluated by linear regression analysis. Furthermore, the time frequency space and regions in time frequency space where the chlorophyll and SST time series show high common power and phase angle were explored and found with Continuous wavelet transform (CWT) and Cross wavelet transform(xwt) methods.. Data and methods.. Data Moderate Resolution Imaging Spectroradiometer (MODIS) is a key instrument aboard the Terra (on board in 999) and Aqua (on board in 00) satellites. MODIS Terra and Aqua are designed to provide measurements in large-scale global dynamics including changes and processes occurring in the oceans, on land, and in the lower atmosphere. The time series data of global chlorophyll concentration and sea surface temperature from 003 to 009 are retrieved from MODIS Aqua/NASA for entire ocean region (Fig.). Chlorophyll concentration data are caculated with MODIS algorithm (OC3M) and averaged in global ocean region. The OC3M algorithm is: log 0 (CHL) = R +.457R R 3.403R 4, where R = log 0 [max(r rs (443), R rs (488))/ R rs (55)].

3 68 J.F. Feng and L. Zhu / Procedia Environmental Sciences 3 (0) Fig.. Global ocean region of MODIS Aqua/NASA ( MODIS Aqua/Daytime sea surface temperature (u) were also averaged in global ocean region... Wavelet analysis method Fourier transform is a mathematical operation that transfer a signal from time domain into its constituent frequencies domain. The main problem with the Fourier transform is the inconsistent treatment of different frequencies. Wavelet analysis attempts to solve these problems by decomposing a times eries into time/frequency space simultaneously and is becoming a common tool for analyzing localized variations of power within a time series. The detailed theory for wavelet analysis has been described by Torrence et al [5]. The Continuous Wavelet Transform (CWT) is a common tool for analyzing localized intermittent oscillations in a time series, and it is very often desirable to examine the relations between two time series. From two CWTs we construct the Cross Wavelet Transform (XWT) which will expose their common power and relative phase in time-frequency space. Also by calculating the phase difference from wavelets cross the whole spectrum, we were able to quantify the lag period between two time series [6]. In this paper, wavelet analysis and wavelet coherence were conducted in MatLab wavelet coherence package and can be downloaded from 3. Results 3.. Changing trends of global ocean chlorophyll and SST The time series data of global ocean chlorophyll and SST from 003 to 009 were showed in Fig. and Fig.3 respectively. With linear regression analysis, we got the changing trends of chlorophyll and SST in Table.. Results showed that the global ocean chlorophyll concentration was increasing while the SST was decreasing in Fig. Time series data of global ocean chlorophyll from 003 to 009

4 J.F. Feng and L. Zhu / Procedia Environmental Sciences 3 (0) Fig.3 Time series data of global SST from 003 to 009 Tab. Global trends in ocean chlorophyll and temperature Slope Intercept Global ocean chlorophyll Global ocean temperature Changes in the frequency domain of global ocean chlorophyll and SST Wavelet analysis showed that both global chlorophyll and SST have the one consistent perodic bands abroud year(fig.4). The global sea surface temperature also have a predominant 0.5 year bands, which is not very clearly for global chlorophyll. There is a change for global chlorophyll power spectrum in 007. The 0.5 year perodic bands became significant. So, there is a stable one year frequency in the global ocean chlorophyll for the whole time period but a half year frequency only in To the SST, there are a stable one year and half year frequency during Period Period Global Chlorophyll /4 /8 /6 /64 / Global SST / 0.5 /4 0.5 /8 /6 /64 / / Fig.4 Continuous wavelet power spectrum of global chlorophyll(top) and SST(bottom) from 003 to 009. The thick black contour designates the 5% significance level against red noise and the cone of influence (COI) where edge effects might distort the picture is shown as a lighter shade

5 630 J.F. Feng and L. Zhu / Procedia Environmental Sciences 3 (0) Cross wavelet analysis of global ocean chlorophyll and SST Cross wavelet transform was used to understand the relation between global ocean chlorophyll and SST. From Fig.5, it s very clearly that there is a stable one year resonance frequency between global ocean chlorophyll and SST. The 0.5 year resonance frequency is not consistent and disappeared in 006. Phase relationship is shown as arrows with in-phase pointing right, anti-phase pointing left. For the stable year frequency, the phase angle between global ocean chlorophyll and SST is 30 before 006. Then, the phase relationship is anti-phase which means that the phase angel is about 80 after 006. For the 0.5 year frequency, the phase relationship is in-phase before 006, but changed to anti-phase after 006. XWT: Global Chlorophyll-Global SST Period / /4 /8 /6 /3 /64 Fig.5 Cross wavelet transform of the global ocean chlorophyll and sea surface temperature time series from The 5% significance level against red noise is shown as a thick contour. The relative phase relationship is shown as arrows (with in-phase pointing right, anti-phase pointing left, and SST leading chlorophyll by 90 pointing straight down). 4. Conclusions From 003 to 009, the global ocean chlorophyll mean concentration increased slightly, while the global sea surface temperature decreased slightly. There are a stable year perodic bands for global chlorophyll concentration and maybe an unstable 0.5 year perodic band after 006. For the global sea surface temperature, there are a stable one year and half year frequency during There are a stable one year resonance frequency and an unstable 0.5 year resonance frequency between global ocean chlorophyll and SST. The phase angle between global ocean chlorophyll and SST increased from 30 to 80 after 006 for the year frequency, and phase relationship changed from in-phase to anti-phase after 006 for 0.5 year frequency. Acknowledgements The work is supported with funds provided by the National Natural Science Foundation of China (No ) and the Fundamental Research Funds for the Central Universities.

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