Monitoring sea level change at Cascais tide gauge

Transcription

1 Journal of Coastal Research SI 64 pg - pg ICS211 (Proceedngs) Poland ISSN Montorng sea level change at Cascas tde gauge C. Antunes IDL Unversty of Lsbon, Lsbon, Portugal cmantunes@fc.ul.pt ABSTRACT Antunes, C., 211. Montorng sea level change at Cascas tde gauge. Journal of Coastal Research, SI 64 (Proceedngs of the 11th Internatonal Coastal Symposum), pg pg. Szczecn, Poland, ISSN Sea level change (SLC) real-tme montorng has beng developed and mproved for the Portuguese tde gauge, based n FTP access to the nternet server of the Portuguese geographcal nsttute. A software applcaton, named MareVB and n development snce 28, gets a 3 mnute stream nput of sea level heght and a 1 mnute stream nput of ar-pressure. Based on a predcted tde model, the sea level heght s compared and analyzed, and storm-surge ampltude s determned, as well as the hgh frequency oscllaton (sechas) due to the storm and tsunam waves. Usng such real-tme SLC data analyss, the applcaton s now runnng as a coastal hazard warnng system, emalng automatc warnngs n real-tme to natonal authortes and to other nsttutons and coworkers, where the levels of coastal hazards of SLC are consdered. A post-processng montorng of SLC s also performed and a tme seres s evaluated and actualzed. Such SLC tme seres enables the statstcs of storm-surges and the determnaton of the sea level rse rate assocated to the global clmate change and other regonal phenomena. The computatonal methodology, the applcaton for real-tme montorng and the statstcs of storm-surges at Cascas for the perod of last 1 years are presented here. ADITIONAL INDEX WORDS: Sea level change, real-tme montorng, storm-surge, Cascas tde gauge INTRODUCTION The modern tde gauge wth an nternet connecton avalable allows a permanent and real-tme montorng for Sea Level Change (SLC), caused by atmospherc and oceanc forcng, namely, the storm-surges and the sesmc and meteorologcal tsunams. Based on adjusted and precse tde models and through an applcaton connected by FTP to a tde gauge, t s possble to montor n real-tme the sea level varaton due to dfferent causes and send emal warnngs to the natonal nsttutons for rescue and cvl protecton. The FTP access to the Cascas and Lagos natonal tde gauges has enabled ths strategy for SLC montorng, where every storm, surge, and even a meteo-tsunam has been detected n Cascas snce the applcaton MareVB started to be developed n 28 at the FCUL (Faculdade de Cêncas da Unversdade de Lsboa). Due to ts capablty to send automatc emal warnngs, ths applcaton has become a coastal hazard warnng system for SLC events. Recently, new facltes have been added to the MareVB applcaton, a warnng nformaton table wth surge elevaton and sea oscllaton values, and a tde graph are created every 2 mnutes and then sent to the FCUL tde webpage (Antunes, 27), enablng the vsualzaton and updatng of data related to SLC at these two Portuguese tde gauges. For each day, a post-processed montorng of the mean SL (MSL) and the respectve surges assocated to the ongong storms s also preformed. Ths pos-processng, done outsde the MareVB applcaton, s manly performed to determne the evoluton of the Sea Level Rse (SLR), although, the daly storm-surge ampltude and respectve statstcs are also determned. Ths statstcs enable the study of the evoluton of such events, n ampltude and frequency, and the possble assocaton to the global clmate change and other eventual regonal forcng phenomena. Ths paper presents the computatonal methodology, as well as the applcaton for real-tme montorng and the emal warnng system. It also presents the statstcs of the storm surges at Cascas along the last 1 years. PRECISE HARMONIC TIDE MODEL The actual ampltude of storm surge, due to meteorologcal effects, s determned by comparng drectly the nstantaneous sea level heght wth a precse harmonc tde model. Ths s the prncple of the MareVB applcaton, where acheved precson of 2 cm of the harmonc tde model for the respectve tde gauge plays a very mportant role, snce t contrbutes for the relablty of the SLC montorng. Such tde model for the Cascas tde gauge was evaluated based on hourly data from 27. The numercal model was determned by harmonc analyss through the least square adjustment (LSA), where the correspondent parametrc model s composed by a maxmum of 37 local harmonc consttuents (ampltude, H, and phase lag, g ), wth the addtonal ampltude of the zero order harmonc consttuent (Z ), correspondng to the actual MSL referred local tde datum. Mathematcally, the harmonc model, accordng to Godn (1972), assumes that the one-dmensonal tme seres of astronomc tde can be expressed by n Z( t) Z f ( t ) H cos ( t t ) V ( t ) u ( t ) g (1) 1 Where, Z(t) s the measured tde heght; f and u are the nodal correctons to ampltude and phase for the consttuents =1,...,n wth frequency : and, V s the astronomcal argument at the ntal nstant t (: of 1 of January of each year). Journal of Coastal Research, Specal Issue 64, 211

2 Montorng sea level change at Cascas tde gauge Ether by harmonc or by spectral analyzes t can be verfed that a certan number of harmonc consttuents are absent or mxed wth the nose data, whch mples only 24 harmonc consttuents to be estmated by LSA at the Cascas tde gauge. In the present case, for each harmonc consttuent, the parameters f and V are known and evaluated for each year, and the long term nodal argument, u, s assumed as a small and neglgble constant. Thus, the lnearzaton of the functon model (1), due to the need of a fast convergence of the unknown local phase arguments, rather than the usual Taylor seres expanson, must follow a varable change approach, such as X H cos( g ); Y H sn( g ) (2) Transformng (1) nto n Z( t) Z A X BY (3) where, A B 1 f cos( ( t t ) V ( t )) (4) f sn( ( t t ) V ( t )) After applyng LSA to the functonal model (3), the orgnal parameters, H and g, can be recovered from the estmates X and Y, by: 2 2 Y H X Y ; g arctg (5) X Beng Hg the respectve varance-covarance matrx obtaned by the followng transformaton T Hg J Hg XY J Hg (6) where J Hg s the Jacobean matrx of (5), a bloc dagonal matrx composed wth the H and g dervatves n respect to X and Y. Ths lnearzaton approach avods the need of a good ntal set of parameters (local harmonc consttuents) close to the fnal adjust soluton of LSA (as t would have happened f a Taylor seres expanson had been used for lnearzaton of the functon model), enablng a fast convergence of the phase component. The strategy that was used to correct data from known effects, to correctly ft functon model (1) to the data, such as ar-pressure and wnd forced surges (atmospherc tde), and the seasonal varaton of MSL, was determnant to acheve a good accuracy and precson nto the tde model. The tde heght was corrected from the nverse barometrc effect (IBE) expressed n equaton (7), usng the theoretcal rate of -1 cm/mbar (whch closely agree wth the Cascas data correlaton), where the mean value of the arpressure n 27 was assumed to be 119 mbar (hpa). Fgure 1. Seasonal varaton of Cascas MSL from 25 to 28; MSL of year 27; and, ftted SA harmonc consttuent. 119 AP( mbar ) Z '( t) Z ( t) (7) 1 The resdual atmospherc effect caused by wnd spread was removed by a numercal resdual functon, based n a daly centred movng average of the prevous remanng resduals (after all effects have been removed), representng a correctve trend functon. Fnally, the seasonal varaton of MSL was ndependently estmated by the annual harmonc consttuent (SA solar annual) adjusted to a seasonal varaton average for a perod of 4 years, from 25 to 28 (Fgure 1), and removed from the tde heght data. The annual harmonc consttuent, SA, estmated outsde of LSA, ncludes n ts own consttuents, the ampltude and the phase tag, all the seasonal varaton of the tdes,.e., the astronomc component (solar annual) and the atmospherc wnd regme nfluence (Antunes and Taborda, 29; Relvas et al., 27). Ths harmonc consttuent s, n ths way, LSA-ndependently estmated and converted to a dependent varable n the LSA model (3). After all the known correcton effects had been appled, the resduals, wth ampltude less than 1 cm, showed up to be a whte nose wth a normal dstrbuton (Gaussan nose). Ths proves that all non-astronomc tdal effects were correctly removed from the tde heght data and that the harmonc tde functonal (1) through expresson (3) was accurately ftted to the data. Therefore, the harmonc model obtaned for the Cascas tde has a daly precson of 2 cm (posteror mean square error of the LSA) and accuracy greater than 5 cm, after removng the IBE. The mean square error for the estmated parameters (5) resulted, by (6), greater than 1 mm, for the ampltudes, and greater than.1º, for the phase arguments. On the respect to the phase, and based on the dfferences of low and hgh tde arrval nstant to the recorded tdes, the tde model was estmated to have less than 3 mnutes of error. Table 1: Man harmonc tde consttuents of Cascas tde gauge. Consttuent Ampltude (m) Phase tag (º) M S N K K O SA The seasonal component ftted through the SA harmonc consttuent shows a great varablty along dfferent years (see Fgures1 and 3). Ths varablty supports the approach that has been used to ft the SA consttuent to a seasonal varaton average of several years (from 25 to 28). The estmated harmonc model and ts error values are daly confrmed, n MareVB applcaton, to be an accurate and precse model for ts purpose, the daly and real-tme montorng of SLC at tde gauges. REAL-TIME MONITORING The applcaton MareVB, based on the receved Internet Protocol (IP) streams of data sent by tde gauges of IGP (Insttuto Geográfco Português), started beng developed at FCUL n the begnnng of 28, to montor n real-tme the storm-surges that reach the Portuguese coast and any unpredctable event of SLC. Journal of Coastal Research, Specal Issue 64, 211

3 Montorng sea level change at Cascas tde gauge Through a FTP connecton, to a selected tde gauge, the MareVB applcaton receves two streams, the water level and the ar-pressure measured at the gauge ste, respectvely, at 3 and 1 mnute rates. The applcaton converts the water level nto tde heght, accordng to the local tde datum (hydrographcal zero = 2.8 m at Cascas), and computes the dfference to the respectve harmonc model heght, resultng n the surge of the SL. Dependng on the ampltude of the surge, and based n pre-defned warnng levels of storm-surge (green, yellow, orange and red), the applcaton sends warnng emals to the natonal nsttutons for rescue and cvl protecton, and then forwarded to a malng lst. Addtonally, and based on the hgh frequency of water level oscllaton, the applcaton, through an emprcal correlaton, can estmate the level of sea waves caused by storm wnds or by any other exctaton source, such as the meteorologcal tsunam detected on 6-7 July 21 (Fgure 3), on the south coast of Portugal manland, or any tsunam generated from a ocean sesmc source. The actual verson of the MareVB applcaton (V3.2) bulds a graphcal chart of the measured and predcted tdes, and the hgh frequency of oscllaton (resonance). Wthn an nterval of 2 mnutes, the tde chart and a fle-report s sent, by FTP, to the predcton tdes webpage of FCUL webste (Antunes, 27), reportng the actual surge sea level warnng, wave condtons, water heght and ar-pressure. On the Fgure 2, the runnng MareVB applcaton wndow s dvded n four man parts: on top-left wndow - the raw data streams sent by the tde gauge; top-rght wndow - the tde table and the warnng levels; centered wndow - processed tde data; and, bottom wndow - the tde chart. Ths fgure shows the montorng of tde gauge of Cascas on the day 6 December 21, where at that nstant a storm-surge of 19 cm was beng observed, whch was caused by a low atmospherc pressure of 17 mbar. On the same day, the oscllaton of hgh frequency ndcates a SNR (sgnal nose rato) of 4.4, wth ampltude sechas of about 7 cm, ndcatng a sea storm wth relatve hgh waves. At that same tme the applcaton had already sent 14 emals of warnngs related to storm-surges and sea state. Fgure 3. Meteo-Tsunam occurred on 6-7 July 21 and detected at Cascas tde gauge by the MareVB applcaton. On 6 to 7 of July 21, an unusual phenomena has occurred on the south-west coast of the Portuguese manland, specfcally a meteorologcal tsunam (Fgure 3) caused by convectve cells, whch rapdly generated hgh pressure varatons, causng local resonance waves that reached, at Lagos tde gauge, 51 cm of ampltude and a perod of 57 mnutes. Unfortunately, by that tme the MareVB was only connected to the Cascas tde gauge (the connecton to the Lagos tde gauge was off), however the MareVB applcaton has behaved wth a good performance to the phenomena, as t lately reached Cascas area. The applcaton sent a frst emal warnng at 1:1 AM of day 7 (yellow warnng) and at 6:43 AM has sent ts frst red warnng level emal, related to hgh frequency oscllaton of SL and sgnfcant ampltude. Ths fact together wth every warnng that s sent whenever there s a stormsurge, makes the MareVB a valuable tool as a coastal warnng system for the Portuguese coast. The applcaton saves several daly data fles, that can lately converted to a database, namely: the sea level heght and arpressure raw data; the computed data wth tde, the tde model, the storm-surge ampltude and the resonance; a warnng data fle to be delvered to the tde webpage of FCUL; and, an emalng log, to record the number, the type and the warnng level of the emals sent each day. DECADAL SEA LEVEL TIME SERIES The daly data fles from the tde gauge of Cascas, wth the water level and the ar-pressure s downloaded each day by FTP. Ths data s merged nto a decadal tme seres of MSL and stormsurge evaluaton data set, from whch a SLR evaluaton statstcs and frequency of storm-surges are determned. Fgure 2. MareVB applcaton wndow, runnng at Tme Seres of Decadal MSL The daly MSL and the ar-pressure mean are determned from daly data and ncluded n a 1 year base seres, to montor SLR and evaluate the surge events assocated to storms (Fgure 4). From ths tme seres, the monthly mean s also evaluated to ncrement and actualze the MSL secular tme seres of the Cascas tde gauge, whch has been gatherng data snce 1882, beng one of the oldest tme seres of SL n the world. Journal of Coastal Research, Specal Issue 64, 211

4 Montorng sea level change at Cascas tde gauge Fgure 4. Tme seres of MSL and MSL corrected from IBE, of last ten years. Top and bottom lnes represent trends of extremes. On daly MSL tme seres, as shown n Fgure 4, the seasonal varatons of SL s perfectly evdent, from whch one can adjust a numercal model to be appled for the predcton of tde models, as show before (Fgure 1). These varatons, wth average ampltude of 14 cm, are manly explaned by the meteorologcal forcng caused by the wnter/summer regme of wnds that cross the west coast (Relvas et al., 27). The summer wnds, comng from N and NW, together wth the Corols effect, mply a movement of the surface waters near the coast towards offshore/west drecton (responsble for the well known upwellng phenomena), generatng a convergence (lowerng) of the water surface to the topographcal seabed (correspondng to the lowest annual MSL of 8 cm). Whle the wnter wnds, comng from S and SW, generate the dvergence (up lftng) of the water surface (correspondng to the hghest annual MSL of 22 cm). If, for any reason, ths regular regme of wnds changes for a whle, one can observe slghtly perturbatons n the behavor of the seasonal varaton. The IBE (correctve term of equaton (7)) s removed from the daly MSL, whch results n a new tme seres of IBE-corrected MSL (Fgure 4). Such data corresponds to a SL seres wthout the nfluence of hgh and medum frequency of the ar-pressure oscllaton. The dfferences between both seres denote the occurrence of surges forced by ar-pressure oscllaton, usually assocated to storms. Ths s dentfed as barometrc surge. The storm-surge, as the total amount of surge (total atmospherc effect), s computed as the dfference between the daly MSL and the average of MSL (now at 15.9 cm above MSL of Cascas 1938), when t s greater than 1 cm. The IBE-corrected MSL tme seres s then used to better estmate the SLR of the present decade as t s shown n Fgure 4 by the centered trend lne, whch n present analyss corresponds to a lnear regresson of the 6-day movng average computed over the IBE-corrected MSL tme seres. New SLR rate and acceleraton The long term tme seres, recorded snce1882, use monthly mean values of the MSL, over whch a 1-year perod movng average representng an estmaton model of the SLR at Cascas tde gauge s computed (Antunes and Taborda, 29). Such tme seres dsables the possblty to estmate the present SLR rate. Snce the movng average must be centered, t would be necessary to have avalable the data for the next 5 to 1 years, whch s presently mpossble. Thus, the present decadal tme seres, as shown n Fgure 4, s an alternatve to enable the estmaton of the present rate of SLR, whch can be compared to the latest decade rate obtaned through the secular tme seres, and nduce an acceleraton of SLR. The tme seres n Fgure 4 show an ncrease of the extreme values along the last 1 years, as shown by the bottom and top trend lnes on the graph. However, these trends represent hgher rates than the lnear regresson computed over the 6-day movng average of the IBE-corrected MSL tme seres (mddle trend lne). Runnng such computaton of relatve SLR evaluaton (overall data trend), the rate of 2.6 mm/yr (.3 mm/yr) was obtaned for the perod of 2 to 29 (Antunes et al., 21), whle consderng the addtonal data of 21 a much hgher rate s obtaned, around 4.8 mm/yr. Ths abnormal ncrease of SLR rate, only n one year of data, s yet to be explaned and ts relablty needs to be confrmed wth data for the next years. However, t can be observed that the MSL n 21 s 7. cm hgher (22.9 cm) than ts decadal average (15.9 cm). Addtonally, the mnmum of annual MSL (seasonal varaton determned by the 6-day movng average over the corrected MSL tme seres), occurrng at the summer perod (Fgure 4), has been ncreasng snce 28, reachng ts hghest value of 16 cm n 21. What s causng such rate rse of SL? Ths s the queston that has been posed lately. Analyzng the ar-pressure tme seres of last ten years, one can ndentfy two man trends, one wth a postve rate of.2 mbar/yr, from 2 to 26, and a second one, from 26 to 21, wth a rate of -.46 mbar/yr. The negatve rate of the present ar-pressure data seres can be responsble for a forcng of the SLR, due to the release of the pressure that was actng n the earler perod of the ar-pressure rse. Ths s a possble explanaton for what s beng observed n the SLR of Cascas tde gauge for the last three years, but addtonal data s requred and correlaton must be analyzed to prove such hypothess. As t was presented n Antunes et al. (21), when comparng the SLR rate of 2.6 mm/yr estmated for the perod of 2-29 wth a 2.1 mm/yr rate for the 199 s decade, an acceleraton of.26 mm/yr 2 s obtaned. However, f we use the seres wth the 21 data ncluded, the acceleraton rses up to.15 mm/yr 2. Such hgh value for SLR acceleraton, explaned by the unexpected SL rate rse of the last three years, s not so unusual, once t has already occurred n the past century, namely, n the 2 s and 7 s decades (Antunes and Taborda, 29). Statstcs of Storm-Surges for the last 1 years The tde gauge data are stored and processed for each day to compute the MSL and ar-pressure daly average values, as explaned before. Wth such daly averages t s determned whether there s or not a storm-surge and, the ampltudes of stormsurge and IBE are computed. Both values are ncluded n the statstcs of the last 1 years, where the frequency s determned to obtan the percentle curve (Fgure 5). For each year, the total number of storm-surge occurred for each percentle s determned Fgure 5. Percentle curve of the daly storm-surge occurred at Cascas tde gauge, snce 21 to 21. Values of SS referred to the MSL reference of Cascas Journal of Coastal Research, Specal Issue 64, 211

5 Montorng sea level change at Cascas tde gauge and charted to obtan an evoluton of the storms that has occurred along ths perod of 1 years. The results have ponted out that the ampltudes of 27 and 33 cm of storm-surge correspond to 9 and 95 percentle, respectvely, and has shown that the last wnter of has been the most sever perod of storms for the last 1 years, snce 44% of the storms of the percentle 9 and 63% of the percentle 95 occurred n ths short perod. Ths percentage values ncrease to 57.1% and 71.4 %, respectvely, f we consder the complete year of 21 (Fgure 6). The analyss of the MSL seres (Fgure 4) shows that, also n the wnter of 29-21, the daly MSL has ht several heght records, namely n December of 29, where the monthly average of SL ht 31.3 cm above the reference of MSL (Cascas 1938), and 33.7 cm n February; where the hghest value of all, snce 1882, has occurred n December of 1989 wth 34.5 cm of monthly MSL heght. Identcal extreme value of MSL has occurred agan at December 21, wth 34.2 cm of heght. LITERATURE CITED Antunes, C., 27. Prevsão de Marés dos Portos Prncpas de Portugal. FCUL Webpages - ~cmantunes /hdrografa/hdro_mares.html Antunes, C. and Taborda, R., 29. Sea level a Cascas Tde Gauge: Data, Analyss and Results. Journal of Coastal Research, SI 56, Proceedngs of the 1th Internatonal Coastal Symposum (Lsbon, Portugal), pp Antunes, C., Taborda, R. and Mendes, V., 21. Analyss of the most recent data of Cascas Tde Gauge. Geophyscal Research Abstract, Vol. 12, EGU , EGU General Assembly 21. Godn, G., The Analyss of Tdes. Unversty of Toronto press, 264 pp. Relvas, P., Barton, E.D., Dubert, J., Olvera, P.B., Pelz, A., da Slva J.C.B and Santos, A. M. P., 27. Physcal Oceanography of the Western Ibera Ecosystem: latest vews and challenges. Progress n Oceanography, 74(2-3), Taborda, R., Andrade, C., Marques, F., Fretas, C., Flpa, R., and Antunes, C., 21. Zonas Costeras. In: PECAC (21). Alterações Clmátcas - Cascas. Plano Estratégco de Cascas face às Alterações Clmátcas. Relatóro Executvo e Integrador. F.D. Santos e R. Aguar (Eds). Câmara Muncpal de Cascas, Cascas. 59 pp. Fgure 6. Number of storm-surges (SS), snce 2, of percentle 9 ad 95%, respectvely, wth ampltudes of 27 and 33 cm. Ths data of storm-surge frequency reveals that the last wnter of was one of the most severng perods of extreme events on the Portuguese coast, consderng the last 1 years. Such results may suggest some knd of correlaton to the on-gong clmate changes or to the present negatve perod of the NAO Index, that needs to be studed. CONCLUSIONS The present paper presents a great tool that has been used to montor the SLC n real tme, whch s very mportant nowadays to help on the coastal hazard management, especally when t can be truly useful for the nsttutons of rescue and cvl protecton, as t has been lately. Another mportant result s the achevement on the accuracy and precson of the harmonc tde models, whch was based n the correct numercal modelng of the non-astronomc tdal components. Ths approach made possble a relable montorng of the SLC n real tme. The actual and permanent montorng of the SLR rate and storm-surge statstcs through dfferent tme seres of MSL s another achevement that has shown a great mportance to help to understand and to manage the mpact of the clmate changes n the Portuguese coastal areas (Taborda et al., 21). ACKNOWLEDGEMENT The author would lke to acknowledge the IGP for allowng full access to the data of Cascas Tde Gauge, as well as the colleague Ana Navarro, who have helped on the wrte revson of the paper. Journal of Coastal Research, Specal Issue 64, 211