P1D.6 IMPACTS OF THE OCEAN SURFACE VELOCITY ON WIND STRESS COEFFICIENT AND WIND STRESS OVER GLOBAL OCEAN DURING

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1 P1D.6 IMPATS OF THE OEAN SURFAE VELOITY ON WIND STRESS OEFFIIENT AND WIND STRESS OVER GLOBAL OEAN DURING Zengan Deng 1* Lian Xie Ting Yu 1 an Kejian Wu 1 1. Physical Oceanography Laboratory, Ocean Uniersity Of hina, Qingao, Shangong, 660, P.R. hina. Marine Earth & Atmospheric Sciences, North arolina State Uniersity, Raleigh, N 7695, U.S.A Abstract: By accounting for the effects of ocean surface elocity (wae-inuce surface rift elocity an current elocity) on the rag coefficient, the spatial istribution of rag coefficient an win stress are compute oer the global ocean uring , using an empirical rag coefficient parameterization formula base on wae steepness an win spee. The global ocean current fiel is generate from the Hybri oorinate Ocean Moel (HYOM) an the waes from Waewatch III (WW). The spatial ariability of the rag coefficient an win stress is analyze. Preliminary results inicate that, the ocean surface rift elocity exerts an important influence on the win stress. The results also show that accounting for the effect of the ocean surface elocity on the win stress can lea to significant improement in the moeling of ocean circulation an air-sea interaction processes. Key wors: win stress, rag coefficient, ocean surface elocity, HYOM, WW, air-sea interaction 1. Introuction The air-sea momentum exchange is of great importance on many oceanic an atmospheric stuies, such as air-sea interaction, climate researches, an ocean moeling. Win stress rag coefficient D, a orresponing author aress: Zengan Deng, Marine Earth & Atmospheric Sciences, North arolina State Uniersity, Raleigh, N 7695, USA engzengan@gmail.com imensionless coefficient escribes the surface roughness, is a key important ariable that reflects the main characters of processes at the air-sea interface, such as momentum exchanges at the sea surface through win stress. In this paper, a relation gien by Guan at el. (004) is introuce to escribe the sea surface rag coefficient. The theory of Guan at el. (004) seres as a unifie approach to reconcile the arious forms of linear parameterization by introucing the wae steepness δ as another parameter besies -m win elocity. Since sea surface roughness is ue mainly to surface waes, take the wae state into account in D parameterization is necessary, an employ the wae steepness δ seres as the representation of the surface wae state is reasonably. Non-imensional ominant wae steepnessδ, a pure wae parameter measuring the wae status, is efine as, δ = Hω P / g, here, H is the significant wae height, ω p is the angular frequency at the winsea spectral peak, an g is graitational acceleration. Both the win an wae information are containe in the parameterization of Guan at el.(004). Traitionally, accoring to the theory of D is emonstrate to hae a clear, mostly linear tren of increase with win spee an is often expresse as a function of win spee for easy operation, while larger scatter in the ata suggeste the other simultaneous influences, such as ocean surface elocity (wae inuce rift elocity an current elocity). Ocean surface elocity influences

2 remain an important contemporary issue in air sea interaction research. Kara et al. (007) reeale that the ocean surface elocity effects on win stress rag coefficient is to reuce is no eniable that by about % globally. It is a multi-factor-epenent coefficient, therefore, to improe the parameterization of,two factors shoul be inclue as well as win spee.one is wae status, which propose aboe, the other is ocean surface elocity. In the region of sea surface elocity exists, it may not be simply only the win elocity that is important for etermining, but the ifference between near-surface win elocity an ocean surface elocity. U is the -m win elocity relatie to the sea surface. Since the real ocean surface is not quiescence, taking the win spee as the relatie elocity is unreasonable. The wae-inuce motion (especially the swell-inuce motion) is a kin of rift, an one may woner the impact of this rift on the air-sea interaction ariables. Pacific to be a function of air-sea elocity ifference. It is reasonably to use following relation: τ = ρ U ( U ) which 1 a c w c w inclues the effects of the ocean surface elocity. Use the improe win stress τ 1 has been proen to lea to significant improements in moeling of the tropical Atlantic Ocean by Pacanowski (1987), an the similar results in simulations of the tropical Pacific Ocean, by Luo et al. (005). In this paper, two fine resolution moels, HYOM an Waewatch III, hae been use to obtain the necessary wae an current ariables. f. alculate the rag coefficient Guan at el.(004) eelope a algorithm as follow, = [ f ( δ ) U ] (1) B B ( δ ) = 0.85 A 1 / δ () where U is -m win spee, δ = Hω P / g is ominant wae steepness, H is significant wae The epenence of on ocean surface rift height, ω p is the angular frequency at the winsea elocity oer the global Ocean will be examine herein. The win stress plays a weightily role in moeling arious win-rien air sea interaction processes incluing sea state preiction, large-scale ocean circulation, an climate eolution. Accurate parameterization of the win stress is neee to optimize the reliability of these moel preictions. Many researches often approximate the surface win stress τ exerte by the atmosphere on the ocean as a function of the -m win elocity: τ = ρ a U U, here U is -m win elocity. This relationship neglects the ocean surface wae elocity U an current elocity w U c epenence inτ. Kelly et al. (001) showe that scatterometer-erie win stress oer the tropical spectral peak. Both A an B are from function gz 0 / u β* B * = A, where 0 z is the ocean surface roughness, u * is the friction elocity of air, an β * is the wae age. A an B are coefficients etermine by obserations. Different alues of A an B hae been propose by arious researchers. Since controersy remains oer the nature of the epenence of roughness on wae age, here we set A = 0.0, B = 0.7 accoring to Sugimori et al. (000). The positie alue of B implies that mature waes are rougher than younger waes, an the negatie correspons to the contrary. Many researches inicate that, δ is completely comprise of wae parameters an it is more suitable than wae age for escribing the wae epenence of rag coefficient.

3 Here, win spee in () is replace by a complex win elocity consiste of win elocity, current elocity an wae elocity. [ f ( δ ) U ] () U U U = = U (4a) U w = U (4b) U c where elocity, = U (4c) w c U is relatie win elocity, U U form HYOM) an c is win is current spee ector(generate U w is wae elocity. (4a) represents the relatie elocity inclues the wae elocity effects, (4b) inclues the effects of current elocity, an (4c) inclues both the effects. Accoring to the theory of Bourassa(006), U w = 0.8V orb (5) V orb =.14H / T ) D ( (6) 0.8. Due to the significant, if not ominant, role of wae status form rag in the win stress, the egree of eelopment of the sea state proies one reason for this phenomenon. Large wae steepness alues exist in this area, so, the waes can not be fully eelope. It is also shoul be notice that, the ecrease of the increase of epenence of with wae steepness is equialent to with wae age. Yet the has not been easily resole in rag coefficient measurements. While a few obserational stuies hae reporte eience of wae steepness epenence in the rag coefficient primarily for short fetch an coastal sites, this behaior has been maske in the few open-ocean stuies that reporte ata for a wie range of wae age. At higher wins where the sea state becomes ominate by the winsea, alues oer the open Ocean are more nearly conform to the relationship for a fully eelope sea. This implies a less rapi increase of where V orb is the orbital elocity of the sea surface with increasing wins. It also inicates that wae δ - base formula will more accurately waes, H is significant wae height, T is ominant wae perio an D is unit mean wae irection ector. All wae ata use here are simulate by WW, an the current ata are output from HYOM.. Results an analysis Fig.7 shows the 0.7 to 0.9 alues arie from with some extreme alues great than 1.5. The region with large alue oes not always correspon to the region that with high win spees. For example, in the westerly region, high wins preails, but is about estimate the in high win conitions. Fig.1 shows the etails of effects of ocean surface wae inuce elocity on to 0.01 aries from approximately, the changes are less than 1% in most parts. In tropical Atlantic area, the change can reaches 1%. Fig. presents the influences of ocean surface current elocity on. is generally between an 0.00, the maximum change reaches 1% at the equatorial Pacific. Fig. presents the influences of both wae inuce elocity an current elocity on. are generally

4 from to In most area, the change is about 1%. Most of the researches in small-scale air-sea interaction are focuse on the air-sie processes, incluing the transfer of momentum an sensible an latent heat. Also, the transportation of macro magnitue processes such as energy input from the win to sea was assume only epens on win spee at a certain height aboe the ocean surface in many conceptual moels. Very few attempts hae been mae to augment or replace win spee with wae or current parameters in air-sea moeling. Howeer, reality is usually far more complex. It is note that ocean waes can break without win forcing by inherent hyroynamic processes in water, such as wae-current interactions an shoaling. As mentione aboe, to obtain a reliable estimation of any reference ariable efine in air-sea interaction fiel, the eents on the water sie incluing the waes an ocean surface elocity are inispensable. Following we iscuss the win stress by taking the wae information an ocean surface elocity effects into consieration. Fig.4 shows the effects of wae inuce elocity on win stress, the τ generally ary from Nm to 0.01 Nm, most of alues are in the range of Nm to Nm, the maximum change can be 0%. In westerly an tropical areas, the change of win stress is larger than other places. Fig.5 presents the alue change of win stress cause by surface current elocity. It is from Nm to Nm. There is obiously a maximum alue region locate at the central tropical Pacific, the percentage of change is close to 0%. The etails of the influences of both wae inuce elocity an current elocity on win stress can be foun in Fig.6. The maximum alues of changes are occurre in the westerly an tropical regions. 4. onclusions The rag coefficient an win stress τ are beliee to play a key role in many air-sea bounary stuies. In orer to unerstan the theory an eolution of these processes, it must be escribe quantitatiely. The question of a possible sea state epenence on the rag coefficient has receie a lot of attention recently. The epenence of on wae age for pure winseas has now been establishe, but in the open ocean where the ariation in wae age is small, this is of seconary importance. The open ocean is ominate by swells, an how these impact the rag coefficient has not yet been establishe. Eience to ate has been contraictory. Employing the wae steepness δ which characterizes the wae eelopment for the case where effectiely in local equilibrium with the relatie win, to moel the, we a surface wae an current elocity besies the win elocity. All the analysis supports a soun conclusion that currents an waes can cause substantial changes in the. This may explain some of the obsere scatter in the measure reporte in the literature. Oer global Ocean, where we place our focus on, the impacts of ocean surface elocity on are to change it by generally ary form The effects of ocean surface elocity on win stress are to change it by generally ary from Nm to 0.01 Nm with the maximum change of 0%. The to inclues the effects of wae an current elocity shoul be introuce to the ocean moeling, an further research is necessary to ientify how this new performance. can improes the moel

5 5. Acknowlegements The authors appreciate support from the National Basic Research Program of hina (Nos. 005B40 an 005B407) an also support from the National Natural Science Founation of hina (Nos ). The wins supplie by the NOAA/NEP ata serer are appreciate. 6. References hanglong Guan an Lian Xie, 004, A Note on the Linear Parameterization of Drag oefficient oer Sea Surface, J. Phys. Oceanogr., 4, Donelan, W. M. Drennan, an K. B. Katsaros, 1997, The Air sea Momentum Flux in Mixe Win Sea an Swell onitions. J. Phys. Oceanogr., 7, Dongliang Zhao, 00, A Note on Wae State Depenence of the Sea-surface Roughness, Proceeings of the Twelfth (00) international offshore an polar engineering conference, Kitakyushu, Japan, May, 6-1 Kara, A. B., E. J.Metzger, an M. A. Bourassa, 007, Ocean urrent an Wae Effects on Win Stress Drag oefficient Oer the Global Ocean, Geophys. Res. Lett., 4, L01604, oi:.9/006gl Sugimori, Y., M. Akiyama, an N. Suzuki, 000, Ocean Measurement an limate Preiction-expectation for Signal Processing (in Japanese). J. Signal Processing, 4, 09. Taylor, P. K., an M. J. Yellan, 001, The Depenence of Sea Surface Roughness on the Height an Steepness of the Waes, J. Phys. Oceanogr., 1, Yellan, M., an P. K. Taylor, 1996: Win Stress Measurements from the Open Ocean. J. Phys. Oceanogr., 6,

6 Fig.1 hanges of alue by the influences of wae inuce elocity. (unit: ) Fig.4 hanges of win stress by the influences of wae inuce elocity. (unit: N / m ) Fig. hanges of alue by the influences of current elocity. (unit: ) Fig.5 hanges of win stress by the influences of current elocity. (unit: N / m ) Fig. hanges of alue by the influences of current elocity an wae inuce elocity. (unit: ) Fig.6 The changes of win stress by the influences of wae inuce elocity an current elocity. (unit: N / m ) Fig year mean without the influences of Ocean surface elocity. (unit: )