two-Way coupled atmosphere-Ocean modeling of the Philex intensive Observational Periods

Abstract : High-resolution coupled atmosphere-ocean simulations of the Philippines show the regional and local nature of atmospheric patterns and ocean response during Intensive Observational Period cruises in January-February 2008 (IOP-08) and February-March 2009 (IOP-09) for the Philippine Straits Dynamics Experiment. Winds were stronger and more variable during IOP-08 because the time period covered was near the peak of the northeast monsoon season. Distinct wind jets between islands and wakes behind islands are common northeast monsoon features that are controlled by winds interacting with island topography. The modeled upperocean flow associated with Philippine straits during IOP-08 exhibits large (> 1 m s-1) westward mean flow through Suriago Strait and highly variable flow through Mindoro Strait. The model shows prominent eddies in the Bohol Sea and Cuyo East Pass that were also observed during the field experiment. A high-resolution nested simulation of the Verde Island Passage finds local wind-driven upwelling that is confirmed by shipboard sea surface temperature measurements and satellite observations of chlorophyll concentration.

, but within the island archipelago, there are finer scales at play in both the atmosphere and ocean that have not received as much attention.These In nearby and surrounding waters, kinematics and dynamics.Although there are other numerical studies of the region (e.g., Metzger and Hurlburt, 1996;Han et al., 2009), the NRL coupled atmosphere-ocean model is a unique tool for simultaneous high-resolution studies of both the meteorology and oceanography of a region.Seasonal monsoon winds are also known to modulate circulation in the adjacent South China Sea (Metzger and Hurlburt, 1996;Qu, 2000).But, local-scale winds also drive important interisland circulations.These local wind characteristics during PhilEx are described by considering the mean and variability of the atmospheric model's 10-m wind.The variability of vector quantities (winds and currents) is concisely summarized by using the eigenvalues and eigenvectors of the vector component's covariance matrix (Emery and Thompson, 1998), which define two principal axes of variability.In this scheme, the axis in the direction of greatest variance, the major principal axis, has a magnitude equal to the standard deviation of the wind or current in that direction.The minor axis is the magnitude and direction of variability that is orthogonal or uncorrelated with the major axis variability.
Mean 10-m winds from the 9-km COAMPS forecasts for January through February 2008 (Figure 1a) clearly show the influence of the northeasterly (winter) monsoon, which is normally strongest at this time of year (Chang, 2004).The mean winds range from 10-12 m s -1 in Luzon Strait (20°N, 121°E) to 5-6 m s -1 in the southern Sulu Sea (7°N, 120°E).A series of wind-speed maxima and minima are noticeable features on the leeward or downwind side of the Philippine Archipelago (see tains (e.g., Gabersek and Durran, 2004).Downstream wakes form from air descent, acceleration, wave breaking, and potential vorticity generation that occur over the islands (Smith et al., 1997).
Wakes can be long and straight when the mountain-generated potential vorticity and Coriolis force are weak (e.g., at low latitudes; Smith et al., 1997;Burk et al., 2003).In other situations, with stronger forcing, vortex shedding can ensue (Smolarkiewicz and Rotunno, 1989).
Island wakes and wind jets in ocean straits are not uncommon and have been observed in the Hawaiian Islands (Smolarkiewicz et al., 1988;Yoshida et al., 2010), Caribbean (Smith et al., 1997); Tsushima Straits (Shimada and Kawamura, 2008) Carpenter and Springer (2005) as a peak region of marine biological diversity and has been designated as a priority marine conservation site by the Philippine government (Ong et al., 2002;Conservation International, 2009).In this biologically productive region, atmospheric forcing may contribute to marine ecosystem dynamics through wind-induced upwelling (Villanoy et al., 2011).
For most of IOP-08, winds blew steadily from east to west over Verde Island Passage.On several occasions during January 2008, the easterly wind briefly intensified (e.g., Pullen et al., 2011).Figure 4a   straits during the northeast monsoon (e.g., Figure 5a), indicating increased biological productivity and suggesting that cooler, nutrient-rich water is being transported upward into the euphotic zone by upwelling.Studies of marine life (Dolar et al., 2006;Deocadez et al., 2008) also find greater species richness and P h i l i P P i N e S t r a i t S D y N a m i c S e x P e r i m e N t two-Way coupled atmosphere-Ocean modeling of the Philex intensive Observational Periods abStr act.High-resolution coupled atmosphere-ocean simulations of the Philippines show the regional and local nature of atmospheric patterns and ocean response during Intensive Observational Period cruises in January-February 2008 (IOP-08) and February-March 2009 (IOP-09) for the Philippine Straits Dynamics Experiment.Winds were stronger and more variable during IOP-08 because the time period covered was near the peak of the northeast monsoon season.Distinct wind jets between islands and wakes behind islands are common northeast monsoon features that are controlled by winds interacting with island topography.The modeled upperocean flow associated with Philippine straits during IOP-08 exhibits large (> 1 m s -1 ) westward mean flow through Suriago Strait and highly variable flow through Mindoro Strait.The model shows prominent eddies in the Bohol Sea and Cuyo East Pass that were also observed during the field experiment.A high-resolution nested simulation of the Verde Island Passage finds local wind-driven upwelling that is confirmed by shipboard sea surface temperature measurements and satellite observations of chlorophyll concentration.iNtrODuctiON The Philippine Archipelago is an intricate array of islands, straits, and semienclosed seas whose broad atmospheric circulation is governed by the rhythm of the East Asian monsoon and steady Pacific trade winds.Philippine oceanic circulation is dominated by the position of the island chain between the South China Sea to the west and the western Pacific Ocean to the east, with its nearby North Equatorial, Kuroshio, and Mindanao currents.Observational and numerical studies of these globalscale phenomena abound scheme.Ocean observations, which include floats, drifters, moorings, satellite sea surface temperatures (SST), conductivity-temperature-depth/expendable bathythermograph (CTD/XBT) profiles, and satellite altimetry, are assimilated by the MVOI-based NRL Coupled Ocean Data Assimilation System (Cummings, 2005).The initial state and lateral boundary conditions for the outermost atmospheric and oceanic nests are from the 1° Navy Operational Global Analysis and Prediction System (NOGAPS) and the operational 1/8° Global NCOM (gNCOM) forecast fields, respectively.During IOP-08 and IOP-09, daily ), which consists of an atmospheric model coupled with the Navy Coastal Ocean Model (NCOM).The atmospheric model is nonhydrostatic with a terrain-following vertical coordinate(Hodur, 1997), while the ocean model features hydrostatic dynamics, generalized curvilinear horizontal coordinates, a bathymetryfollowing vertical coordinate, and a free surface(Barron et al., 2006).Air-toocean two-way coupling is accomplished through the community-based Earth System Modeling Framework (ESMF; Earth System Modeling, 2010), which allows each model to use flexible, generalized grids and a configurable air-sea coupling interval, typically every 10 to 15 minutes.Atmospheric physical parameterizations for surface fluxes use bulk aerodynamic formulae based on the TOGA-COARE (Tropical Ocean Global Atmosphere-Coupled Ocean Atmosphere Response Experiment) algorithms (Fairall et al., 1996), a level 2.5 Mellor-Yamada boundary layer representation, short-and long-wave radiation parameterization, and moist processes, including explicit microphysical equations for rain, snow, cloud water, cloud ice, and large-scale stresses from the Pacific trade winds are responsible for the North Equatorial Current and the resulting sea level differences between the western Pacific and the South China Sea.

Figure
Figure 1a in Gordon et al., 2011, for locations of major islands and passages).The most prominent wind jets occur (from north to south) at Verde Island Passage (13°30'N, 120°50'E), which separates Luzon and Mindoro islands, and Tablas Strait (12°20'N, 121°40'E), which separates Mindoro and Panay.In Tablas Strait, mean winds exceed 12 m s -1 and the jet extends nearly 200 km to the southwest.There are prominent wind maxima or tip jets (Doyle and Shapiro, 1999) at the northern and southern flanks of Mindoro Island.Wind jets are also present in the Panay Gulf/Guimaras Strait (10°20'N, 122°20'E) and Dipolog Strait (8°50'N, 123°10'E), which lies between the islands of Mindanao and Negros.Minima in the mean wind velocities occur on the downwind side of each of the islands: Luzon, Mindoro, Panay, Negros, and Mindanao.These wind jets and associated leeside wakes are caused by the airflow over the mountainous terrain of the Philippine Archipelago.Along this mountainous island chain, several peaks in the Cordillera Central on Luzon exceed 2700 m in elevation, and the highest point in the Philippines, Mount Apo on Mindanao, is more than 2900 m above sea level.The enhanced wind flow in the straits is driven by along-gap pressure gradients that are a , Aleutians, Sandwich Islands, Madagascar, and numerous other island regions.It is remarkable that wind variability (Figure 1b) is largest in the wake regions, where unsteady conditions may exist.By contrast, wind variability has relative minima in the wind jet regions where the flow is less influenced by topography and hence less turbulent.In the two most prominent wake regions, those downwind from Luzon and Mindoro islands, the largest variability is nearest the western coast, and the major axis of variability is oriented in a distinctly onshore direction, which likely is associated with strong but intermittent downslope winds that penetrate to the ocean surface and interrupt the wake signature.Another obvious area of large variability is the region to the northwest of Luzon where the monsoon winds are channeled into the South China Sea through Luzon Strait.In this region, the largest wind Paul W. May (paul.may.ctr@nrlmry.navy.mil) is Research Scientist, Computer Science Corporation, Monterey, CA, USA.James D. Doyle is Head, Mesoscale Modeling Section, Marine Meteorology Division, Naval Research Laboratory, Monterey, CA, USA.Julie D. Pullen is Director, Maritime Security Laboratory, Stevens Institute of Technology, Hoboken, NJ, USA.Laura T. David is Professor, Marine Science Institute, University of the Philippines Diliman, Quezon City, Philippines.variability is associated with strong mean winds, and the orientation of the variability is parallel to the wind.Further south along Luzon, the mean wind direction is relatively unchanged, but the preferred wind variability orientation trends in the alongshore direction where it is, in some cases, at right angles to the mean wind direction.The mean 10-m winds and wind variability from the COAMPS forecasts that cover the PhilEx IOP-09 period are

Figure 1 .
Figure 1.The coupled Ocean/atmosphere mesoscale Prediction System (cOamPS) 10-m mean wind (left panels) and mean variability (right panels) from the 9-km computational grids for the two intensive Observational Periods (iOPs).every fifth wind vector and variability cross is plotted.(a) Wind vectors with color-shaded wind speed (m s -1 ) for January to February 2008.(b) Variability vectors with color-shaded variability magnitude (m 2 s -2 ) for 2008.(c) Wind vectors plotted over color-shaded wind speed (m s -1 ) for February to march 2009.(d) Variability vectors plotted over color-shaded variability magnitude (m 2 s -2 ) for 2009.
shows details of the vector wind field in the passage and along the northern coast of Mindoro Island on January 31 (at the same time as the SAR image in Figure 2a), following one of the periods of increased wind.In this figure (an enlargement of the model fields from Figure 2b), winds reach a maximum of ~ 12 m s -1 along the north coast of Mindoro and over 15 m s -1 off the northwest shore of Mindoro.The strong, steady winds along the north coast of Mindoro are a manifestation of the Verde Island Passage topographic wind jet and favor offshore Ekman transport with subsequent nearshore upwelling of deeper water.As an indication of an upwelling response to increased surface wind, surface waters along the north coast of Mindoro cooled noticeably on January 26 in the 1-km resolution ocean model.The cooler temperatures are evident along the coast for several days following the wind pulse.The minimum temperature, 1°C colder than ambient, was found between January 27 and January 29 in the model (Figure 4c).SST data collected during IOP-08-1 for January 30-31 clearly show that the ship is sailing through cooler water along the northern coast of Mindoro (Figure 4d).The zonal temperature gradient in the observations is located to the east of that in the model, and observed temperatures along the ship's track are 0.5-1.0°Cwarmer than the model, but the basic pattern of colder temperatures is reinforced by the in situ SST.Additional evidence for upwelling in the passage during IOP-08 comes from MODIS Aqua satellite ocean color (Pacific Islands Fisheries Science Center, 2010).A composite image of chlorophyll a concentration for January 25-31, 2008, highlights the influence of upwelling on the phytoplankton biomass north of Mindoro (Figure 4b).Vertical temperature profiles from the model, which are in reasonable agreement with CTD measurements in the area, suggest that the upwelling is confined to the upper 60 m of the water column.Elevated chlorophyll concentrations are frequently seen in Verde Island Passage and other Philippine

Figure 5 .
Figure 5. ( a) chlorophyll concentration (mg m -3 ) along a ship's transit of Verde island Passage a few days after the wind intensification in early January 2008.(b) hovmöller diagram of SSt along 120°50'e (temperature as a function of time and latitude) showing time and spatial extent (offshore) of upwelling in Verde island Passage.