Analyses of the wind-driven response of tropical oceans

Numerical and analytical models are used to study the upper-ocean response to surface wind stress estimates from the tropical Atlantic and Pacific Oceans. These models are used to identify regions of important variability in the wind field, analyze the oceanic response, and demonstrate the applicability of remotely sensed vector wind stress and altimetry data. Both model and XBT depictions of the mean seasonal cycle, 1979 to 1981, were analyzed along the major ship tracks in the western, central, and eastern tropical Pacific. Model solutions were also used to address array design questions in observing system simulation experiments. Subsequent analyses of the 1982 to 1983 solutions will be performed with respect to differences from the mean seasonal cycle 1979 to 1981, as well as, differences in the three wind products

Variability in Equatorial Pacific sea surface topography during the verification phase of the Topex/Poseidon mission

As part of the verification phase of the TOPEX/POSEIDON mission, 10-day gridded fields of altimeter data derived from TOPEX geophysical data records are compared with 10-day gridded fields of dynamic height derived from more than 60 moorings of the Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) array in the Equatorial Pacific ocean. Acess to TAO data in real time permits the first 500 days of the TOPEX/POSEIDON mission to be placed in the context of complementary, in situ measurements of surface winds, sea surface temperatures, and upper ocean thermal structure, as well as the time history of these variables prior to lauch. Analysis of the space-time structure in the TOPEX and TAO surface topography data indicates sea level variability primarily due to equatorial Kelvin wave activity generated by intense windbursts west of the date line in association with the 1991-1993 El Nino. Cross correlations between the two data sets are generally >0.7, with RMS differences 5 cm north of the equator in the Central and Eastern Pacific. (Résumé d'auteur

Observations and wind-forced model simulations of the mean seasonal cycle in tropical Pacific sea surface topography

We examine simulations of the mean seasonal cycle in the tropical Pacific using a multiple vertical mode linear numerical model forced with three different surface wind stress products average over the period 1979-1981. The model is run to equilibrium for each four vertical modes, and results are summed. Simulated mean seasonal cycles in dynamic height and sea level are then compared with observed variations based on expendable bathythermograph and island tide gauge data averaged over the same 1979-1981 period. All simulations show characteristic features of the mean meridional ridge-through structure in surface topography. However, north and south equatorial ridges at 20°N and 20°S are much higher than those observed, only weak equatorial ridges are generated near 4°N, and none of the simulations exhibits a significant equatorial trough. These discrepancies are due principally to limitations in model physics in the wind forcing. Observed and modeled mean seasonal variations in surface height are of the order of a few centimeters

A model study of potential sampling errors due to data scatter around expendable bathythermograph transects in the Tropical Pacific

We describe a series of sampling sensitivy experiments to examine potential errors due to data scatter around expendable bathythermograph (XBT) transects in the tropical Pacific. We use a linear, multiple vertical mode model forced with three differrent monthly mean wind stress sets for the period 1979-1983. The model is sampled along approximately straight lines of grid points corresponding to the mean positions of XBT tracks in the eastern, central, and western Pacific and then sampled again at the dates and location of actual XBT casts for 1979-1983. Model dynamic heights are calculated with a resolution of 1° of latitude and 1 month, then processed to a monthly mean seasonal cycle and anomalies associated with the 1982-1983 El Nino. When results are compared for the two methods of sampling, the model indicates that data scattered zonally around XBT transects in general can lead to about 2 dyn cm error in dynamic height (equivalent to a 10-m error in model pycnocline displacement) in composite sections of XBT data

Investigating the Toxicity of Nonylphenol in Juvenile Faxonius propinquus Crayfish

Nonylphenol (NP) is a commonly used chemical that accumulates in aquatic environments and negatively impacts aquatic life. Previous studies in our lab have shown that NP can reduce olfaction, reproduction, and molting frequency in adult crayfish. Although lethal doses of NP in adult crayfish have been determined, no studies have determined the concentrations in which NP is lethal to juveniles. We hypothesized that juveniles are more susceptible to the effects of NP than adults. Male and female juveniles weighing 3.00g or less were isolated for 2 days prior to a 24-hour exposure to various concentrations of NP. Results indcate that 100% of juvenile crayfish exposed at 0.05 ug/L survived, 75% survived at 0.1 ug/L, 62.5% at 0.125 ug/L, and 0% at or above 0.15 ug/L, indicating that lethal effects occur at very low concentrations. Further studies will investigate the effects of NP on juvenile physiology and could offer insight for other aquatic species exposed to NP during their lifespans

A simple mechanism for the climatological midsummer drought along the Pacific coast of Central America

© ATMOSFERA, 2013. This article is posted here by permission of ATMOSFERA for personal use, not for redistribution. The definitive version was published in Atmósfera 26 (2013): 261-281.The global distribution, seasonal evolution, and underlying mechanisms for the climatological midsummer drought (MSD) are investigated using a suite of relatively high spatial and temporal resolution station observations and reanalysis data with particular focus on the Pacific coast of Central America and southern Mexico. Although the MSD of Central America stands out in terms of spatial scale and coherence, it is neither unique to the Greater Caribbean Region (GCR) nor necessarily the strongest MSD on Earth based on an objective analysis of several global precipitation data sets. A mechanism for the MSD is proposed that relates the latitudinal dependence of the two climatological precipitation maxima to the biannual crossing of the solar declination (SD), driving two peaks in convective instability and hence rainfall. In addition to this underlying local mechanism, a number of remote processes tend to peak during the apex of the MSD, including the North American monsoon, the Caribbean low-level jet, and the North Atlantic subtropical high, which may also act to suppress rainfall along the Pacific coast of Central America and generate interannual variability in the strength or timing of the MSD. However, our findings challenge the existing paradigm that the MSD owes its existence to a precipitation-suppressing mechanism. Rather, aided by the analysis of higher-temporal resolution precipitation records and considering variations in latitude, we suggest the MSD is essentially the result of one precipitation-enhancing mechanism occurring twice.The authors gratefully acknowledge funding from the NOAA Climate Program Office (CPO) Modeling, Analysis, Predictions, and Projections (MAPP) Program, under awards NA10OAR0110239 to the Woods Hole Oceanographic Institution, NA10OAR4310253 to the University of Maryland, and NA10OAR4310252 to Columbia University

A Numerical Simulation of the Mean Water Pathways in the Subtropical and Tropical Pacific Ocean

A reduced-gravity, primitive-equation, upper-ocean general circulation model is used to study the mean water pathways in the North Pacific subtropical and tropical ocean. The model features an explicit physical representation of the surface mixed layer, realistic basin geometry, observed wind and heat flux forcing, and a horizontal grid-stretching technique and a vertical sigma coordinate to obtain a realistic simulation of the subtropical/tropical circulation. Velocity fields, and isopycnal and trajectory analyses are used to understand the mean flow of mixed layer and thermocline waters between the subtropics and Tropics. Subtropical/tropical water pathways are not simply direct meridional routes; the existence of vigorous zonal current systems obviously complicates the picture. In the surface mixed layer, upwelled equatorial waters flow into the subtropical gyre mainly through the midlatitude western boundary current (the model Kuroshio). There is additionally an interior ocean pathway, through the Subtropical Countercurrent (an eastward flow across the middle of the subtropical gyre), that directly feeds subtropical subduction sites. Below the mixed layer, the water pathways in the subtropical thermocline essentially reflect the anticyclonic gyre circulation where we find that the model subtropical gyre separates into two circulation centers. The surface circulation also features a double-cell pattern, with the poleward cell centered at about 30°N and the equatorward component contained between 15° and 25°N. In addition, thermocline waters that can be traced to subtropical subduction sites move toward the Tropics almost zonally across the basin, succeeding in flowing toward the equator only along relatively narrow north–south conduits. The low-latitude western boundary currents serve as the main southward circuit for the subducted subtropical thermocline water. However, the model does find a direct flow of thermocline water into the Tropics through the ocean interior, confined to the far western Pacific (away from the low-latitude western boundary currents) across 10°N. This interior pathway is found just to the west of a recirculating gyre in and just below the mixed layer in the northeastern Tropics. This equatorward interior flow and a flow that can be traced directly to the western boundary are then swept eastward by the deeper branches of the North Equatorial Countercurrent, finally penetrating to the equator in the central and eastern Pacific. Most of these results are consistent with available observations and recently published theoretical and idealized numerical experiments, although the interior pathway of subtropical thermocline water into the Tropics found in this experiment is not apparent in other published numerical simulations. Potential vorticity dynamics are useful in explaining the pathways taken by subtropical thermocline water as it flows into the Tropics. In particular, a large-scale zonally oriented “island” of homogenous potential vorticity, whose signature is determined by thin isopycnal layers in the central tropical Pacific along about 10°N, is dynamically linked to a circulation that does not flow directly from the subtropics to the Tropics. This large-scale potential vorticity feature helps to explain the circuitous pathways of the subducted subtropical thermocline waters as they approach the equator. Consequently, waters must first flow westward to the western boundary north of these closed potential vorticity contours and then mostly move southward through the low-latitude western boundary currents, flow eastward with the North Equatorial Countercurrent, and finally equatorward to join the Equatorial Undercurrent in the thermocline