Roger Lukas

TOGA COARE : the Coupled Ocean-Atmosphere Response Experiment

Despite significant progress in the Tropical Ocean–Global Atmosphere (TOGA) program, a number of major hurdles remain before the primary objective, prediction of the variability of the coupled ocean-atmosphere system on time scales of months to years, can be achieved. Foremost among these hurdles is understanding the physics that maintains and perturbs the western Pacific warm pool, the region of the warmest sea surface temperature in the open oceans, which coexists with the largest annual precipitation and latent heat release in the atmosphere. Even though it is believed that the warm pool is a “center of action” for the El Nino-Southern Oscillation (ENSO) phenomena in the ocean and the atmosphere, successful simulation of the warm pool has remained an elusive goal. To gain a clear understanding of global climate change, the ENSO phenomenon, and the intraseasonal variability of the coupled atmosphere–ocean system, it is clear that a better specification of the coupling of the ocean and the atmosphere is ...

The mixed layer of the western equatorial Pacific Ocean

The mixed layer of the western equatorial Pacific and its thermodynamics are poorly known because of a general lack of data. Conductivity-temperature-depth (CTD) profiles from the recent Western Equatorial Pacific Ocean Circulation Study (WEPOCS) cruises have been analyzed for various measures of the upper layer and mixed layer thickness, using criteria which depend on vertical gradients of temperature, salinity, and density. From 434 profiles, the average mixed layer depth in the western equatorial Pacific during the two WEPOCS cruises was 29 m, which is about a factor of 3 shallower than had previously been thought. The mean depth of the top of the thermocline was found to be 64 m, so there is a nearly isothermal layer that is deeper than the mixed layer. This discrepancy is attributable to salinity stratification. It is hypothesized that the waters in this “barrier” layer between the bottom of the mixed layer and the top of the thermocline are formed to the east of the WEPOCS region, and subducted below the shallow and lighter mixed layer waters found in the west. Under light wind conditions, there was a tendency for warm and thin layers to form at the sea surface as a result of diurnal heating; however, there did not appear to be any nighttime maximum to the mixed layer depth associated with convective overturn due to cooling. This contrast with the central Pacific may be caused by the influence of salinity on the thermodynamics of the mixed layer. A strong westerly wind burst was observed during WEPOCS II, and apparently the mixed layer nearly doubled in depth while cooling by more than 1°C. Evidence of downwelling near the equator, and upwelling off the equator, was seen in the distribution of temperature, salinity, and density in the meridional section along 143°E, which was occupied immediately following the wind event. This event was apparently strong enough to erode through the salinity-stratified layer and into the thermocline, resulting in the observed cooling. The results of this study suggest that except during strong wind events, entrainment cooling may not be an important component of the heat budget of the western Pacific warm pool. This has potentially important implications for the El Nino/Southern Oscillation (ENSO) phenomenon.

The Hawaii Ocean Time-series (HOT) program: Background, rationale and field implementation

Abstract Long-term ocean observations are needed to gain a comprehensive understanding of natural habitat variability as well as global environmental change that might arise from human activities. In 1988, a multidisciplinary deep-water oceanographic station was established at a site north of Oahu, Hawaii, with the intent of establishing a long-term ( > 20 years) data base on oceanic variability. The primary objective of the Hawaii Ocean Time-series (HOT) program is to obtain high-quality time-series measurements of selected oceanographic properties, including: water mass structure, dynamic height, currents, dissolved and particulate chemical constituents, biological processes and particulate matter fluxes. These data will be used, in part, to help achieve the goals of the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) research programs. More importantly, these data sets will be used to improve our description and understanding of ocean circulation and ocean climatology, to elucidate further the processes that govern the fluxes of carbon into and from the oceans, and to generate novel hypotheses. These are necessary prerequisites for developing a predictive capability for global environmental change.

Observations of the Mindanao Current during the western equatorial Pacific Ocean circulation study

The Western Equatorial Pacific Ocean Circulation Study (WEPOCS) III expedition was conducted from June 18 through July 31, 1988, in the far western equatorial Pacific Ocean to observe the low-latitude western boundary circulation there, with emphasis on the Mindanao Current. This survey provides the first quasi-synoptic set of current measurements which resolve all of the important upper-ocean currents in the western tropical Pacific. Observations were made of the temperature, salinity, dissolved oxygen, and current profiles with depth; of water mass properties including transient tracers; and of evolving surface flows with a dense array of Lagrangian drifters. This paper provides a summary of the measurements and a preliminary description of the results. The Mindanao Current was found to be a narrow, southward-flowing current along the eastward side of the southern Philippine Islands, extending from 14°N to the south end of Mindanao near 6°N, where it then separates from the coast and penetrates into the Celebes Sea. The current strengthens to the south and is narrowest at 10°N. Direct current measurements reveal transports in the upper 300 m increasing from 13 Sv to 33 Sv (1 Sverdrup = 1 × 106 m3 s−1) between 10°N and 5.5°N. A portion of the Mindanao Current appears to recurve cyclonically in the Celebes Sea to feed the North Equatorial Countercurrent, merging with waters from the South Equatorial Current and the New Guinea Coastal Undercurrent. Another portion of the Mindanao Current appears to flow directly into the NECC without entering the Celebes Sea. The turning of the currents into the NECC is associated with the Mindanao and Halmahera eddies.

Physical and biogeochemical modulation of ocean acidification in the central North Pacific

Atmospheric carbon dioxide (CO2) is increasing at an accelerating rate, primarily due to fossil fuel combustion and land use change. A substantial fraction of anthropogenic CO2 emissions is absorbed by the oceans, resulting in a reduction of seawater pH. Continued acidification may over time have profound effects on marine biota and biogeochemical cycles. Although the physical and chemical basis for ocean acidification is well understood, there exist few field data of sufficient duration, resolution, and accuracy to document the acidification rate and to elucidate the factors governing its variability. Here we report the results of nearly 20 years of time-series measurements of seawater pH and associated parameters at Station ALOHA in the central North Pacific Ocean near Hawaii. We document a significant long-term decreasing trend of −0.0019 ± 0.0002 y−1 in surface pH, which is indistinguishable from the rate of acidification expected from equilibration with the atmosphere. Superimposed upon this trend is a strong seasonal pH cycle driven by temperature, mixing, and net photosynthetic CO2 assimilation. We also observe substantial interannual variability in surface pH, influenced by climate-induced fluctuations in upper ocean stability. Below the mixed layer, we find that the change in acidification is enhanced within distinct subsurface strata. These zones are influenced by remote water mass formation and intrusion, biological carbon remineralization, or both. We suggest that physical and biogeochemical processes alter the acidification rate with depth and time and must therefore be given due consideration when designing and interpreting ocean pH monitoring efforts and predictive models.

Seasonal and interannual variability of the North Equatorial Current, the Mindanao Current, and the Kuroshio along the Pacific western boundary

Along the Philippine coast in the western Pacific, the North Equatorial Current (NEC) bifurcates into the northward flowing Kuroshio and the southward flowing Mindanao Current. Using both the linear, time-dependent Sverdrup theory and a high-resolution, nonlinear reduced-gravity model, this study investigated the changes in the NEC-Mindanao Current-Kuroshio (NMK) system induced by large-scale surface wind forcings. Using the Florida State University monthly wind stress data from 1961 through 1992, we show that the seasonal bifurcation of the NEC occurs at the northernmost position in October and the southernmost position in February. While the meridional migration of the basin-wide trade wind has a relatively small effect in shifting the bifurcation latitude (by about 100 km), the monsoonal wind along the low-latitude western Pacific is effective in inducing a large northward excursion of the NECs bifurcation in the fall season. On the interannual timeseale, the positive wind stress curl of the trade wind tends to intensify and shifts the zero wind stress curl line northward prior to El Nino-Southern Oscillation (ENSO) events. With a lag of about 1 year this shift induces the bifurcation of the NEC to occur at a higher latitude. During the La Nina years the NEC generally bifurcates at a lower latitude. No significant seasonal fluctuations are found in the transport of the NEC near the Philippine coast. Seasonal changes in the Mindanao Current and the Kuroshio are, however, significant, and their transports tend to fluctuate 180° out of phase, due to the different speeds of the baroclinic Rossby waves at their respective latitudes. The Kuroshio (the Mindanao Current) has a seasonal minimum (maximum) transport in fall when the NEC bifurcates at the seasonally northernmost latitude. The interannual changes in the inflow NEC are largely controlled by the basin-wide, wind stress curl anomalies. While the quasi-biennial changes are confined only to the southern limb of the NMK system, signals with ENSO timescales are found to influence the midlatitude, subtropical circulation via the Kuroshio.

Tropical Pacific Near-Surface Currents Estimated from Altimeter, Wind, and Drifter Data

Tropical surface currents are estimated from satellite-derived surface topography and wind stress using a physically based statistical model calibrated by 15 m drogue drifters. The model assumes a surface layer dominated by steady geostrophic and Ekman dynamics. Geostrophy varies smoothly from a β plane formulation at the equator to an ƒ plane formulation in midlatitude, with the transition occurring at ∼2°–3° latitude. The transition is treated with a Gaussian weight function having a meridional decay scale that is found to be approximately the Rossby radius (∼2.2° latitude). The two-parameter Ekman model represents drifter motion relative to wind stress, with downwind flow along the equator and turning with latitude. Velocities computed from satellite data are evaluated statistically against drifter velocities and equatorial current moorings. Examples of the geostrophic and Ekman flow fields in the western Pacific during a westerly wind burst in late December 1992 depict a strong eastward flow and equatorial convergence. A comparison between December 1996 and June 1997 illustrates the basin-wide reversal of equatorial surface flow during the onset of the 1997 El Nino.

Source waters of the Pacific Equatorial Undercurrent

Abstract Hydrographic and direct current measurements were made north and east Papua New Guinea in June–August 1985 and January–February 1986 as part of the Western Equatorial Pacific Ocean Circulation Study (WEPOCS). Analyses of the data indicate that the major portion of the water in the Equatorial Undercurrent at its beginning north of Papua New Guinea is supplied from the south by a narrow western boundary undercurrent (New Guinea Coastal Undercurrent) transporting high-salinity, low-tritium, high-oxygen, low-nutrient water from the Solomon Sea northwestward along the north coast of Papua New Guinea through the Vitiaz Strait. The New Guinea Coastal Undercurrent has a maximum speed of 40–70 cm s −1 at a depth of about 200 m. It is a permanent feature despite the reversals of the wind and the surface current during the period of the northwest monsoon in austral summer. Its transport through the Vitiaz Strait is as high as 8 × 10 6 m 3 s −1 , which is of the same magnitude as the Equatorial Undercurrent transport at 143°E. The New Guinea Coastal Undercurrent revealed by the WEPOCS data is consistent with the low-latitude equatorward western boundary current implied in a calculation of the Sverdrup transport based on the observed wind-stress distribution for the tropical Pacific Ocean. High-salinity, low-tritium, low -oxygen, high -nutrient water which flows westward into the Bismarck Sea passing north of the Solomon Islands is entrained into the Equatorial Undercurrent north of New Ireland and returns to the east, resulting in a down-stream increae in the Undercurrent transport. Low-salinity, high-tritium, high-nutrient water of eastern North Pacific origin also contributes to the Equatorial Undercurrent in its source area west of the WEPOCS region. However, there is no evidence that northern waters are being continuously entrained into the Undercurrent in the WEPOCS region.

The Bifurcation of the North Equatorial Current in the Pacific

Abstract A new climatology using historical temperature and salinity data in the western Pacific is constructed to examine the bifurcation of the North Equatorial Current (NEC). Integrating dynamically calculated circulation from the sea surface to 1000 m and combining it with surface Ekman transport, it is shown that the bifurcation of the NEC occurs at the southernmost position (14.8°N) in July and the northernmost position (about 17.2°N) in December. This annual signal lags behind the seasonal meridional migration of the zero zonally integrated wind stress curl line by 4–5 months but corresponds pretty well with the local Ekman pumping associated with the Asian monsoon winds. The bifurcation latitude of the NEC is depth dependent. On the annual average, it shifts from about 13.3°N near the surface to north of 20°N at depths around 1000 m. There is a time lag of 1–2 months from the sea surface to the subsurface (300–700 m) for the annual cycle. Below 700 m, the bifurcation of the NEC approaches as far n...

Surface Buoyancy Forcing and the Mixed Layer of the Western Pacific Warm Pool: Observations and 1D Model Results

Abstract The broad, shallow body of warm (>29°C) water found in the western tropical Pacific Ocean plays an important role in the coupled ocean-atmosphere dynamics and thermodynamics associated with the El Nino-Southern Oscillation phenomenon. Thus, it is important to understand the processes that maintain and perturb that warm pool. Measurements from a buoy moored in the center of the warm pool during the TOGA Coupled Ocean-Atmosphere Response Experiment show that the exchange of mass between the ocean and atmosphere is as important as the exchange of heat. Rain forms a shallow, buoyant layer that does not mix with the water below except during infrequent strong wind events. Using a one-dimensional mixed layer model, it is demonstrated that the rate of local precipitation governs the mixed layer depth and can thus alter the rates of change in sea surface temperature during both warming and cooling periods. The observed mixed layer depth in the warm pool is at a depth that allows for maximum warming by ca...