David Halpern

The Tropical Ocean‐Global Atmosphere observing system: A decade of progress

A major accomplishment of the recently completed Tropical Ocean-Global Atmosphere (TOGA) Program was the development of an ocean observing system to support seasonal-to-interannual climate studies. This paper reviews the scientific motivations for the development of that observing system, the technological advances that made it possible, and the scientific advances that resulted from the availability of a significantly expanded observational database. A primary phenomenological focus of TOGA was interannual variability of the coupled ocean-atmosphere system associated with El Nino and the Southern Oscillation (ENSO).Prior to the start of TOGA, our understanding of the physical processes responsible for the ENSO cycle was limited, our ability to monitor variability in the tropical oceans was primitive, and the capability to predict ENSO was nonexistent. TOGA therefore initiated and/or supported efforts to provide real-time measurements of the following key oceanographic variables: surface winds, sea surface temperature, subsurface temperature, sea level and ocean velocity. Specific in situ observational programs developed to provide these data sets included the Tropical Atmosphere-Ocean (TAO) array of moored buoys in the Pacific, a surface drifting buoy program, an island and coastal tide gauge network, and a volunteer observing ship network of expendable bathythermograph measurements. Complementing these in situ efforts were satellite missions which provided near-global coverage of surface winds, sea surface temperature, and sea level. These new TOGA data sets led to fundamental progress in our understanding of the physical processes responsible for ENSO and to the development of coupled ocean-atmosphere models for ENSO prediction.

Observations of 20-day period meridional current oscillations in the upper ocean along the Pacific Equator

Abstract Prominent oscillations of the meridional current, with a mean period of approximately 20 days, have been observed in the upper ocean over several years from May 1979 to October 1985 using moored current measurements along the Pacific equator at 95°, 110°, 124°,140°W and 152°W, as well as off (but near) the equator at 110° and 140°W. The fluctuations are relatively narrowband (±0.005 cpd) in frequency. A 95% statistically significant peak in power spectra of meridional current occurred at 110°, 124° and 140°W, but not at 95° and 152°W where the spectral peaks were smaller. The dominant wave period decreased by about 4% from 110° to 140°W. Maximum amplitude was measured at 124°W; the amplitude above 80 m was maximum at the equator and decreased poleward from the equator. At 15 m the annual averaged root-mean-square amplitude was about 20.5 cm s−1, and individual peak-to-trough values reached 150 cm s−1. The wave amplitude decreased with depth and the wave was essentially confined to the upper 80 m....

Variability in the upper ocean during MILE. Part I: The heat and momentum balances

Abstract The Mixed Layer Experiment (MILE) was an examination of the upper ocean carried out near Ocean Weather Station P during a 20-day period in the autumn of 1977 characterized by two wind events. In this paper the variability of temperature and velocity observed at two moorings are described and related to the surface heat flux and wind forcing. A one-dimensional upper layer heat budget is found to close acceptably and it is shown that this success depends on having well-sampled temperature records and a method of accounting for vertical velocities in the seasonal thermocline. A similar budget of momentum is also reasonably accurate, but it is necessary to account for the effects of quasi-geostrophic velocities in the upper layer inferentially. Internal waves are found to be a major source of variability, dominating velocity records even at 5-m depth, well within the mixed layer. In the seasonal thermocline there is a high-frequency cut-off of internal wave energy well below the local buoyancy frequency. High-frequency internal wave ‘events’ are identified in the seasonal thermocline. Shear in the upper layer, and its response to wind, have some features in accord with models taking slab flow in the mixed layer, but the agreement is far from complete. Observations in the thermocline indicate that velocity differences in the vertical are maintained only if there is sufficient density stratification to make the Richardson number exceed a critical value.

Observations of the Deepening of the Wind-Mixed Layer in the Northeast Pacific Ocean

Abstract Measurements of winds, currents and temperature are used to describe the response of the upper ocean in the northeast Pacific to the passage of an August 1971 synoptic-scale meteorological disturbance. The experiment was designed so that at the beginning of the 32-day study the uppermost two current meters were located in the upper isothermal layer, a third current meter was placed at the top of the seasonal thermocline, and the fourth current meter was located near the bottom of the seasonal thermocline. Thermistors attached to a multi-conductor cable were placed in the mixed layer and in the thermocline region. Before the onset of the storm the thickness of the mixed layer was about 15 m. The storm produced a more homogeneous temperature distribution above 20 m with a lower average temperature, higher temperature values below 20 m, and a thicker (25 m) mixed layer. The heat content of the upper layer changed little (<±5%) as the mixed layer deepened. The storm generated large currents and verti...

Heat Budget of the Upper Ocean Under Light Winds

Abstract The heat budget of a small region (∼2 km×2 km) near the coast of northwest Africa was investigated during a 3-day period in March 1974 when light winds (∼2 m s−1) occurred. Horizontal advection and diffusion of heat were negligible, and the local change in heat content resulted from the net radiative and evaporative flux at the sea surface. Diurnal heating produced surface temperature ranges of 0.9, 1.1 and 1.4°C for each of the 3 days. The exponential decrease of the diurnal heat wave with depth was used to estimate the vertical eddy thermal conductivity of the upper 10 m; values of approximately 10−3 m−2 s−1 were obtained.

Somali Jet in the Arabian Sea, El Nino, and India Rainfall

Abstract Interannual variations of the Somali Jet in the Arabian Sea during 1988–99 were linked to El Nino and La Nina episodes and to India west coast rainfall. Onset dates and monthly mean strengths of the Somali Jet were described with Special Sensor Microwave Imager surface wind speeds. Each year the Somali Jet formed in a similar area in the western Arabian Sea, and always before the onset of monsoon rainfall in Goa. The average date of Somali Jet onset was two days later in El Nino events in comparison with La Nina conditions. Monthly mean strength of the Somali Jet was 0.4 m s−1 weaker during El Nino episodes than during La Nina intervals. When the monthly mean intensity of the Somali Jet was above (below) normal, there was an excess (deficit) of rainfall along the Indian west coast.

Description of Wind and of Upper Ocean Current and Temperature Variations on the Continental Shelf off Northwest Africa during March and April 1974

Abstract Measurements of winds and of near-surface temperatures and currents made during March and April 1974 on the continental shelf off northwest Africa were extremely time-dependent. Alternating land and sea breezes were well-developed and produced temperature and current fluctuations in the uppermost 15 m. Time-averaged speed of the surface current (28 an s−1) was much larger than the geostrophic current computed from the density field over the shelf. Approximately 60% of the variance of the current measurements occurred at frequencies less than the inertial period. Inertial and tidal period currents were large. Water stratification was very weak and tidal internal gravity wave motions were not detected. During a coastal upwelling event the Ekman transport, the offshore transport and the onshore transport were nearly equivalent, and the vertical eddy viscosity coefficient over the upper 10 m was about 125 cm2 s−1.

Comparison of tropical pacific temperature and current simulations with two vertical mixing schemes embedded in an ocean general circulation model and reference to observations

The Pacanowski-Philander (PP) and Mellor-Yamada (MY) parameterization models of vertical mixing by turbulent processes were embedded in the Geophysical Fluid Dynamics Laboratory high-resolution ocean general circulation model of the tropical Pacific Ocean. All other facets of the numerical simulations were the same. Simulations were made for the 1987–1988 period. At the equator the MY simulation produced near-surface temperatures more uniform with depth, a deeper thermocline, a deeper core speed of the Equatorial Undercurrent, and a South Equatorial Current with greater vertical thickness compared with that computed with the PP method. Along 140°W, between 5°N and 10°N, both simulations were the same. Moored-buoy current and temperature observations had been recorded by the Pacific Marine Environmental Laboratory at three sites (165°E, 140°W, 110°W) along the equator and at three sites (5°N, 7°N, 9°N) along 140°W. Simulated temperatures were lower than those observed in the near-surface layer and higher than those observed in the thermocline. Temperature simulations were in better agreement with observations compared to current simulations. At the equator, PP current and temperature simulations were more representative of the observations than MY simulations.

Onset of the Somali Jet in the Arabian Sea during June 1997

The National Aeronautics and Space Administration scatterometer surface wind vectors are used to describe the rapid onset of the Somali Jet throughout the Arabian Sea. In June 1997 the time of Somali Jet onset varied over the Arabian Sea, with June 17–18 the average time. The Somali Jet appeared first in the western Arabian Sea, expanded over 2 weeks to encompass the Arabian Sea, and produced a threefold increase in surface wind convergence in the eastern Arabian Sea. The onset time of the 12 m s−1 isotach preceded by 3–4 days the onset of monsoon rainfall in Goa. For Somali Jet wind speeds above 10 m s−1 the 2 day 1° × 1° sea surface temperature decreased 0.5°C per 1 m s−1 increase in collocated wind speed. The Somali Jet created a north-south distribution of Ekman pumping and suction in the central Arabian Sea to enhance the eastward surface current by 0.1 m s−1. The Somali Jet doubled the southward Ekman transport across the southern boundary of the Arabian Sea. In June 1997, when the most intense El Nino episode of the century was in its onset phase, the southward Ekman transport across the southern boundary of the Arabian Sea was one half that observed since 1992.

Annual and interannual variations of phytoplankton pigment concentration and upwelling along the Pacific equator

The following variables along the Pacific equator from 145°E to 95°W were employed: surface layer phytoplankton pigment concentrations derived from Nimbus 7 coastal zone color scanner (CZCS) measurements of ocean color radiances; vertical velocities simulated at the 90-m bottom of the euphotic layer from a wind-driven ocean general circulation model; and nitrate concentrations estimated from model-simulated temperature. The upward flux of nitrate into the euphotic layer was calculated from the simulated vertical motion and nitrate concentration. The CZCS-derived phytoplankton pigment concentration was uniform from 175° to 95°W. Longitudinal profiles of upwelling, phytoplankton biomass, and 90-m nitrate flux were of different shapes. The small annual cycles of the phytoplankton pigment concentration and nitrate flux were in phase: increased phytoplankton biomass was associated with increased upward nitrate flux, but the phase was not consistent with the annual cycles of the easterly wind or of the upwelling intensity. Variation of phytoplankton pigment concentration was greater during El Nino than during the annual cycle. The substantially reduced phytoplankton pigment concentration observed during El Nino was associated with smaller upward nitrate flux. Phytoplankton biomass during non-El Nino conditions was not related to nitrate flux into the euphotic layer.