ALBERT

Description: Oklahoma Mesonetwork data are used to illustrate important atmospheric features that are not well shown by the usual synoptic data. For example, some shifts of wind from south to north that are shown as cold fronts on synoptic charts are not cold fronts by any plausible definition. As previously discussed by Fred Sanders and others, such errors in analysis can be reduced by knowledge of the wide variety of weather phenomena that actually exists, and by more attention to temperatures at the earth's surface as revealed by conventional synoptic data. Mesoscale data for four cases reinforce previous discussions of the ephemeral nature of fronts and deficiencies in the usual analyses of cold fronts. One type of misanalyzed case involves post-cold-frontal boundary layer air that is warmer than the prefrontal air. A second type is usually nocturnal, with a rise of local temperature during disruption of an inversion and a wind shift with later cooling that accompanies advection of a climatological gradient of temperature.

Description: The tropical oceans have long been recognized as the most important region for large-scale ocean–atmosphere interactions, giving rise to coupled climate variations on several time scales. During the Tropical Ocean Global Atmosphere (TOGA) decade, the focus of much tropical ocean research was on understanding El Niño–related processes and on development of tropical ocean models capable of simulating and predicting El Niño. These studies led to an appreciation of the vital role the ocean plays in providing the memory for predicting El Niño and thus making seasonal climate prediction feasible. With the end of TOGA and the beginning of Climate Variability and Prediction (CLIVAR), the scope of climate variability and predictability studies has expanded from the tropical Pacific and ENSO-centric basis to the global domain. In this paper the progress that has been made in tropical ocean climate studies during the early years of CLIVAR is discussed. The discussion is divided geographically into three tropical ocean basins with an emphasis on the dynamical processes that are most relevant to the coupling between the atmosphere and oceans. For the tropical Pacific, the continuing effort to improve understanding of large- and small-scale dynamics for the purpose of extending the skill of ENSO prediction is assessed. This paper then goes beyond the time and space scales of El Niño and discusses recent research activities on the fundamental issue of the processes maintaining the tropical thermocline. This includes the study of subtropical cells (STCs) and ventilated thermocline processes, which are potentially important to the understanding of the low-frequency modulation of El Niño. For the tropical Atlantic, the dominant oceanic processes that interact with regional atmospheric feedbacks are examined as well as the remote influence from both the Pacific El Niño and extratropical climate fluctuations giving rise to multiple patterns of variability distinguished by season and location. The potential impact of Atlantic thermohaline circulation on tropical Atlantic variability (TAV) is also discussed. For the tropical Indian Ocean, local and remote mechanisms governing low-frequency sea surface temperature variations are examined. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, this study focuses on the active role of ocean dynamics in a seasonally locked east–west internal mode of variability, known as the Indian Ocean dipole (IOD). Influences of the IOD on climatic conditions in Asia, Australia, East Africa, and Europe are discussed. While the attempt throughout is to give a comprehensive overview of what is known about the role of the tropical oceans in climate, the fact of the matter is that much remains to be understood and explained. The complex nature of the tropical coupled phenomena and the interaction among them argue strongly for coordinated and sustained observations, as well as additional careful modeling investigations in order to further advance the current understanding of the role of tropical oceans in climate.

Description: High-resolution satellite observations are used to investigate air–sea interaction over the eastern Pacific warm pool. In winter, strong wind jets develop over the Gulfs of Tehuantepec, Papagayo, and Panama, accelerated by the pressure gradients between the Atlantic and Pacific across narrow passes of Central American cordillera. Patches of cold sea surface temperatures (SSTs) and high chlorophyll develop under these wind jets as a result of increased turbulent heat flux from the ocean and enhanced mixing across the base of the ocean mixed layer. Despite a large decrease in SST (exceeding 3°C in seasonal means), the cold patches associated with the Tehuantepec and Papagayo jets do not have an obvious effect on local atmospheric convection in winter since the intertropical convergence zone (ITCZ) is located farther south. The cold patch of the Panama jet to the south, on the other hand, cuts through the winter ITCZ and breaks it into two parts. A pronounced thermocline dome develops west of the Gulf of Papagayo, with the 20°C isotherm only 30 m deep throughout the year. In summer when the Panama jet disappears and the other two wind jets weaken, SST is 0.5°C lower over this Costa Rica Dome than the background. This cold spot reduces local precipitation by half, punching a hole of 500 km in diameter in the summer ITCZ. The dome underlies a patch of open-ocean high chlorophyll. This thermocline dome is an ocean dynamic response to the positive wind curls south of the Papagayo jet, which is optimally oriented to excite ocean Rossby waves that remotely affect the ocean to the west. The meridionally oriented Tehuantepec and Panama jets, by contrast, only influence the local thermocline depth with few remote effects on SST and the atmosphere. The orographical-triggered air–sea interaction described here is a good benchmark for testing high-resolution climate models now under development.

Description: Tropical instability waves (TIWs) within a half-degree of the equator in the Pacific Ocean have been consistently observed in meridional velocity with periods of around 20 days. On the other hand, near 5°N, TIWs have been observed in sea surface height (SSH), thermocline depth, and velocity to have periods near 30 days. Tropical Atmosphere–Ocean (TAO) Project moored equatorial velocity and temperature time series are used to investigate the spatial and temporal structure of TIWs during 3 years of La Niña conditions from 1998 through 2001. Along 140°W, where the TIW temperature and velocity variabilities are at their maxima, these variabilities include two distinct TIWs with periods of 17 and 33 days, rather than one broadbanded process. As predicted by modeling studies, the 17-day TIW variability is shown to occur not only in meridional velocity at the equator, but also in subsurface temperature at 2°N and 2°S, while the 33-day TIW variability is observed primarily in subsurface temperature at 5°N. These two TIWs, respectively, are shown to have characteristics similar to a Yanai wave/surface-trapped instability and an unstable first meridional mode Rossby wave. One implication of such a description is that the velocity variability on the equator is not directly associated with the dominant 33-day variability along 5°N.

Description: An ocean GCM, interpreted in light of linear models and sparse observations, is used to diagnose the dynamics of the annual cycle of circulation in the western boundary current system of the southwest Pacific Ocean. The simple structure of annual wind stress curl over the South Pacific produces a large region of uniformly phased, stationary thermocline depth anomalies such that the western subtropical gyre spins up and down during the year, directing flow anomalies alternately toward and away from the boundary at its northern end, near 10°S. The response of the western boundary currents is to redistribute these anomalies northward toward the equator and southward to the subtropical gyre, a redistribution that is determined principally by linear Rossby processes, not boundary dynamics. When the subtropical gyre and South Equatorial Current (SEC) are strong (in the second half of the year), the result is both increased equatorward transport of the New Guinea Coastal Current and poleward transport anomalies along the entire Australian coast. Because of this opposite phasing of boundary current anomalies across 10°S, annual migration of the bifurcation point of the total SEC, near 18°S in the mean, has no significance regarding variability of transport from subtropics to equator.

Description: The South Equatorial Current (SEC), the westward branch of the South Pacific subtropical gyre, extends from the equator to 30°S at depth. Linear ocean dynamics predict that the SEC forms boundary currents on the eastern coasts of the South Pacific islands it encounters. Those currents would then detach at the northern and southern tips of the islands, and cross the Coral Sea in the form of jets. The Fiji Islands, the Vanuatu archipelago, and New Caledonia are the major topographic obstacles on the SEC pathway to the Australian coast. Large-scale numerical studies, as well as climatologies, suggest the formation of three jets in their lee: the north Vanuatu jet (NVJ), the north Caledonian jet (NCJ), and the south Caledonian jet (SCJ), implying a bifurcation against the east coast of each island. The flow observed during the SECALIS-2 cruise in December 2004 between Vanuatu and New Caledonia is presented herein. An inverse box model is used to provide quantitative transport estimates with uncertainties and to infer the pathways and boundary current formation. For that particular month, the 0–2000-m SEC inflow was found to be 20 ± 4 Sv (1 Sv ≡ 106 m3 s−1) between Vanuatu and New Caledonia. Of that, 6 ± 2 Sv bifurcated to the south in a boundary current against the New Caledonia coast (the Vauban Current), and the remainder exited north of New Caledonia, feeding the NCJ. The flow is comparable both above and below the thermocline, while complex topography, associated with oceanic eddy generation, introduces several recirculation features. To the north, the NCJ, which extends down to 1500 m, was fed not only by the SEC inflow, but also by waters coming from the north, which have possibly been recirculated. To the south, a westward current rounds the tip of New Caledonia. A numerical simulation suggests a partial continuity with the deep extension of the Vauban Current (this current would then be the SCJ) while the hydrographic sections are too distant to confirm such continuity.

Description: The seasonal change in the development of Aleutian low pressure systems from early fall to early winter is analyzed using a combination of meteorological reanalysis fields, satellite sea surface temperature (SST) data, and satellite wind data. The time period of the study is September–December 2002, although results are shown to be representative of the long-term climatology. Characteristics of the storms were documented as they progressed across the North Pacific, including their path, central pressure, deepening rate, and speed of translation. Clear patterns emerged. Storms tended to deepen in two distinct geographical locations—the Gulf of Alaska in early fall and the western North Pacific in late fall. In the Gulf of Alaska, a quasi-permanent “notch” in the SST distribution is argued to be of significance. The signature of the notch is imprinted in the atmosphere, resulting in a region of enhanced cyclonic potential vorticity in the lower troposphere that is conducive for storm development. Later in the season, as winter approaches and the Sea of Okhotsk becomes partially ice covered and cold, the air emanating from the Asian continent leads to enhanced baroclinicity in the region south of Kamchatka. This corresponds to enhanced storm cyclogenesis in that region. Consequently, there is a seasonal westward migration of the dominant lobe of the Aleutian low. The impact of the wind stress curl pattern resulting from these two regions of storm development on the oceanic circulation is investigated using historical hydrography. It is argued that the seasonal bimodal input of cyclonic vorticity from the wind may be partly responsible for the two distinct North Pacific subarctic gyres.

Description: Near-surface shear in the Pacific cold tongue front at 2°N, 140°W was measured using a set of five moored current meters between 5 and 25 m for nine months during 2004–05. Mean near-surface currents were strongly westward and only weakly northward (∼3 cm s−1). Mean near-surface shear was primarily westward and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was subtracted from the observed shear, the residual ageostrophic currents relative to 25 m were northward and had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a vertically uniform front. According to this “frontal Ekman” model, the ageostrophic Ekman spiral is forced by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A diurnal jet was observed that was on average 12 cm s−1 stronger at 5 m than at 25 m, even though daytime stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night than daytime and decreases with depth. A “generalized Ekman” model is also developed that assumes that viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the features of the observed mean shear and is valid both in frontal regions and at the equator.

Description: Large-amplitude sea surface height (SSH) variations with a wave period of 70 days and a wavelength of about 700 km are detected in the Coral Sea centered along ∼16°S and in the North Fiji Basin along ∼18°S. These mesoscale eddy signals have a well-defined annual cycle with a maximum in December/January and a minimum in May/June. As the southeasterly trades intercept the island mountains of Vanuatu and Fiji, they generate localized wind stress curl dipoles, which in turn induce the eastward-flowing Coral Sea Countercurrent (CSCC) and Fiji Basin Countercurrent (FBCC) in the Coral Sea and the North Fiji Basin, respectively. The high eddy variability band along 16°S is hypothesized in this study as resulting from barotropic instability due to the lateral shear between the eastward-flowing CSCC and its neighboring westward-flowing North Caledonian and North Vanuatu jets (NCJ and NVJ). Based on stability analyses on the background NCJ–CSCC–NVJ system, it is found that the most unstable mode of the system has an equivalent barotropic structure and is insensitive to the specification of the background flow’s vertical shear. This barotropic instability hypothesis is also supported by observations; specifically, the Reynolds stresses inferred from the observed SSH data are tilted against the meridional shear of the background NCJ–CSCC–NVJ flow. As the intensity of the NCJ–CSCC–NVJ shear fluctuates with the season, the growth rate of instability changes, resulting in seasonal amplitude modulations in the observed 70-day eddy signals.