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Abstract
In two recent papers, the frequency of separation of rings from the Loop Current in the Gulf of Mexico was studied; the authors used similar data but obtained remarkably different results for the primary rate of ring shedding. In this paper the time between successive rings for the last 22 known ring events since 1973 are examined. Using a histogramlike technique that does not involve a surrogate variable but deals directly with the ring events themselves, two primary modes are found. The one at a period near 8–9 mo has slightly (but not significantly) more power than the one near 13–14 mo. The uncertainty in the periods of these peaks is estimated to be ∼0.3 mo from measurement uncertainties and an additional ∼0.3 mo from the natural variability of the process. If the high resolution available from a 20-year record were not maintained, it would be possible to smooth the present result heavily (in frequency space) and obtain the ∼11 mo peak reported by Maul and Yukovich.
Abstract
In two recent papers, the frequency of separation of rings from the Loop Current in the Gulf of Mexico was studied; the authors used similar data but obtained remarkably different results for the primary rate of ring shedding. In this paper the time between successive rings for the last 22 known ring events since 1973 are examined. Using a histogramlike technique that does not involve a surrogate variable but deals directly with the ring events themselves, two primary modes are found. The one at a period near 8–9 mo has slightly (but not significantly) more power than the one near 13–14 mo. The uncertainty in the periods of these peaks is estimated to be ∼0.3 mo from measurement uncertainties and an additional ∼0.3 mo from the natural variability of the process. If the high resolution available from a 20-year record were not maintained, it would be possible to smooth the present result heavily (in frequency space) and obtain the ∼11 mo peak reported by Maul and Yukovich.
Abstract
The coherence of sea level is examined between a number of widely distributed stations chosen from those with the longest datasets, such as at San Francisco, where the data has been recorded since 1855. The sea-level signals in the eastern Pacific appear to be dominated by propagating Rossby waves, so that the variability, which has periods of 5–8 years, (e.g., between San Francisco and Honolulu), is coherent, but out of phase by several years. A surprising finding is that sea level is coherent on opposite sides of the Atlantic at periods near 6 years, but this is suspected to be the result of direct atmospheric forcing rather than of wave propagation. At the longest periods detectable—40–50 years—the sea-level signals have amplitudes of 5–15 cm and are “visually coherent" between the west coasts of the United States and Europe. The amplitude of these extremely long-period signals is the same as the apparent “rise of sea level over the the past century," although the rate of rise from these fluctuations is larger. Because there is so much variability at extremely long periods, the sea-level data must be treated carefully in space as well as in the time to avoid contaminating the “sea-level rise" signals with propagating signals. If the data were adjusted, or corrected for these signals, the signal-to-noise ratio might be substantially improved, allowing better estimates of the observed rise of sea level, but the forcing mechanisms are not well known at the longer periods. Until the data are so corrected, changes in the rate of rise of sea level on time scales of 10–50 years can not be distinguished from the background “noise.".
Abstract
The coherence of sea level is examined between a number of widely distributed stations chosen from those with the longest datasets, such as at San Francisco, where the data has been recorded since 1855. The sea-level signals in the eastern Pacific appear to be dominated by propagating Rossby waves, so that the variability, which has periods of 5–8 years, (e.g., between San Francisco and Honolulu), is coherent, but out of phase by several years. A surprising finding is that sea level is coherent on opposite sides of the Atlantic at periods near 6 years, but this is suspected to be the result of direct atmospheric forcing rather than of wave propagation. At the longest periods detectable—40–50 years—the sea-level signals have amplitudes of 5–15 cm and are “visually coherent" between the west coasts of the United States and Europe. The amplitude of these extremely long-period signals is the same as the apparent “rise of sea level over the the past century," although the rate of rise from these fluctuations is larger. Because there is so much variability at extremely long periods, the sea-level data must be treated carefully in space as well as in the time to avoid contaminating the “sea-level rise" signals with propagating signals. If the data were adjusted, or corrected for these signals, the signal-to-noise ratio might be substantially improved, allowing better estimates of the observed rise of sea level, but the forcing mechanisms are not well known at the longer periods. Until the data are so corrected, changes in the rate of rise of sea level on time scales of 10–50 years can not be distinguished from the background “noise.".
Abstract
In the course of archiving positions of the edge of the Loop Current from satellite infrared (IR) data, we have found a substantial amount of energy at periods in the “wind-driven band.” Using a technique patterned after that of Price et al., we constructed a series of new datasets of IR positions at a variety of angles relative to the daily wind. Using data for a period of November–May, we find that the IR fluctuations are coherent with wind, and are at an angle of 80° to the right. Our IR data do not resolve periods shorter than ∼10 days realiably, but motions of ∼12–16 days are well resolved. These findings show that the wind-coherent motion of the surface IR signal is associated with the Ekman transport of the upper mixed layer.
Abstract
In the course of archiving positions of the edge of the Loop Current from satellite infrared (IR) data, we have found a substantial amount of energy at periods in the “wind-driven band.” Using a technique patterned after that of Price et al., we constructed a series of new datasets of IR positions at a variety of angles relative to the daily wind. Using data for a period of November–May, we find that the IR fluctuations are coherent with wind, and are at an angle of 80° to the right. Our IR data do not resolve periods shorter than ∼10 days realiably, but motions of ∼12–16 days are well resolved. These findings show that the wind-coherent motion of the surface IR signal is associated with the Ekman transport of the upper mixed layer.
Abstract
Results of land leveling do not agree with oceanographers’ expectations concerning coastal slopes of sea level. Recent studies have shown that along the west coast of the United State this discrepancy can be explained by the vertical movement of leveling benchmarks. On the east coast, where the movement of benchmarks is not expected, land leveling suggests that sea level should rise as much as 30 cm from Miami, Florida, to Charleston, South Carolina. However, oceanographers find that sea level falls, from south to north, approximately 15 cm along the shoreward edge of the Gulf Stream. The discrepancy between these findings could be explained if oceanographic effects that support large alongshore slopes, and that arise primarily on the continental shelf, could be identified. We examine coastal wind stress and runoff as two such forcing mechanisms. To do this, we derive a statistical model of the alongshore change of sea level between tide gauges, based on the alongshore momentum equation. The computed mean sea level differences are only 1–2 cm between pairs of tide gauges, and the signs change from one pair of stations to the next. The conclusion is that the forcing mechanisms that we have studied cannot explain the slopes found by land leveling.
The model is able to predict the seasonal variation of slope between Charleston and Fernandina and between Atlantic City and Norfolk. The observed differences (∼8 cm) compare well with the model-computed differences between these sites. Thus, along these sections of coastline, close to shore, we conclude that the seasonal variations of slope seem to be forced largely by coastal winds. On the other hand, only half of the seasonal range observed between Fernandina and Miami and between Norfolk and Charleston could be modeled as effects of wind stress and runoff. The effects of other forcing mechanisms may be more important in these areas.
Abstract
Results of land leveling do not agree with oceanographers’ expectations concerning coastal slopes of sea level. Recent studies have shown that along the west coast of the United State this discrepancy can be explained by the vertical movement of leveling benchmarks. On the east coast, where the movement of benchmarks is not expected, land leveling suggests that sea level should rise as much as 30 cm from Miami, Florida, to Charleston, South Carolina. However, oceanographers find that sea level falls, from south to north, approximately 15 cm along the shoreward edge of the Gulf Stream. The discrepancy between these findings could be explained if oceanographic effects that support large alongshore slopes, and that arise primarily on the continental shelf, could be identified. We examine coastal wind stress and runoff as two such forcing mechanisms. To do this, we derive a statistical model of the alongshore change of sea level between tide gauges, based on the alongshore momentum equation. The computed mean sea level differences are only 1–2 cm between pairs of tide gauges, and the signs change from one pair of stations to the next. The conclusion is that the forcing mechanisms that we have studied cannot explain the slopes found by land leveling.
The model is able to predict the seasonal variation of slope between Charleston and Fernandina and between Atlantic City and Norfolk. The observed differences (∼8 cm) compare well with the model-computed differences between these sites. Thus, along these sections of coastline, close to shore, we conclude that the seasonal variations of slope seem to be forced largely by coastal winds. On the other hand, only half of the seasonal range observed between Fernandina and Miami and between Norfolk and Charleston could be modeled as effects of wind stress and runoff. The effects of other forcing mechanisms may be more important in these areas.
Abstract
The most energetic events in the circulation of the Gulf of Mexico are the separation of large anticyclonic rings from the Loop Current. Building on previous work, the authors examine all the apparent rings since July 1973. This new dataset includes the satellite altimetry since 1992, providing a set of 34 consecutive ring formations. The primary advantage of altimetry is that the data remain available in the summer. One finding is that the ambiguity of whether or not a ring has separated is reduced, but not eliminated; the uncertainty with which separation “events” can be specified remains approximately 4 weeks, even with nearly continuous data. Primary peaks in the distribution of separation intervals are found at 6 and 11 months with a smaller peak at 9 months. If the spectrum is smoothed heavily enough, a peak in the distribution can be formed nearer 12 months, but this near-annual peak is a result more of the smoothing than of the data.
Abstract
The most energetic events in the circulation of the Gulf of Mexico are the separation of large anticyclonic rings from the Loop Current. Building on previous work, the authors examine all the apparent rings since July 1973. This new dataset includes the satellite altimetry since 1992, providing a set of 34 consecutive ring formations. The primary advantage of altimetry is that the data remain available in the summer. One finding is that the ambiguity of whether or not a ring has separated is reduced, but not eliminated; the uncertainty with which separation “events” can be specified remains approximately 4 weeks, even with nearly continuous data. Primary peaks in the distribution of separation intervals are found at 6 and 11 months with a smaller peak at 9 months. If the spectrum is smoothed heavily enough, a peak in the distribution can be formed nearer 12 months, but this near-annual peak is a result more of the smoothing than of the data.
Abstract
Variability of sea level on the offshore side of the Gulf Stream has been estimated with a wind-forced numerical model. The difference in sea level between the model and coastal tide gauges therefore provides an estimate of variability of the Gulf Stream. These results can be compared with direct measurements of transport; the agreement is surprisingly good. Transport estimates are then made for sections offshore of four major tide stations along the U.S. East Coast. When data since World War II are used, the spectrum of sea level at the coast appears to peak at periods of ∼150–250 mo. The difference signal (ocean minus coast), however, which the authors interpret as transport variability, has a weakly red spectrum. Power decreases at somewhat less than f −1 at periods just less than ∼500 months but decreases strongly at periods less than ∼150 months. The low-frequency variability arises primarily from the influx of open ocean Rossby waves. The large variance at low frequencies suggests that measurements of the transport of western boundary currents do not have many degrees of freedom; measurements made many years apart may vary substantially because of this localized variability. Sea level at the coast is coherent over long distances, but the incoming Rossby wave radiation from the open ocean has a relatively short north–south scale. These results emphasize that transport measured at one location along the coast may be incoherent with transport at locations only ∼200 km away. As a result, measurements at one location will in general not be representative of transport along the entire coast.
Abstract
Variability of sea level on the offshore side of the Gulf Stream has been estimated with a wind-forced numerical model. The difference in sea level between the model and coastal tide gauges therefore provides an estimate of variability of the Gulf Stream. These results can be compared with direct measurements of transport; the agreement is surprisingly good. Transport estimates are then made for sections offshore of four major tide stations along the U.S. East Coast. When data since World War II are used, the spectrum of sea level at the coast appears to peak at periods of ∼150–250 mo. The difference signal (ocean minus coast), however, which the authors interpret as transport variability, has a weakly red spectrum. Power decreases at somewhat less than f −1 at periods just less than ∼500 months but decreases strongly at periods less than ∼150 months. The low-frequency variability arises primarily from the influx of open ocean Rossby waves. The large variance at low frequencies suggests that measurements of the transport of western boundary currents do not have many degrees of freedom; measurements made many years apart may vary substantially because of this localized variability. Sea level at the coast is coherent over long distances, but the incoming Rossby wave radiation from the open ocean has a relatively short north–south scale. These results emphasize that transport measured at one location along the coast may be incoherent with transport at locations only ∼200 km away. As a result, measurements at one location will in general not be representative of transport along the entire coast.
Abstract
The Bermuda tide gauge record extends back to the early 1930s. That sea level fluctuations there are highly coherent with dynamic height from hydrographic data has two interesting implications. First, it should contain information about the low-frequency circulation of the Atlantic. Furthermore, because dynamic height contains information on heat storage, it might, on the limited timescales accessible in the record, also contain clues about climate.
A simple model of wind forcing of the Atlantic from the African coast to Bermuda uses the Levitus mean density data to estimate the long Rossby wave speed as a function of longitude. Sea level and thermocline variability estimated this way are in remarkably good agreement with observations at periods of more than a few years duration. The peak-to-peak sea level signal is ∼18 cm, which is nearly 25% of the slope across the Gulf Stream at this latitude. The model results suggest that the variability is largest somewhat to the east of Bermuda; fluctuations of ∼10 cm extend as far east as ∼35°W.
One surprising result is that at the longest periods in the COADS data, the wind curl has a double-peak structure in longitude. That is, there is a significant amount of power on the eastern side of the ocean as well as near Bermuda. Therefore, it is essential to use the full horizontal resolution of the wind data; using the mean curl across the Atlantic turns out not to be a good way to estimate thermocline variability. One might wonder if the wind data are reliable at these long periods were it not for the good agreement between the results and observed sea level. The power in wind variability increases out to ∼500 months, although with little statistical reliability. Sea level variability however, appears to peak at somewhat shorter periods. Although it is pushing the resolution of the data, this result is a limitation imposed by the basin width scale. The power in the model ocean's response to wind forcing is nearly an order of magnitude larger during the first half of the record (1952–69) than during the second (1970–86).
It is likely that significant changes in buoyancy forcing by the atmosphere are coherent with changes in wind. Nevertheless, these results suggest that the variability in sea level—and so in deep temperature—can perhaps be accounted for without invoking changes in stored heat of the deep ocean.
Abstract
The Bermuda tide gauge record extends back to the early 1930s. That sea level fluctuations there are highly coherent with dynamic height from hydrographic data has two interesting implications. First, it should contain information about the low-frequency circulation of the Atlantic. Furthermore, because dynamic height contains information on heat storage, it might, on the limited timescales accessible in the record, also contain clues about climate.
A simple model of wind forcing of the Atlantic from the African coast to Bermuda uses the Levitus mean density data to estimate the long Rossby wave speed as a function of longitude. Sea level and thermocline variability estimated this way are in remarkably good agreement with observations at periods of more than a few years duration. The peak-to-peak sea level signal is ∼18 cm, which is nearly 25% of the slope across the Gulf Stream at this latitude. The model results suggest that the variability is largest somewhat to the east of Bermuda; fluctuations of ∼10 cm extend as far east as ∼35°W.
One surprising result is that at the longest periods in the COADS data, the wind curl has a double-peak structure in longitude. That is, there is a significant amount of power on the eastern side of the ocean as well as near Bermuda. Therefore, it is essential to use the full horizontal resolution of the wind data; using the mean curl across the Atlantic turns out not to be a good way to estimate thermocline variability. One might wonder if the wind data are reliable at these long periods were it not for the good agreement between the results and observed sea level. The power in wind variability increases out to ∼500 months, although with little statistical reliability. Sea level variability however, appears to peak at somewhat shorter periods. Although it is pushing the resolution of the data, this result is a limitation imposed by the basin width scale. The power in the model ocean's response to wind forcing is nearly an order of magnitude larger during the first half of the record (1952–69) than during the second (1970–86).
It is likely that significant changes in buoyancy forcing by the atmosphere are coherent with changes in wind. Nevertheless, these results suggest that the variability in sea level—and so in deep temperature—can perhaps be accounted for without invoking changes in stored heat of the deep ocean.
Abstract
Tidal and meteorological records at stations in the eastern Gulf of Mexico have been studied. The sea-level response is a maximum for winds along the coast and varies symmetrically with angle. The coherence is maximum at periods of 4–10 days. The horizontal coherence of sea level is high out to 500 km for 4–10 day periods. The horizontal coherence for wind (measured at coastal stations) is high out to at least 500 km. The amplitude of the response of sea level to winds is larger by a factor of 4 here, on a broad shelf, than on the Oregon coast, which is more narrow by about the same ratio. A response of ∼16 cm is induced by ∼4 m s−1 wind. This response, or transfer function, is uniform over the spectral range (4–100 days). The sea-level response to the longshore wind stress is not linear, but to the power 0.8 ± 0.1, and is attributed to the relatively low tidal currents in this region. The large horizontal coherences of wind and sea level imply broad longshore flows extending 500 km or more along the coast. Over 85% of the variance between 4 days and 3 years is contained in fluctuations with periods less than 3 months. A longshore slope of sea level is observed; in the 4–10 day band this slope can be explained by longshore variation in the width of the shelf. A mean longshore slope of ∼0.6 × 10−7 is found, and it may be caused by the (weak) mean winds. Freely propagating coastal trapped waves are found in a narrow band near 0.18 cpd.
Abstract
Tidal and meteorological records at stations in the eastern Gulf of Mexico have been studied. The sea-level response is a maximum for winds along the coast and varies symmetrically with angle. The coherence is maximum at periods of 4–10 days. The horizontal coherence of sea level is high out to 500 km for 4–10 day periods. The horizontal coherence for wind (measured at coastal stations) is high out to at least 500 km. The amplitude of the response of sea level to winds is larger by a factor of 4 here, on a broad shelf, than on the Oregon coast, which is more narrow by about the same ratio. A response of ∼16 cm is induced by ∼4 m s−1 wind. This response, or transfer function, is uniform over the spectral range (4–100 days). The sea-level response to the longshore wind stress is not linear, but to the power 0.8 ± 0.1, and is attributed to the relatively low tidal currents in this region. The large horizontal coherences of wind and sea level imply broad longshore flows extending 500 km or more along the coast. Over 85% of the variance between 4 days and 3 years is contained in fluctuations with periods less than 3 months. A longshore slope of sea level is observed; in the 4–10 day band this slope can be explained by longshore variation in the width of the shelf. A mean longshore slope of ∼0.6 × 10−7 is found, and it may be caused by the (weak) mean winds. Freely propagating coastal trapped waves are found in a narrow band near 0.18 cpd.
Abstract
Three weeks of current-meter, wind and sea-level data off Cedar Key, Florida are analyzed. Currents and sea level are found to be coherent with alongshore wind stress in the “synoptic” band (∼0.05–0.25 cycle per day) and to lag it by approximately half a day. Little coherence is found with cross-shelf wind stress.
At the inshore mooring (22 m depth) currents are nearly barotropic for these winter 1978 data. A linear parameterization of bottom stress in the barotropic alongshore current leads to a bottom friction parameter r of 0.01–0.02 cm s−1 using coastal wind stress. No significant steady alongshore slope is found during this short interval. The dominant momentum balance in the alongshore direction is between wind and bottom stress. The offshore frictional length scale (Csanady, 1978) is estimated to be 75–100 km, which implies a seaward extent to a depth of about 30 m.
At the offshore mooring (44 m depth) there is vertical shear between the currents at 9 and 39 m. The upper cross-shelf components, which is large relative to that at the inshore mooring, is consistent with Ekman transport while the lower record shows a return flow. The u, v velocity components correlate significantly at the offshore mooring and lead to an upper layer ūv̄ gradient on the order of 10−5 cm2 s−2 between the arrays (75 km separation).
The sea-level fluctuations are consistent with a geostrophic balance in the cross-shelf momentum equation with a length scale of 170 km (approximately equal to the shelf width).
Abstract
Three weeks of current-meter, wind and sea-level data off Cedar Key, Florida are analyzed. Currents and sea level are found to be coherent with alongshore wind stress in the “synoptic” band (∼0.05–0.25 cycle per day) and to lag it by approximately half a day. Little coherence is found with cross-shelf wind stress.
At the inshore mooring (22 m depth) currents are nearly barotropic for these winter 1978 data. A linear parameterization of bottom stress in the barotropic alongshore current leads to a bottom friction parameter r of 0.01–0.02 cm s−1 using coastal wind stress. No significant steady alongshore slope is found during this short interval. The dominant momentum balance in the alongshore direction is between wind and bottom stress. The offshore frictional length scale (Csanady, 1978) is estimated to be 75–100 km, which implies a seaward extent to a depth of about 30 m.
At the offshore mooring (44 m depth) there is vertical shear between the currents at 9 and 39 m. The upper cross-shelf components, which is large relative to that at the inshore mooring, is consistent with Ekman transport while the lower record shows a return flow. The u, v velocity components correlate significantly at the offshore mooring and lead to an upper layer ūv̄ gradient on the order of 10−5 cm2 s−2 between the arrays (75 km separation).
The sea-level fluctuations are consistent with a geostrophic balance in the cross-shelf momentum equation with a length scale of 170 km (approximately equal to the shelf width).
Abstract
The separation of anticyclonic rings is studied using a 12-level primitive equation numerical model of the western North Atlantic. The “Gulf Stream Formation Region” model is based on the Bryan-Cox-Semtner code, and uses ¼ degree horizontal resolution. The eastern boundary of the model, near the mid-Atlantic Ridge, is forced by a “pumps and baffles” region to have the appropriate temperature and salinity structure, vertical shear, and total transport. The model is closed by a solid northern wall at 36°N and is forced by steady winds. In the results presented here, large rings separate from the Loop Current in the Gulf of Mexico at periods near 30 weeks. The separation of a single typical ring is shown in detail. The most striking feature is that the separation is not a single spectacular event but a long, gradual process involving recirculation between the ring and the main flow for many weeks after the time at which one would, on the basis of standard observational evidence, normally believe the ring to be completely separated. There is no clear point during the separation sequence at which one can point to the horizontal velocity pattern and say “the ring has just separated.”
This is the first modeling study focusing on the Gulf of Mexico that resolves the vertical structure of the currents with more than two degrees of freedom and the first that includes the sills at the Yucatan and Florida straits in a realistic way. The model velocities are lower than those observed in the ocean, but the fundamental idea of the ring-shedding process seems realistic. These results suggest an unexpected complexity in the circulation patterns. The flow in the deeper levels of the model consists of a rich field of vortexlike and wavelike features that travel in company with the upper anticyclone. They travel to the west at a greater speed than the upper anticyclone, and they have substantial north-south motions. They fill the deep basin and interact with the bottom topography. The ring behavior is completely consistent with the observations of Lewis and Kirwan; the deep flow is in keeping with the analysis of Hamilton.
Abstract
The separation of anticyclonic rings is studied using a 12-level primitive equation numerical model of the western North Atlantic. The “Gulf Stream Formation Region” model is based on the Bryan-Cox-Semtner code, and uses ¼ degree horizontal resolution. The eastern boundary of the model, near the mid-Atlantic Ridge, is forced by a “pumps and baffles” region to have the appropriate temperature and salinity structure, vertical shear, and total transport. The model is closed by a solid northern wall at 36°N and is forced by steady winds. In the results presented here, large rings separate from the Loop Current in the Gulf of Mexico at periods near 30 weeks. The separation of a single typical ring is shown in detail. The most striking feature is that the separation is not a single spectacular event but a long, gradual process involving recirculation between the ring and the main flow for many weeks after the time at which one would, on the basis of standard observational evidence, normally believe the ring to be completely separated. There is no clear point during the separation sequence at which one can point to the horizontal velocity pattern and say “the ring has just separated.”
This is the first modeling study focusing on the Gulf of Mexico that resolves the vertical structure of the currents with more than two degrees of freedom and the first that includes the sills at the Yucatan and Florida straits in a realistic way. The model velocities are lower than those observed in the ocean, but the fundamental idea of the ring-shedding process seems realistic. These results suggest an unexpected complexity in the circulation patterns. The flow in the deeper levels of the model consists of a rich field of vortexlike and wavelike features that travel in company with the upper anticyclone. They travel to the west at a greater speed than the upper anticyclone, and they have substantial north-south motions. They fill the deep basin and interact with the bottom topography. The ring behavior is completely consistent with the observations of Lewis and Kirwan; the deep flow is in keeping with the analysis of Hamilton.
