Dennis A. Mayer

Tropical Atlantic sea surface temperature variability and its relation to El Niño-Southern Oscillation

Past analyses of tropical Atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics, across the Intertropical Convergence Zone (ITCZ). By analyzing an improved 43-year (1950-1992) record of SST (Smith et al., 1996) and other data derived from the Comprehensive Ocean-Atmosphere Data Set (COADS), it is shown that the regions north and south of the ITCZ are statistically independent of each other at the seasonal to interannual timescales dominating the data, confirming the conclusions of Houghton and Tourre (1992). Some dipole behavior does develop weakly during the boreal spring season, when there is a tendency for SST anomaly west of Angola to be opposite of that in the tropical North Atlantic. It is further shown that tropical Atlantic SST variability is corre- lated with Pacific E1 Nifio-Southern Oscillation (ENSO) variability in several regions. The ma- jor region affected is the North Atlantic area of NE trades west of 40oW along 10oN - 20oN and extending into the Caribbean. There, about 50-80% of the anomalous SST variability is associ- ated with the Pacific ENSO, with Atlantic warmings occurring 4-5 months after the mature phases of Pacific warm events. An analysis of local surface flux fields derived from COADS data shows that the ENSO-related Atlantic warmings occur as a result of reductions in the surface NE trade wind speeds, which in turn reduce latent and sensible heat losses over the region in ques- tion, as well as cooling due to entrainment. This ENSO connection is best developed during the boreal spring following the most frequent season of maximum ENSO anomalies in the Pacific. A region of secondary covariability with ENSO occurs along the northern edge of the mean ITCZ position and appears to be associated with northward migrations of the ITCZ when the North At- lantic warmings occur. Although easterly winds are intensified in the western equatorial Atlantic in response to Pacific warm events, they do not produce strong local changes in SST. Contrary to expectations from studies based on equatorial dynamics, these teleconnected wind anomalies do not give rise to significant correlations of SST in the Gulf of Guinea with the Pacific ENSO. As the teleconnection sequence matures, strong SE trades at low southern latitudes follow the de- velopment of the North Atlantic SST anomaly and precede by several months the appearance of weak negative SST anomalies off Angola and stronger positive anomalies extending eastward from southern Brazil along 15o-30oS.

How ubiquitous is the dipole relationship in tropical Atlantic sea surface temperatures

Several kinds of analysis are applied to sea surface temperature anomalies (SSTA) (1856–1991) to determine the degree to which SSTA of opposite sign in the tropical North and South Atlantic occur. Antisymmetric (“dipole”) configurations of SSTA on basin scales are not ubiquitous in the tropical Atlantic. Unless the data are stratified by both season and frequency, inherent dipole behavior cannot be demonstrated. Upon removing the global El Nino-Southern Oscillation signal in SSTA (which is symmetric between the North and South Atlantic) from the data, the regions north or south of the Intertropical Convergence Zone have qualitatively different temporal variabilities and are poorly correlated. Dipole configurations do occur infrequently (12–15% of the time), but no more so than expected by chance for stochastically independent variables. Nondipole configurations that imply significant meridional SSTA gradients occur much more frequently, nearly half of the time. Cross-spectral analysis of seasonally averaged SSTA indices for the North and South Atlantic show marginally significant coherence with antisymmetric phase in two period bands: 8–12 years for the boreal winter-spring and 2.3 years for the boreal summer-fall. Antisymmetric coherence is optimal for a small subregion west of Angola in the South Atlantic, with respect to SSTA of basin scale in the tropical North Atlantic. Dipole variability, even where optimal, explains only a small fraction of the total variance in tropical Atlantic SSTA (<7%).

Multiyear variability in the near‐surface temperature structure of the midlatitude western North Atlantic Ocean

Between 1966 and 1995, subsurface temperature data have been collected in the western North Atlantic Ocean using expendable bathythermographs. Data coverage is sparse in both time and space, but evidence for decadal variability in the upper 400 m of the water column is found. The data were averaged by month onto a 2° of latitude by 4° of longitude grid. Thirty-one quadrangles in the region bounded by 17°N and 43°N and 78°W and 66°W have sufficient data to provide consistent results. Anomaly time series at 0, 100, 200, 300, and 400 m were estimated by subtracting a mean monthly climatology. The individual records were detrended and filtered to highlight the longer-period signals. The analysis resulted in 25-year records (1969–1993) for study. Within the thermocline of the subtropical gyre and the Gulf Stream at 100 and 200 m, periods of predominately positive temperature anomaly end in 1971, 1982, and 1990, while periods of negative anomaly end in 1976 and 1985. Only the events ending in 1971, 1976, and 1990 are in the majority of the records at 300 and 400 m. Most of the events also appear in the sea surface temperature (SST) records but are somewhat masked by significant noise at the surface. Meridional-vertical temperature sections through the subtropical gyre show that transitions from negative to positive anomaly events are characterized by a deepening of the isotherms throughout the section and transitions from positive to negative events by a rising of the isotherms. Significant lateral migration of the axis of the Gulf Stream, although possibly masked by the 2° averaging, is not necessary to explain either type of event. The transitions in the SST and 100-m temperature time series occur at essentially the same time as the transitions in an index of the North Atlantic Oscillation (NAO) that has also been detrended (i.e., 1971, 1976, 1980, 1984, 1988). The 1971, 1976, and 1988 NAO events are also observed at 300 and 400 m as described earlier. Periods of positive subsurface temperature anomaly are coincidental with periods of positive NAO index, and periods of negative subsurface temperature anomaly are coincidental with periods of negative NAO index. Thus earlier results showing connections between the NAO and western Atlantic SST at decadal timescales are now extended to at least 400 m in the water column. Trends were computed from the individual 25-year records. The trends at all depths are predominately negative north of 38°N and positive south of 38°N. Inferences from the horizontal distribution of the trends and results from earlier studies suggest that the 1969–1993 period may be a phase of a 30- to 50-year signal observed in the northern Atlantic since the beginning of the century.

Subtropical Atlantic Climate Studies: Introduction

This report is an introduction to the accompanying collection of reports that present the results of a 2-year period of intensive monitoring of the Florida Current. Both direct observing systems (ship-deployed current profilers and moored current meters) and indirect observing systems (coastal tide gauge stations, bottom pressure gauge arrays, a submarine cable, acoustic arrays, and radar installations) were used to measure temperature and volume transport.

West Florida Shelf mean circulation observed with long‐term moorings

[1]xa0The mean circulation on the West Florida Continental Shelf is described using long-term current measurements. Bounded by the Florida peninsula to the east and the Gulf of Mexico to the west, the West Florida Continental Shelf mean flow is oriented approximately along-isobath and southward. The mean velocity vectors veer systematically with depth, shoreward over shallow water and seaward over deeper water. This polarization change implies that the mean flow is upwelling over shallow water and downwelling seaward from the inner shelf. Such a well-organized, three-dimensional coastal ocean circulation pattern, revealed by an unprecedented set of observations, and explained on the basis of wind forcing and density field adjustment, has important implications for both fisheries and red tide occurrences.

Transition regions and their role in the relationship between sea surface height and subsurface temperature structure in the Atlantic Ocean

Expendable bathythermograph (XBT) profiles and TOPEX/Poseidon altimeter data (T/P) are compared for the years 1993 through 1997 to determine how much can be understood about water column variability from XBTs given only sea height anomalies (SHA) from T/P. Our focus is on the annual cycle along two well sampled XBT sections in the Atlantic Ocean from 10°S to 40°N. Regions of transition are identified that separate the mid-latitudes where surface buoyancy fluxes dominate the forcing of sea level, from those in the equatorial region where thermocline effects dominate. Zones of transition occur in the vicinity of troughs where small fluctuations in SHA belie the true nature of water column variability. Here, surface and thermocline variability tend to cancel each other. Thus, the character of SHA in transition regions emphasizes how important direct observations can be in interpreting satellite altimetric observations correctly when both surface and thermocline variability are important but are compensating in nature.

Sea Level Variation as an Indicator of Florida Current Volume Transport: Comparisons with Direct Measurements

Sea level measurements from tide gauges at Miami, Florida, and Cat Cay, Bahamas, and bottom pressure measurements from a water depth of 50 meters off Jupiter, Florida, and a water depth of 10 meters off Memory Rock, Bahamas, were correlated with 81 concurrent direct volume transport observations in the Straits of Florida. Daily-averaged sea level from either gauge on the Bahamian side of the Straits was poorly correlated with transport. Bottom pressure off Jupiter had a linear coefficient of determination ofr2 = 0.93, and Miami sea level, when adjusted for weather effects, had r2 = 0.74; the standard errors of estimating transports were � 1.2 x 106 and � 1.9 x 106 cubic meters per second, respectively. A linear multivariate regression, which combined bottom pressure, weather, and the submarine cable observations between Jupiter and the Bahamas, had r2 = 0.94 with a standard error of estimating transport of � 1.1 x 106 cubic meters per second. These results suggest that a combination of easily obtained observations is sufficient to adequatelv monitor the daily volume transport fluctuations of the Florida Current.

Near-Inertial Internal Waves Observed on the Outer Shelf in the Middle Atlantic Bight in the Wake of Hurricane Belle

Abstract On 10 August 1976 Hurricane Belle passed rapidly over the highly stratified shelf of the New York Bight. Records from Aanderaa current-meter moorings show that the response to the hurricane depended strongly on bathymetry. At deeper stations (∼70 m depth), intense, first-mode, internal near-inertial oscillations were generated at frequencies ∼1% less than the local inertial frequency. At shallower stations (∼50 m depth), only weak, heavily damped second-mode oscillations were observed in the current records, with no corresponding inertial signals in temperature. In the Hudson Shelf Valley, inertial motion occurred only near the surface. This was probably due to topographic effects. The divergence and curl of the wind stress contributed equally to the forcing. The response at the deeper stations is consistent with Geislers (1970) theory for the open ocean in which a hurricane leaves a wake of internal-inertial oscillations if it travels faster than the internal phase speed and if its horizontal s...

An Analysis of Subtidal Current Fluctuations in the Middle Atlantic Bight

Abstract Subtidal current fluctuations in the Middle Atlantic Bight are examined from current-meter data collected in 1975 and 1976. Spectral analysis provides evidence for both locally wind-forced response and free waves that propagate downshelf3 which are not correlated with the local wind. A simple empirical model has been constructed to fit two linearly independent plane waves to the observed current spectra. Application of the model to the current data obtained at a pair of stations in the New York Bight during the period of 26 October 1975 to 4 April 1976 indicates that the two waves propagate in opposite directions along the coast, and with the additional evidence from rotary-coefficient calculations, it is suggested that they correspond to the forced and free waves speculated upon earlier. The noise level is a free parameter in the model and is determined by adjusting the phase speed of the forced wave to the translation speed of the observed wind field. This gives a 526 km day−1 phase speed for t...

Current Meter Observations on the Continental Slope at Two Sites in the Eastern Gulf of Mexico

Abstract Current-meter observations obtained at two sites on the continental slope of the eastern Gulf of Mexico, at nominal positions of 29°N, 88°W (the Mobile site) and 27.5°N, 85.5°W (the Tampa site) are presented. Data were collected at three levels at Mobile (90,190 and 980 m) from July 1977 through August 1978 and at four levels at Tampa (150, 250, 550 and 950 m) from June 1978 through June 1979. At 90 and 190 m, the flow at Mobile was on the average to the east. Sustained periods of flow to the west were observed during the summer 1977 and spring 1978. During the periods of eastward flow, the wind was generally out of the north and during the periods of westward flow, the wind was out of the east. The flow at the top meter at Tampa was on the average to the west, in the same direction as the average wind. At both sites, the motions are perturbed by events associated with the Loop Current. These events make it difficult to define any seasonal variability in the upper layers. The flow at the bottom m...