Showing papers in "Journal of Climate in 1991"

Transient Responses of a Coupled Ocean–Atmosphere Model to Gradual Changes of Atmospheric CO2. Part I. Annual Mean Response

Abstract: This study investigates the response of a climate model to a gradual increase or decrease of atmospheric carbon dioxide The model is a general circulation model of the coupled atmosphere-ocean-land surface system with global geography and seasonal variation of insulation To offset the bias of the coupled model toward settling into an unrealistic state, the fluxes of heat and water at the ocean-atmosphere interface are adjusted by amounts that vary with season and geography but do not change from one year to the next Starting from a quasi-equilibrium climate, three numerical time integrations of the coupled model are performed with gradually increasing, constant, and gradually decreasing concentration of atmospheric carbon dioxide It is noted that the simulated response of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where vertical mixing of water penetrates very deeply However, in most of the Northern Hemisphere an

Amazonian Deforestation and Regional Climate Change

Abstract: Large-scale conversion of tropical forests into pastures or annual crops could lead to changes in the climate. We have used a coupled numerical model of the global atmosphere and biosphere (Center for Ocean-Land- Atmosphere GCM) to assess the effects of Amazonian deforestation on the regional and global climate. We found that when the Amazonian tropical forests were replaced by degraded grass (pasture) in the model, there was a significant increase in the mean surface temperature (about 2.5°C) and a decrease in the annual evapo-transpiration (30% reduction), precipitation (25% reduction), and runoff (20% reduction) in the region. The differences between the two simulations were greatest during the dry season. The deforested case was associated with larger diurnal fluctuations of surface temperature and vapor pressure deficit; such effects have been observed in existing deforested arms in Amazonia. The calculated reduction in precipitation was larger than the calculated decrease in evapotranspirat...

A Simplified Biosphere Model for Global Climate Studies

Abstract: A comprehensive analysis of the simple biosphere model (SIB) of Sellers et al. (1986) is performed in an effort to bridge the gap between the typical hydrological treatment of the land surface biosphere and the conventional general circulation model treatment, which is specified through a single parameter. Approximations are developed that stimulate the effects of reduced soil moisture more simply, maintaining the essence of the biophysical concepts utilized in SIB. Comparing the reduced parameter biosphere with those from the original formulation in a GCM and a zero-dimensional model shows the simplified version to reproduce the original results quite closely.

The Pacific/North American Teleconnection Pattern and United States Climate. Part I: Regional Temperature and Precipitation Associations

Abstract: The Pacific/North American (PNA) teleconnection index, a measure of the strength and phase of the PNA teleconnection pattern, is related to the variations of the surface climate of the United States from 1947 through 1982 for the autumn, winter, and spring months when the PNA is a main mode of Northern Hemisphere midtropospheric variability. The results demonstrate that the PNA index is highly correlated with both regional temperature and precipitation. The strongest, most extensive correlations between the index and temperature are observed in winter, but large areas of the country show important associations during the spring and autumn as well. Although the centers of highest correlation migrate systematically with changes in the circumpolar vortex over the course of the annual cycle, the southeastern and northwestern parts of the United States possess consistently high index-temperature correlations. Correlations between the PNA index and precipitation are weaker and less extensive than those...

The Use of General Circulation Models to Predict Regional Climatic Change

Abstract: Equilibrium simulations using the best-available general circulation models to estimate the sensitivity of the climate to a doubling of the atmospheric carbon dioxide concentration are in broad general agreement that the global annual average surface air temperature would increase 2.5 to 4.5 K. However, at finer spatial scales, the range of changes in temperature and precipitation predicted by different computer models is much broader. Many shortcomings are also apparent in the model simulations of the present climate, indicating that further model improvements are needed to achieve reliable regional and seasonal projections of the future climatic conditions.

The effect of snow cover on the climate

Abstract: Large-scale snow cover anomalies are thought to cause significant changes in the diabatic heating of the earth's surface in such a way as to produce substantial local cooling in the surface temperatures. This theory was tested using the GISS 3-D GCM (General Circulation Model). The results of the GCM experiment showed that snow cover caused only a short term local decrease in the surface temperature. In the surface energy budget, reduction in absorbed shortwave radiation and the increased latent heat sink of melting snow contributed to lower temperatures. However, all the remaining heating terms contribute to increasing the net heating over a snow covered surface. The results emphasize the negative feedback which limits the impact of snow cover anomalies over longer time scales.

Climate Diagnostics from Global Analyses: Conservation of Mass in ECMWF Analyses

Abstract: The extent to which mass is conserved in European Centre for Medium Range Weather Forecasts (ECMWF) analyses archived on pressure surfaces is examined from two perspectives and with two different datasets. The data used come from the WMO archive of 7-level, twice daily initialized analyses and from the WCRP archive of 14-level, four-times daily uninitialized analyses. The first perspective, which considers the equation of continuity in pressure coordinates locally in three dimensions, reveals spurious residuals in the equation of up to 100% of the size of the divergence term, with largest errors of 60% to 100% in the tropics. In this case the horizontal velocity and vertical p-velodty (ω) fields are checked for consistency. Modest improvements occur when 14 versus 7 levels are used. The second perspective considers the vertical integral in which the surface pressure tendency should balance the total mass convergence into a column, and thus does not involve the omega fields. The latter reveals tha...

The Interaction of Multiple Time Scales in the Tropical Climate System

Abstract: The spatial distribution and evolution of variability of near-global SST and SLP data in the quasi-biennial (QB) and 3–7 year low-frequency (LF) period bands are investigated and described. The largest signals in both bands are in the tropics. The near-equatorial characteristics of the QB in the SLP field are those of a quasi-progressive wave while the LF variation in the same field is closer to the standing wave. Both bands show the traditional Southern Oscillation pattern. The SST variability in both bands is essentially that of El Nino. It is shown that ENSO is partially due to a nonlinear interaction between the two frequency bands. Both bands appear important to the ENSO cycle. The current work could not establish conclusively that if either was the fundamental mode, although there is weak evidence favoring the QB mode. The QB signal described here is essentially the ENSO signal and does not seem to be simply related to the stratospheric QBO. Calculations suggest the tropospheric QB describe...

Nonlinear Influence of Mesoscale Land Use on Weather and Climate

Abstract: It is shown that the influence of mesoscale landscape spatial variability on the atmosphere must be parameterized or explicitly modeled in larger-scale atmospheric model simulations including general circulation models. The mesoscale fluxes of heat that result from this variability are shown to be of the same order of magnitude but with a different vertical structure than found for the turbulent fluxes. These conclusions are based on experiments in which no phase changes of water were permitted. To parameterize surface thermal inhomogeneities, the influence of landscape must be evaluated using spectral analysis or an equivalent procedure. To include the nonlinear contribution of each scale, numerical model simulations for the range of observed surface and overlying atmospheric conditions must be performed.

A further extension of the Tahiti-Darwin SOI, early ENSO events and Darwin pressure

Abstract: An extension of the Tahiti minus Darwin Southern Oscillation Index (SOI) from 1882 back to 1876 is reported following the recovery of early Darwin mean sea-level pressure data spanning the period 1865–81. As a result, we are able to compare, for the first time, the major 1877–78 and 1982–83 ENSO events on the basis of this commonly used index. Early Darwin and Jakarta data are also examined in terms of a measure of the Australian response to documented El Nino and/or ENSO events in 1866, 1868, 1871, 1873, 1874 and 1875. The SOI during the 1877–78 ENSO event has a similar temporal response to that in 1982–83, but the index is slightly weaker than in the recent event. Examination of documentary evidence confirms the severity of the drought conditions that affected the Australian continent during the 1877–78 ENSO, and shows that this response is in line with the wider Indo-Pacific impacts reported in the literature. Earlier El Nino phases in 1868 and 1873 are not resolved distinctly in either the Da...

Predictability of a coupled ocean-atmosphere model

Abstract: A study is presented to determine the limits on the predictability of the coupled ocean-atmosphere system. Following the classical methods developed for atmospheric predictability studies, the model used is one of the simplest that realistically reproduces many of the important features of the observed interannual variability of sea surface temperature in the tropical Pacific Ocean when forced by observed wind stresses. As no reasonable analysis is available for all the fields, initial conditions for these prediction experiments were taken from a model control run in which the ocean model was forced by the observed surface winds. The atmospheric component of the coupled model is not capable of accurately simulating the large-scale components of the observed wind stress.

Continental-Scale Simulation of the Antarctic Katabatic Wind Regime

Abstract: Katabatic winds are a common feature of the lower Antarctic atmosphere. Although these drainage flows are quite shallow, there is increasing evidence that the low-level circulations are an important component in establishing large-scale tropospheric motions in the high southern latitudes. Three-dimensional numerical simulations of the Antarctic katabatic wind regime and attendant tropospheric circulations have been conducted over the entire continent to depict the topographically forced drainage patterns in the near-surface layer of the atmosphere. Results of the simulation enable a mapping of katabatic wind potential and identification of coastal regions which may experience anomalously intense katabatic winds. A large upper-level cyclonic circulation forms rapidly in response to the evolving katabatic wind structure in the lower atmosphere, suggesting that the drainage circulations are an important component in prescribing the resulting resulting circumpolar vortex. These results imply that som...

Sensitivity of Simulated Climate to Model Resolution

Abstract: Dynamical measures of the climate (e.g., winds, eddy fluxes) simulated by a general circulation model are compared at different horizontal and vertical resolutions for the December, January, and February period. The simulations of the troposphere are found to improve significantly as the horizontal resolution increases in the range of spectral truncations from T21 to T63. Little sensitivity is found to changes in vertical resolution between about 2.8 km and 0.7 km vertical grid spacing. The improvements in the Southern Hemisphere troposphere are greater than in the Northern Hemisphere as the horizontal resolution increases. The eddy momentum fluxes and kinetic energies in both hemispheres increase monotonically with horizontal resolution. At T63, the Southern Hemisphere winds, eddy fluxes, and eddy kinetic energies agree favorably with observations, while serious discrepancies are present at lower resolutions. In the Northern Hemisphere, the eddy momentum flux at T63 is slightly larger than obser...

Soil Moisture: Empirical Data and Model Results.

Abstract: A unique dataset of soil moisture in the upper 1-m soil layer at sites with natural plant cover in the Soviet Union is compared to simulations of soil moisture for the present climate by the Geophysical Fluid Dynamics Laboratory, Oregon State University, and United Kingdom Meteorological Office general circulation models. It is found that the present-day soil moisture regime is not well simulated by these models. Delworth and Manabe's hypothesis that the spectrum of time variations in soil moisture in the upper 1-m layer corresponds to a first-order Markov process with a decay time of the correlation function equal to the ratio of field capacity to potential evapotranspiration is empirically confirmed with this dataset. Analysis of measurement data over the 1972–1985 period reveals that the long-term trends of soil moisture north of 50°N are mainly due to increasing precipitation during this period of the same scale (1–3 cm/10 yr). The seasonal structure does not correspond to the “summer contine...

Spring and Summer 1988 Drought over the Contiguous United States—Causes and Prediction

Abstract: This paper deals primarily with the 1988 summer drought over much of the contiguous United States and its generation from conditions during the preceding spring. Both the spring and summer environment are described in terms of hemispheric flow patterns in midtroposphere, temperature and precipitation anomalies, and sea surface temperature anomalies. Conditions in March were especially indicative of the ensuing drought, since a model routinely employed in long-range forecasting showed that the March circulation would most likely be followed by a hot dry April, May, and June over much of the nation—a pattern which persisted into early summer. This result suggests that the initiation of the drought was rooted in extratropical climate variations, an alternative hypothesis to one which attributes the persistent drought-producing circulation to oceanic and atmospheric conditions in the tropics. In many respects the summer drought of 1988 was similar to earlier great droughts of the Great Plains, althou...

Predictability of a Coupled Ocean–Atmosphere Model

Abstract: This paper presents an analysis of a coupled ocean-atmosphere model used to study ENSO (El Nino–Southern Oscillation). Our interest here is in the growth of initial error: that is, the predictability of ENSO. The analysis proceeds by constructing a linear model that optimally fits the behavior of the original nonlinear coupled model. By construction, this approximate linear model has only a few degrees of freedom. Because the linear model is so much smaller than the original, it is possible to understand it in much finer detail, indirectly offering insight into the properties and behavior of the original model. As it turns out, even linear models with only a few degrees of freedom can have rather elaborate and surprising short-term error behavior. It has been shown that if a system is not self-adjoint that there is a possibiliiy of error growth in a mode completely unrelated to the classic nation of a fastest growing linearly unstable mode. This holds for simple linear models as well. The work he...

Evidence of Secular Variations in Indian Monsoon Rainfall–Circulation Relationships

Abstract: Detailed correlation analysis of the all-India monsoon rainfall and mean sea-level seasonal pressure at Bombay (19°N, 73°E) up to three lags on either side of the monsoon wren during the last 30 years (1951–80) indicates a systematic relationship. The winter-to-premonsoon (March, April, May–Deceinber, January, February; MAM–DJF) seasonal pressure tendency at Bombay shows a correlation coefficient (CC) of −0.70 (significant at 0.1% level) with the Indian monsoon rainfall. Further examination of this relationship over a long period of 144 years (1847–1990), using sliding correlation analysis, reveals some interesting features. The sliding CCs were positive before 1870, negative during 1871–1900, positive in the years 1901–40, and again negative later on, showing systematic turning points around the years 1870, 1900, and 1940. In light of other corroborative evidence, these climatic regimes can be identified as “meridional monsoon” periods during 1871–1900 and after 1940, and as “zonal monsoon” peri...

On ENSO Physics

Abstract: Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period. The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation. The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient. The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.

Infrared radiation parameterizations in numerical climate models

Abstract: This study presents various approaches to parameterizing the broadband transmission functions for utilization in numerical climate models. One-parameter scaling is applied to approximate a nonhomogeneous path with an equivalent homogeneous path, and the diffuse transmittances are either interpolated from precomputed tables or fit by analytical functions. Two-parameter scaling is applied to parameterizing the carbon dioxide and ozone transmission functions in both the lower and middle atmosphere. Parameterizations are given for the nitrous oxide and methane diffuse transmission functions.

A Model of Sea Level Rise Caused by Ocean Thermal Expansion

Abstract: Warming of the atmosphere as a result of an increased concentration of greenhouse gases is expected to lead to a significant rise is global sea level. We present estimates of the component of this sea level rise caused by thermal expansion of the ocean. These estimates are based on the idea that the upper layers of the main gyres of the ocean are ventilated by the subduction of water at higher latitudes and its subsequent equatorward and downward flow into the main thermocline along surfaces of constant “density”. In this mechanism, heat enters the ocean by an advection process rather than by vertical diffusion, as in previous estimates of the component of sea level rise that is caused by thermal expansion. After the heat initially enters the subtropical gyres by subduction, it is then redistributed to preserve gradients of the depth-integrated pressure field, by an adjustment involving low vertical-mode baroclinic waves. Estimates of historical sea level rise based on this simple ventilation sch...

Space and Time Scales of Global Tropospheric Moisture

Abstract: Radiosonde data from a global 118-station network are used to determine the spatial and temporal scales of variability of tropospheric water vapor. Various sources of possible error and bias in the data are analyzed. Changes in instrumentation at U.S. stations are shown to have a considerable influence on the record; information on comparable changes in other countries is not readily available. Mean monthly data are shown to be acceptable at tropical nations but not at high-latitude stations, where the nonlinear dependence of saturation vapor pressure on temperature, coupled with large temperature ranges, leads to biases of up to 10% in mean monthly specific humidity. A series of three empirical orthogonal function analyses (for the tropics, North America, and the globe) of specific humidity at the surface, 850-mb, 700-mb, and 500-mb levels is presented. All three show evidence of a shift in the specific humidity field in the winter of 1976/77, with generally lower values from the beginning of th...

Estimating probabilities of hurricane wind speeds using a large-scale empirical model

Abstract: A new method is presented for estimating the probability of exceeding a given wind speed in 1 year at any given location in the Atlantic tropical cyclone basin. The method is especially appropriate for wind speeds with return periods of 100 years or more, for which on-the-spot data are inadequate. The relative intensity of a tropical cyclone at any point in time is the actual central pressure drop divided by the greatest possible central pressure drop that mean seasonal climatic conditions allow. The empirical distribution of relative intensity as a function of time is derived from all Atlantic cyclone data. By combining this with information about the distribution of time from cyclone inception to closest approach to the site, and other steps, probabilities for various wind speeds at the site are obtained. Unlike other methods, this one does not attempt to fit data to any extreme value distribution, and it performs all integrations by discretizing and summing explicitly, never by simulation. The...

A Reexamination of the Mechanism of the Semiannual Oscillation in the Southern Hemisphere

Abstract: The local pressure changes associated with the twice-annual contraction/intensification and expansion/weakening of the circumpolar trough of low pressure around Antarctica, termed the semiannual oscillation (SAO), was the dominant signal in the annual cycle at mid and high southern latitudes before 1979 The mechanism, as shown by Van Loon (1967), arises from diVerent response to the surface heat budget over the polar continent and the midlatitude ocean It has subsequently been shown that in most years since 1979 the SAO has weakened considerably Evidence is presented here from surface temperature data, 500 mb temperatures from a station pair and zonal mean 500 mb temperatures from the NCAR/NCEP reanalyses to show that a warming trend since 1979 has not been evenly distributed through the year at each latitude Thus an anomalous change in the temperature gradient between 50°S and 65°S, with peaks in roughly May and November, has modulated the mechanism that produces the SAO, with its peaks in March and September Consequently, the magnitude of the SAO has decreased in the more recent period

Secular trend of surface temperature at an elevated observatory in the Pyrenees

Abstract: Surface temperature was measured at the Pic du Midi de Bigorre, 2862 m MSL, from the foundation of the Observatory in 1878 until the closing of the meteorological station in 1984. After testing the homogeneity of the series with the annual mean temperatures in western Europe and in southwestern France, the period 1882–1970 was retained for trend analysis. The mean annual temperature increased 0.83°C during the 89-yr period. This increase is the sum of a very significant increase in the daily minimum temperature (+ 2.11°C) and a decrease in the maximum temperature (− 0.45°C). In consequence, the most dramatic change in the temperature regime was the difference between maximum and minimum; this decreased from 8.05°C in 1882 to 5.49°C in 1970. A mean increase is observed in all seasons, but, as for western Europe, it is stronger in spring and fall than in winter and summer. Analysis of cloudiness data for the same period shows a 15% increase in annual mean cloudiness and also significant year-to-yea...

Springtime Soil Moisture, Natural Climatic Variability, and North American Drought as Simulated by the NCAR Community Climate Model 1

Abstract: Previous results concerning the role that summertime soil moisture reductions can play in amplifying or maintaining North American droughts are extended to include the role of springtime soil moisture reductions and the role that natural climatic variability, as expressed in soil moisture, can play. General circulation model (GCM) simulations with the NCAR Community Climate Model have been made with initial desert-like soil moisture anomalies imposed on 1 May and on 1 March. The May simulation maintained the imposed anomaly throughout the summer, while in the March simulation the anomaly was ameliorated within one month. Thus, the timing of soil moisture reductions may be crucial. A 10-year model control integration with prescribed sea surface temperatures yielded 1 year with late spring and summer soil moisture values similar to those of the 1 May anomaly simulation. This suggests that occasional widespread North American droughts may be an inherent feature of at least the GCM employed for this study. The results also demonstrate the important role played by moisture transport from the Gulf of Mexico in modulating or ameliorating drought conditions for much of the south-central United States, a topic that requires considerable further investigation.

Fossil Fuel and Biomass Burning Effect on Climate—Heating or Cooling?

Abstract: The basic theory of the effect of pollution on cloud microphysics and its global implications is applied to compare the relative effect of a small increase in the consumption rate of oil, coal, or biomass burning on cooling and heating of the atmosphere. The characteristics of and evidence for the SO2 induced cooling effect are reviewed. This perturbation analysis approach permits linearization, therefore simplifying the analysis and reducing the number of uncertain parameters. For biomass burning the analysis is restricted to burning associated with deforestation. Predictions of the effect of an increase in oil or coal burning show that within the present conditions the cooling effect from oil and coal burning may range from 0.4 to 8 times the heating effect.

Low-frequency changes in El Niño-Southern oscillation

Abstract: Although there are indications from numerical models that El Nino-Southern Oscillation (ENSO) may be an internal mode of the coupled Pacific ocean-atmosphere system, sensitive to climatic background parameters, it has not yet been possible to find significant changes in ENSO variability between the Little Ice Age and the present. Yet a number of authors have found qualitative indications in anecdotal and proxy records of shorter, century-scale variations in the return-interval statistics for El Nino episodes. To objectively determine what nonstationarities exist, we statistically examine the El Nino occurrences since 1525, compiled by Quinn et al. We have stratified the return intervals both for strong events and for all events according to two null hypotheses. 1) return intervals are stationary over periods of 200–500 years, and 2) the intervals are stationary on a centenary time scale, between epochs of contrasting solar variability. Two-parameter Weibull distributions are fit to subsamples of ...

Florida Citrus Freezes and Polar Anticyclones in the Great Plains

Abstract: Severe Florida citrus freezes since 1880 are identified and described in terms of the horticultural damage, overall frequency of occurrence, and association with polar anticyclone outbreaks in the plains of southern Canada and the United States. The most severe “advective” freezes are associated with strong cold anticyclones having tracks southward across the plains to Texas with subsequent northeastward movement. Other anticyclones move in a track somewhat east of this and ultimately pass over Florida or the eastern Gulf of Mexico. Over 80% of the worst Florida citrus freezes are associated with anticyclones with central pressures in excess of 1045 mb moving along these paths. However, anticyclones of similar intensity with more zonally oriented paths across higher latitudes are associated with minor citrus damage. The major freezes tend to be clustered in time in the 1890s and since 1977. On interdecadal time scales, the recent freezes are not linked to higher winter mean pressure in the northe...

Humidity profiles over the ocean

Abstract: The variabilities of atmospheric humidity profile over oceans from daily to interannual time scales were examined using 9 years of daily and semidaily radiosonde soundings at island stations extending from the Arctic to the South Pacific. The relative humidity profiles were found to have considerable temporal and geographic variabilities, contrary to the prevalent assumption. Principal component analysis on the profiles of specific humidity were used to examine the applicability of a relation between the surface-level humidity and the integrated water vapor; this relation has been used to estimate large-scale evaporation from satellite data. The first principal component was found to correlate almost perfectly with the integrated water vapor. The fractional variance represented by this mode increases with increasing period. It reaches approximately 90 percent at two weeks and decreases sharply, below one week, down to approximately 60 percent at the daily period. At low frequencies, the integrated water vapor appeared to be an adequate estimator of the humidity profile and the surface-level humidity. At periods shorter than a week, more than one independent estimator is needed.

The Annual Cycle of Satellite-derived Sea Surface Temperature in the Southwestern Atlantic Ocean

Abstract: The annual cycle of sea surface temperature (SST) in the southwestern Atlantic Ocean was estimated using four years (July 1984-July 1988) of NOAA Advanced Very High Resolution Radiometer observations. High resolution satellite observations at 1-km space and daily time resolution were grided at 100-km space and 5-day time intervals to develop an analysis dataset for determination of low frequency SST variability. The integral time scale, a measure of serial correlation, was found to vary from 40 to 60 days in the domain of interest. The existence of superannual trends in the SST data was investigated, but conclusive results could not be obtained. The annual cycle (and, in particular, the annual harmonic) explains a large proportion of the SST variability. The estimated amplitude of the cycle ranges between 5 deg and 13 deg C throughout the study area, with minima in August-September and maxima in February. The resultant climatology is compared with an arbitrary 5-day satellite SST field, and with the COADS/ICE SST climatology. It was found that the higher resolution satellite-based SST climatology resolves boundary current structure and has significantly better structural agreement with the observed field.