Atlantic Ocean Sea Surface Temperature Research Articles

Long Lead-Time Forecasting of U.S. Streamflow Using Partial Least Squares Regression

Pacific and Atlantic Ocean sea surface temperatures (SSTs) were used as predictors in a long lead-time streamflow forecast model in which the partial least squares regression (PLSR) technique was used with over 600 unimpaired streamflow stations in the continental United States. Initially, PLSR calibration (or test) models were developed for each station, using the previous spring-summer Pacific (or Atlantic) Ocean SSTs as predictors. Regions were identified in the Pacific Northwest, Upper Colorado River Basin, Midwest, and Atlantic states in which Pacific Ocean SSTs resulted in skillful forecasts. Atlantic Ocean SSTs resulted in significant regions being identified in the Pacific Northwest, Midwest, and Atlantic states. Next, streamflow stations were selected in the Columbia River Basin, Upper Colorado River Basin, and Mississippi River Basin and a PLSR cross-validation model (i.e., forecast) was developed. The results of the PLSR cross-validation model for each station varied with linear error in probab...

Impact of Desert Dust Radiative Forcing on Sahel Precipitation: Relative Importance of Dust Compared to Sea Surface Temperature Variations, Vegetation Changes, and Greenhouse Gas Warming

Abstract The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model. The model simulations are conducted either forced by the observed sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant precipitation changes are restricted to over North Africa. These changes are considered to be due to the cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase of desert dust when allowing dust to respond to changes in simulated climate, even including changes in vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the direct radiative forcing by the increase of North African dust can explain up to 30% of the observed precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact. The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation.

The middle Pleistocene transition: characteristics, mechanisms, and implications for long-term changes in atmospheric pCO 2

The emergence of low-frequency, high-amplitude, quasi-periodic (∼100-kyr) glacial variability during the middle Pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system. This middle Pleistocene transition (MPT) began 1250 ka and was complete by 700 ka. Its onset was accompanied by decreases in sea surface temperatures (SSTs) in the North Atlantic and tropical-ocean upwelling regions and by an increase in African and Asian aridity and monsoonal intensity. During the MPT, long-term average ice volume gradually increased by ∼50 m sea-level equivalent, whereas low-frequency ice-volume variability experienced a 100-kyr lull centered on 1000 ka followed by its reappearance ∼900 ka, although as a broad band of power rather than a narrow, persistent 100-kyr cycle. Additional changes at 900 ka indicate this to be an important time during the MPT, beginning with an 80-kyr event of extreme SST cooling followed by the partial recovery and subsequent stabilization of long-term North Atlantic and tropical ocean SSTs, increasing Southern Ocean SST variability primarily associated with warmer interglacials, the loss of permanent subpolar sea-ice cover, and the emergence of low-frequency variability in Pacific SSTs and global deep-ocean circulation. Since 900 ka, ice sheets have been the only component of the climate system to exhibit consistent low-frequency variability. With the exception of a near-universal organization of low-frequency power associated with marine isotope stages 11 and 12, all other components show an inconsistent distribution of power in frequency-time space, suggesting a highly nonlinear system response to orbital and ice-sheet forcing. Most hypotheses for the origin of the MPT invoke a response to a long-term cooling, possibly induced by decreasing atmospheric pCO 2. None of these hypotheses, however, accounts for the geological constraint that the earliest Northern Hemisphere ice sheets covered a similar or larger area than those that followed the MPT. Given that the MPT was associated with an increase in ice volume, this constraint requires that post-MPT ice sheets were substantially thicker than pre-MPT ice sheets, indicating a change in subglacial conditions that influence ice dynamics. We review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline Precambrian Shield bedrock became exposed by glacial erosion of a thick mantle of regolith. This exposure of a high-friction substrate caused thicker ice sheets, with an attendant change in their response to the orbital forcing. Marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the MPT. If this carbon came from terrigenous sources, an increase in atmospheric pCO 2 would be likely, which is inconsistent with evidence for widespread cooling, Apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith. A more likely source of organic carbon and nutrients (which would mitigate pCO 2 rise) is from shelf and upper slope marine sediments, which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the MPT. Modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long-term silicate weathering rates, in good agreement with marine Sr and Os isotopic records. We use a carbon cycle model to show that a post-MPT increase in silicate weathering rates would lower atmospheric pCO 2 by 7–12 ppm, suggesting that the attendant cooling may have been an important feedback in causing the MPT.

Relationships between Pacific and Atlantic ocean sea surface temperatures and U.S. streamflow variability

An evaluation of Pacific and Atlantic Ocean sea surface temperatures (SSTs) and continental U.S. streamflow was performed to identify coupled regions of SST and continental U.S. streamflow variability. Both SSTs and streamflow displayed temporal variability when applying the singular value decomposition (SVD) statistical method. Initially, an extended temporal evaluation was performed using the entire period of record (i.e., all years from 1951 to 2002). This was followed by an interdecadal‐temporal evaluation for the Pacific (Atlantic) Ocean based on the phase of the Pacific Decadal Oscillation (PDO) (Atlantic Multidecadal Oscillation (AMO)). Finally, an extended temporal evaluation was performed using detrended SST and streamflow data. A lead time approach was assessed in which the previous year's spring‐summer season Pacific Ocean (Atlantic Ocean) SSTs were evaluated with the current water year continental U.S. streamflow. During the cold phase of the PDO, Pacific Ocean SSTs influenced streamflow regions (southeast, northwest, southwest, and northeast United States) most often associated with El Niño–Southern Oscillation (ENSO), while during the warm phase of the PDO, Pacific Ocean SSTs influenced non‐ENSO streamflow regions (Upper Colorado River basin and middle Atlantic United States). ENSO and the PDO were identified by the Pacific Ocean SST SVD first temporal expansion series as climatic influences for the PDO cold phase, PDO warm phase, and the all years analysis. Additionally, the phase of the AMO resulted in continental U.S. streamflow variability when evaluating Atlantic Ocean SSTs. During the cold phase of the AMO, Atlantic Ocean SSTs influenced middle Atlantic and central U.S. streamflow, while during the warm phase of the AMO, Atlantic Ocean SSTs influenced upper Mississippi River basin, peninsular Florida, and northwest U.S. streamflow. The AMO signal was identified in the Atlantic Ocean SST SVD first temporal expansion series. Applying SVD, first temporal expansions series were developed for Pacific and Atlantic Ocean SSTs and continental U.S. streamflow. The first temporal expansion series of SSTs and streamflow were strongly correlated, which could result in improved streamflow predictability.

A multimodel study of the twentieth‐century simulations of Sahel drought from the 1970s to 1990s

In this paper, we evaluate the performance of 19 coupled general circulation models (CGCMs) in twentieth‐century simulations of the Sahel during the 1970s to 1990s. Correlation, regression, and cluster analyses are applied to observations and model outputs including Sahel monthly precipitation, evaporation, soil moisture, and sea surface temperature (SST). We find that only eight CGCMs (hit models) produce a reasonable Sahel drought signal, while seven CGCMs (miss models) produce excessive rainfall over the Sahel during the observed drought period. Even the model with the highest prediction skill of the Sahel drought could only predict the increasing trend of severe drought events but not the beginning and duration of the events. From analyses of the statistical characteristics of the hit and miss models, we conclude that a good simulation of the Sahel drought requires (1) a strong coupling between Sahel rainfall and Indian Ocean SST, with warm (cold) SST identified with Sahel drought (flood), (2) a significant coupling between Sahel rainfall and the Atlantic Ocean SST, with a warm equatorial Atlantic and cold extratropical North Atlantic coexisting with Sahel drought, and vice versa, and (3) a robust land surface feedback with strong sensitivity of precipitation and land evaporation to soil moisture. These three characteristics constitute sufficient conditions for a good simulation of Sahel drought in CGCMs.

Multidecadal modulation of El Niño–Southern Oscillation (ENSO) variance by Atlantic Ocean sea surface temperatures

Observations suggest a possible link between the Atlantic Multidecadal Oscillation (AMO) and El Niño–Southern Oscillation (ENSO) variability, with the warm AMO phase being related to weaker ENSO variability. A coupled ocean‐atmosphere model is used to investigate this relationship and to elucidate mechanisms responsible for it. Anomalous sea surface temperatures (SSTs) associated with the positive AMO lead to change in the basic state in the tropical Pacific Ocean. This basic state change is associated with a deepened thermocline and reduced vertical stratification of the equatorial Pacific ocean, which in turn leads to weakened ENSO variability. We suggest a role for an atmospheric bridge that rapidly conveys the influence of the Atlantic Ocean to the tropical Pacific. The results suggest a non‐local mechanism for changes in ENSO statistics and imply that anomalous Atlantic ocean SSTs can modulate both mean climate and climate variability over the Pacific.

Changes in precipitation and temperature extremes in Central America and northern South America, 1961–2003

In November 2004, a regional climate change workshop was held in Guatemala with the goal of analyzing how climate extremes had changed in the region. Scientists from Central America and northern South America brought long‐term daily temperature and precipitation time series from meteorological stations in their countries to the workshop. After undergoing careful quality control procedures and a homogeneity assessment, the data were used to calculate a suite of climate change indices over the 1961–2003 period. Analysis of these indices reveals a general warming trend in the region. The occurrence of extreme warm maximum and minimum temperatures has increased while extremely cold temperature events have decreased. Precipitation indices, despite the large and expected spatial variability, indicate that although no significant increases in the total amount are found, rainfall events are intensifying and the contribution of wet and very wet days are enlarging. Temperature and precipitation indices were correlated with northern and equatorial Atlantic and Pacific Ocean sea surface temperatures. However, those indices having the largest significant trends (percentage of warm days, precipitation intensity, and contribution from very wet days) have low correlations to El Niño–Southern Oscillation. Additionally, precipitation indices show a higher correlation with tropical Atlantic sea surface temperatures.

The effect of Atlantic sea surface temperature dipole mode on hurricanes: Implications for the 2004 Atlantic hurricane season

Results from this study indicate that the dipole mode of tropical Atlantic Ocean sea surface temperature (SST) anomalies is correlated with the overall activity of hurricanes as well as with the annual hurricane landfall frequency along the southeast coast of the United States. The tropical Atlantic SST dipole mode could affect hurricanes in at least three ways: 1) modulating the weather in West Saharan Africa; 2) influencing the local SST and hence the atmosphere‐ocean environment in the hurricane main development region; 3) coupling with the tropical and subtropical atmospheric circulation that controls the steering of hurricanes. The warm tropical North Atlantic and cool tropical South Atlantic waters are likely one of the main causes for Florida's hurricane havoc in 2004.

Wavelet Analysis of Variability, Teleconnectivity, and Predictability of the September–November East African Rainfall

Abstract By applying wavelet analysis and wavelet principal component analysis (WPCA) to individual wavelet-scale power and scale-averaged wavelet power, homogeneous zones of rainfall variability and predictability were objectively identified for September–November (SON) rainfall in East Africa (EA). Teleconnections between the SON rainfall and the Indian Ocean and South Atlantic Ocean sea surface temperatures (SST) were also established for the period 1950–97. Excluding the Great Rift Valley, located along the western boundaries of Tanzania and Uganda, and Mount Kilimanjaro in northeastern Tanzania, EA was found to exhibit a single leading mode of spatial and temporal variability. WPCA revealed that EA suffered a consistent decrease in the SON rainfall from 1962 to 1997, resulting in 12 droughts between 1965 and 1997. Using SST predictors identified in the April–June season from the Indian and South Atlantic Oceans, the prediction skill achieved for the SON (one-season lead time) season by the nonlinear model known as artificial neural network calibrated by a genetic algorithm (ANN-GA) was high [Pearson correlation ρ ranged between 0.65 and 0.9, Hansen–Kuipers (HK) scores ranged between 0.2 and 0.8, and root-mean-square errors (rmse) ranged between 0.4 and 0.75 of the standardized precipitation], but that achieved by the linear canonical correlation analysis model was relatively modest (ρ between 0.25 and 0.55, HK score between −0.05 and 0.3, and rmse between 0.4 and 1.2 of the standardized precipitation).

Predicting rainfall in the Dutch Caribbean—more than El Niño?

AbstractA strong lagged relationship between El Niño–Southern Oscillation (ENSO) and rainfall in the main rain season (October–January) on the leeward islands of Aruba, Curaçao and Bonaire is found. It can easily be used for skilful seasonal predictions, with an anomaly correlation coefficient r ≈ 0.6 at lag 4 months on historical data. The other two seasons, February–May and June–September, also show correlations with ENSO that can be exploited for predictions, r = 0.4 to 0.5. In the February–May dry season there is also a lagged correlation with sea‐surface temperature (SST) in the Pacific Ocean off the Central American coast that can be used to increase the forecast skill. A June–September small rains season correlation to equatorial Atlantic Ocean SST is absent in earlier data. Most of these results are also applicable to other stations in northern South America. Regressions with the circulation show that the main intermediate factors are upper‐level divergence and vorticity, and at lower levels a veering of the trade winds. This modifies the descending limb of the sea–continent breeze circulation that is responsible for the dry zone off the coast. Copyright © 2002 Royal Meteorological Society.

Pollen-inferred precipitation time-series from equatorial mountains, Africa, the last 40 kyr BP

Palaeoclimatic estimates of mean annual rainfall in the equatorial highlands of Central East Africa have been established for the last 40 kyr. The values are inferred from nine fossil pollen sequences, collected from six peat bogs located between 2° and 4°S latitude, in the forest belt, from 1800 to 2240 m a.s.l. The transformation of pollen data into climatic parameters is achieved by the best analogues statistical method, using a modern pollen data set of East and Central Africa and calibration by meteorological data. The climatic reconstructions are first performed for each individual sequences. They are transformed into time-series using 88 of the 125 available radiocarbon dates (including 41 AMS dates). The synthesis of the results is presented as a single curve, illustrating the precipitation values obtained for 682 dated stratigraphic layers plotted on a radiocarbon timescale. The precipitation changes are presented at about a century resolution during the Holocene, and about a millennium for the glacial period. The most characteristic feature of the synthetic curve is the changing variability through time. During the last glacial period (30 to 15 kyr BP interval), our results indicate a ca. 450 mm/yr (32%) precipitation decrease relative to the present. Two maxima decrease periods are registered at ca. 19 kyr BP (700 mm/yr, ca. 45%) and between 18 and 16 kyr BP (608 mm/yr, ca. 42%). Several abrupt positive peaks of higher precipitation are noticed pre 35 kyr BP and ca. 22–20 kyr BP. During the lower Holocene (10–7 kyr BP), the mean calculated precipitation estimate is 30 mm/yr (2%) below the present-day value, with several abrupt positive shifts; the maximum at ca. 8 kyr BP reaches 600 mm/yr (42%). Great variations between low and high precipitation values are expressed post ca. 4 kyr BP. These results are discussed in light of other climatic indicators extracted either from the same sedimentary record, isotopes, microfauna, or from other geological palaeoclimatic proxy in the concerned region, and pattern of the monsoon record in marine cores. Anthropogenic disturbance on past vegetation, as well as humidity signal as a better response of forest to climatic forcing, and possible CO 2 effect need more investigation in the future. We feel confident that the greater variability at the century scale registered in the equatorial highlands provides a good explanation for increased aridity in the desertic region of Sudan and Egypt during the last 4 kyr. The possibility that such greater variability could be induced by high variability or instability of the Atlantic Ocean or global sea surface temperature instability should be explored.

Some aspects of climate variability in the north east Ethiopian highlands - Wollo and Tigray

This paper presents a review of climate variability in the northeast Ethiopian Highlands, particularly Wollo and Tigray, during the last 10000 years (the Holocene) and an analysis of rainfall variability during the historical period. To date little work has been done on climate reconstruction in Tigray and Wollo, however, a number of ongoing studies in the area using palaeosoil analysis, archaeological techniques and lake level reconstruction will help reveal the magnitude of past climate variability in the region. On recent time scales, rainfall over Wollo is characterised by a distinctive bi-modal pattern with Belg rains in April-May preceding the main wet season Krempt, July to September, which is typical of the study region along much of the eastern escarpment. Further north over Tigray the Belg rains are less marked and the main Krempt rains dominate the seasonal pattern. Only three stations in the region, Dessie, Combolcha and Mekele, possess relatively unbroken records back to the 1950s. In all three, 1984 stands out as the driest year on record due to very low rainfall during the Krempt season, whilst 1999's Belg season was the driest on record. Although the succession of dry years between the late 1970s and late 1980s produced the driest decade in the Ethiopian Highlands this century there is no evidence for a long-term trend or change in the region's annual rainfall regime. Rainfall during the 1990s has seen a return to more humid conditions since the dry 1980s. The paper ends with a discussion of the major influences on rainfall variability over the Ethiopian Highlands: the El Niño-Southern Oscillation - El Niño events tend to be associated with lower than average rainfall; Atlantic, Indian and Pacific Ocean sea surface temperatures play a varying role in different seasons; the strength of the Indian Summer Monsoon; and the frequency of several tropical depressions over the Southwest Indian Ocean. SINET: Ethiopian Journal of Science Vol. 23, No. 2 (December 2000), pp. 139-161 Key words/phrases: Climate, El Niño, Ethiopian Highlands, Holocene, rainfall variability

Forecasting discharge in Amazonia using artificial neural networks

The Amazon, located in northern South America, is the world's largest river basin, and covers an area of about 6.5 million km2. The observed interannual variability in precipitation and water availability during its main discharge season has been shown to be influenced by Pacific and Atlantic Ocean sea surface temperatures (SSTs). However, the links between large-scale atmospheric motion and local and regional runoff patterns are essentially complex and still not fully understood. The processes involved are believed to be highly non-linear, spatially and temporally variable, and not easily described by physical or conceptual models. Artificial neural networks (NN) were trained to forecast discharge, one and two seasons in advance, at ten river sites in Amazonia from Pacific and Atlantic Ocean SST anomalies. The NN with an input layer of eight neurons, one hidden layer with 20 neurons and a one-neuron output layer was trained using back-propagation with momentum and gradient descendent. Results confirmed that different oceanic regions have distinct influences on different parts of the Amazonian basin. Better forecasts for basins in the northern part of Amazonia were obtained from Pacific Ocean SST and from Atlantic Ocean SST for basins in the southern part. Correlation coefficients between observed and estimated discharge (validation) were as high as 0.76 at some of the sites studied. The inclusion of precipitation as input improved the forecast for sites where NN did not perform well with training by SST only as input. The results obtained during this study corroborate and improve results obtained previously by means of linear statistical methods. Copyright © 2000 Royal Meteorological Society

Validation of Atlantic Ocean Sea Surface Temperatures Measured by theERS-1Along Track Scanning Radiometer

Abstract During the period from October 1991 to May 1992 the royal research ship Bransfield made its annual voyage from the United Kingdom to Antarctica and back. Whenever the measurement swath of the Along Track Scanning Radiometer (ATSR) on the ERS-1 satellite passed over the ship, the in situ skin and bulk sea surface temperatures (SSTs) were measured as well as profiles of atmospheric temperature and humidity. A total of seven cases were obtained where surface observations indicated cloud-free conditions during a satellite overpass. All the cases were in the tropical and subtropical Atlantic Ocean between 37°N and 45°S. The SSTs obtained from the ATSR nadir-view brightness temperatures were all colder than the skin SST by more than 0.5 K. The use of dual-view data made the agreement with the in situ measurements better in all but one case. In two cases agreement to better than 0.3 K could be obtained between the skin and ATSR SST when the dual-view brightness temperatures were used. In the other cases...

Northern hemisphere atmospheric response to changes of atlantic ocean SST on decadal time scales: a GCM experiment

Analyses indicate that the Atlantic Ocean sea- surface temperature (SST) was considerably colder at the beginning than in the middle of the century. In pa- rallel, a systematic change in the North Atlantic sea- level pressure (SLP) pattern was observed. To find out whether the SST and SLP changes analyzed are consis- tent, which would indicate that the SST change was real and not an instrumental artifact, a response experiment with a low-resolution (T21) atmospheric GCM was per- formed. Two perpetual January simulations were ducted, which differ solely in the Atlantic Ocean (40 ° S-60 ° N) SST: the cold simulation utilizes the SSTs for the period 1904-1913; the warm simulation uses the SSTs for the period 1951-1960. Also, a con- trol run with the model's standard SST somewhat be- tween the cold and warm SST was made. For the response analysis, a rigorous statistical approach was taken. First, the null hypothesis of identical horizontal distributions was subjected to a multivariate signifi- cance test. Second, the level of recurrence was esti- mated. The multivariate statistical approaches are based on hierarchies of test models. We examined three different hierarchies: a scale-dependent hierarchy based on spherical harmonics (S), and two physically motivated ones, one based on the barotropic normal modes of the mean 300 hPa flow (B) and one based on the eigenmodes of the advection diffusion operator at 1000 hPa (A). The intercomparison of the cold and warm experiments indicates a signal in the geostrop-

ESTIMATION OF PERIODICALLY VARYING MEANS AND STANDARD DEVIATIONS IN TIME SERIES DATA

Abstract. Models which account for seasonal changes in mean and standard deviation in time series which are serially correlated are discussed. Full Gaussian maximum likelihood estimation of the parameters specifying the mean function, of the parameters specifying the standard deviation function, and of the parameters specifying the stationary serial correlation structure is discussed and the asymptotic distribution derived without the assumption of Gaussian data. A commonly used method which estimates the three components separately, and uses Fourier series parameterisation, is also reviewed and the asymptotic distribution for these estimates is also derived. Both of these have asymptotic covariances which depend upon third and fourth cumulants of the underlying distribution. When these vanish (e.g., Gaussian case) then a simple revision of the inefficient estimates yields estimates which are asymptotically equivalent to the Gaussian estimates. An application to a series of monthly Atlantic Ocean Sea surface temperatures is given to illustrate this last method.

Tropospheric Mean Temperature and Its Relationship to the Oceans and Atmospheric Aerosols

Abstract Multiple-regression analyses of changes in tropospheric mean temperature as predictands and Pacific, Atlantic and Indian Ocean sea surface temperatures and atmospheric aerosol concentrations as predictors show that large fractions of the variances of the tropical, Northern Hemispheric and Southern Hemispheric extratropical tropospheric temperatures may be explained by fluctuations in ocean surface temperatures and atmospheric aerosols. The sensitivity of the tropical, Northern Hemisphere and Southern Hemisphere extratropical tropospheric temperatures to the various predictors are estimated. To improve the precision of the estimates in the presence of serial correlations in the variables we used a generalized least-squares procedure to obtain the regression models.