Catherine Gautier, Peter Peterson and Charles Jones
Institute for Computational Earth System Science
University of California
Santa Barbara, CA 93106-3060
USA
Phone: (805)893-4912
World Water Resources Vol 8, No. 4 pg 505-514
1996
Keywords: 1) Global Water Cycle; 2) Evaporation; 3) Precipitation; 4) Oceanic Transport; 5) Interannual Climate Variability
The exchange of moisture and heat fluxes across the ocean-atmosphere interface exerts a strong influence on the oceanic and atmospheric circulations, and therefore on the maintenance of the climate system equilibrium. Observational measurements of these fluxes over large areas of the ocean's surface are limited by the lack of in-situ data. This paper reports our research efforts to estimate the freshwater budget and freshwater oceanic transport using remotely sensed data. Six years (1988-1993) of surface evaporation estimated with satellite and in-situ data are combined with satellite-derived precipitation to compute the freshwater budget and freshwater oceanic transport. The interannual variability of the freshwater budget and oceanic transport estimates are examined for two contrasting events: the La Niña of 1988-89 and the El Niño condition during 1991-92, one of the longest El Niño episodes on record. Possible implications for future climate change are discussed.
Natural and possibly anthropogenic changes in the earth's climate have challenged scientists to achieve a thorough understanding of the mechanisms operating in the climate system. In this respect, scientists have long recognized the fundamental role of the global oceans in regulating the energy balance and the hydrological cycle of the planet. The ocean's influence in the climate system is basically represented by three mechanisms: exchange of carbon dioxide and other greenhouse gases with the atmosphere, exchange of momentum, heat and freshwater with the atmosphere and storage and transport of heat (Houghton et al., 1991).
The exchange of heat and water between the ocean and the atmosphere over climatic time scales create density gradients which drive a global oceanic circulation. This global thermohaline circulation is commonly known as the "ocean conveyor belt". This plays an important role in the redistribution of heat in the earth system. For a comprehensive review the reader is referred to Broecker (1991) and Trenberth (1992). Some observational studies show evidence that changes in the thermohaline circulation may be linked to strong climate changes. During glacial events, for example, flow in the lower branch of the thermohaline circulation in the Atlantic Ocean, known as the North Atlantic Deep Water flow (NADW), was significantly reduced, surface waters were depleted from nutrients, and deep ocean waters were colder than normal (Boyle and Keigwin, 1987).
Since the atmosphere exhibits a much larger natural variability on short time scales than the oceans, monitoring processes of air-sea interaction and their effect in the global oceanic circulation provide a unique opportunity to detect upcoming climate variations such as global warming associated with the greenhouse effect (The Ocean Observing System Development Panel, 1995). In contrast, our knowledge of the annual and interannual variability of the heat and moisture fluxes over large expanses of the global oceans, and particularly the southern oceans, has been historically limited by the lack of available in-situ measurements. In this context, satellite observations represent the only way to achieve adequate coverage in time and space necessary to fully characterize that variability. In addition, observations of moisture and energy fluxes across the air-sea interface are essential to validate climate models used to predict the long term evolution of the planet Earth.
Over the past several years, we have developed methods to estimate surface fluxes of solar radiation and latent heat using multi-sensor satellite observations (Gautier and Landsfeld, 1995; Jourdan and Gautier, 1994). Recently, we have extended the data record of our satellite estimation of surface evaporation to a six-year period. Precipitation, the other important term in the freshwater budget, is now routinely estimated by different research groups. Here we use the precipitation generated by the Goddard Space Flight Center (USA). The objective of this paper is to describe interannual variations in satellite estimates of freshwater balance and freshwater oceanic transport during the period 1988-1993. To do so, we examine the large-scale variations occurred in the ocean-atmosphere interface during two contrasting periods: 1988-89 and 1991-92. The first period is associated with a La Niña event, the largest event since 1977. The second period is associated with the warm El Niño conditions that lasted from 1990-95, one of the longest El Niño events on record.
The precipitation fields are retrieved from the Special Sensor Microwave Imager (SSM/I) data (Wilheit et al., 1991). The algorithm is based on an iterative process that fits the parameters of a probability distribution function of the rainfall intensity to the actual histogram of brightness temperatures measured in two different channels (19 and 22 GHz).The precipitation fields consist of monthly averages from 50° N to 50° S with an original resolution of 5°x5° latitude/longitude. The precipitation field has been resampled to 1 degree resolution in order to agree with the evaporation data resolution. The data record for the precipitation and all other fields extend from January 1988 through December 1993.
The satellite estimates of surface evaporation are based on the method developed by Jourdan and Gautier (1994). In summary, the inputs to the model are: total precipitable water and surface wind speeds from SSM/I (Wentz, 1992) and sea surface temperature analyses from the National Meteorological Center (Optimal Interpolation Scheme). The final product from our model consists of a blended analysis of the satellite only computation and in-situ parameters from the Comprehensive Ocean-Atmosphere Date Set (COADS) (Woodruff et al., 1997). For comparison purposes, this paper also includes evaporation estimates based solely on COADS observations.
The oceanic freshwater budget (FWB) is defined as the difference between the sink, evaporation (E), and the sources, precipitation (P) and runoff (R), of freshwater in the oceans. It is not balanced everywhere at the surface, but globally precipitation and runoff compensate the loss of water due to evaporation. On the other hand, when the freshwater transport (FWT) is known across a given latitude, the integration of the FWB gives FWT across any zonal section further south in the ocean:
An initial value of FWT is required to start the integration. Since the satellite estimates of FWB are available only below 50° N, the present study uses the following FWT values to start the integration. From the north pole to 50° N, the climatological values of Baumgartner and Reichel (1975) (hereafter B&R) are used. In addition, the FWT through the Bering Strait determined by Wijffels et al. (1992), 0.774 Sv (Sv = 106 m3 s-1), is added as northward transport into the Pacific Ocean, and included as southward transport from the Atlantic Ocean.
In order to examine interannual variations in the FWB and FWT, two twelve months periods are chosen for analysis : May 1988 - April 1989 and August 1991 - July 1992. The first period characterizes the peak of the 1988-89 La Niña event, while the second period includes the warm conditions associated with the 1990-95 El Niño.
The surface freshwater balance, E - P, for the period 1988-93, Figure 1a, shows the expected large-scale climatological features. Precipitation exceeds evaporation along the Intertropical Convergence Zone (ITCZ), over the maritime continent in the eastern Indian and western Pacific Oceans and over the western boundary currents in Asia and North America. By contrast, evaporation exceeds precipitation over the subtropical high pressures in both hemispheres. For comparison, Figure 1b shows the budget E - P, where E is computed with COADS data only and P is estimated with satellite data. A good agreement of the large-scale patterns between both fields is observed. The magnitudes, however, disagree in some regions, specially in the subtropical high pressure systems, where the satellite/COADS blended estimation of E is larger than the estimation using COADS only. In addition, the sparse number of in-situ observations in the southern oceans is rather apparent (Figure 1b).
The time variability of E - P during 1988-93 is seen in the time/longitude section along the equator (Figure 2). The most noticeable change is the sign reversal in the freshwater balance in the central Pacific Ocean. Evaporation exceeds precipitation by about 200 cm year-1 during the La Niña conditions of 1988-89, whereas precipitation exceeds evaporation by about 200 cm year-1 during the El Niño of 1991-92.
Figure 3 shows the zonal average of E - P for the period 1988-93 using the blended satellite/COADS estimates for E (solid curve), as well as COADS data only estimates for E (dashed curve). Except for some disagreement near the equatorial regions in all oceans, the blended satellite/COADS estimates capture the variations described by in-situ observations. The disagreement near the equatorial region is probably associated with a decrease in the number of in-situ observations.
The blended satellite/COADS estimates of freshwater transport during 1988-93 for each ocean basin is shown in Figure 4. For comparison purposes, we also included the freshwater transport estimated with evaporation from COADS data and the climatology of B&R. In the Indian Ocean (Figure 4a), the transport is near zero from 40° N to 10° S and northward from 20° S to 50° S, whereas in the Pacific Ocean (Figure 4b) the transport is northward and increases towards the southern hemisphere. In contrast, the freshwater transport in the Atlantic Ocean (Figure 4c) is southward and decreases from north to south. Although all three estimates of the oceanic freshwater transport have the same general pattern, important differences exist. In general, differences on the order of 0.2-0.4 Sv are observed between the satellite, COADS and B&R estimates. In the Pacific Ocean, however, the differences in the freshwater transports computed with satellite and COADS data and B&R are much larger, on the order of 0.5 Sv at 10° N and increasing southward.
In order to investigate interannual variations in the freshwater balance and oceanic transport during the La Niña (1988-89) and El Niño (1991-92) years, the long term mean for the period 1988-93 was subtracted from the fields of E - P prior to the computation of the freshwater transport. The spatial patterns of the anomalous surface freshwater balance, E - P, for the La Niña (1988-89) and El Niño (191-92) years are shown in Figure 5. As discussed before, a sign reversal in E - P is observed in the central Pacific Ocean during these two contrasting events, from about 75 cm year-1 during 1988-89 to -75 cm year-1 in 1991-92.
Some interesting features in the anomalous freshwater transports during La Niña and El Niño years are observed (Figures 6 and 7, respectively). In the Indian Ocean, anomalous northward freshwater transport is observed during both La Niña (Figure 6a) and El Niño conditions (Figure 7a), although the latitudinal maxima change. On the other hand, south of the equator in the Pacific Ocean, anomalous northward freshwater transport (positive anomaly) is observed during the La Niña event (Figure 6b), whereas anomalous southward freshwater transport (negative anomaly) is observed during the El Niño episode (Figure 7b). Note also that the maximum anomalies during both events are comparable (300 103 m-3 s-1). Although the spatial contrast between La Niña and El Niño conditions are more evident in the Pacific Ocean (Figure 5), the freshwater transport in the Atlantic Ocean also shows some sensitivity. During the La Niña event (Figure 6c) anomalous northward (positive anomaly) freshwater transport is observed, whereas anomalous southward transport occurs during El Niño event. In general, there is good agreement between the freshwater transport estimates from satellite and COADS. A significant disagreement between the two estimates is observed during the La Niña conditions in the Atlantic Ocean (Figure 6c).
This paper reports the data record extension of our satellite estimation of surface evaporation. The satellite derived estimates of precipitation and evaporation have been used to compute the freshwater budget and freshwater oceanic transport. The extension of the data record allows us to investigate interannual variations in the freshwater budget and oceanic transport. This is accomplished by examining two contrasting events in the data record: the La Niña of 1988-89 and the El Niño conditions during 1991-92. The variations in the large-scale circulation associated with these two contrasting periods indicate that significant changes occur in the freshwater budget over all oceans. These changes are significant enough to induce variations in the freshwater water transport over the Pacific and Atlantic Oceans, and the observed trends are in general consistent with the trends obtained using evaporation from in-situ observations. Indeed, recent observational and modeling studies suggest that freshwater flux anomalies are important in determining the vertical structure of the upper ocean during El Niño and La Niña conditions (for additional discussion, see Schneider and Barnett, 1995). Since the satellite estimation of freshwater budget offer a more complete spatial coverage than in situ-observations, satellite derived freshwater budget may serve as a useful diagnostic tool of the El Niño/Southern Oscillation (ENSO) phenomenon.
The dramatic changes in the freshwater transport shown between El Niño and La Niña conditions have most likely influenced only the upper ocean, since the response time of the deep ocean is too slow to be affected on a interannual basis. However, if one of these conditions, El Niño or La Niña, were to last for an anomalously long period of time, as was observed during the 1990-95 period with conditions similar to El Niño, then marked changes in the ocean thermohaline circulation could occur and strongly modified the earth's climate. The possibility of El Niño and La Niña cycle lengthening has been speculated in association with greenhouse gases changes or with decadal variability of the climate system (Trenberth and Hoar, 1996). This could imply a stronger role for large surface freshwater flux variability on the thermohaline circulation, and the climate system in general. Data sets such as the one presented here will then be crucial to understand and possibly predict long-term changes associated with large surface freshwater flux variability.
This study was supported by research grants from the National Aeronautics and Space Administration (NASA - NAGW-2460), the National Science Foundation (NSF - ATM-93-19483), and the Jet Propulsion Laboratory (JPL-959177). The authors thank Dr. Tim Liu and Dr. Didier Jourdan for their helpful support and contribution throughout this study. The precipitation data was kindly provided by Dr. Al Chang from NASA/GSFC. The sea surface temperature analyses are produced by the National Meteorological Center (NMC-USA). COADS data was provided by the National Center for Atmospheric Research (NCAR). NCAR is sponsored by the National Science Foundation.
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