A43A-0031 1340h
ENSO Impact on Precipitation Variability Over the Mid-Atlantic States
We have performed Spectral and Empirical Mode Decomposition (EMD) analyses on daily rainfall data from five rain gauges in Virginia and have indications of substantial ENSO signatures in the data. This study extends our previous work to include ~100 years of monthly rainfall compiled by water divisions in the Mid-Atlantic region. Trend, spectral, and EMD analyses are performed on the water division data. An Empirical Orthogonal Analysis is performed on the data set to delineate major mode of variability. Correlation and coherence analyses between the rainfall time series and climate indices such as the Southern Oscillation index, North Atlantic Oscillation, North Pacific Oscillation, and the Arctic Oscillation are also performed. Such analysis will extract predictive information on the rainfall patterns and hence allow response to mitigate interannual climate changes brought about by climate events such as the Niño/La Nina for the Mid-Atlantic regions. This work is part of our effort to support the Virginia Access/Middle Atlantic Geospatial Information Consortium VAccess/MAGIC) to provide information to water resource managers and environmental monitoring agencies. The VAccess/MAGIC is a distributed, remote sensing data, information and services, virtual center to support the advancement and transfer of remote sensing data and associated technologies for local, state, and regional application user communities.
A43A-0032 1340h
The Effects of La Nina on the Distribution and Transport of Trace Gases and Aerosols over Southern Africa
At present there is relatively little understanding regarding the behavior of trace gases and aerosols and their resultant long-range transport pathways during high phase El Nino Southern Oscillation (ENSO) years for the region of southern Africa. Atmospheric chemical measurements and atmospheric profiles taken as a part the Southern African Regional Science Initiative (SAFARI 2000) were conducted in the austral winter of 2000 during what was a relatively strong La Nina year. Differences in the frequency of synoptic-scale circulation patterns as a result of changes in Walker Cell positioning led to changes in distribution and transport of trace gases and aerosols for this La Nina year as compared with measurements taken in the region during a previous El Nino year austral winter campaign (SAFARI-92). The climatic mechanisms for these changes and the resultant differences in circulation patterns from the local to synoptic scale are examined through the description of the transport and distribution of aerosols and trace gases. Despite many differences on the synoptic level and climatic level, vertical stability, a key indicator of the nature of the southern African atmosphere, remained relatively the same for both phases of ENSO. The implications of these differences and similarities are examined and explained using the mechanisms of ENSO and Quasi-Biennial Oscillation (QBO) teleconnections and interactions.
A43A-0033 1340h
ENSO-mediated climate-tree growth relationships in the southeastern United States
We use a mechanistic model of tree-ring formation to evaluate the cause of a shift in climate-tree growth relationships observed after 1976 in the southeastern United States. Principal component analysis demonstrates that the leading EOFs of our simulated (n=8) and real (n=10) chronologies are well correlated (r=0.61, p$<$0.0001) and that the model has substantial skill in reproducing actual regional patterns of tree growth as controlled by climate. Modeled growth rates show a shift toward greater sensitivity to summer drought after 1976. This simulated regional response is consistent with that of our actual tree-ring data from North Carolina. After 1976, narrow ring width years in both our actual and simulated tree-ring series are associated with ENSO warm phase-like anomalies in Pacific sea surface temperatures. These results collectively suggest that summer drought, associated with more frequent ENSO activity, has become an important control on forest growth in the region since the mid-1970s.
A43A-0034 1340h
Mechanisms of the Remote Tropical Precipitation Reduction During El Niño
During El Niño phases, negative rainfall anomalies are evident across many portions of the tropics outside of the Pacific (hereinafter ``the remote tropics''). In this paper, we make use of an intermediate level complexity model (the Quasi-equilibrium Tropical Circulation Model or QTCM) and its ``single column'' analogue (the Single Column QTCM or SCQTCM) to investigate the mechanisms underlying the remote tropical precipitation reduction during El Niño. Specifically, we focus on the ``equilibrated precipitation response'', i.e., the precipitation anomalies that occur after the remote tropics has adjusted to a time-invariant El Niño forcing. In both the QTCM and SCQTCM, widespread remote tropical precipitation reductions are simulated, although there are some differences in the magnitude and spatial patterns of the precipitation response between the two models. Using the SCQTCM---which represents the remote tropics as a set of disconnected columns in which both tropospheric temperature and horizontal circulation are imposed---we show that anomalous tropospheric temperature forcing (as opposed to anomalous horizontal circulation) accounts for the largest fraction of the precipitation reduction. Moreover, through application of spatially uniform and nonuniform anomalous tropospheric temperature forcing scenarios to the SCQTCM, we find that only the latter produces an El Niño-like distribution of precipitation anomalies. By contrast, the uniform forcing scenario produces a distinctly different pattern of anomalies, namely enhanced precipitation over deep convective zones and decreased precipitation elsewhere. That the SCQTCM produces an El Niño-like anomalous precipitation field when forced with tropospheric temperature alone points to the potentially significant role of tropospheric temperature as a generator of the equilibrium precipitation response to El Niño. However, it appears that nonuniformities (gradients) in tropospheric temperature, rather than the uniform warming of temperature itself, are crucial to the establishment of the remote tropical precipitation anomalies during El Niño.
A43A-0035 1340h
The Role of Tropospheric Temperature in the ENSO-driven Surface Temperature Variability over the Remote Tropics
Previous observational and modeling studies have shown the importance of wind speed and the shortwave influence of cloud cover in mediating the Niño warming overthe remote tropical oceans via the `atmospheric bridge'. We argue - based on simulations with a single column model and with an atmospheric GCM coupled to a thermodynamic slab ocean model - that atmospheric boundary layer humidity effects, and also the influence of net LW, can also be significant influences. The relative strengths of each surface flux influence differ from region to region, indicating the complicating influence of regional climate processes. However, we argue thattropospheric temperature plays a central role in the El Nino-related remote surface temperature warming by i) communicating the ENSO influence to the remote tropics; and ii) setting the equilibrium surface temperature response over moist convective regions, through the convective quasi-equilibrium constraint linking planetary boundary layer moist static energy to the free tropospheric temperature. This constraint offers a simple explanation why all remote moist convective regions (over ocean and land) warm during and after the Niño peak, despite the region-dependent nature of the surface flux response.
A43A-0036 1340h
Indian Summer Monsoon Rainfall and Its Link With ENSO and the Indian Ocean
The all-India summer monsoon rainfall (ISMR), which is defined as the rainfall received during June to September over India, has a large impact on the agriculture and related economic activities of the region. The prediction of interannual variability of ISMR is thus a matter of great concern to society. It has been known that the El Nino/Southern Oscillation (ENSO) phenomenon is the most important external forcing of ISMR. However, ENSO does not always explain the interannual variability of ISMR. Recent studies have suggested that in addition to ENSO, the state of the Indian Ocean also influences the interannual variability of ISMR. However, most of these studies examine the relationships using the data of recent decades only. In this study, we examine the relationship between the state of equatorial Indian Ocean, ENSO and ISMR using data from 1881 to 1998. We particularly focus on the zonal wind anomalies at the equatorial Indian Ocean (hereafter EQWIN) and the SST anomalies over the Indian Ocean as the indices that represent the condition of Indian Ocean. During this period, the chosen ENSO index (NINO3) is moderately correlated with ISMR (the simultaneous correlation coefficient:-0.57). Although the EQWIN poorly correlates with ISMR, the linear reconstruction of ISMR based on a multiple regression on the ENSO and EQWIN indices correlates with the ISMR stronger than with the ENSO only regression. Using contingency tables we find that the negative association between the categories of ISMR and EQWIN is significant during "warm" (El Niño) and "neutral" ENSO events but not during La Ni\~{n}a events. The Indian Ocean SSTA demonstrates positive association with ISMR during the El Niño events but not during La Ni\~{n}a and neutral ENSO; thus the warmer Indian Ocean relates to the reduction of El Niño effects on ISMR.
A43A-0037 1340h
The Direct Effect of Summertime ENSO Conditions on the South Asian Monsoons: Barotropic and Baroclinic Teleconnection Mechanisms
Two distinct teleconnection mechanisms explaining the interannual covariability of northern hemisphere summertime ENSO conditions and the South Asian monsoons are presented. First, summertime convection over the far eastern Pacific Ocean is shown to excite stationary barotropic Rossby waves that propagate eastward and enter the North African-Asian jet. These disturbances are associated with colder upper tropospheric potential temperatures within the North African-Asian jet, which reduce the upper tropospheric thermal gradient between the Asian landmass and equatorial Indian Ocean. This diminution of the thermal gradient reduces monsoon overturning circulation and associated Indian monsoon rainfall, and explains the long-observed summertime negative correlation between ENSO and the Indian monsoons. Second, east of peninsular India, monsoon rainfall is additionally affected by convection over Indonesia, which increases during La Nina events. This Indonesian convection produces a baroclinic response within the tropics that increases subsidence and reduces monsoon rainfall over Bangladesh and Myanmar. During El Nino events, when convection moves east of Indonesia to the central and eastern equatorial Pacific, the baroclinic response induced by this convection does not extend far enough west to produce subsidence and suppression of monsoon rainfall over Bangladesh and Myanmar. Consequently, summer season El Nino conditions have two effects on the South Asian monsoons east of the Indian peninsula: 1) stationary barotropic Rossby waves reduce monsoon rainfall by diminishing the upper tropospheric land-sea thermal gradient; 2) stationary baroclinic Rossby waves fail to reach Bangladesh and Myanmar and therefore favor monsoon rainfall. These two offsetting effects appear to explain the poor correlation between ENSO and the South Asian monsoons east of India.
A43A-0038 1340h
Impact of Indian Ocean sea surface temperature on developing Niño
Prior to the 1976-77 climate shift (1950-76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basin-wide warming during boreal fall of the developing phase of most Niños whereas after the shift (1977-99) they had an east-west asymmetry, a consequence of Niño being associated with the Indian Ocean Dipole/Zonal Mode. In this study, we investigate the possible impact of these contrasting SST patterns on the on-going Niño, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), the latter used to identify basic processes. Specifically, analyses of reanalysis products during the Niño onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. Our study focuses on determining influence of Indian Ocean SST on these changes. A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical West Pacific and the subsequent development of the South China Sea anticyclone being particularly sensitive to Indian Ocean SST anomalies. Prior to the climate shift, the basin-wide Indian-Ocean SST anomalies generate an atmospheric Kelvin wave associated with easterly flow over the equatorial western-central Pacific, thereby weakening the westerly anomalies associated with the developing Niño. In contrast, after the shift the east-west contrast in Indian-Ocean SST anomalies does not generate a significant Kelvin-wave response, and there is little effect on the Niño-induced westerlies. The LBM solutions confirm the AGCM's results.
A43A-0039 1340h
An Investigation of the Role of Internal Atmospheric Variability in ENSO
The NCAR atmospheric general circulation model (CCM3) is coupled to a Zebiak-Cane type of reduced gravity ocean model (RGO) to study the role of atmospheric internal variability in ENSO. A novel noise- filter is developed and applied to the coupled model simulations. The filter effectively reduces the impact of the internal atmospheric variability on coupled ENSO dynamics, and thus allows a systematic examination of the effect of internal atmospheric variability on coupled ENSO dynamics. A set of numerical experiments was conducted with and without the noise filter. First, in a long control simulation where the filter is not employed we show that the coupled CCM3-RGO model generates ENSO variability that has many statistical properties similar to the observed one, including the power spectrum and seasonal phase locking. We then conducted a set of experiments where the filter is applied to surface wind stresses and surface heat fluxes jointly and separately. These experiments allow a closer look at the relative importance of dynamic vs. thermodynamic stochastic forcing in coupled ENSO dynamics, and provide a means to test various stochastic forcing mechanisms, such as the seasonal footprinting mechanism. The results show that the internal atmospheric variability plays a crucial role in maintaining ENSO variability. Without the presence of the internal atmospheric variability, not only is the variance of the ENSO substantially reduced, but also its statistical characteristics are altered. The physical mechanisms of how the atmospheric internal variability affects ENSO evolution are explored and will be discussed in some detail.
A43A-0040 1340h
Surface Water Circulation in the Indonesian Seas as Reconstructed by High-Resolution Coral Radiocarbon Records
The Indonesian Seas are a main conduit for the redistribution (and modification) of heat and freshwater between the Pacific and Indian Oceans. Through strong coupling between the ocean and atmosphere, variations in the transport and mixing in this region is thought to play an important role in ENSO and the Indonesian/Asian Monsoon on seasonal to decadal timescales. To further elucidate surface water processes including seasonal transport of water from the North Pacific, we have generated $\sim$bimonthly coral-based pre to post-bomb $^{14}$C time-series from Makassar and Lombok Straits, and Bunaken in the Sulawesi Sea. The keystone record is a 115-year time-series from the Makassar Straits. In the pre-bomb (pre-1955) era from 1890 to 1954 surface waters in the Makassar straits display a small and variable seasonal signal (10-15\permil). After 1954 the radiocarbon record increases rapidly, in response to the increased atmospheric $^{14}$C content caused by nuclear weapons testing. From 1954 to 1986 the record displays clear seasonal variability from 15 to 60\permil and the post-bomb peak (163\permil) occurred in 1974. The seasonal cycle of radiocarbon can be attributed to variations of surface waters passing through South Makassar Strait. Southern Makassar experiences two distinct seasons, the Northwest Monsoon which is responsible for the high Austral summer radiocarbon (North Pacific waters) and the Southeast Monsoon that flushes back a mixture of low (South Pacific and an inferred upwelling altered) radiocarbon water from the Banda Sea. The Lombok Strait radiocarbon record displays seasonal influences of Indian Ocean waters and North Pacific waters, while the Bunaken radiocarbon record reflects local influences (upwelling) on the North Pacific water source.