The Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere-ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of processes often more independent of the tropics than is observed. Finally, it is suggested that the assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into the past with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for the effects of these different processes, which only partly represent the direct forcing of the atmosphere by North Pacific Ocean SSTs.

From observational analysis a Pacific mode of variability in the intertropical convergence zone (ITCZ)/cold tongue region is identified that possesses characteristics and interpretation similar to the dominant ''meridional'' mode of interannual-decadal variability in the tropical Atlantic. The Pacific and Atlantic meridional modes are characterized by an anomalous sea surface temperature (SST) gradient across the mean latitude of the ITCZ coupled to an anomalous displacement of the ITCZ toward the warmer hemisphere. Both are forced by trade wind variations in their respective northern subtropical oceans. The Pacific meridional mode exists independently of ENSO, although ENSO nonlinearity projects strongly on it during the peak anomaly season of boreal spring. It is suggested that the Pacific and Atlantic modes are analogous, governed by physics intrinsic to the ITCZ/ cold tongue complex.

Midlatitude atmospheric variability is identified as a particularly effective component of the stochastic forcing of ENSO. This forcing is realized via a seasonal footprinting mechanism (SFM), in which the tropical atmosphere is forced during the spring and summer by SST anomalies generated by midlatitude atmospheric variability during the previous winter. The strong relationship between the SFM and ENSO may serve to enhance ENSO predictability and supports the view that ENSO is linearly stable in nature. * Joint Institute for the Study of the Atmosphere and Ocean Contribution Number 901. 1 Due to the strong similarities between the current results and methods, and the results presented in a heirarchy of CGCMs by Vimont et al. (2003), the reader may wish to refer to that manuscript.

Abstract.A

[1] We present intriguing evidence that the majority of El Niño events over the past four decades are preceded by a distinctive sea-surface warming and southwesterly wind anomaly in the vicinity of the Intertropical Convergence Zone (ITCZ) during the boreal spring. This phenomenon, known as the Meridional Mode (MM), is shown to be intrinsic to the thermodynamic coupling between the atmosphere and ocean. The MM effectively acts as a conduit through which the extratropical atmosphere influences ENSO. Modeling results further suggest that the MM plays a vital role in the seasonal phase-locking behavior of ENSO. The findings provide a new perspective for understanding the important role of thermodynamic ocean-atmosphere feedback in ENSO and may have profound implications for ENSO prediction, particularly the unresolved issue of the spring predictability barrier.

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El Niñ o events typically lead to delayed rainfall and decreased rice planting in Indonesia's main rice-growing regions, thus prolonging the hungry season and increasing the risk of annual rice deficits. Here we use a risk assessment framework to examine the potential impact of El Niñ o events and natural variability on rice agriculture in 2050 under conditions of climate change, with a focus on two main rice-producing areas: Java and Bali. We select a 30-day delay in monsoon onset as a threshold beyond which significant impact on the country's rice economy is likely to occur. To project the future probability of monsoon delay and changes in the annual cycle of rainfall, we use output from the Intergovernmental Panel on Climate Change AR4 suite of climate models, forced by increasing greenhouse gases, and scale it to the regional level by using empirical downscaling models. Our results reveal a marked increase in the probability of a 30-day delay in monsoon onset in 2050, as a result of changes in the mean climate, from 9 -18% today (depending on the region) to 30 -40% at the upper tail of the distribution. Predictions of the annual cycle of precipitation suggest an increase in precipitation later in the crop year (April-June) of Ϸ10% but a substantial decrease (up to 75% at the tail) in precipitation later in the dry season (July-September). These results indicate a need for adaptation strategies in Indonesian rice agriculture, including increased investments in water storage, drought-tolerant crops, crop diversification, and early warning systems.empirical downscaling models ͉ risk assessment A gricultural production in Indonesia is strongly influenced by annual and interannual variations in precipitation, caused by the Austral-Asia monsoon and El Niño-Southern Oscillation (ENSO) dynamics. Indonesia consistently experiences dry climatic conditions and droughts during the warm phase of the ENSO cycle (El Niño), with significant consequences for agricultural output, rural incomes, and staple food prices (1, 2). The year-to-year dynamics of ENSO and precipitation over the archipelago have been well studied (3-6), as have various links between ENSO, crop production, and famines in different parts of the country (7-10). Over the longer run, rising concentrations of greenhouse gases will likely create additional climate impacts on Indonesian agriculture. The combined forces of climate variability and climate change could have a dramatic effect on agricultural production in Indonesia and other tropical countries.Here, we present a framework for assessing the risks of climate change for Indonesian rice agriculture, drawing on the observational record of interannual variability in precipitation and production and on projections of climate change. We focus on precipitation, rather than temperature, because the links between precipitation and production in Indonesia are significant and well documented. Our earlier work (1) showed that ENSO has been the primary determinant of year-to-year variation in Indonesian rice outp...

[1] Connections between the Atlantic Meridional Mode (AMM) and seasonal hurricane activity are investigated. The AMM, a dynamical ''mode'' of variability intrinsic to the tropical coupled ocean-atmosphere system, is strongly related to seasonal hurricane activity on both decadal and interannual time scales. The connection arises due to the AMM's relationship with a number of local climatic conditions that all cooperate in their influence on hurricane activity. Further analysis indicates that the Atlantic Multi-decadal Oscillation (AMO) can excite the AMM on decadal time scales. As such, it is suggested that the AMO's influence on seasonal hurricane activity manifests itself through the AMM. This relationship between the AMM, AMO, and seasonal hurricane activity refocuses our understanding of how climate variations relate to seasonal hurricane activity in the Atlantic, and offers an improved framework beyond purely thermodynamic arguments that relates hurricanes to large-scale climate variations.