wavelength, longer period MRG wave basic states, with the vertical mode number increasing as the square of the MRG wave period.An appendix deals with the case of zonally long and intermediate wavelength MRG waves, where a weak instability regime causes a moderate adjustment involving resonant triad interactions without leading to jet formation. For eastward phase propagating waves, adjustment does not lead to significant angular momentum redistribution.

Eastward currents in the thermocline and subthermocline layers of the tropical Atlantic Ocean are described using observations obtained during two boreal summer cruises covering the whole equatorial basin. The analysis focuses on the eastward evolution of zonal currents in the thermocline and sub‐thermocline. The Equatorial Undercurrent is not found close to the African coast in boreal summer 2000, maybe because of interannual variability. As observed in the Pacific, the South and North Equatorial Undercurrents are shown to shift poleward from west to east, and the two cruises indicate that the North Equatorial Undercurrent does not enter the Gulf of Guinea. The South and North Intermediate Countercurrents are variable with longitude and, contrarily to the observations in the eastern equatorial Pacific, are not found in the east of the Guinea basin.

[1] The nature of the Pacific Decadal Oscillation (PDO) is investigated based upon analyses of sea surface temperature observations over the last century. The PDO is suggested to be comprised of a 20 year quasi-periodic oscillation and a lower frequency component with a characteristic timescale of 60 years. The 20 year quasi-periodic oscillation is clearly identified as a phase locked signal at the eastern boundary of the Pacific basin, which could be interpreted as the signature of an ocean basin mode. We demonstrate that the 60 year component of the PDO is strongly time-lag correlated with the Atlantic Multidecadal Oscillation (AMO). On this timescale the AMO is shown to lead the PDO by approximately 13 years or to lag the PDO by 17 years. This relation suggests that the AMO and the 60 year component of the PDO are signatures of the same oscillation cycle. Citation: d'Orgeville, M., and W.

This study aims to assess the impact of climate change on water resources in a large watershed within the Laurentian Great Lakes region, using the fully integrated surface‐subsurface model HydroGeoSphere. The hydrologic model is forced with an ensemble of high‐resolution climate projections from the Weather Research and Forecasting (WRF) model. The latter has been extended with an interactive lake model (FLake) to capture the effect of the Great Lakes on the regional climate. The WRF ensemble encompasses two different moist physics configurations at resolutions of 90, 30, and 10 km, as well as four different initial and boundary conditions, so as to control for natural climate variability. The integrated hydrologic model is run with a representative seasonal cycle, which effectively controls natural climate variability, while remaining computationally tractable with a large integrated model. However, the range of natural variability is also investigated, as are the impacts of climate model resolution and bias correction. The two WRF configurations show opposite climate change responses in summer precipitation, but similar responses otherwise. The hydrologic simulations generally follow the climate forcing; however, due to the memory of the subsurface, the differences in summer propagate throughout the entire seasonal cycle. This results in a set of dry scenarios with reduced streamflow and water availability year‐round and a set of wet scenarios with increased streamflow for all times excluding the spring peak, which does not increase. Most of the analysis focuses on streamflow, but changes in the seasonal cycle of baseflow and groundwater recharge are also analyzed.

This paper presents initial results from new velocity observations in the eastern part of the equatorial Atlantic Ocean from a moored current-meter array. During the "EQUALANT" program (1999-2000), a mooring array was deployed around the equator near 10°W that recorded one year of measurements at various depths. Horizontal velocities were obtained in the upper 60 m from an upward-looking acoustic Doppler current profiler (ADCP) and at 13 deeper levels from current meters between 745 and 1525 m. To analyze the quasiperiodic variability observed in these records, a wavelet-based technique was used. Quasiperiodic oscillations having periods between 5 and 100 days were separated into four bands: 5-10, 10-20, 20-40, and 40-100 days. The variability shows (i) a strong seasonality (the first half of the series is dominated by larger periods than the second one) and (ii) a strong dependence with depth (some oscillations are present in the entire water column while others are only present at certain depths). For the oscillations that are present in the entire water column the origin of the forcing can be traced to the surface, while for the others the question of their origin remains open. Phase shifts at different depths generate vertical shears in the horizontal velocity component with relatively short vertical scales. This is especially visible in long-duration events (>100 days) of the zonal velocity component. Comparison with a simultaneous lowered acoustic Doppler current profiler (LADCP) section suggests that some of these flows may be identified with equatorial deep jets. A striking feature is a strong vertical shear lasting about 7 months between 745 and 1000 m. These deep current-meter observations would then imply a few months of duration for the jets in this region

[1] The University of Victoria Earth System Climate Model is used to investigate the effects of changes in Southern Hemisphere Westerlies (SHW) on atmospheric CO 2 . It is shown that a northward shift of the SHW and a decrease of their amplitude have the same qualitative effect on deep ocean carbon storage, which increases because of a deceleration of the bottom meridional overturning circulation. A southward shift or a strengthening of the SHW has the opposite effect. However, latitudinal shifts of the SHW and changes in their amplitude are not equivalent in terms of atmospheric CO 2 . In particular, while doubling the SHW amplitude increases atmospheric CO 2 by 36 ppm and halving reduces it by 20 ppm, the latitudinal shifts (north-or southward) have no significant impact on atmospheric CO 2 . These different CO 2 responses are due to different dynamical responses of the upper ocean circulation which, in the case of latitudinal shifts, produce a carbon storage change opposite to the one observed for the deep ocean. In all experiments, the changes in the biological carbon pump in response to a redistribution of the nutrients by the modified oceanic circulation remain small. Ultimately, the atmospheric CO 2 response depends on the control the SHW exert on both the ventilation of the deep ocean and the depth of the upper ocean pycnocline.

Accurate identification of the impact of global warming on water resources in major river systems represents a significant challenge to the understanding of climate change on the regional scale. Here, dynamically downscaled climate projections for western Canada are presented, and impacts on hydrological variables in two major river basins, the Fraser and Athabasca, are discussed. These regions are both challenging because of the complexity of the topography and important because of the economic activity occurring within them. To obtain robust projections of future conditions, and to adequately characterize the impact of natural variability, a small initial condition ensemble of independently downscaled climate projections is employed. The Community Earth System Model, version 1 (CESM1), is used to generate the ensemble, which consists of four members. Downscaling is performed using the Weather Research and Forecasting Model, version 3.4.1 (WRF V3.4.1), in a nested configuration with two domains at 30-and 10-km resolution, respectively. The entire ensemble was integrated for a historical validation period and for a mid-twenty-first-century projection period [assuming representative concentration pathway 8.5 (RCP8.5) for the future trajectory of greenhouse gases]. The projections herein are characterized by an increase in winter precipitation for the mid-twentyfirst-century period, whereas net precipitation in summer is projected to decrease, due to increased evapotranspiration. In the Fraser River basin, a shift to more liquid precipitation and earlier snowmelt will likely reduce the seasonal variability of runoff, in particular the spring freshet. In the Athabasca River basin, winter precipitation and snowmelt may increase somewhat, but increasing evapotranspiration may lead to reduced streamflow in late summer.

The response of the deep equatorial ocean to an oscillatory baroclinic western boundary current is investigated in a continuously stratified primitive equation model. The symmetry of the current about the equator is such that mixed Rossby-gravity (MRG) waves are excited in the western part of the equatorial ocean. Depending on the forcing frequency, short to long scale (when compared to equatorial Rossby radius) monochromatic MRG waves are selected. The subsequent MRG wave destabilization generally leads to a much higher vertical mode response than the forced MRG wave mode. In a channel, short MRG waves are destabilized by shear instability (Hua et al., 2007). In a basin, the destabilization occurs in the vicinity of the western boundary and leads to the formation of finite amplitude, nonlinear jets in the entire equatorial basin. The space and time pattern of the jets correspond to low-frequency oscillating equatorial basin-modes, the period of which is set by the dominant vertical mode of the response. The vertical scale of the jets is a function mainly of the forcing period and is independent of the forcing vertical mode, as long as the excited MRG waves are in a sufficiently short regime to be unstable. As a result, an oscillatory western boundary current leads to a permanent equatorial zonal circulation, unlike a steady western boundary current. But most importantly, MRG wave destabilization appears to be a plausible formation mechanism for the observed Equatorial Deep Jets. The spatial and dynamical characteristics of the zonal circulation achieved with a 60-day forcing period are indeed compatible with the observations in the Atlantic Ocean.