James A. Renwick

Blocking over the South Pacific and Rossby Wave Propagation

Abstract Atmospheric blocking events over the South Pacific are investigated using a 39-yr record of 500-hPa height fields from the NCEP–NCAR reanalysis dataset. The analysis extends earlier work using a 16-yr record and confirms that the occurrence of blocking over the southeast Pacific is strongly modulated by the ENSO cycle during austral spring and summer. Comparison of results at 500 hPa with the 300-hPa meridional wind component showed that blocking events are associated with large-scale wave trains lying across the South Pacific from the region of Australia to southern South America. Similar wave trains are evident in both hemispheres in singular value decomposition analyses between 300-hPa meridional wind components and tropical Pacific outgoing longwave radiation (OLR) anomalies. The hypothesis that the divergence associated with tropical OLR anomalies forces an extratropical wave response that results in enhanced blocking over the southeast Pacific was tested using a linearized, barotropic vorti...

Relationships between North Pacific Wintertime Blocking, El Niño, and the PNA Pattern

Abstract The frequency of persistent high-latitude ridging events (commonly referred to as blocking) over the North Pacific–Alaskan region is investigated using a 44-winter record of daily 500-mb height fields. The winters are stratified in accordance with the phase of the El Nino–Southern Oscillation (ENSO) cycle and according to the sign of the seasonally averaged PNA index. It is found that the occurrence of blocking in the Bering Strait region is sensitive to the averaged polarity of the PNA pattern but is even more sensitive to the phase of the ENSO cycle. Sixty-nine percent more days of blocking are observed during winters occurring during the cool phase of ENSO, compared to those occurring during the warm phase. ENSO-related differences in blocking frequency are found to be associated with changes to both the mean and variance of the circulation over the North Pacific. The variance of geopotential heights on timescales corresponding to the lifetime of blocking events is found to be higher over the ...

ENSO-Related Variability in the Frequency of South Pacific Blocking

Abstract Interannual variability in the frequency of atmospheric blocking events over the southern Pacific Ocean is analyzed in terms of variations in the El Nino–Southern Oscillation (ENSO) cycle, using a 16-yr record of Southern Hemisphere 500-hPa height fields. The number of days of blocking tends to increase on average during the warm phase of the ENSO cycle, particularly over the southeast Pacific during the southern spring and summer. Over the southeast Pacific between September and February, more than twice as many days of blocking are observed on average during El Nino events than during neutral or La Nina conditions. Changes in the frequency of days of blocking are found to be related to changes in the mean circulation and more strongly to changes in the variance of circulation over the South Pacific.

Low-Frequency Variability of Southern Hemisphere Sea Level Pressure and Weather System Activity

Abstract This study examines the month-to-month variations in the tracks of Southern Hemisphere weather systems and their relation to low-frequency circulation variability. Cyclones and anticyclones are identified and tracked from ECMWF analyses during 1980–94 via an automated method and the principal patterns of variation identified by EOF analysis of monthly track density anomaly fields. Only the first three EOFs of cyclone track density involving about one-third of the total track variance were distinguishable from noise. Spatial patterns derived from both unrotated and rotated EOF analysis were not reproducible on subsets of the data, pointing to secular changes in the variance structure of the cyclone dataset. An increase in cyclone numbers over the Southern Ocean during the 1980s suggested that detection of small-scale cyclones is sensitive to changes in data coverage and analysis procedure, as associated changes in the mean circulation were small during this period. EOFs of anticyclone track data w...

Southern Hemisphere Circulation and Relations with Sea Ice and Sea Surface Temperature

Abstract Relationships on the seasonal timescale between Southern Hemisphere 500-hPa height, sea surface temperature, and Antarctic sea ice variability have been investigated using NCEP–NCAR reanalyses, NCEP sea surface temperatures, and Met Office sea surface temperature and sea ice data. The dominant region of interannual variability in the Southern Hemisphere circulation, over the southeast Pacific Ocean, is found to be related to ENSO variability in tropical Pacific sea temperatures, as shown in a number of earlier papers. It is also related to Antarctic sea ice variability, where an out-of-phase relationship is found between sea ice extent in the central Pacific and in the southwest Atlantic Ocean. Sea ice extent is enhanced in one region when the atmospheric flow anomaly is equatorward, presumably through a combination of anomalous heat flux and direct advection. At the same time, the atmospheric flow anomaly in the other region tends to be poleward, resulting in a poleward retreat in the sea ice ed...

Can the Increase in the Eddy Length Scale Under Global Warming Cause the Poleward Shift of the Jet Streams

AbstractThe question of whether an increase in the atmospheric eddy length scale may cause a poleward shift of the midlatitude jet streams is addressed. An increase in the length scale of the eddy reduces its zonal phase speed and so causes eddies to dissipate farther from the jet core. If the eddy dissipation region on the poleward flank of the jet overlaps with the eddy source latitudes, shifting this dissipation to higher latitudes will alter which latitudes are a net source of baroclinic eddies, and hence the eddy-driven jet stream may shift poleward. This behavior does not affect the equatorward flank of the jet in the same way because the dissipation region on the equatorward flank is well separated from the source latitudes. An experiment with a barotropic model is presented in which an increase in the length scale of a midlatitude perturbation results in a poleward shift in the acceleration of the zonal flow. Initial investigations indicate that this behavior is also important in both observationa...

Seasonal Zonal Asymmetries in the Southern Annular Mode and Their Impact on Regional Temperature Anomalies

AbstractThe Southern Hemisphere annular mode (SAM) is the dominant mode of climate variability in the extratropical Southern Hemisphere. Representing variations in pressure and the corresponding changes to the circumpolar zonal flow, it is typically thought of as an “annular” or ringlike structure. However, on seasonal time scales the zonal symmetry observed in the SAM in monthly or annual mean data is much less marked. This study further examines the seasonal changes in the SAM structure and explores temperature signals across the Southern Hemisphere that are strongly tied to the asymmetric SAM structure.The SAM asymmetries are most marked in the Pacific sector and in austral winter and spring, related to changes in the jet entrance and exit regions poleward of 30°S. Depending on the season, the asymmetric SAM structure explains over 25% of the variance in the overall SAM structure and has strong connections with ENSO or zonal wavenumber 3. In austral summer and autumn the SAM has been becoming more zona...

The impact of climate fluctuation on food availability and reproductive performance of the planktivorous red‐billed gull Larus novaehollandiae scopulinus

1. Using 41 years of data, we examined annual variations in the reproductive performance of the red-billed gull at the Kaikoura Peninsula, New Zealand and related these to changes in climate, oceanography and the availability of the planktonic euphausiid Nyctiphanes australis, the birds principal food. 2. In 1994 the population began to decline, and between 1983 and 2003 it was reduced by 51%. Adult non-breeders comprised an average of 32% of the population between 1983 and 1994, but following the decline they averaged only 18%. The age at recruitment changed markedly following the population decline: 27% of 2-year-old males bred for the first time prior to the decline, whereas the corresponding figure after the decline was 38%. The proportion of females commencing to breed as a 3-year-old was not significantly different before or after the decline. 3. An increase in the availability of euphausiids increased the likelihood of breeding and the recruitment of young individuals, caused earlier laying and resulted in an increase in the condition of adults, egg volume of gulls laying two egg clutches, clutch size and fledging success. 4. The relationship between food availability and the number of pairs that bred, laying date, clutch size and fledging success was significantly different prior to and after the population decline. The underlying cause appears to be a compensatory density-dependent mechanism that reduced interspecific competition for food. 5. The relative abundance of N. australis was correlated positively with the Southern Oscillation Index (SOI) and the frequency of occurrence of NE winds. The proportion of non-breeders and mean laying date were correlated negatively with the SOI, and mean egg volume of two egg clutches correlated positively with the SOI. 6. These results emphasize that availability of adult euphausiids is critical for red-billed gulls. We hypothesize that high primary productivity of inshore water near Kaikoura in winter, linked to a stable water column of coastal water and upwelling of additional dissolved inorganic nutrients, increases the availability of adult euphausiids to the red-billed gull as long as the coastal water is not replaced by offshore subtropical water intrusions of warmer, low-nutrient water.

Hemispheric-Scale Seasonality of the Southern Annular Mode and Impacts on the Climate of New Zealand

Abstract The seasonality of the southern annular mode (SAM) and the resulting impacts on the climate variability of New Zealand (NZ) are investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wavenumber 2–3 variability. This is consistent with seasonal variations in the mean state, and the authors argue that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy-driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behavior are discussed. For the NZ region this seasonality implies that fluctuations in the SAM are associated with a zonal wind speed anomaly during summer but a more meridional wind speed anomaly during winter. This behavior is well captured by temperature and rainfall station dat...

Potential Predictability of Seasonal Means Based on Monthly Time Series of Meteorological Variables

Abstract Based only on monthly mean data, an analysis of variance method is proposed for decomposing the interannual atmospheric variability in seasonal-mean time series into components related to “weather noise” and to slowly varying boundary forcing and low-frequency internal dynamics. The “potential predictability” is then defined as the fraction of the total interannual variance accounted for by the latter two components. A study using synthetic data showed that the method proposed here is comparable in performance to conventional methods requiring daily data. The technique was applied to gridded global data of monthly surface temperature, 500-hPa height, and 300-hPa wind in order to examine the geographical and seasonal dependencies of their potential predictability. For all the variables, the highest potential predictability tends to be found in the Tropics, where seasonal anomalies in the atmosphere are strongly coupled with the underlying sea surface temperature anomalies and the weather noise com...