Abstract

Relationships between Gulf of California moisture surges and tropical cyclones (TCs) in the eastern Pacific basin are examined. Standard surface observations are used to identify gulf surge events at Yuma, Arizona, for a multiyear (July–August 1979–2001) period. The surges are related to TCs using National Hurricane Center 6-hourly track data for the eastern Pacific basin. Climate Prediction Center (CPC)- observed daily precipitation analyses and the NCEP Regional Reanalysis are used to examine the relative differences in the precipitation, atmospheric circulation, and moisture fields for several categories of surge events, including those that are directly related to TCs, indirectly related to TCs, and not related to TCs.

It is shown that the response to the surge in the southwestern United States and northwestern Mexico is strongly discriminated by the presence or absence of TCs. Surges related to TCs tend to be associated with much stronger and deeper low-level southerly flow, deeper plumes of tropical moisture, and wetter conditions over the core monsoon region than surges that are unrelated to TCs. The response to the surge is also strongly influenced by the proximity of the TC to the Gulf of California (GOC) region. Tropical cyclones that track toward the GOC region exert a stronger, more direct influence on Yuma surges than those that track away from the GOC.

Abstract

Interannual variability of the summer monsoon in the southwestern United States is controlled by various ocean- and land-based conditions (e.g., SST, soil moisture, and snow cover) that provide sources of memory of antecedent climate anomalies such as ENSO. It is hypothesized that this interannual variability is also modulated by decade-scale fluctuations in the North Pacific SSTs.

The following observations have been made in support of this hypothesis. First, the summer precipitation regime is dominated by a continental-scale precipitation pattern characterized by an out-of-phase relationship between precipitation in the southwestern United States and that in the Great Plains of the United States. Second, interannual fluctuations in the onset date of the monsoon in the southwestern United States are significantly correlated with interannual fluctuations in the intensity of summer rainfall in this region such that early monsoons are often very wet and late monsoons tend to be dry. Third, wet (dry) monsoons in the southwestern United States often follow winters characterized by dry (wet) conditions in the southwestern United States and wet (dry) conditions in the northwestern United States. Finally, interannual variability of the summer monsoon in the southwestern United States is modulated by long-term (decade scale) fluctuations in the North Pacific SSTs. The mechanism relating the North Pacific SST pattern to interannual variability in the summer monsoon appears to be via the impact of variations in the Pacific jet on West Coast precipitation regimes during the preceding winter. Multiyear fluctuations in the North Pacific SST pattern are consistent with multiyear fluctuations in the atmospheric circulation and in the West Coast precipitation regimes during Northern Hemisphere (NH) winter, hence with multiyear variability in the summer monsoon state. Influences on the summer monsoon during the preceding winter and spring are tied together using appropriate SST indices that capture decade-scale variability in the North Pacific during NH winter and interannual variability in the eastern tropical Pacific during NH spring. The results suggest that decadal variability in the North Pacific SSTs may be an important factor in determining long-term periods of summertime drought or rainy conditions both in the southwestern United States and in the Great Plains of the United States.

Abstract

Time series of seasonal-, monthly, and pentad-mean precipitation are subjected to empirical orthogonal function analysis, regression analysis, and compositing techniques to study the principal modes of interannual and intraseasonal variability of the North American Monsoon System (NAMS). The leading principal component (PC) from the summertime seasonal-mean data is associated with El Niño–Southern Oscillation (ENSO) variability while the leading PC from the pentad-mean data is associated with 30–60-day intraseasonal (Madden–Julian) oscillations (MJOs). The leading PC from the monthly mean data is a hybrid of the two above-mentioned modes, capturing aspects of both. The leading PCs are used as reference time series for regressions and composites that reveal the structure of the principal modes and their manifestation in the NAMS. The leading PCs are also used to estimate the fraction of the variance of summer precipitation that is explained by ENSO and by interannual variations of MJO activity.

ENSO-related impacts on the NAMS are linked to meridional adjustments of the ITCZ. In its positive polarity, the leading PC of interannual variability is associated with warm (ENSO) episodes and is characterized by an expansion of the ITCZ toward the south, increased precipitation in a zonally oriented band just north of the equator, and decreased precipitation over Mexico and portions of the Caribbean. MJO-related impacts on the NAMS are linked to more regional meridional adjustments in the precipitation pattern over the eastern tropical Pacific. In its positive polarity, the leading PC of intraseasonal variability is associated with an intensification and northward adjustment of the precipitation pattern in the eastern tropical Pacific, with increased precipitation over the warm pool to the west of Mexico and over portions of Mexico and the southwestern United States. Both the interannual and intraseasonal modes have well-defined, but distinct, sea level pressure and surface wind signatures in the eastern tropical Pacific. These features extend to the middle troposphere and are capped by circulation features in the opposite sense in the upper troposphere.

The relationship of the MJO to the NAMS is examined in more detail using the leading PC of intraseasonal variability and an objective procedure to identify the phase of MJO events. The leading PC is strongly related to the eastward progression of centers of enhanced (reduced) convection around the global Tropics on intraseasonal timescales. Notably, a strong relationship between the leading mode of intraseasonal variability of the NAMS, the MJO, and the points of origin of tropical cyclones in the Pacific and Atlantic basins is also present.

Abstract

This study examines the role of synoptic-scale eddies during the development of persistent anticyclonic height anomalies over the central North Pacific in a general circulation model under perpetual January conditions. The GCM replicates the basic characteristics of the evolution of the anomaly patterns found in observations. The life cycle is characterized by the rapid establishment of the major anomaly center and considerably longer maintenance and decay phases, which include the development of downstream anomaly centers. The simulation also shows a realistic evolution of synoptic-scale activity beginning with enhanced activity off the east coast of Asia prior to onset, followed by a northward shift of the Pacific storm track, which lasts throughout the maintenance phase. The initial enhancement of synoptic-scale eddy activity is associated with a large-scale cyclonic anomaly that develops over Siberia several days prior to the onset of the main anticyclonic anomaly over the central North Pacific. The observations, however, show considerable interdecadel variability in the details of the composite onset behavior; it is unclear whether this variability is real or whether it reflects differences in the data assimilation systems.

The role of the time mean flow and synoptic-scale eddies in the development of the persistent Pacific anomalies is studied within the context of a kinetic energy budget in which the flow is decomposed into the time-mean, low-frequency (timescales longer than 10 days), and synoptic (timescales less than 6 days) components. The budget, which is carried out for the simulation at 500 mb, shows that the initial growth of the persistent anticyclonic anomalies is associated with barotropic conversions of energy, with approximately equal contributions coming from the mean flow and the synoptic-scale eddies. After onset the barotropic conversion from the mean flow dominates, whereas the decay phase is associated with baroclinic processes within the low-frequency flow.

Abstract

The nature of low-frequency variations in synoptic-eddy activity over the North Pacific is examined in a general circulation model (GCM). A comparison with observations reveals that the GCM produces realistic time mean and low-frequency synoptic-eddy forcing of the 200-mb zonal wind. In the time mean, this forcing, which is computed as the divergence of the extended Eliassen–Palm (E–P) flux, shows an east-west dipole structure that tends to reduce the zonal wind over the western North Pacific and tends to enhance it to the east. This structure is consistent with the general picture of the life cycle of baroclinic waves, which show strong upward and eastward propagation in the western and central Pacific and meridional propagation to the east. The western and central Pacific synoptic-eddy forcing is dominated by the convergence of the baroclinic component of the E–P flux divergence, while over the eastern Pacific the divergence of the barotropic component is important. The dominant component of the low-frequency “envelope” (periods > 10 days) of synoptic-eddy forcing, computed as the first empirical orthogonal function (EOF) modulates the time mean synoptic-eddy forcing. This modulation is associated with low-frequency changes in the intensity of the synoptic-eddy activity and is only weakly tied to fluctuations in the low-frequency flow.

Composites of the hemispheric distribution of synoptic-eddy forcing in the GCM, based on the extremes of the dominant Pacific EOF, show a seesaw behavior with enhanced eddy forcing in the North Pacific basin associated with suppressed forcing in the North Atlantic basin, and vice versa. The link between the Pacific and the Atlantic basins appears to be due to the presence of eastward-traveling baroclinic wave packets that travel around the globe with a period of about 10 days. Some evidence is found for a similar behavior in the observations.

Abstract

Evidence is presented from a composite analysis of a 14-year general circulation model simulation, that persistent North Pacific (PNP) circulation anomalies during boreal winter are part of a larger-scale meridional development extending into the Tropics and the Southern Hemisphere. Lagged composites suggest that the development is initiated over the tropical Pacific by anomalous convection (characterized by an east-west dipole structure centered at the date line) one to two weeks prior to the extratropical onset time. Relatively weak wave trains. extending from the region of anomalous convection into the extratropics, appear to set the stage for the subsequent rapid development of the PNP anomalies. After onset, the PNP anomalies extend into the Tropics and enhance moisture transports that tend to supply moisture to, and thus reinforce, the associated tropical precipitation anomalies. The mature stage is characterized by a strong coupling between hemispheres, including twin low-level cyclonic (anticyclonic) circulations straddling the equator with westerly (easterly) wind “bursts” on their equatorward flanks. The tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations reminiscent of the Madden–Julian oscillation.

Results from a similar composite analysis of a shorter (5 year) assimilated atmospheric dataset are generally consistent with the simulated results, despite the substantially smaller sample size. The assimilation, however, positions the tropical heating dipole farther west, in better agreement with previous observational studies of intraseasonal tropical extratropical teleconnections. As a consequence. the pre-onset extratropical “response” to the tropical anomalies in the simulation has significant phase errors. The remarkably similar evolution in the extratropics after onset suggests that the tropical forcing acts primarily as a catalyst for the development of the PNP anomalies and that the most useful predictors of PNP events may lie not in the extratropics but in the tropical western and central Pacific.

Abstract

Changes in observed daily precipitation over the conterminous United States between two 30-yr periods (1950–79 and 1980–2009) are examined using a 60-yr daily precipitation analysis obtained from the Climate Prediction Center (CPC) Unified Raingauge Database. Several simple measures are used to characterize the changes, including mean, frequency, intensity, and return period. Seasonality is accounted for by examining each measure for four nonoverlapping seasons. The possible role of the El Niño–Southern Oscillation (ENSO) cycle as an explanation for differences between the two periods is also examined. There have been more light (1 mm ≤ P < 10 mm), moderate (10 mm ≤ P < 25 mm), and heavy (P ≥ 25 mm) daily precipitation events (P) in many regions of the country during the more recent 30-yr period with some of the largest and most spatially coherent increases over the Great Plains and lower Mississippi Valley during autumn and winter. Some regions, such as portions of the Southeast and the Pacific Northwest, have seen decreases, especially during the winter. Increases in multiday heavy precipitation events have been observed in the more recent period, especially over portions of the Great Plains, Great Lakes, and Northeast. These changes are associated with changes in the mean and frequency of daily precipitation during the more recent 30-yr period. Difference patterns are strongly related to the ENSO cycle and are consistent with the stronger El Niño events during the more recent 30-yr period. Return periods for both heavy and light daily precipitation events during 1950–79 are shorter during 1980–2009 at most locations, with some notable regional exceptions.

Abstract

The authors have documented the relationship between tropical convection and precipitation regimes in the western United States. Circulation patterns associated with precipitation regimes are described and physical mechanisms are proposed. Contributions from intraseasonal and interannual bands are examined.

When enhanced convection is located in the western Pacific, dry conditions in the Southwest (SW) and wet conditions in the Pacific Northwest (PNW) are observed. Fluctuations in both intraseasonal and interannual bands contribute to the rainfall variability. Enhanced convection in the western Pacific is accompanied by suppressed convection in the central Pacific. The associated Rossby wave vorticity source (S) anomalies keep the Pacific jet west of 150°W. A westward shift of the storm track to the North Pacific also contributes to dry conditions in the SW and wetness in the PNW. When enhanced tropical convection is located near 150°E, substantial contributions to outgoing longwave radiation anomalies are from fluctuations in the intraseasonal band. A wave train extends from the convective area in the Tropics to North America, where negative 200-hPa streamfunction anomalies are consistent with wetness in California and dry conditions in the PNW.

When tropical convection is enhanced in the central Pacific from the date line to 135°W, most contributions are from the interannual band. The positive S anomalies associated with an enhanced local Hadley cell extending from the North Pacific to California are responsible in part for the eastward shift of the subtropical jet. The storm track moves southeast and is consistent with wet conditions in the SW.

Abstract

Relationships between sea surface temperatures (SSTs) in the Gulf of California (GoC) and moisture surge events during the North American Monsoon Experiment (NAME) were examined using the multiplatform-merged daily SST data and the final NCEP North American Regional Reanalysis.

When tropical storms pass through the southern end of the gulf, they often trigger moisture surge events. When surge events pass through the gulf, SSTs in the GoC decrease. Evidence indicates that changes in SSTs in the GoC are caused by the surge events associated with tropical storms.

The intensification of surface winds and changes in humidity and temperature near the surface associated with surge events increase evaporation and decrease the latent heat entering the gulf. Rainfall originating from the peaks and foothills of the Sierra Madre Occidental (SMO) propagates westward into the GoC. Cloudiness increases over the GoC during the surge events, preventing shortwave radiation from reaching the oceanic surface. The net radiation entering the ocean reduces, and less heat into the ocean leads to the cooling of SSTs in the gulf.

Abstract

An alert classification system for the ENSO cycle is introduced. The system includes watches, advisories, and a five-class intensity scale for warm and cold phases of the ENSO cycle. A watch is issued when conditions are favorable for the formation of an El Niño or La Niña within the next 6 months. An advisory is issued when El Niño or La Niña conditions are present, based on NOAA’s operational definitions. The intensity scale, referred to as the ENSO Intensity Scale (EIS), is used for operational and retrospective assessments of the intensity of warm (El Niño) and cold (La Niña) episodes, without being prescriptive concerning ENSO-related anomalies or impacts. The Climate Prediction Center’s (CPC’s) monthly Climate Diagnostics Bulletin and ENSO Diagnostic Discussions will serve as the primary vehicles for disseminating real-time information concerning the ENSO alert status to the scientific community and public at large. An objective method that relates the EIS to anomalies is used to assess the effects of warm and cold episodes. The method is illustrated using precipitation in the global Tropics and subtropics and in the conterminous United States. The methodology is quite general and can be used to relate the ENSO cycle to other quantities.