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Daniel R. Cayan

Abstract

Empirical relationships between the sea surface temperature (SST) and surface air temperatures (SAT) are examined on monthly, seasonal and annual time scales for Marsden square areas in the North Pacific and the North Atlantic. On these time scales SST and SAT have roughly the same variance throughout the sample region. They are well correlated (contemporaneously) with warm seasons and months having slightly higher correlations than cold ones. For the most part, the spatial patterns and temporal changes in these statistics are similar between the North Atlantic and North Pacific.

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Daniel R. Cayan

Abstract

An important part of the water supply in the western United States is derived from runoff fed by mountain snowmelt Snow accumulation responds to both precipitation and temperature variations, and forms an interesting climatic index, since it integrates these influences over the entire late fall-spring period. Here, effects of cool season climate variability upon snow water equivalent (SWE) over the western part of the conterminous United States are examined. The focus is on measurements on/and 1 April, when snow accumulation is typically greatest. The primary data, from a network of mountainous snow courses, provides a good description of interannual fluctuations in snow accumulations, since many snow courses have records of five decades or more. For any given year, the spring SWE anomaly at a particular snow course is likely to be 25%–60% of its long-term average. Five separate regions of anomalous SWE variability are distinguished, using a rotated principal components analysis. Although effects vary with region and with elevation, in general, the anomalous winter precipitation has the strongest influence on spring SWE fluctuations. Anomalous temperature has a weaker effect overall, but it has great influence in lower elevations such as in the coastal Northwest, and during spring in higher elevations. The regional snow anomaly patterns are associated with precipitation and temperature anomalies in winter and early spring. Patterns of the precipitation, temperature, and snow anomalies extend over broad regional areas, much larger than individual watersheds. These surface anomalies are organized by the atmospheric circulation, with primary anomaly centers over the North Pacific Ocean as well as over western North America. For most of the regions, anomalously low SWE is associated with a winter circulation resembling the PNA pattern. With a strong low in the central North Pacific and high pressure over the Pacific Northwest, this pattern diverts North Pacific storms northward, away from the region. Both warm and cool phases of El Niño-Southern Oscillation tend to produce regional patterns with out-of-phase SWE anomalies in the Northwest and the Southwest.

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Daniel R. Cayan

Abstract

The influence of the atmospheric circulation on monthly anomalies of ocean surface latent and sensible heat fluxes is explored. The fluxes are estimated using bulk formulas applied to a set of about four decades of marine observations over 1946–1986. Monthly averaging over 5° “square” reduces errors contained in individual observations. The focus is on behavior of the flux anomalies over the relatively well-sampled North Atlantic and North Pacific ocean during winter, when the latent and sensible components are large and the incoming shortwave radiative flux is low.

In the North Atlantic and North Pacific (north of about 15°N), flux anomalies are partially caused by local variations in the monthly mean wind direction. In these extratropical regions, largest positive anomalies occur during northerly to northwesterly winds in response to advection of humidity and temperature from north to south and also to favored directions experiencing strong wind speeds. In the tropics, there is little relationship between the direction and the latent and sensible flux anomalies, since horizontal gradients of humidity and temperature are weak and the wind direction is relatively steady.

The most convincing connection between the wind and the flux anomalies is not local, but rather has basin scales associated with the monthly atmospheric circulation. In the North Atlantic and North Pacific, dominant atmospheric circulation modes, represented as empirical orthogonal functions of the sea level pressure (SLP) anomaly, have systematic patterns of the anomalies of wind speed (w), surface saturation humidity–air humidity difference (Δq), and sea surface temperature–air temperature difference (ΔT); these produce large-scale patterns in the latent and sensible fluxes. In the extratropics, a major negative SLP anomaly tends to have positive w anomalies to its south and negative w anomalies to its north, while Δq (and ΔT) anomalies lie to the west and east of the low, apparently because of meridional air advection. The resultant flux anomalies are shifted meridionally and zonally about the SLP centers, with enhanced sea-to-air fluxes to the southwest and diminished fluxes to the east of an anomalous low. Regions of increased monthly mean fluxes tend to have larger than normal intramonthly variability in the fluxes. Months with strong monthly atmospheric circulation anomalies frequently exhibit combined latent and sensible flux anomalies with magnitudes exceeding 50 W m−2 over several hundred kilometer regions.

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Daniel R. Cayan
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Daniel R. Cayan

Abstract

A part of the large-scale thermodynamic forcing of the upper ocean is examined by relating monthly anomalous latent and sensible heat flux to changes in sea surface temperature (SST) anomalies over the North Atlantic and North Pacific. The fluxes are estimated using bulk formulas from a set of about four decades of marine observations from the COADS dataset from 1946 to 1986. Monthly anomalies are constructed by removing the long-term monthly means. The latent and sensible flux anomalies are strongly correlated over most of the ocean, so they are considered together as a sum.

The heat flux estimates contain large spatial-scale anomalies consistent with both atmospheric circulation anomalies and with month-to-month changes (tendencies) in monthly SST anomalies. The monthly flux anomalies and the SST anomaly tendency are significantly correlated over much of the oceans, with anomalous positive/negative fluxes associated with anomalous cooling/warming. The connection between the flux and the SST tendency anomalies is strongest in the extratropics during the cool season when the latent and sensible fluxes and their variability are greatest, and the radiative fluxes are weakest.

While the heat flux forcing of the SST anomalies operates locally, the flux and SST tendency anomalies are organized over spatial scales that often span major portions of the North Atlantic and North Pacific. For each basin, canonical correlations expose large-scale, collocated anomaly patterns in the two fields. These patterns reflect the control exerted by the large-scale atmospheric circulation, inferred from sea level pressure (SLP) modes. Evidence for this result is the strong similarity in the configuration of anomalous flux and SST tendency patterns in their association with major SLP modes. Typical flux anomalies of 50 W m−2 are associated with monthly SST anomaly changes of order 0.2°C. The surface-layer thickness inferred from a simplified model relating the flux anomalies to the temperature anomalies of a slab ocean is consistent in magnitude and seasonal cycle with the observed mixed-layer depth in middle latitudes.

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Daniel R. Cayan
and
John O. Roads

Abstract

Monthly accumulations of area-averaged precipitation along the West Coast of the United States are related to estimates of local circulation parameters. The annual cycle as well as anomalous components of these quantities are compared. A strong annual cycle in most of the circulation parameters reflects the influence of the large-scale circulation on the annual variation of the precipitation field. For the anomalous monthly components, especially in winter, high correlations are found between precipitation and sea-level pressure or 70 kPa height. Other circulation parameters are also significantly correlated with the precipitation. These include the zonal and meridional wind components and the advection of relative vorticity.

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Jessica D. Lundquist
and
Daniel R. Cayan

Abstract

The diurnal cycle in streamflow constitutes a significant part of the variability in many rivers in the western United States and can be used to understand some of the dominant processes affecting the water balance of a given river basin. Rivers in which water is added diurnally, as in snowmelt, and rivers in which water is removed diurnally, as in evapotranspiration and infiltration, exhibit substantial differences in the timing, relative magnitude, and shape of their diurnal flow variations. Snowmelt-dominated rivers achieve their highest sustained flow and largest diurnal fluctuations during the spring melt season. These fluctuations are characterized by sharp rises and gradual declines in discharge each day. In large snowmelt-dominated basins, at the end of the melt season, the hour of maximum discharge shifts to later in the day as the snow line retreats to higher elevations. Many evapotranspiration/infiltration-dominated rivers in the western states achieve their highest sustained flows during the winter rainy season but exhibit their strongest diurnal cycles during summer months, when discharge is low, and the diurnal fluctuations compose a large percentage of the total flow. In contrast to snowmelt-dominated rivers, the maximum discharge in evapotranspiration/infiltration-dominated rivers occurs consistently in the morning throughout the summer. In these rivers, diurnal changes are characterized by a gradual rise and sharp decline each day.

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Daniel R. Cayan
and
Arthur V. Douglas

Abstract

Trends of surface temperature at rapidly growing urban sites during the last three to five decades are compared to those at non-urban sites, temperatures at 70 kPa, and sea surface temperature at a coastal Pacific station. Significant urban heat island effects have apparently taken hold, with urban-affected temperature increases of 1 to 2°C common over this period. In contrast, the trend of the non-urban records has been distinctly smaller over this period. The urban warming appears to be predominantly a nighttime phenomenon, with minimum temperatures displaying considerably more increase than maximum temperatures. No uniform seasonal preference for this increase emerged from these stations. Because of this increase, the distribution of observed temperatures shows a marked warm bias at several of the urban sites during recent years.

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Michael D. Dettinger
and
Daniel R. Cayan

Abstract

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year.

Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate cause of the trend toward warmer winters in California is a concurrent, long-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishable from natural atmospheric variability. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central North Pacific Ocean that is teleconnected to California temperatures. Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures.

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Shyh-Chin Chen
and
Daniel R. Cayan

Abstract

Behavior of regional precipitation and temperature over the West Coast of the United States was examined in a long perpetual winter simulation from a simplified global general circulation model. The model, a simplified version of the U.S. National Weather Service global operational forecast model, was run over a series of 568 winters, complete with geopotential, precipitation, and near-surface temperature. In spite of the fixed climatologica boundary conditions, the simulated winter-mean precipitation and temperature anomalies have a fairly realistic low-frequency regional variability. Both synoptic-scale events and seasonal average behavior are produced quite realistically by the model. Like observations, the regional surface variations can be related to the large-scale low-frequency circulation.

Four regional temperature/precipitation extreme—namely, warm/dry, cool/wet, cool/dry, and warm/wet—can be identified from the simulated winter-mean time series over the West Coast. Associated with these four regional extremes, model Northern Hemisphere 500-rnb height composites exhibit distinct planetary-scale circulation patterns. An empirical orthogonal function analysis further reveals that the first and third modes of the 500-mb height anomalies am primary contributors to these four regional extremes. The first mode largely governs the regional temperature variation, whereas the third mode largely determines the precipitation variation.

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