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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
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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

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

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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Alexander Gershunov and Daniel R. Cayan

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By matching large-scale patterns in climate fields with patterns in observed station precipitation, this work explores seasonal predictability of precipitation in the contiguous United States for all seasons. Although it is shown that total seasonal precipitation and frequencies of less-than-extreme daily precipitation events can be predicted with much higher skill, the focus of this study is on frequencies of daily precipitation above the seasonal 90th percentile (P90), a variable whose skillful prediction is more challenging. Frequency of heavy daily precipitation is shown to respond to ENSO as well as to non-ENSO interannual and interdecadal variability in the North Pacific.

Specification skill achieved by a statistical model based on contemporaneous SST forcing with and without an explicit dynamical atmosphere is compared and contrasted. Statistical models relating the SST forcing patterns directly to observed station precipitation are shown to perform consistently better in all seasons than hybrid (dynamical–statistical) models where the SST forcing is first translated to atmospheric circulation via three separate general circulation models and the dynamically computed circulation anomalies are statistically related to observed precipitation. Skill is summarized for all seasons, but in detail for January–February–March, when it is shown that predictable patterns are spatially robust regardless of the approach used. Predictably, much of the skill is due to ENSO. While the U.S. average skill is modest, regional skill levels can be quite high. It is also found that non-ENSO-related skill is significant, especially for the extreme Southwest and that this is due mostly to non-ENSO interannual and decadal variability in the North Pacific SST forcing.

Although useful specification skill is achieved by both approaches, hybrid predictability is not pursued further in this effort. Rather, prognostic analysis is carried out with the purely statistical approach to analyze P90 predictability based on antecedent SST forcing. Skill at various lead times is investigated and it is shown that significant regional skill can be achieved at lead times of several months even in the absence of strong ENSO forcing.

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Edwin Sumargo and Daniel R. Cayan

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This study investigates the spatial and temporal variability of cloudiness across mountain zones in the western United States. Daily average cloud albedo is derived from a 19-yr series (1996–2014) of half-hourly Geostationary Operational Environmental Satellite (GOES) images. During springtime when incident radiation is active in driving snowmelt–runoff processes, the magnitude of daily cloud variations can exceed 50% of long-term averages. Even when aggregated over 3-month periods, cloud albedo varies by ±10% of long-term averages in many locations. Rotated empirical orthogonal functions (REOFs) of daily cloud albedo anomalies over high-elevation regions of the western conterminous United States identify distinct regional patterns, wherein the first five REOFs account for ~67% of the total variance. REOF1 is centered over Northern California and Oregon and is pronounced between November and March. REOF2 is centered over the interior northwest and is accentuated between March and July. Each of the REOF/rotated principal components (RPC) modes associates with anomalous large-scale atmospheric circulation patterns and one or more large-scale teleconnection indices (Arctic Oscillation, Niño-3.4, and Pacific–North American), which helps to explain why anomalous cloudiness patterns take on regional spatial scales and contain substantial variability over seasonal time scales.

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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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Jerome Namias, Xiaojun Yuan, and Daniel R. Cayan

Abstract

North Pacific monthly sea surface temperature (SST) anomalies are more persistent than a first-order Markev process, often lasting for more than 5 months. Sea surface temperature persistence undergoes an annual cycle that is attributable to the depth of the surface mixed layer and to the annual cycle of focing. For a given lag, the pattern correlation is minimum when it involves SST during the summer months and maximum when it involves SST during the winter months. Average winter SST anomalies that have exhibited greatest persistence during the last four decades have been negative in the central North Pacific and positive along the West Coast but antipersistent SST anomalies have not confermed to a repeated pattern. The atmospheric 700 mb height anomalies associated with high persistence SST cases indicate that strong SST persistence is associated with long-lasting atmospheric anomaly patterns. For highly persistent January SST anamalies, 700 mb anomalies often last from December through February. The high persistence 700 mb anomalies tend to be negative over the east-central North Pacific and positive over North America, with strong teleconnections. This pattern translates to strengthened westerlies over the subtropics and weakened westerlies in middle latitudes across the North Pacific—a zonal wind profile that is nearly opposite to that which appeared in low persistency SST cases.

Over the four decades since 1947, North Pacific SST persistence has undergone substantial multiyear variability, and has increased significantly since the beginning of this record. Related low-frequency fluctuations as well as linear trends, have occurred in the zonal mean subtropical westerlies across the North Pacific and in related large-scale atmospheric indices, the PNA pattern and the Southern Oscillation Index.

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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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