11:00 AM | Global sea ice areal extent running above normal and experiencing a recent spike

[Global sea ice anomaly versus the 1979-2008 mean; recent spike pushes sea ice areal extent to nearly 1 million square kilometers above the norm)

Discussion

Overall summary The Great Lakes has received a lot of attention lately for its tremendous build-up of ice this winter – and it still stands at historically high levels for ice coverage (below) for this time of year - but the northern hemisphere as a whole remains at below-normal levels for sea ice areal extent by about 566,000 square kilometers (versus 1979-2008 mean). On the other hand, the southern hemisphere sea ice areal extent continues to be at or near record high levels for this time of year at around 1,342,000 square kilometers above the norm and this has boosted global sea ice to above-normal levels on the order of 1 million square kilometers. In fact, there has actually been a spike in recent weeks (above) with respect to global sea ice areal extent to these levels which have been seen only rarely in the past several years.

[Great Lakes ice coverage remains above 50%; well above all years back to the winter season of 1980/1981]

Northern Hemisphere Sea Ice The northern hemisphere sea ice areal extent is still below-normal for this time of year although it has gained significantly in the past several weeks relative-to-normal and remains well above the lowest points of the past few years. The northern hemisphere sea ice areal extent has generally trended lower since the mid 1990’s to mostly below-normal levels since the turn of the century. In the past several years, however, there has been a leveling off of the trend line in terms of sea ice areal extent at those below-normal levels. In the time period before the mid 1990’s, the sea ice extent was generally above-normal dating back to 1979.

This directional change in trend that developed during the mid 1990’s correlates quite well with a northern Atlantic Ocean sea surface temperature cycle that is tracked by meteorologists through an index called the Atlantic Multidecadal Oscillation (AMO). Indeed, the Atlantic Ocean has a significant impact on northern hemisphere sea ice and the AMO index flipped in phase during the mid 1990’s from negative (cold) to positive (warm) and the trend changed at that point in time. Once the northern Atlantic Ocean sea surface temperatures flip back to cooler-than-normal values – perhaps in the next few years - the northern hemisphere sea ice areal extent should return to the normal or above-normal levels seen prior to the mid 1990’s.

Southern Hemisphere Sea Ice The southern hemisphere sea ice areal extent continues its recent impressive run at record or near record high levels for this time of year when compared to all prior years in the satellite record-keeping era which began in 1979. This remarkable period of increasing sea ice areal extent in this part of the world has actually been occurring for the past few years with only a few brief exceptions to that overall upward trend. Back in 2011, the southern hemisphere sea ice areal extent was at below-normal levels, but it is currently running well above-normal at levels seen only a couple of times dating back to the late 1970’s (all data courtesy University of Illinois "cryosphere" web site with data originating from NOAA/NCEP Snow and Ice Data Center).

The video discussion (below) details the current ice coverage situation in the Great Lakes, the northern and southern hemispheres and across the globe.

Video

httpv://youtu.be/Vwtt3SjMOFw

1:30 PM | Tornado threat increases tomorrow in the mid-Mississippi Valley, but so far the numbers this year are way down

Discussion

Tomorrow, the mid-Mississippi Valley will have a threat for severe storms and tornadoes and it may very well be the greatest threat of tornadoes that we have seen all year; however, the overall nationwide numbers so far this year are way down and this continues a downward trend that began a few years ago. In fact, the number of nationwide tornadoes through the end of March as reported by NOAA’s Storm Prediction Center (SPC) was 70 and this is the lowest amount in the “January through March” time period of any of the past ten years. The average number of tornadoes for the first three months of the year are 243 (based on the period from 2005-2013). The yearly nationwide tornado totals for the last two years (943 in 2013, 1119 in 2012) were the lowest annual amounts of any of the past ten years according to NOAA’s SPC.

The overriding reason for the low number of tornadoes so far this year as well as for the beginning three months of last year has to do with the fact that the persistent cold and snowy weather pattern in much of the central and eastern U.S. basically squashed the threat for severe weather. A pronounced southward dip in the polar jet stream frequently pushed cold air masses into the Gulf of Mexico and this prevented deep, moisture-laden warm air from that region to flow northward into the southern U.S. – an important, and generally necessary, ingredient for the generation of tornadoes.

As far as tomorrow is concerned, a strong upper level trough will swing out of the western states into the middle of the country and combine with an influx of warm, moisture-rich air from the Gulf of Mexico to increase the chances for severe weather and tornadoes in the mid-Mississippi Valley region. Specifically, this general threat area on Thursday will likely run from northern Louisiana to Missouri with the greatest threat perhaps centered on the state of Arkansas. The threat area for severe weather will shift eastward on Friday into the Ohio Valley albeit in a slightly weakened state.

11:30 AM | ***Baseball is back and so is the "Home Run Weather" app!

Discussion

It has long been contemplated that weather conditions have an effect on the distance a baseball can travel. More home runs are seemingly hit on hot days or on days with the wind blowing out.

Who can forget the many games at Wrigley Field that have featured numerous home runs as the wind raced out towards Waveland Avenue? Mike Schmidt cranked four home runs on just such a day at Wrigley Field in April of 1976. That game featured nine home runs and 34 total runs and a wind blowing out strongly ahead of a cold front. Not surprisingly, most of the games with four home runs hit by an individual player have occurred with temperatures of at least 80 degrees or with a strong wind blowing out. Conversely, most people would agree that fewer home runs are hit on cold days or with the wind blowing in. The old Candlestick Park in San Francisco frequently offered such weather.

The distance that a baseball travels is indeed impacted by atmospheric conditions. In general, the less dense the air is, the farther a baseball can travel. Humidity plays a crucial role in air density. Air with higher humidity is actually less dense than drier air. This may be contrary to perception and many baseball fans have no doubt heard baseball announcers incorrectly use the phrase “heavy humid air” on a hot summer night. Dry air is mostly comprised of diatomic oxygen and nitrogen (i.e. O2 and N2) whereas water vapor (H2O) is composed of one oxygen atom and two hydrogen atoms and the moist air has a lower overall atomic mass than dry air. Thus, at a constant temperature, the more water vapor that displaces the other gases, the less dense the air will become.

Additionally, hot air is less dense than cold air and higher altitude air is less dense than air at sea level. It is for this reason that so many home runs were hit in Colorado before the humidor was put into place. The elevated humidity in the humidor that stores baseballs for the Rockies home games effectively reduces the distance that a ball will travel in multiple ways: 1) by adding slightly to its weight through absorption of water, 2) by causing the size of the ball to increase slightly which increases air drag and 3) by reducing its “bounciness” factor.

Utilizing sophisticated math and physics, meteorologists and software engineers at The SI Organization, Inc. have investigated this topic which mixes science and baseball. And the results of their efforts are available in a free, real-time baseball weather application called Home Run Weather. Developed for the iPhone and Android devices, the app relates live temperature, atmospheric pressure, humidity, field orientation, wind direction, wind speed and the drag coefficient of a baseball to the user to determine if local weather conditions, for any big league park, are favorable for home runs being hit. Twenty-four hour forecasts are available in addition to live weather.

Two approaches were combined to arrive at a “home run favorability” index, which the app displays on a scale of 0 to 10 (least-to-most favorable). The first approach was to analyze actual weather conditions and home run data over several seasons (Citizens Bank Park was chosen as the venue for this study). The second approach uses a theoretical, physics-based model that determines the distance a ball will travel based on the temperature, relative humidity and atmospheric pressure.

The Citizens Bank Park study yielded some interesting, but perhaps not too surprising, findings. First, temperatures and dew points had a clear trend line relationship with home runs, as generally more home runs were hit in hot and humid air than colder, less-humid air. It was also found that 13-percent more home runs were hit when the wind was blowing out than other wind conditions. Additionally, 6-percent more home runs were hit in the daytime as compared to nighttime games.

A full major league season of testing on the app index produced very encouraging results with respect to the average number of home runs hit per game. The low home run favorability index values (0-3) had an average of 1.38 home runs per game for the full season. The moderate index values (4-7) had an average of 1.95. And the high values (8-10) had an average of 2.47 home runs per game. In addition, the home run favorability index correlated very well with average total runs scored per game. The low home run favorability index values (0-3) had an average of 7.07 total runs per game for the season. The moderate index values (4-7) had an average of 8.08. And the high values (8-10) had an average of 9.45 total runs per game.

Whether you're a fan at the park who's interested in a home run forecast, a spirited fantasy owner or baseball analyst, Home Run Weather should provide some useful information and conversation material. For more details, visit the "Home Run Weather" page on The SI Weather website (just click on the baseball).

3:00 PM | An amazing Atlantic Ocean storm!!

Discussion

One of the worst storms in years continues to pound away at eastern New England and the Canadian Maritime provinces with blizzard conditions and exceedingly high waves in some sections. Winds have been clocked as high as 101 mph earlier this afternoon at a buoy some 20 miles off the Maine coastal town of Jonesport. Earlier in the day, there were wind gusts as high as 82 mph in Nantucket, Massachusetts along with wind-whipped heavy snow of up to 10 inches. These wind gusts may have been the highest recorded in Nantucket since the superstorm of March 1993. Waves as high as 45 feet are beginning to batter Nova Scotia, Canada and its capitol city, Halifax, is all but shut down for the day. This storm intensified at quite astonishing rates from late last night into this morning with an amazing drop in atmospheric pressure of 43 millibars in less than 24 hours. This easily meets the criteria of rapid intensification known to crazy meteorologists as “bombogenesis” which only requires a drop of 24 millibars in 24 hours. The latest pressure reading of the storm is around 964 millibars (and still falling) and this is equivalent to a category 3 hurricane.

2:00 PM | Stratospheric warming and the MJO: two separate signals that are far apart on our planet suggest the colder-than-normal weather will continue in the central and eastern U.S. right into April

Discussion

Overview It looks like our colder-than-normal weather of recent months in the Mid-Atlantic region will continue into the month of April. Two separate signals that are far apart on the planet suggest cold weather will indeed continue in the central and eastern U.S. right into the month of April. The first signal is coming from a tropical disturbance known as the Madden Julian Oscillation (MJO) and the second signal is coming from the highest part of the atmosphere over the North Pole.

Madden Julian Oscillation (MJO)

Background information on the MJO The MJO is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is a large-scale coupling between atmospheric circulation and tropical deep convection. The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics. Furthermore, the MJO influences both precipitation and surface temperature patterns across the US. Specifically, one significant impact of the MJO over the U.S. during the northern hemisphere winter is an increase in the frequency and intensity of cold air outbreaks across the central and eastern US.

MJO Phases Research has found that the location of the MJO, or phase, is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram illustrates the progression of the MJO index through different phases, which generally coincide with locations along the equator around the globe. When the index is within the center circle, the MJO is considered weak, meaning it is difficult to discern. Outside of this circle, the index is stronger and will usually move in a counter-clockwise direction as the MJO moves from west to east. The very latest European model MJO index forecast propagates the MJO from its current "phase 1" location in "phases 2 and 3" as we progress through the remainder of the month of March (follow green line in figure above). Phases 2 and 3 for the MJO index typically signal colder-than-normal temperatures this time of year in the central and eastern U.S. (see circled areas below in "phases 2 and 3").

Sudden Stratospheric Warming (SSW)

Overview Another way to monitor the potential for Arctic air outbreaks in the northern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events in the region of the North Pole have been found to set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from central Canada into the central and eastern U.S. Indeed, there appears to be a significant stratospheric warming event in progress right now over the North Pole that could prolong winter-like conditions across the central and eastern U.S. as we end with March and begin the new month of April (current stratospheric temperature pattern in figure below centered on the North Pole).

SSW Consequences During the winter months in the lower polar stratosphere, temperatures on average are below minus 70 degrees Celsius. The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50 degrees in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. In recent SSW events, the polar vortex has split into two pieces and that opened the floodgates for Arctic air to move southward. In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes. This doesn’t mean that each and every day following an SSW event will be below normal as that will not be the case. However, it does suggest that, based on historical similarities, we could be looking at an overall below-normal temperature pattern in the central and eastern U.S. continuing right into the month of April.

2:00 PM | An upcoming polar stratospheric warming event could prolong winter-like conditions in the central and eastern U.S.

Discussion

One of the ways to monitor the potential for Arctic air outbreaks in the northern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events in the region of the North Pole have been found to set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from central Canada into the central and eastern U.S. Indeed, there appears to be a significant stratospheric warming event in the offing over the next 10 days or so (above) centered over the North Pole that could prolong winter-like conditions across the central and eastern U.S. as we progress through March and into the month of April.

During the winter months in the lower polar stratosphere, temperatures on average are below minus 70 degrees Celsius. The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50 degrees in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. In recent SSW events, the polar vortex has split into two pieces and that opened the floodgates for Arctic air to move southward. In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes. The entire process from the initial warming of the stratospheric at high latitudes to the cooling in the troposphere at middle latitudes can take several weeks to unfold. This doesn’t mean that each and every day following an SSW event will be below normal as that will not be the case. However, it does suggest that, based on historical similarities, we could be looking at an overall below-normal temperature pattern in the central and eastern U.S. continuing well into the month of April. Indeed, the very latest NCEP Couple Forecast System (CFS) temperature anomaly forecast (below) for the month of April is colder-than-normal for much of the central and the eastern U.S.

1:15 PM | The coldest December/January/February in 35 years for the contiguous U.S. and March is off to an amazingly cold start

[Niagara Falls has frozen over for the second time this winter; courtesy Reuters]

Discussion

December through February The 3-month winter period of December, January and February was the coldest in the last 35 years across the contiguous United States as measured by the U.S. Historical Climatology Network (USHCN). In fact, according to USHCN data, this was the 10th coldest 3-month winter period ever for the contiguous U.S. going back to the late 1800’s (below). The last time nationwide temperatures averaged this low in the December, January and February time frame was during the winter of 1978-1979 which happened to immediately follow two other very cold winters of 1976-1977 and 1977-1978. The cold weather this winter season has been the most dramatic “relative-to-normal” across the Great Lakes and Upper Midwest with some impressive results. For example, Chicago registered its 3rd coldest winter ever in the 3-month time period of December through February with the most days ever having its low temperature at or below zero. Additionally, the ice cover extent on the Great Lakes is at a record high for the month of March and quite close to the all-time record high. Finally, a rarity has occurred at Niagara Falls where the water has frozen over for a second time this winter season (above).

March The month of March has begun in much the same fashion as its three preceding months – namely, much colder-than-normal in much of the U.S. There have been numerous all-time record low temperatures set for the month of March that include the following:

-Atlantic City, New Jersey at 2 degrees (3/4) -Dover, Delaware at 6 degrees (3/4) -Charlottesville, Virginia at 1 degrees (3/4) -Dulles Airport, Virginia at -1 degrees (3/4) -Baltimore, Maryland at 4 degrees (3/4) [this broke the March record that held for the city of Baltimore since 1873] -Kansas City, Missouri at -3 degrees (3/3)

In addition, the following locations have experienced their coldest days ever in the month of March (i.e., the coldest high temperature in March):

-Little Rock, Arkansas at 28 degrees (3/3) -International Falls, Minnesota at -9 degrees (3/1) -Erie, Pennsylvania at 9 degrees (3/3) -Kansas City, Missouri at 5 degrees (3/2)

Finally, the snow cover across the Lower 48 states according to NOAA's National Ice Center is at ~54% which is the highest level in 10 years at this late date in the winter season.

Detailed description of the USHCN data The United States Historical Climatology Network (USHCN) is a high-quality data set of daily and monthly records of basic meteorological variables from 1218 observing stations across the 48 contiguous United States. Daily data include observations of maximum and minimum temperature, precipitation amount, snowfall amount, and snow depth; monthly data consist of monthly-averaged maximum, minimum, and mean temperature and total monthly precipitation. Most of these stations are U.S. Cooperative Observing Network stations located generally in rural locations, while some are National Weather Service First-Order stations that are often located in more urbanized environments. The USHCN has been developed over the years at the National Oceanic and Atmospheric Administration's (NOAA) National Climatic Data Center (NCDC) to assist in the detection of regional climate change. Furthermore, it has been widely used in analyzing U.S. climate. The period of record varies for each station. USHCN stations were chosen using a number of criteria including length of record, percent of missing data, number of station moves and other station changes that may affect data homogeneity, and resulting network spatial coverage.

1:10 PM | More on the incredible cold winter weather including the latest on the Great Lakes record ice cover extent and some dazzling winter phenomena

[Ice caves at "Apostle Islands National Lakeshore" in northern Wisconsin (Lake Superior); photo taken by Brian Peterson, Minneapolis Star Tribune, via Associated Press]

Discussion

The incredible winter cold and the Great Lakes This winter has brought some incredible cold to much of the eastern two-thirds of the nation, but nowhere has it been more persistent than in the Upper Midwest/Great Lakes region of the country. As a result, the Great Lakes have now reached a record ice cover extent for this late in the season at 90.5% (as of 3/2). The all-time record for ice cover on the Great Lakes is 95% set back in February 1979 (records date back to 1973). That all-time record is certainly within reach as brutal cold air continues to dominate the scene in the eastern two-thirds of the country which will very likely contribute to additional icing on the Great Lakes. In fact, the average 2-meter temperature this morning at 8AM (ET) across the continental U.S. was 19.9°F which may very well be the coldest reading ever this late in the season (source WeatherBell Analytics using NCEP RTMA 2.5 km analysis data).

Currently, Lake Ontario has the least ice cover of the five lakes at 38.2%, but that is more than three times its normal, and it has increased dramatically in recent days. Ontario has a reputation for never freezing as it quite deep so it retains heat longer than the other four lakes. Also, the Niagara River feeds water into Lake Ontario from Lake Erie, providing agitation which keeps the water's surface from freezing. Here are the latest ice cover percentages for all five lakes: Erie 95.4%, Superior 95.0%, Huron 94.5%, Michigan 90.1% and Ontario at 38.2%.

Dazzling winter weather phenomena One of the positive by-products of this bitter cold winter has been the fact that the extensive ice cover has allowed hikers to visit “ice caves” accessible by foot for the first times in many years. For example, tens of thousands of people in northern Wisconsin have taken advantage of this year’s deep freeze to hike across Lake Superior to visit the caves at the western end of the “Apostle Islands National Lakeshore” (see above). These caves were carved out of sandstone by the waves from Lake Superior, the largest by volume and surface area of all five Great Lakes. In addition, an amazing picture from near Mackinac Island, Michigan shows this "frozen" wave on Lake Huron (below).

Philly's chance at history Locally, the Philadelphia region has a chance to make some history late tonight with respect to the bitter cold Arctic air that is currently gripping the region. The record low for tomorrow’s date at Philly Int’l Airport is 7°F and the all-time low temperature for the month of March is 4°F – both within the realm of possibility. Many suburban locations to the north and west of the city will drop down to near 0 degrees by early tomorrow, but milder air will return by the end of the week with highs rebounding to the 40’s.

3:00 PM | The outlook for March - continued cold and snowy

Discussion

Overview It looks like our colder-than-normal weather pattern of recent months in the Mid-Atlantic region will continue on average right through the month of March. One signal that suggests cold weather will indeed continue in March in the eastern U.S. comes from a tropical disturbance known as the Madden Julian Oscillation (MJO). In addition, support for a colder-than-normal month of March comes from a NOAA seasonal climate forecast model called the Coupled Forecast System version 2 (CFSv2).

Background information on the Madden Julian Oscillation (MJO) The MJO is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is a large-scale coupling between atmospheric circulation and tropical deep convection. The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics. Furthermore, the MJO influences both precipitation and surface temperature patterns across the US. Specifically, one significant impact of the MJO over the U.S. during the northern hemisphere winter is an increase in the frequency and intensity of cold air outbreaks across the eastern US.

MJO Phases Research has found that the location of the MJO, or phase, is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram illustrates the progression of the MJO index through different phases, which generally coincide with locations along the equator around the globe. When the index is within the center circle, the MJO is considered weak, meaning it is difficult to discern. Outside of this circle, the index is stronger and will usually move in a counter-clockwise direction as the MJO moves from west to east. The very latest European model MJO index forecast (below) propagates the MJO from phase 8 in early March to phases 1, 2 and 3. All of these particular phases of the MJO (i.e., 8, 1, 2 and 3) typically result in a colder-than-normal temperature pattern for the eastern US (see “temperature composites” figure centered on February-March-April).

CFSv2 The latest monthly forecast from NOAA’s Coupled Forecast Model (v2) strongly suggests the colder-than-normal weather pattern of recent months will continue (see below) throughout the eastern two-thirds of the nation. This model is updated on a daily basis and it has done a very good job in its temperature forecasts when reasonably close to event time as we are now with March only a few days away. As has been the case throughout the winter months, the core of the coldest air “relative to normal” will likely be centered from the Northern Plains to the Upper Midwest according to the model forecast, but an incredibly large area across much of Canada and the U.S. will be below-normal for the month of March.

How about snow As far as snow is concerned, the combination of the expected colder-than-normal temperatures along with a newly activated southern branch of the jet stream (e.g., California storms) will quite likely produce above-normal snowfall for the Mid-Atlantic region during the month of March. Philadelphia has actually had 3 straight months (December, January and February) featuring monthly snowfall totals in the top ten for the given month using records dating all the way back to the 1880’s. The 3 consecutive months of top ten monthly snowfall amounts has not happened in Philly since January-February-March of 1978. There has never been a winter with 4 consecutive months ending up in the top ten for that given month so if it happens in March - certainly a possibility given the overall pattern - it would be a first for Philadelphia. As far as seasonal totals are concerned, Philly is in 3rd place on their all-time list some 20 inches shy of the seasonal record set in the winter of 2009-2010 with 58.4 inches so far this season - this record of 78.7" is also within reach given the current overall pattern.

11:00 AM | The California Drought and the Role of the Pacific Ocean

Discussion

Overview Solar and oceanic cycles are the most important drivers of all weather and climate on our planet and the most important ocean of all is the Pacific. This large body of water covers about a third of the planet’s surface and is bigger than all of the Earth’s land masses combined. Both the Pacific and Atlantic Oceans go through sea surface temperature phases that are characterized as cool (negative) and warm (positive). While there are other important factors that contribute to drought conditions in the western U.S. (e.g., hours of sunshine, humidity levels), the given temperature phase of the Pacific Ocean has historically been found to be a major factor in precipitation trends just as it has contributed greatly to global temperature trends. Indeed, the on-going California drought appears to be a natural consequence of the current cool phase of the Pacific Ocean which typically produces an overall dry weather pattern in the western U.S. and, if history is any guide, it is likely to persist for years to come.

Pacific Decadal Oscillation (PDO) The Pacific Decadal Oscillation (PDO) is a climate index based upon long-term patterns of variation in sea surface temperature of the north Pacific. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of about 20°N. During a warm (positive) phase, the west Pacific becomes cool and part of the eastern ocean warms; during a cool (negative) phase, the opposite occurs. These phases result from the direction of winter winds in the northern Pacific: winter winds blowing chiefly from the southwest result in warmer conditions in the northern California Current (CC); conversely, when winds blow primarily from the north, upwelling occurs both in the open ocean and at the coast, leading to cooler conditions in the northern CC.

The PDO temperature phase in the north Pacific tends to have an impact on the shorter-term sea surface temperature cycles of the tropical Pacific in the equatorial region. Specifically, warm phases of the PDO are generally associated with stronger and more numerous El Nino (warmer-than-normal) events in the tropical Pacific and weaker and fewer La Nina (colder-than-normal) episodes. During cold phases of the PDO, La Nina tends to dominate El Nino in the tropical Pacific. In the past several years, during the current cool phase of the PDO, La Nina conditions have indeed dominated the scene in the tropical Pacific region.

Warm and cool phases of the PDO can persist for decades, usually about 20 to 30 years. A warm phase occurred from 1925 to 1946, a cool phase from 1947 to 1976, and then another warm phase from 1977 to 1998. Currently, the Pacific Ocean is in a cool PDO phase that began around the turn of the century and the recent years of 2008 and 2012 actually exhibited the most negative PDO index values since the 1950’s [PDO Index plot below].

PDO Index plot: Time series of shifts in sign of the Pacific Decadal Oscillation (PDO), 1925 to present. Values are averaged over the months of May through September. Red bars indicate positive (warm) years; blue bars negative (cool) years. Note that 2008 and 2012 were the most negative values recorded since 1956.

The PDO and precipitation anomalies and trends Precipitation anomaly charts are shown below for the U.S. with primarily wetter-than-normal conditions seen on a nationwide basis during the warm PDO phase between 1977 and 1998 (top) and generally drier-than-normal conditions experienced during the cold PDO phase that lasted from 1947 to 1976 (bottom). Note the significant dry weather pattern seen across California during the last cold PDO phase with precipitation nearly two inches below the long term average in much of the state.

Precipitation trends in the southwestern U.S. have been shown to have direct correlation to PDO phases in the Pacific Ocean. The plot below displays the PDO index (red) versus the annual precipitation amounts in the southwestern U.S. (blue) all the way back to the year 1900 [source WSI Energy Weather]. In general, when the PDO was in a warm phase, the annual precipitation amounts in the southwestern U.S. tended to climb and in times of a cold PDO phase they typically dropped. This is supporting evidence that the on-going California drought is likely to continue on average as long as the Pacific Ocean’s cool PDO phase persists.

[credit to WSI Energy Weather]