NOAA's 33rd Climate Diagnostics and Prediction Workshop/CLIVAR Drought Workshop (CDPW) October 20-24, 2008 in Lincoln, Nebraska Hydrodynamics of the Caribbean Low-level Jet and its Relationship to Drought K.H. Cook and E.K. Vizy Jackson School of Geosciences University of Texas at Austin

Seen in the NARR domain, the CLLJ is a prominent feature, with the highest low-level zonal wind speeds in the domain July, 925 hPa, m/s Caribbean low-level jet (CLLJ)

Cuba Hispaniola Puerto Rico Venezuela Columbia Zonal Wind Speed July, 925 hPa, m/s

Seasonality of the CLLJ Zonal wind speed (m/s) Month U wind averaged 80-60W and 12-18N, 925 hPa

Seasonality of the CLLJ Zonal wind speed (m/s) Month U wind averaged 80-60W and 12-18N, 925 hPa 0 m/s to -10 m/s (easterly)

Seasonality of the CLLJ Zonal wind speed (m/s) Month U wind averaged 80-60W and 12-18N, 925 hPa Strongest in June/July Strong in DJF

Seasonality of the CLLJ Zonal wind speed (m/s) Month U wind averaged 80-60W and 12-18N, 925 hPa Weak in May Weakest in October

October through April CLLJ largely disconnected from the flow (and moisture) entering the US and northern Mexico May through September Caribbean and Great Plains low-level jet system

There are also significant seasonal differences in the elevation and vertical structure of the CLLJ.

Zonal wind cross section in latitude and height at 70W January May AprilMarch February June

Zonal wind cross section in latitude and height at 70W Note that the minimum in Sept/Oct at 925 hPa shown previously is partly due to this elevation change in boreal fall July DecemberNovember September August October

Diurnal Cycle July 925 hPa Local time at CLLJ region ~ GMT – 5 hr Daytime CLLJ ~ 2m/s weaker than the nighttime jet. Secondary minimum at 4 AM LT

Purpose What controls the CLLJ on seasonal and diurnal time scales? What is the relationship of the CLLJ to drought …. over the central U.S.? …. over central America and southern Mexico?

Dynamics of the CLLJ Examine the horizontal momentum balance to understand (1) Diurnal cycle of the jet (2) Differences between seasons Start with July ….

First-order v-momentum balance: geostrophy

Note the diurnal cycle of the height gradient, weakening through the night and building up during the day.

Late Night: 4 AM LT Afternoon: 4 PM LT The diurnal cycle of the meridional height gradient is caused by the diurnal cycle of heating over far northern South America.

This diurnal cycle in height gradient does not directly generate the CLLJ’s diurnal cycle. The diurnal cycle of the jet has a different shape and the jet weakens during the day. Instead, the diurnal heating and cooling over northern South America generates meridional acceleration.

Daytime: Land warms and geopotential heights fall. This strengthens the meridional geopotential height gradients and leads to onshore flow (southward, negative local acceleration). Night: Offshore flow and northward (positive) acceleration. This solenoid circulation sets up because of the space scales involved and because of the low latitude – Coriolis accelerations are relatively weak.

July, u-momentum equation

The July u-momentum balance is more complicated than the v-momentum balance The only small terms are meridional and vertical advection.

Strong zonal geopotential height gradients associated with low heights over Central America/ southern Mexico to the west and the NASH to the east.

Strong westward frictional accelerations associated with the easterly jet.

jet decelerates Should have the weakest jet speeds at ~ 3 PM, and a secondary minima at ~3 AM. (We saw this above.)

jet decelerates The minimum at ~ 3 PM occurs mainly because of the Coriolis force, but also because the zonal pressure gradient force weakens. The northerly off-shore flow anomaly is strongest at 10 AM, after which it begins to weaken.

jet decelerates The minimum at ~ 3 AM is related to a weakening of the zonal pressure gradient force.

How is the momentum balance different in other months, and what does this tell us about how the seasonal cycle works?

Magnitude GPLLJ Magnitude CLLJ Seasonality of the CLLJ Jet has its maximum in July (which we just examined), and has minima in May and Oct, with a secondary max in December/January

V-momentum balance is about the same as in July, i.e., to first order geostrophic. So we need to understand the seasonality of meridional height gradients.

For example, why are the meridional geopotential height gradients similar in magnitude and, especially, sign in January and July, and weak in May and October?

Even though the CLLJ is strong in both January and July, it has very different downstream connections in those months.

October through April CLLJ largely disconnected from the flow (and moisture) entering the US and northern Mexico May through September Caribbean and Great Plains low-level jet system

The u-momentum equation is different because the Coriolis force in January is negative, while it was (mostly) positive in July.

925 hPa geopotential heights and winds ONDJFMA MJJAS V < 0 V > 0

925 hPa geopotential heights and winds Difference

What is the relationship between the CLLJ and drought?

1. Is the CLLJ anomalous during dry years in the central US and/or Central America? 2. Is precipitation anomalous during strong or weak CLLJ years?

Examine the NARR for to identify dry years in these regions Central U.S, W and 30-50N, for JJA 1980, 1988, 2006 are anomalously dry

In June 1988, the CLLJ is quite weak at ~½ its climatological value. But not In 1980 and 2006.

In July 1980 the CLLJ is weak by ~¼ of its climatological value. But not in 1988 or 2006 – its even strong in 2006.

In August, the CLLJ is weak in 1980 and 1988, but strong in We don’t find a consistent relationship between the CLLJ and the 3 years of central US drought.

For S. Mexico/Central America? Only one year (2003) is identifiable as a drought year in the NARR.

1. Is the CLLJ anomalous during dry years in the central US and/or Central America? 2. Is precipitation anomalous during strong or weak CLLJ years?

Define a CLLJ Index 925 hPa u; JJA; W and N

Standard Deviations from the JJA mean zonal wind Stronger CLLJ Weaker CLLJ

Drought Years in the U.S. Stronger CLLJ Weaker CLLJ

Drought Year in Central America Stronger CLLJ Weaker CLLJ

Correlation between the CLLJ strength and JJA rainfall

Negative correlation at the 99% CI Positive correlation at the 99% CI

Correlation between the CLLJ strength and JJA rainfall Strongest, most coherent responses over land are along the Pacific coast of southern Latin America, over the Caribbean Islands, and the eastern Yucatan.

Correlation between the CLLJ strength and JJA rainfall There is not a significant correlation between the summer CLLJ strength and summer rainfall over the Central US

Maybe we need to look at smaller timescales to better understand ….

Define Strong and Weak CLLJ events on monthly timescale Monthly CLLJ Standardized Anomaly

Correlation between the monthly CLLJ index and monthly rainfall anomalies Strong positive correlation between springtime rainfall in the southeastern U.S. and the strength of the CLLJ

May precipitation (shaded; mm/day) Vertically integrated water vapor transport (x 10 5 kg/m) ~ 40% increase ~ 29% decrease

May 925 hPa Heights (m) & Wind (m/s)

Correlation of monthly CLLJ strength to monthly rainfall anomalies Strength of CLLJ and Central American rainfall are negatively correlated during the months when the CLLJ and the GPLLJ are connected (MJJAS) Hypothesis: Dry conditions in Central America lead to warming and an enhancement of the zonal geopotential height gradient, adding to the positive meridional flow over the Caribbean and further depriving the region of moisture.

Conclusions A solenoid circulation (land/sea breeze) is generated by diurnal heating and cooling over northern South America, and Coriolis forces acting on this meridional flow contribute to the diurnal cycle of the CLLJ. Meridional height gradients and the CLLJ are strong in January because of low heights over northern South America, and in July because of the position of the NASH. When Central America/southern Mexico warm in the spring, the resulting zonal height gradients introduce northerly flow across the western Caribbean and direct moisture flux into the Gulf of Mexico.

Central American rainfall is negatively correlated with the strength of the CLLJ during the months when the CLLJ and the GPLLJ are connected (MJJAS). There is a strong positive correlation between springtime rainfall in the southeastern U.S. and the strength of the CLLJ There is no significant, systematic correlation between the summer CLLJ strength and summer rainfall over the Central US (although the CLLJ was quite weak in the summer of 1988) Relationships between the CLLJ and Regional Rainfall

The End

On diurnal time scales - when the height gradient and Coriolis forces do not balance, meridional acceleration occurs. Non-linear terms (advection) and friction approximately balance, are smaller than the local acceleration term, and don’t explain diurnal variations.

Close up of the region showing the jet is related to the strong meridional geopotential height gradients between the northern coast of South America and the Caribbean Islands (Cuba, Hispaniola, Puerto Rico) Low over northern South America North Atlantic subtropical high, and its eastward extension (Bermuda high) Geopotential height lines pinch together, And the flow associated with these high Meridional gradients is the CLLJ

Difference between the drought ”climatology” (3 central US drought years) and the 29 year NARR climatology 900 hPa U JJA CLLJ is a lot more narrow and weaker in June, not much change in July; in all three months it is anomalously weak over the Yucatan/eastern Caribbean

So how consistent is this response in each of the 3 years identified as drought in the NARR time period?

Diurnal Cycle July 925 hPa Meridional wind has a very different diurnal signal. (Hint: geostrophic processes are not controlling diurnal variations.)

Coriolis force is playing a role in the diurnal cycle.

Strong westward frictional accelerations associated with the easterly jet. Coriolis force is playing a role in Contributing to

The diurnal cycle of the Coriolis force is related to the land/sea breeze. Note that the Coriolis force changes sign, consistent with the land-sea breeze super- imposed over a weak northward flow. Coriolis and zonal advection terms are comparable during much of the day.

Positive zonal acceleration, which decelerates the zonal jet, occurs from 10 PM until 3 AM, and resumes at about 5 AM until 2 PM. The jet begins to strengthen after 2 PM until 10 PM (du/dt<0), consistent with the max at 10 PM, Due to decreases in Coriolis (land/sea) Which are amplified by a decrease in Zonal advection and then supported after 1 PM by a strengthening of the zonal height Gradient due to the warming of the land To the west (Central America).

Jet strengthens

Related to sensible heating over central America In the early spring – maybe orographic effect? Heights fall over central America, setting up a zonal Geopotential height gradient and, by geostrophy, Positive (northward) meridional accelerations Draw boxes – 3D – showing balance of forces in each season In temperature u-momentum equation; maybe it will relate to Why the jet is stronger in July than in Jan – or whatever